Polyvinyl alcohol polymer, powder of polyvinyl alcohol polymer, additive for oil well cement, cement slurry, method for producing polyvinyl alcohol polymer, method for producing powder of polyvinyl alcohol polymer, and method for producing cement slurry

Abstract

Provided is a polyvinyl alcohol polymer having good fluid loss reduction performance under high temperature conditions.　The present invention provides: a polyvinyl alcohol polymer which is a saponified product of a copolymer of a raw material monomer containing a vinyl ester monomer and a compound A represented by formula (1); and a method for producing the polyvinyl alcohol polymer.

Description

Polyvinyl alcohol polymer, polyvinyl alcohol polymer powder, additive for oil well cement, cement slurry, method for producing polyvinyl alcohol polymer, method for producing polyvinyl alcohol polymer powder, and method for producing cement slurry

[0001] The present invention relates to a polyvinyl alcohol-based polymer, a polyvinyl alcohol-based polymer powder, an additive for oil well cement, a cement slurry, a method for producing a polyvinyl alcohol-based polymer, a method for producing a polyvinyl alcohol-based polymer powder, and a method for producing a cement slurry.

[0002] Oil well cement, used in cementing oil wells, gas wells, steam wells for geothermal power generation, etc., is filled into the gap between the steel pipe (casing) and the well to protect the steel pipe. The loss of water content from the cement slurry due to high pressure during injection and underground heat is generally referred to as "fluid loss." Because fluid loss impairs the fluidity of the cement slurry and its strength after hardening, a fluid loss reducing agent is usually added to the cement slurry.

[0003] One example of a known fluid loss reducing agent is an additive for oil well cement containing polyvinyl alcohol (hereinafter also referred to as PVA). In recent years, as shale gas wells in particular have been mined deeper, the temperature conditions have become more severe, and attempts have been made to use modified PVA derived from a copolymer obtained by copolymerizing a vinyl ester monomer with another polymer (Patent Document 1).

[0004] International Publication No. 2019 / 163490

[0005] <<First Aspect>> However, such modified PVAs have a variety of usage methods and conditions, and new modified PVAs are needed. For example, modified PVAs having a crosslinked structure derived from a polyfunctional comonomer are needed to achieve a fluid loss reduction effect. On the other hand, modified PVAs having a crosslinked structure tend to have high viscosity and poor mixability during preparation of cement slurry, and as a corresponding property, low viscosity aqueous solutions are sometimes needed.

[0006] The present invention has been made in view of the above circumstances, and provides a polyvinyl alcohol polymer whose aqueous solution has low viscosity and exhibits good fluid loss reducing properties under high temperature conditions.

[0007] <<Second Aspect>> However, there are various methods and conditions for using such modified PVA, and a new modified PVA is desired. For example, an aqueous solution of the modified PVA may be required to have a high viscosity.

[0008] The present invention has been made in view of the above circumstances, and provides a polyvinyl alcohol polymer which has a high viscosity in aqueous solution and exhibits good fluid loss reducing properties under high temperature conditions.

[0009] <<Third Viewpoint>> However, although Patent Document 1 exhibited good fluid loss reduction performance, it had a problem in that gel-like deposits were likely to form on a production apparatus when a polyvinyl alcohol-based polymer (modified PVA) was produced by copolymerizing a vinyl ester-based monomer with a polyfunctional comonomer such as triallyl isocyanurate (TAIC).

[0010] The present invention has been made in view of the above circumstances, and provides a method for producing a polyvinyl alcohol-based polymer, which can produce a polyvinyl alcohol-based polymer having good fluid loss reduction performance and which produces little gel-like deposits on a production apparatus.

[0011] <<First Aspect>> As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved when the polyvinyl alcohol-based polymer is a modified PVA using a specific compound as a comonomer and has a viscosity-average degree of polymerization within a specific range, and have thus completed the present invention.

[0012] <<Second Aspect>> As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved when the polyvinyl alcohol-based polymer is a modified PVA using a specific compound as a comonomer and has a viscosity-average degree of polymerization within a specific range, and have thus completed the present invention.

[0013] <<Third Viewpoint>> As a result of extensive investigations to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved when a vinyl ester-based monomer is polymerized with a specific compound as a comonomer to produce a polyvinyl alcohol-based polymer having a viscosity-average degree of polymerization within a specific range, and have thus completed the present invention.

[0014] The following invention is provided: <<First Aspect>> [1-1] A saponified copolymer of raw material monomers containing a vinyl ester monomer and a compound 1-A represented by the following formula (1-1): A polyvinyl alcohol-based polymer having a viscosity-average degree of polymerization of 2000 to 5000. [1-2] The polyvinyl alcohol-based polymer according to [1-1], having a degree of saponification of 80.0 to 99.9 mol %. [1-3] The polyvinyl alcohol-based polymer according to [1-1] or [1-2], containing 0.001 to 1.0 mol % of structural units derived from Compound 1-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer. [1-4] The polyvinyl alcohol-based polymer according to any one of [1-1] to [1-3], wherein the vinyl ester-based monomer comprises vinyl acetate. [1-5] The polyvinyl alcohol-based polymer according to any one of [1-1] to [1-4], wherein the average particle size calculated by cumulant analysis of a particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C is 50.0 to 120.0 nm. [1-6] The polyvinyl alcohol-based polymer according to any one of [1-1] to [1-5], wherein the proportion of particles having a size of 400 nm or more is 10% or less in a particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C. [1-7] The polyvinyl alcohol-based polymer according to any one of [1-1] to [1-6], wherein, when a 10% by mass aqueous solution of the polyvinyl alcohol-based polymer is filtered through a 100 mesh (0.154 mm mesh) filter and the aqueous solution is measured, the slope of a logarithmic plot of the storage modulus G' (30°C) in the angular frequency range of 1.0 to 10.0 rad / s is 1.2 to 1.5, the maximum value of the angular frequency-dependent loss tangent (tan δ) is 10.0 to 100.0, and the minimum value of the angular frequency-dependent loss tangent (tan δ) is 3.0 to 8.0. [1-8] The polyvinyl alcohol-based polymer according to any one of [1-1] to [1-7], wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer has a viscosity at 20°C of 30 to 80 mPa s.[1-9] A powder of the polyvinyl alcohol-based polymer according to any one of [1-1] to [1-8], wherein in a particle size distribution, the content of particles having a particle size of less than 75 μm is 30 mass% or less and the content of particles having a particle size of 500 μm or more is 10 mass% or less. [1-10] An additive for oil well cement, comprising the polyvinyl alcohol-based polymer according to any one of [1-1] to [1-8]. [1-11] A cement slurry, comprising the polyvinyl alcohol-based polymer according to any one of [1-1] to [1-8].

[0015] <<Second Aspect>> [2-1] A saponified copolymer of raw material monomers containing a vinyl ester monomer and a compound 2-A represented by the following formula (2-1): A polyvinyl alcohol-based polymer having a viscosity-average degree of polymerization of 1,800 to 5,000. [2-2] The polyvinyl alcohol-based polymer according to [2-1], having a degree of saponification of 80.0 to 99.9 mol %. [2-3] The polyvinyl alcohol-based polymer according to [2-1] or [2-2], containing 0.001 to 1.0 mol % of structural units derived from Compound 2-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer. [2-4] The polyvinyl alcohol-based polymer according to any one of [2-1] to [2-3], wherein the vinyl ester-based monomer comprises vinyl acetate. [2-5] The polyvinyl alcohol-based polymer according to any one of [2-1] to [2-4], wherein the average particle size calculated by cumulant analysis of a particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at 25° C. is 50.0 to 120.0 nm. [2-6] The polyvinyl alcohol-based polymer according to any one of [2-1] to [2-5], wherein the proportion of particles having a size of 400 nm or more is 10% or less in a particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at 25° C. [2-7] The polyvinyl alcohol-based polymer according to any one of [2-1] to [2-6], wherein the viscosity of a 4 mass% aqueous solution of the polyvinyl alcohol-based polymer at 20° C. is 300 to 30,000 mPa s. [2-8] A powder of the polyvinyl alcohol-based polymer according to any one of [2-1] to [2-7], wherein in a particle size distribution, the content of particles having a particle size of less than 75 μm is 30 mass% or less and the content of particles having a particle size of 500 μm or more is 10 mass% or less. [2-9] An additive for oil well cement, comprising the polyvinyl alcohol-based polymer according to any one of [2-1] to [2-7]. [2-10] A cement slurry, comprising the polyvinyl alcohol-based polymer according to any one of [2-1] to [2-7].

[0016] <<Third Aspect>> [3-1] A method for producing a polyvinyl alcohol-based polymer, comprising: a polymerization step of polymerizing raw material monomers including a vinyl ester-based monomer and a compound 3-A represented by the following formula (3-1) to obtain a copolymer; and a saponification step of saponifying the copolymer to obtain a polyvinyl alcohol-based polymer, A production method in which, in the formula (3-1), R is a linear, branched, or cyclic alkyl group, and the viscosity-average degree of polymerization of the polyvinyl alcohol-based polymer is 2000 to 5000. [3-2] The production method according to [3-1], wherein the polyvinyl alcohol-based polymer has a degree of saponification of 80.0 to 99.9 mol %. [3-3] The production method according to [3-1] or [3-2], wherein the polyvinyl alcohol-based polymer contains 0.001 to 1.0 mol % of structural units derived from Compound 3-A relative to 100 mol % of structural units derived from vinyl ester-based monomers. [3-4] The production method according to any one of [3-1] to [3-3], wherein the vinyl ester-based monomer includes vinyl acetate. [3-5] The manufacturing method according to any one of [3-1] to [3-4], wherein the average particle size calculated by cumulant analysis of the particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C is 50.0 to 120.0 nm. [3-6] The manufacturing method according to any one of [3-1] to [3-5], wherein the proportion of particles having a size of 400 nm or more is 10% or less in the particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C. [3-7] The manufacturing method according to any one of [3-1] to [3-6], wherein R is a linear, branched, or cyclic alkyl group having 1 to 18 carbon atoms. [3-8] The manufacturing method according to any one of [3-1] to [3-7], wherein the viscosity of a 4 mass% aqueous solution of the polyvinyl alcohol-based polymer is 30 to 80 mPa s. [3-9] A method for producing a powder of a polyvinyl alcohol-based polymer, comprising a pulverization step of pulverizing a polyvinyl alcohol-based polymer obtained by the production method according to any one of [3-1] to [3-8] to obtain the powder, wherein in the particle size distribution of the powder, the content of particles having a particle size of less than 75 μm is 30 mass% or less and the content of particles having a particle size of 500 μm or more is 10 mass% or less. [3-10] A method for producing a cement slurry, comprising a step of mixing the polyvinyl alcohol-based polymer obtained by the production method according to any one of [3-1] to [3-8] with a cement raw material to obtain the cement slurry.

[0017] <<First Aspect>> The polyvinyl alcohol-based polymer of the present invention has a low viscosity in aqueous solution and exhibits good fluid loss reducing performance under high temperature conditions, and therefore can be used as a fluid loss reducing agent. As a fluid loss reducing agent, the polyvinyl alcohol-based polymer can be preferably used because it requires little stirring power when mixed with water, cement, etc., and is easily mixed.

[0018] <<Second Aspect>> The polyvinyl alcohol polymer of the present invention has a high viscosity in aqueous solution and exhibits good fluid loss reducing performance under high temperature conditions, and therefore can be used as a fluid loss reducing agent.

[0019] <<Third Aspect>> The method for producing a polyvinyl alcohol-based polymer of the present invention can produce a polyvinyl alcohol-based polymer having good fluid loss reduction performance, and produces little gel-like deposits on the production equipment.

[0020] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently. Furthermore, in the following embodiments, elements not specified in the claims are optional elements and can be omitted. Any number of "0"s (for example, one or two) may be added to the end of numerical values ​​disclosed in the following description. For example, one or two "0"s may be added after "1.4" to make it "1.40" or "1.400".

[0021] <<First Aspect>> 1-1. Polyvinyl Alcohol-Based Polymer A polyvinyl alcohol-based polymer according to one embodiment of the present invention is a modified PVA (modified polyvinyl alcohol, hereinafter also simply referred to as “PVA”), which is a saponified copolymer of raw material monomers including a vinyl ester-based monomer and a compound 1-A represented by the following formula (1-1):

[0022]

[0023] In the above formula (1-1), R is a linear, branched, or cyclic alkyl group having 1 to 7 carbon atoms. R is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. The number of carbon atoms in R is preferably 1 to 3, more preferably 1 to 2, and even more preferably 1. The number of carbon atoms in R is specifically, for example, 1, 2, 3, 4, 5, 6, or 7, and may be within a range between any two of the values ​​exemplified here. Specific examples of R that is a linear alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, and a heptyl group, and a methyl group is preferred.

[0024] As the compound 1-A, the compound represented by the above formula (1-1) may be used alone or in combination of two or more. Preferably, the compound 1-A includes a compound in which R is a methyl group, and more preferably, the compound 1-A is a compound in which R is a methyl group.

[0025] The structural unit derived from compound 1-A (hereinafter also referred to as "structural unit α") is resistant to decomposition during the saponification reaction, and such a structural unit contributes to improving the fluid loss reduction performance under high-temperature conditions and reducing the viscosity of the aqueous solution.

[0026] The polyvinyl alcohol polymer is a saponified product obtained by further saponifying a copolymer (hereinafter also referred to as an "intermediate copolymer") obtained by copolymerizing a vinyl ester monomer with a raw material monomer containing the compound 1-A represented by the above formula (1-1). Some or all (depending on the degree of saponification) of the structural units derived from the vinyl ester monomer contained in the intermediate copolymer (vinyl ester monomer units) are converted into structural units having hydroxyl groups (hydroxyl group-containing vinyl units) through hydrolysis of the ester groups by saponification.

[0027] Furthermore, at least a portion of the structural units α may be crosslinked with crosslinkable structural units such as other structural units α (for example, in copolymerization of the intermediate copolymer). The intermediate copolymer and the polyvinyl alcohol-based polymer may have a crosslinked structure at least in part of the structural units α. The polyvinyl alcohol-based polymer has, for example, a hydroxyl group-containing vinyl unit, a vinyl ester-based monomer unit, and the structural unit α, and has a crosslinked structure at least in part of the structural units α.

[0028] Examples of the vinyl ester monomer include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, and these may be used alone or in combination of two or more. Preferably, the vinyl ester monomer includes vinyl acetate, and more preferably, the vinyl ester monomer is vinyl acetate.

[0029] The polyvinyl alcohol-based polymer preferably contains 0.001 to 1.0 mol %, more preferably 0.01 to 0.5 mol %, and even more preferably 0.05 to 0.3 mol % of structural units derived from compound 1-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer. The content of the structural units derived from Compound 1-A in the polyvinyl alcohol polymer is, for example, 0.001, 0.01, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.35, 0.40, 0.45, 0.50, or 1.0 mol%, based on 100 mol% of the structural units derived from the vinyl ester monomer contained in the polyvinyl alcohol polymer, and may be within a range between any two of the numerical values ​​exemplified here.

[0030] The content of the structural unit derived from Compound 1-A in the polyvinyl alcohol polymer (the copolymerization amount of Compound 1-A) can be calculated, for example, based on the results of NMR measurement, trace nitrogen determination, etc. A calculation method based on NMR measurement in the case of using diallyl methyl isocyanurate will be specifically explained in the examples described later.

[0031] The polyvinyl alcohol polymer preferably contains 90 to 100 mol %, more preferably 95 to 100 mol %, and even more preferably 99 to 100 mol % of structural units derived from vinyl ester monomers and structural units derived from Compound 1-A in total, based on 100 mol % of structural units contained in the polyvinyl alcohol polymer. Specific examples of the total content of structural units derived from vinyl ester monomers and structural units derived from Compound 1-A in 100 mol % of structural units contained in the polyvinyl alcohol polymer include 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, 99.9, and 100 mol %, and may be within a range between any two of the numerical values ​​exemplified here.

[0032] The intermediate copolymer may further be copolymerized with other monomers copolymerizable with the vinyl ester monomer and compound 1-A, within the scope of not impairing the effects of the present invention. Examples of other monomers include α-olefin monomers such as ethylene and propylene; (meth)acrylic acid alkyl ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; unsaturated amide monomers such as (meth)acrylamide and N-methylolacrylamide; unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; and alkyl (methyl, ethyl, propyl) carboxylic acids of unsaturated carboxylic acids. anhydrides of unsaturated carboxylic acids such as maleic anhydride; salts of unsaturated carboxylic acids with sodium, potassium, ammonium, or the like; glycidyl group-containing monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; sulfonic acid group-containing monomers such as 2-acrylamido-2-methylpropanesulfonic acid or salts thereof; phosphate group-containing monomers such as acid phosphooxyethyl methacrylate and acid phosphooxypropyl methacrylate; alkyl vinyl ether monomers; and the like.

[0033] The viscosity-average degree of polymerization of the polyvinyl alcohol polymer is preferably 2000 to 5000, more preferably 2200 to 3000. If the viscosity-average degree of polymerization is too low, the fluid loss reduction effect may be insufficient. If the viscosity-average degree of polymerization is too high, the viscosity of the aqueous solution may be high. Furthermore, if the viscosity-average degree of polymerization is too high, the viscosity of the cement slurry may be high, which may increase the pump power required for slurry transport and increase pressure loss. Specific examples of the viscosity-average degree of polymerization are 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3500, 4000, 4500, and 5000, and may be within a range between any two of the values ​​exemplified here.

[0034] The "viscosity average degree of polymerization" is a value calculated from the intrinsic viscosity [η] (g / dL) measured at 30°C using an Ostwald viscometer using ion-exchanged water as a solvent in accordance with JIS K 6726:1994 "3.7 Average degree of polymerization" by the following formula (A): log(P)=1.613×log([η]×10 ４ / 8.29) ... (A) where P represents the viscosity average degree of polymerization.

[0035] The saponification degree of the polyvinyl alcohol polymer is preferably 80.0 to 99.9 mol%, more preferably 90 to 99.5 mol%. When this range is satisfied, the hydrophobicity of the remaining ester groups and the action of intermolecular hydrogen bonds of the polyvinyl alcohol polymer fall within a preferred range, resulting in a sufficient fluid loss reduction effect. Specific examples of the saponification degree include 80.0, 85.0, 86.0, 87.0, 88.0, 89.0, 89.5, 90.0, 90.5, 91.0, 91.5, 92.0, 92.5, 93.0, 93.5, 94.0, 94.5, 95.0, 95.5, 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0, 99.5, and 99.9 mol%, and may be within a range between any two of the values ​​exemplified here.

[0036] The "saponification degree" can be calculated by measuring in accordance with JIS K 6726:1994 "3.5 Saponification degree".

[0037] The polyvinyl alcohol polymer preferably has an average particle size of 50.0 to 120.0 nm, more preferably 59.0 to 80.0 nm, as calculated by cumulant analysis of the particle size distribution obtained by performing dynamic light scattering measurement on a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer at 25°C. When used as an additive for oil well cement, if the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 50 nm or more, the polyvinyl alcohol polymer is less likely to flow out of the cement slurry, improving fluid loss reduction performance. If the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 120 nm or less, little gel-like deposits are generated during production. Specific examples of the average particle size include 50.0, 55.0, 55.5, 56.0, 56.5, 57.0, 57.5, 58.0, 58.5, 59.0, 59.5, 60.0, 60.5, 61.0, 61.5, 62.0, 62.5, 63.0, 63.5, 64.0, 64.5, 65.0, 65.5, 66.0, 66.5, 67.0, 67.5, 68.0, 68.5, 69.0, 69.5, 70.0, 80.0, 90.0, 100.0, 110.0, and 120.0 nm, and may be within a range between any two of the values ​​exemplified here. The particle size distribution can be adjusted, for example, by adding an alcohol such as methanol in the polymerization step or by the amount of copolymerization of Compound 1-A.

[0038] The polyvinyl alcohol polymer preferably has a proportion of particles having a size of 400 nm or more of 10% or less, more preferably 5% or less, in a particle size distribution obtained by subjecting a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer to dynamic light scattering measurement at 25° C. Specific examples of the proportion of particles having a size of 400 nm or more include 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10%, and may be within a range between any two of the values ​​exemplified here.

[0039] When a 10% by mass aqueous solution of the polyvinyl alcohol polymer is filtered through a 100 mesh (0.154 mm mesh) filter and the resulting aqueous solution is measured, the slope of a logarithmic plot of the storage modulus G' (30°C) in the angular frequency range of 1.0 to 10.0 rad / s (slope of G') is preferably 1.2 to 1.5. Furthermore, the maximum value of the angular frequency-dependent loss tangent (tan δ) is preferably 10 to 100, more preferably greater than 10.0, for example, 11.0 to 50.0. Furthermore, the minimum value of the angular frequency-dependent loss tangent (tan δ) is preferably 3.0 to 8.0, more preferably 3.5 to 5.0. Viscoelastic properties can be adjusted, for example, by the amount of copolymerization or degree of polymerization of compound 1-A.

[0040] The viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol polymer at 20°C is preferably 30 to 75 mPa·s, more preferably 35 to 70 mPa·s. The viscosity is measured at a shear rate of 10 s in accordance with the rotational viscometer method described in JIS K6726:1994 and JIS K 7117:1999. －１ The viscosity can be calculated as follows. Specifically, the viscosity may be, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 mPa·s, and may be within a range between any two of the values ​​exemplified here. Various methods can be used to mix a polyvinyl alcohol-based polymer with cement. For example, cement, a polyvinyl alcohol-based polymer, and optionally other additives may be dry-blended, and the resulting mixture may be stirred with water to obtain a slurry, which may then be transported to an oil well. In this case, if the viscosity of the aqueous solution, and therefore the viscosity of the polyvinyl alcohol-based polymer, is low, the stirring power required for mixing is small, making mixing easier and reducing pressure loss. Furthermore, when preparing a cement slurry, the polyvinyl alcohol-based polymer may first be mixed with water and then mixed with cement. Even in such cases, a low viscosity aqueous solution of the polyvinyl alcohol-based polymer may be required from the standpoint of mixability (stirring power) and suppression of air entrapment. Therefore, a polyvinyl alcohol-based polymer that satisfies the above viscosity range is preferred.

[0041] The polyvinyl alcohol-based polymer has a 300 mesh (0.045 mm mesh) filter penetration rate of preferably 95% by mass or more, more preferably 99 to 100% by mass, of a 1.0% by mass aqueous solution of the polyvinyl alcohol-based polymer, calculated as solid content. It is preferable that the polyvinyl alcohol-based polymer does not contain excessively large gel particles, from the viewpoint of the strength of the cement after hardening and the production of the polyvinyl alcohol-based polymer.

[0042] <Applications> The polyvinyl alcohol-based polymer can be used as an additive for oil well cement, and an additive for oil well cement according to one embodiment of the present invention comprises the polyvinyl alcohol-based polymer. Such an additive for oil well cement is suitable as an additive for oil well cement used in cementing oil wells, gas wells, steam wells for geothermal power generation, etc.

[0043] Cementing, which is performed when drilling a well, is the process of injecting cement into the gap between the drilled well and a steel pipe inserted into it. A widely used cementing method involves mixing cement and various additives in a dry state, then pumping the mixture into a slurry using high-pressure water. The oil well cement additive of the present invention reduces the loss of water content from the cement slurry during cementing (i.e., reduces fluid loss), making it possible to maintain the fluidity of the cement slurry. If fluid loss is large, the fluidity of the cement slurry is lost, making it difficult to perform sufficient cementing.

[0044] Fluid loss is a physical property of oil well cement defined by the American Petroleum Institute (API). The fluid loss test method is described in Recommended Practice for Testing Well Cements, API Recommended Practice 10B-2, April 2013.

[0045] The fluid loss of the polyvinyl alcohol polymer measured by this test method under conditions of 0.5% bwoc and 80°C is, for example, 50 cc or less, preferably 46 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 20 cc or more. Furthermore, the fluid loss under conditions of 0.8% bwoc and 110°C (high temperature conditions) is, for example, 90 cc or less, preferably 60 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 30 cc or more.

[0046] <Method for Producing Polyvinyl Alcohol-Based Polymer> The method for producing a polyvinyl alcohol-based polymer is not particularly limited, but may include, for example, a polymerization step and a saponification step.

[0047] In the polymerization step, raw material monomers including a vinyl ester monomer and compound 1-A are polymerized to obtain an intermediate copolymer. The polymerization method for the raw material monomers including a vinyl ester monomer and compound 1-A is not particularly limited, and known polymerization methods such as solution polymerization, suspension polymerization, and bulk polymerization can be used. Because of ease of operation and the ability to use the same solvent as in the saponification step, it is preferable to use a solution polymerization method in alcohol. It is particularly preferable to use methanol as the alcohol. Diluting the mixture by adding an alcohol such as methanol makes it easier to obtain the particle size distribution described above.

[0048] The amount of alcohol added may be, for example, 30.0 to 80.0 parts by mass, and preferably 40.0 to 70.0 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0049] The amount of compound 1-A added may be, for example, 0.10 to 0.40 parts by mass, and preferably 0.20 to 0.25 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0050] The amount of the vinyl ester monomer added relative to the total amount of the raw material monomers (100% by mass) may be, for example, 95.0 to 99.9% by mass, preferably 99.0 to 99.5% by mass, and more preferably 99.6 to 99.0% by mass.

[0051] The conversion rate of the vinyl ester monomer may be, for example, 40.0 to 70.0%, and preferably 50.0 to 60.0%.

[0052] The polymerization initiator may be, but is not particularly limited to, azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile; peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; percarbonate compounds such as di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; and perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate, which may be used alone or in combination.

[0053] In the saponification step, the intermediate copolymer is saponified to obtain a saponified product. While the method for saponifying the intermediate copolymer is not particularly limited, a simple and preferred method involves dissolving the intermediate copolymer in alcohol and then saponifying it with an alkali (e.g., sodium hydroxide, etc.). The alcohol used is not particularly limited, but examples include methanol, ethanol, and butanol. Among these, methanol is preferred because it facilitates solvent recovery and reuse, reducing production costs. The saponification step may also include a neutralization step using acetic acid, etc. In one example, when an alkali is added to a methanol solution of the intermediate copolymer, the paste becomes gel-like as the saponification progresses. This gel-like product (which may become a block gel-like product) is crushed with a cutter, neutralized by adding acetic acid, washed, and dried (the solvent removal step described below) to obtain granular particles. These granular particles can be crushed (the crushing step and classification step described below) to obtain a powder.

[0054] The method for producing a polyvinyl alcohol-based polymer may further include a solvent removal step, in which the solvent can be removed by, for example, heat drying.

[0055] 1-2. Polyvinyl Alcohol-Based Polymer Powder In the polyvinyl alcohol-based polymer powder according to one embodiment of the present invention, the content of particles having a particle size of less than 75 μm (under 75 μm sieve) is preferably 30% by mass or less, more preferably 8% by mass or less, in the particle size distribution. Furthermore, the content of particles having a particle size of 500 μm or more (over 500 μm sieve) is preferably 10% by mass or less, more preferably 0.05% by mass or less, in the particle size distribution. If the polyvinyl alcohol-based polymer powder contains a large number of excessively large particles, the strength of the hardened cement decreases. The particle size distribution can be measured by sieving using sieves with 75 μm and 500 μm mesh openings in accordance with JIS Z 8815:1994.

[0056] A powder having such a particle size distribution can be obtained by pulverizing the polyvinyl alcohol polymer obtained by the above-described manufacturing method (e.g., heat-dried) (pulverization step) and classifying the pulverized powder (classification step). The pulverization method is not particularly limited, and known pulverization techniques can be used. Suitable pulverization devices include rotary blade pulverizers, turbo mill pulverizers, turbo disk mill pulverizers, turbo cutter pulverizers, jet mill pulverizers, impact pulverizers, hammer pulverizers, and vibrating pulverizers. An ACM pulverizer, for example, can be used as the pulverization device. The classification method is not particularly limited, and includes methods using a sieve (such as a vibrating sieve) and methods utilizing the difference in sedimentation velocity or travel distance of particles in a fluid such as air or water. Pulverization devices such as an ACM pulverizer are also equipped with a classification function using an airflow classifier, and classification may be performed using such a device, for example.

[0057] 1-3. Cement Slurry A cement slurry according to one embodiment of the present invention may contain the polyvinyl alcohol-based polymer. The cement slurry is, for example, a composition containing the polyvinyl alcohol-based polymer, cement, and water. The cement slurry according to one embodiment may also contain the additive for oil well cement. The cement slurry is, for example, a composition containing the additive for oil well cement, cement, and water.

[0058] The cement slurry may contain other additives as needed.

[0059] The method for adding the polyvinyl alcohol polymer or oil well cement additive to the cement slurry is not particularly limited, and conventional methods such as mixing the polymer with dry cement beforehand, mixing the polymer when forming a cement slurry, or mixing the polymer with premixed water and then mixing the polymer with cement can be used.

[0060] The content of the polyvinyl alcohol polymer in the cement slurry is, for example, 0.01 to 30% bwoc, preferably 0.05 to 10% bwoc, and more preferably 0.1 to 5% bwoc. The term "by weight of cement" (bwoc) refers to the weight of the additive in dry form added to the cement composition based on the cement solids content only.

[0061] <<Second Aspect>> 2-1. Polyvinyl Alcohol-Based Polymer A polyvinyl alcohol-based polymer according to one embodiment of the present invention is a modified PVA (modified polyvinyl alcohol, hereinafter also simply referred to as “PVA”), which is a saponified copolymer of raw material monomers including a vinyl ester-based monomer and a compound 2-A represented by the following formula (2-1):

[0062]

[0063] In the above formula (2-1), R is a linear, branched, or cyclic alkyl group having 8 to 18 carbon atoms. R is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. The number of carbon atoms in R is preferably 10 to 14, more preferably 12 to 14, and even more preferably 14. Specific examples of the number of carbon atoms in R include 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18, and may be within a range between any two of the values ​​exemplified here. Specific examples of the linear alkyl group R include an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, and an octadecyl group, with a tetradecyl group being preferred.

[0064] As the compound 2-A, the compound represented by the above formula (2-1) may be used alone or in combination of two or more. Preferably, the compound 2-A includes a compound in which R is a tetradecyl group, and more preferably, the compound 2-A is a compound in which R is a tetradecyl group.

[0065] The structural unit derived from compound 2-A (hereinafter also referred to as "structural unit α") is resistant to decomposition during the saponification reaction, and such a structural unit contributes to improving the fluid loss reduction performance under high-temperature conditions and increasing the viscosity of the aqueous solution.

[0066] The polyvinyl alcohol polymer is a saponified product obtained by further saponifying a copolymer (hereinafter also referred to as an "intermediate copolymer") obtained by copolymerizing a vinyl ester monomer with a raw material monomer containing the compound 2-A represented by the above formula (2-1). Some or all (depending on the degree of saponification) of the structural units derived from the vinyl ester monomer contained in the intermediate copolymer (vinyl ester monomer units) are converted into structural units having hydroxyl groups (hydroxyl group-containing vinyl units) through hydrolysis of the ester groups by saponification.

[0067] Furthermore, at least a portion of the structural units α may be crosslinked with crosslinkable structural units such as other structural units α (for example, in copolymerization of the intermediate copolymer). The intermediate copolymer and the polyvinyl alcohol-based polymer may have a crosslinked structure at least in part of the structural units α. The polyvinyl alcohol-based polymer has, for example, a hydroxyl group-containing vinyl unit, a vinyl ester-based monomer unit, and the structural unit α, and has a crosslinked structure at least in part of the structural units α.

[0068] Examples of the vinyl ester monomer include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, and these may be used alone or in combination of two or more. Preferably, the vinyl ester monomer includes vinyl acetate, and more preferably, the vinyl ester monomer is vinyl acetate.

[0069] The polyvinyl alcohol-based polymer preferably contains 0.001 to 1.0 mol %, more preferably 0.01 to 0.5 mol %, and even more preferably 0.05 to 0.3 mol % of structural units derived from compound 2-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer. The content of the structural units derived from compound 2-A in the polyvinyl alcohol polymer is, for example, 0.001, 0.01, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.35, 0.40, 0.45, 0.50, or 1.0 mol%, based on 100 mol% of the structural units derived from the vinyl ester monomer contained in the polyvinyl alcohol polymer, and may be within a range between any two of the numerical values ​​exemplified here.

[0070] The content of the structural unit derived from Compound 2-A in the polyvinyl alcohol polymer (copolymerization amount of Compound 2-A) can be calculated, for example, based on the results of NMR measurement, trace nitrogen determination, etc. A calculation method based on NMR measurement in the case of using diallyl tetradecyl isocyanurate will be specifically explained in the examples described later.

[0071] The polyvinyl alcohol polymer preferably contains 90 to 100 mol %, more preferably 95 to 100 mol %, and even more preferably 99 to 100 mol % of structural units derived from vinyl ester monomers and structural units derived from Compound 2-A in total, based on 100 mol % of structural units contained in the polyvinyl alcohol polymer. Specific examples of the total content of structural units derived from vinyl ester monomers and structural units derived from Compound 2-A in 100 mol % of structural units contained in the polyvinyl alcohol polymer include 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, 99.9, and 100 mol %, and may be within a range between any two of the numerical values ​​exemplified here.

[0072] The intermediate copolymer may further contain other monomers copolymerizable with the vinyl ester monomer and compound 2-A, as long as the effects of the present invention are not impaired. Examples of such other monomers include α-olefin monomers such as ethylene and propylene; (meth)acrylic acid alkyl ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; unsaturated amide monomers such as (meth)acrylamide and N-methylolacrylamide; unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; and alkyl (methyl, ethyl, propyl) carboxylic acids of unsaturated carboxylic acids. anhydrides of unsaturated carboxylic acids such as maleic anhydride; salts of unsaturated carboxylic acids with sodium, potassium, ammonium, or the like; glycidyl group-containing monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; sulfonic acid group-containing monomers such as 2-acrylamido-2-methylpropanesulfonic acid or salts thereof; phosphate group-containing monomers such as acid phosphooxyethyl methacrylate and acid phosphooxypropyl methacrylate; alkyl vinyl ether monomers; and the like.

[0073] The viscosity-average degree of polymerization of the polyvinyl alcohol polymer is preferably 1800 to 5000, more preferably 2200 to 3000. If the viscosity-average degree of polymerization is too low, the fluid loss reduction effect may be insufficient. Specific examples of the viscosity-average degree of polymerization include 1800, 1900, 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3500, 4000, 4500, and 5000, and may be within a range between any two of the values ​​exemplified here.

[0074] For the "viscosity average degree of polymerization", see the measurement and calculation method described in 1-1 above.

[0075] The saponification degree of the polyvinyl alcohol polymer is preferably 80.0 to 99.9 mol%, more preferably 90 to 99.5 mol%. When this range is satisfied, the hydrophobicity of the remaining ester groups and the action of intermolecular hydrogen bonds of the polyvinyl alcohol polymer fall within a preferred range, resulting in a sufficient fluid loss reduction effect. Specific examples of the saponification degree include 80.0, 85.0, 86.0, 87.0, 88.0, 89.0, 89.5, 90.0, 90.5, 91.0, 91.5, 92.0, 92.5, 93.0, 93.5, 94.0, 94.5, 95.0, 95.5, 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0, 99.5, and 99.9 mol%, and may be within a range between any two of the values ​​exemplified here.

[0076] The "saponification degree" can be calculated by measuring in accordance with JIS K 6726:1994 "3.5 Saponification degree".

[0077] The polyvinyl alcohol polymer preferably has an average particle size of 50.0 to 120.0 nm, more preferably 70.0 to 100.0 nm, as calculated by cumulant analysis of the particle size distribution obtained by performing dynamic light scattering measurement on a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer at 25°C. When used as an additive for oil well cement, if the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 50 nm or more, the polyvinyl alcohol polymer is less likely to flow out of the cement slurry, improving fluid loss reduction performance. If the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 120 nm or less, little gel-like deposits are generated during production. Specific examples of the average particle size include 50.0, 55.0, 55.5, 56.0, 56.5, 57.0, 57.5, 58.0, 58.5, 59.0, 59.5, 60.0, 60.5, 61.0, 61.5, 62.0, 62.5, 63.0, 63.5, 64.0, 64.5, 65.0, 65.5, 66.0, 66.5, 67.0, 67.5, 68.0, 68.5, 69.0, 69.5, 70.0, 80.0, 90.0, 100.0, 110.0, and 120.0 nm, and may be within a range between any two of the values ​​exemplified here. The particle size distribution can be adjusted, for example, by adding an alcohol such as methanol in the polymerization step or by the amount of copolymerization of compound 2-A.

[0078] The polyvinyl alcohol polymer preferably has a proportion of particles having a size of 400 nm or more of 10% or less, more preferably 6% or less, in a particle size distribution obtained by subjecting a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer to dynamic light scattering measurement at 25° C. Specific examples of the proportion of particles having a size of 400 nm or more include 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10%, and may be within a range between any two of the values ​​exemplified here.

[0079] The viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol polymer at 20°C is preferably 300 to 30,000 mPa·s, more preferably 1,000 to 20,000 mPa·s. The viscosity is measured at a shear rate of 10 s in accordance with the rotational viscometer method described in JIS K6726:1994 and JIS K 7117:1999. －１ Specific examples of the viscosity include 300, 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000, 20000, 21000, 22000, 23000, 24000, 25000, 26000, 27000, 28000, 29000, and 30000 mPa·s, and may be within a range between any two of the values ​​exemplified here. Deeper oil wells have higher temperatures, and therefore, there is a demand for the effect of reducing fluid loss at higher temperatures. Cement slurries containing polyvinyl alcohol-based polymers tend to have a higher fluid loss reduction effect when the viscosity is high, and a polyvinyl alcohol-based polymer (oil well cement additive) that can increase the viscosity of cement compositions is desired. Therefore, for example, a high viscosity when dissolved in water, i.e., a viscosity of 300 mPa·s, is a preferred corresponding characteristic. On the other hand, if the viscosity of the cement slurry is too high, the pressure loss during slurry transfer will be too high, so a viscosity of 30,000 mPa·s or less is a preferred corresponding characteristic.

[0080] The polyvinyl alcohol-based polymer has a 300 mesh (0.045 mm mesh) filter penetration rate of preferably 95% by mass or more, more preferably 99 to 100% by mass, of a 1.0% by mass aqueous solution of the polyvinyl alcohol-based polymer, calculated as solid content. It is preferable that the polyvinyl alcohol-based polymer does not contain excessively large gel particles, from the viewpoint of the strength of the cement after hardening and the production of the polyvinyl alcohol-based polymer.

[0081] <Applications> The polyvinyl alcohol-based polymer can be used as an additive for oil well cement, and an additive for oil well cement according to one embodiment of the present invention comprises the polyvinyl alcohol-based polymer. Such an additive for oil well cement is suitable as an additive for oil well cement used in cementing oil wells, gas wells, steam wells for geothermal power generation, etc.

[0082] Cementing, which is performed when drilling a well, is the process of injecting cement into the gap between the drilled well and a steel pipe inserted into it. A widely used cementing method involves mixing cement and various additives in a dry state, then pumping the mixture into a slurry using high-pressure water. The oil well cement additive of the present invention reduces the loss of water content from the cement slurry during cementing (i.e., reduces fluid loss), making it possible to maintain the fluidity of the cement slurry. If fluid loss is large, the fluidity of the cement slurry is lost, making it difficult to perform sufficient cementing.

[0083] For details about fluid loss, see the explanation in 1-1 above.

[0084] The fluid loss of the polyvinyl alcohol polymer measured by this test method under conditions of 0.5% bwoc and 80°C is, for example, 50 cc or less, preferably 46 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 20 cc or more. Furthermore, the fluid loss under conditions of 0.8% bwoc and 110°C (high temperature conditions) is, for example, 90 cc or less, preferably 60 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 30 cc or more.

[0085] <Method for Producing Polyvinyl Alcohol-Based Polymer> The method for producing a polyvinyl alcohol-based polymer is not particularly limited, but may include, for example, a polymerization step and a saponification step.

[0086] In the polymerization step, raw material monomers including a vinyl ester monomer and compound 2-A are polymerized to obtain an intermediate copolymer. The polymerization method for the raw material monomers including a vinyl ester monomer and compound 2-A is not particularly limited, and known polymerization methods such as solution polymerization, suspension polymerization, and bulk polymerization can be used. Because of ease of operation and the ability to use the same solvent as in the saponification step, it is preferable to use a solution polymerization method in alcohol. It is particularly preferable to use methanol as the alcohol. Diluting the mixture by adding an alcohol such as methanol makes it easier to obtain the particle size distribution described above.

[0087] The amount of alcohol added may be, for example, 15.0 to 60.0 parts by mass, and preferably 20.0 to 40.0 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0088] The amount of compound 2-A added may be, for example, 0.20 to 0.50 parts by mass, and preferably 0.30 to 0.45 parts by mass, per 100 parts by mass of the vinyl ester monomer.

[0089] The amount of the vinyl ester monomer added relative to the total amount of the raw material monomers (100% by mass) may be, for example, 95.0 to 99.9% by mass, preferably 99.0 to 99.5% by mass, and more preferably 99.6 to 99.0% by mass.

[0090] The conversion rate of the vinyl ester monomer may be, for example, 30.0 to 70.0%, and specifically may be, for example, 30.0, 35.0, 40.0, 45.0, 50.0, 55.0, 60.0, 65.0, or 70.0%, and may be within a range between any two of the values ​​exemplified here.

[0091] The polymerization initiator may be, but is not particularly limited to, azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile; peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; percarbonate compounds such as di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; and perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate, which may be used alone or in combination.

[0092] In the saponification step, the intermediate copolymer is saponified to obtain a saponified product. While the method for saponifying the intermediate copolymer is not particularly limited, a simple and preferred method involves dissolving the intermediate copolymer in alcohol and then saponifying it with an alkali (e.g., sodium hydroxide, etc.). The alcohol used is not particularly limited, but examples include methanol, ethanol, and butanol. Among these, methanol is preferred because it facilitates solvent recovery and reuse, reducing production costs. The saponification step may also include a neutralization step using acetic acid, etc. In one example, when an alkali is added to a methanol solution of the intermediate copolymer, the paste becomes gel-like as the saponification progresses. This gel-like product (which may become a block gel-like product) is crushed with a cutter, neutralized by adding acetic acid, washed, and dried (the solvent removal step described below) to obtain granular particles. These granular particles can be crushed (the crushing step and classification step described below) to obtain a powder.

[0093] The method for producing a polyvinyl alcohol-based polymer may further include a solvent removal step, in which the solvent can be removed by, for example, heat drying.

[0094] 2-2. Polyvinyl Alcohol-Based Polymer Powder In the polyvinyl alcohol-based polymer powder according to one embodiment of the present invention, the content of particles having a particle size of less than 75 μm (under 75 μm sieve) is preferably 30% by mass or less, more preferably 8% by mass or less, in the particle size distribution. Furthermore, the content of particles having a particle size of 500 μm or more (over 500 μm sieve) is preferably 10% by mass or less, more preferably 0.05% by mass or less, in the particle size distribution. If the polyvinyl alcohol-based polymer powder contains a large number of excessively large particles, the strength of the hardened cement decreases. The particle size distribution can be measured by sieving using sieves with 75 μm and 500 μm mesh openings in accordance with JIS Z 8815:1994.

[0095] A powder having such a particle size distribution can be obtained by pulverizing the polyvinyl alcohol polymer obtained by the above-described manufacturing method (e.g., heat-dried) (pulverization step) and classifying the pulverized powder (classification step). The pulverization method is not particularly limited, and known pulverization techniques can be used. Suitable pulverization devices include rotary blade pulverizers, turbo mill pulverizers, turbo disk mill pulverizers, turbo cutter pulverizers, jet mill pulverizers, impact pulverizers, hammer pulverizers, and vibrating pulverizers. An ACM pulverizer, for example, can be used as the pulverization device. The classification method is not particularly limited, and includes methods using a sieve (such as a vibrating sieve) and methods utilizing the difference in sedimentation velocity or travel distance of particles in a fluid such as air or water. Pulverization devices such as an ACM pulverizer are also equipped with a classification function using an airflow classifier, and classification may be performed using such a device, for example.

[0096] 2-3. Cement Slurry A cement slurry according to one embodiment of the present invention may contain the polyvinyl alcohol-based polymer. The cement slurry is, for example, a composition containing the polyvinyl alcohol-based polymer, cement, and water. The cement slurry according to one embodiment may also contain the additive for oil well cement. The cement slurry is, for example, a composition containing the additive for oil well cement, cement, and water.

[0097] The cement slurry may contain other additives as needed.

[0098] The method for adding the polyvinyl alcohol polymer or oil well cement additive to the cement slurry is not particularly limited, and conventional methods such as mixing the polymer with dry cement beforehand, mixing the polymer when forming a cement slurry, or mixing the polymer with premixed water and then mixing the polymer with cement can be used.

[0099] The content of the polyvinyl alcohol polymer in the cement slurry is, for example, 0.01 to 30% bwoc, preferably 0.05 to 10% bwoc, and more preferably 0.1 to 5% bwoc. The term "by weight of cement" (bwoc) refers to the weight of the additive in dry form added to the cement composition based on the cement solids content only.

[0100] <<Third Aspect>> 3-1. Method for Producing Polyvinyl Alcohol-Based Polymer A method for producing a polyvinyl alcohol-based polymer according to one embodiment of the present invention includes a polymerization step and a saponification step.

[0101] In the polymerization step, a copolymer is obtained by polymerizing raw material monomers including a vinyl ester monomer and a compound 3-A represented by the following formula (3-1).

[0102]

[0103] In the above formula (3-1), R is a linear, branched, or cyclic alkyl group. R is preferably a linear or branched alkyl group, and more preferably a linear alkyl group. The number of carbon atoms in R is preferably 1 to 18, more preferably 1 to 16, and even more preferably 1 to 14. Specific examples of the number of carbon atoms in R include 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18, and may be within a range between any two of the numerical values ​​exemplified here. Specific examples of R which is a linear alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, and an octadecyl group, and preferably a methyl group or a tetradecyl group.

[0104] As the compound 3-A, the compound represented by the above formula (3-1) may be used alone or in combination of two or more. Preferably, the compound 3-A includes one or more compounds in which R is selected from a methyl group and a tetradecyl group, and more preferably, the compound 3-A is a compound in which R is a methyl group or a tetradecyl group.

[0105] By using such compound 3-A as a comonomer, it is possible to reduce the generation of gel during the production process (particularly the polymerization step), and to reduce the amount of gel-like deposits on production equipment such as polymerization vessels.

[0106] Examples of the vinyl ester monomer include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, and these may be used alone or in combination of two or more. Preferably, the vinyl ester monomer includes vinyl acetate, and more preferably, the vinyl ester monomer is vinyl acetate.

[0107] The polymerization method for the vinyl ester monomer and the raw material monomers including Compound 3-A is not particularly limited, and known polymerization methods such as solution polymerization, suspension polymerization, and bulk polymerization can be used. It is preferable to use a solution polymerization method in alcohol because of ease of operation and the ability to use the same solvent as in the saponification step. It is particularly preferable to use methanol as the alcohol. Diluting the mixture by adding an alcohol such as methanol makes it easier to obtain the particle size distribution described above.

[0108] The amount of alcohol added may be, for example, 15.0 to 80.0 parts by mass, preferably 20.0 to 70.0 parts by mass, per 100 parts by mass of the vinyl ester monomer, specifically, for example, 15.0, 20.0, 25.0, 30.0, 35.0, 40.0, 45.0, 50.0, 55.0, 60.0, 65.0, 70.0, 75.0, or 80.0 parts by mass, and may be within a range between any two of the values ​​exemplified here.

[0109] The amount of Compound 3-A added may be, for example, 0.10 to 0.40 parts by mass, preferably 0.20 to 0.25 parts by mass, relative to 100 parts by mass of the vinyl ester monomer. Specific examples of the amount of Compound 3-A added relative to 100 parts by mass of the vinyl ester monomer include 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 parts by mass, and may be within a range between any two of the values ​​exemplified here.

[0110] The amount of the vinyl ester monomer added relative to 100% by mass of the total amount of raw material monomers may be, for example, 95.0 to 99.9% by mass, preferably 99.0 to 99.5% by mass, and more preferably 99.6 to 99.0% by mass. Specific examples of the amount of the vinyl ester monomer added relative to 100% by mass of the total amount of raw material monomers added include 95.0, 96.0, 97.0, 98.0, 99.0, 99.1, 99.2, 99.3, 99.4, 99.5, 99.6, 99.7, 99.8, and 99.9% by mass, and may be within a range between any two of the values ​​exemplified here.

[0111] The conversion rate of the vinyl ester monomer may be, for example, 30.0 to 70.0%, and specifically may be, for example, 30.0, 35.0, 40.0, 45.0, 50.0, 55.0, 60.0, 65.0, or 70.0%, and may be within a range between any two of the values ​​exemplified here.

[0112] The polymerization initiator may be, but is not particularly limited to, azo compounds such as azobisisobutyronitrile, azobis-2,4-dimethylvaleronitrile, azobis(4-methoxy-2,4-dimethylvaleronitrile), azobisdimethylvaleronitrile, and azobismethoxyvaleronitrile; peroxides such as acetyl peroxide, benzoyl peroxide, lauroyl peroxide, acetylcyclohexylsulfonyl peroxide, and 2,4,4-trimethylpentyl-2-peroxyphenoxyacetate; percarbonate compounds such as di-n-propyl peroxydicarbonate, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, and diethoxyethyl peroxydicarbonate; and perester compounds such as t-butyl peroxyneodecanate, α-cumyl peroxyneodecanate, and t-butyl peroxyneodecanate, which may be used alone or in combination.

[0113] In the saponification step, the intermediate copolymer is saponified to obtain a saponified product. While the method for saponifying the intermediate copolymer is not particularly limited, a simple and preferred method involves dissolving the intermediate copolymer in alcohol and then saponifying it with an alkali (e.g., sodium hydroxide, etc.). The alcohol used is not particularly limited, but examples include methanol, ethanol, and butanol. Among these, methanol is preferred because it facilitates solvent recovery and reuse, reducing production costs. The saponification step may also include a neutralization step using acetic acid, etc. In one example, when an alkali is added to a methanol solution of the intermediate copolymer, the paste becomes gel-like as the saponification progresses. This gel-like product (which may become a block gel-like product) is crushed with a cutter, neutralized by adding acetic acid, washed, and dried (the solvent removal step described below) to obtain granular particles. These granular particles can be crushed (the crushing step and classification step described below) to obtain a powder.

[0114] The method for producing a polyvinyl alcohol-based polymer may further include a solvent removal step, in which the solvent can be removed by, for example, drying by heating or distillation under reduced pressure.

[0115] The polyvinyl alcohol-based polymer obtained by the above-described production method is a modified PVA (modified polyvinyl alcohol, hereinafter also simply referred to as "PVA"), which is a saponified copolymer of raw material monomers including a vinyl ester-based monomer and a compound 3-A represented by the following formula (3-1):

[0116] The structural unit derived from compound 3-A contained in the polyvinyl alcohol polymer (hereinafter also referred to as "structural unit α") is preferred because it is less susceptible to decomposition during the saponification reaction.

[0117] The polyvinyl alcohol polymer is a saponified product obtained by further saponifying a copolymer (hereinafter also referred to as an "intermediate copolymer") obtained by copolymerizing a vinyl ester monomer with a raw material monomer containing the compound 3-A represented by the above formula (3-1). Some or all (depending on the degree of saponification) of the structural units derived from the vinyl ester monomer contained in the intermediate copolymer (vinyl ester monomer units) are converted into structural units having hydroxyl groups (hydroxyl group-containing vinyl units) through hydrolysis of the ester groups by saponification.

[0118] Furthermore, at least a portion of the structural units α may be crosslinked with crosslinkable structural units such as other structural units α (for example, in copolymerization of the intermediate copolymer). The intermediate copolymer and the polyvinyl alcohol-based polymer may have a crosslinked structure at least in part of the structural units α. The polyvinyl alcohol-based polymer has, for example, a hydroxyl group-containing vinyl unit, a vinyl ester-based monomer unit, and the structural unit α, and has a crosslinked structure at least in part of the structural units α.

[0119] Examples of the vinyl ester monomer include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, and these may be used alone or in combination of two or more. Preferably, the vinyl ester monomer includes vinyl acetate, and more preferably, the vinyl ester monomer is vinyl acetate.

[0120] The polyvinyl alcohol-based polymer preferably contains 0.001 to 1.0 mol %, more preferably 0.01 to 0.5 mol %, and even more preferably 0.05 to 0.3 mol % of structural units derived from compound 3-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer. The content of the structural units derived from Compound 3-A in the polyvinyl alcohol polymer is, for example, 0.001, 0.01, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.35, 0.40, 0.45, 0.50, or 1.0 mol%, based on 100 mol% of the structural units derived from the vinyl ester monomer contained in the polyvinyl alcohol polymer, and may be within a range between any two of the numerical values ​​exemplified here.

[0121] The content of the structural unit derived from Compound 3-A in the polyvinyl alcohol polymer (copolymerization amount of Compound 3-A) can be calculated, for example, based on the results of NMR measurement, trace nitrogen determination, etc. A calculation method based on NMR measurement in the case of using diallyl methyl isocyanurate and diallyl tetradecyl isocyanurate will be specifically explained in the examples described later.

[0122] The polyvinyl alcohol polymer preferably contains 90 to 100 mol %, more preferably 95 to 100 mol %, and even more preferably 99 to 100 mol % of structural units derived from vinyl ester monomers and structural units derived from Compound 3-A in total, based on 100 mol % of structural units contained in the polyvinyl alcohol polymer. Specific examples of the total content of structural units derived from vinyl ester monomers and structural units derived from Compound 3-A in 100 mol % of structural units contained in the polyvinyl alcohol polymer include 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, 99.9, and 100 mol %, and may be within a range between any two of the numerical values ​​exemplified here.

[0123] The intermediate copolymer may further contain other monomers copolymerizable with the vinyl ester monomer and compound 3-A, provided that the effects of the present invention are not impaired. Examples of such other monomers include α-olefin monomers such as ethylene and propylene; (meth)acrylic acid alkyl ester monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; unsaturated amide monomers such as (meth)acrylamide and N-methylolacrylamide; unsaturated carboxylic acid monomers such as (meth)acrylic acid, crotonic acid, maleic acid, itaconic acid, and fumaric acid; and alkyl (methyl, ethyl, propyl) carboxylic acids of unsaturated carboxylic acids. anhydrides of unsaturated carboxylic acids such as maleic anhydride; salts of unsaturated carboxylic acids with sodium, potassium, ammonium, or the like; glycidyl group-containing monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; sulfonic acid group-containing monomers such as 2-acrylamido-2-methylpropanesulfonic acid or salts thereof; phosphate group-containing monomers such as acid phosphooxyethyl methacrylate and acid phosphooxypropyl methacrylate; alkyl vinyl ether monomers; and the like.

[0124] The viscosity-average degree of polymerization of the polyvinyl alcohol polymer is preferably 2000 to 5000, more preferably 2200 to 3000. If the viscosity-average degree of polymerization is too low, the fluid loss reduction effect may be insufficient. If the viscosity-average degree of polymerization is too high, the viscosity of the cement slurry may increase, which may result in increased pump power and pressure loss during slurry transport. Specific examples of the viscosity-average degree of polymerization include 2000, 2100, 2200, 2300, 2400, 2500, 2600, 2700, 2800, 2900, 3000, 3500, 4000, 4500, and 5000, and may be within a range between any two of the values ​​exemplified here.

[0125] For the "viscosity average degree of polymerization", see the measurement and calculation method described in 1-1 above.

[0126] The degree of saponification of the polyvinyl alcohol polymer is preferably 80.0 to 99.9 mol %, more preferably 90.0 to 99.5 mol %. When this range is satisfied, the hydrophobicity of the remaining ester groups and the action of hydrogen bonds between the molecules of the polyvinyl alcohol polymer fall within preferred ranges, resulting in a sufficient fluid loss reduction effect. Specific examples of the saponification degree are 80.0, 85.0, 86.0, 87.0, 88.0, 89.0, 89.5, 90.0, 90.5, 91.0, 91.5, 92.0, 92.5, 93.0, 93.5, 94.0, 94.5, 95.0, 95.5, 96.0, 96.5, 97.0, 97.5, 98.0, 98.5, 99.0, 99.5, and 99.9 mol%, and may be within a range between any two of the values ​​exemplified here.

[0127] The "saponification degree" can be calculated by measuring in accordance with JIS K 6726:1994 "3.5 Saponification degree".

[0128] The polyvinyl alcohol polymer preferably has an average particle size of 50.0 to 120.0 nm, more preferably 59.0 to 80.0 nm, as calculated by cumulant analysis of the particle size distribution obtained by performing dynamic light scattering measurement on a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer at 25°C. When used as an additive for oil well cement, if the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 50 nm or more, the polyvinyl alcohol polymer is less likely to flow out of the cement slurry, improving fluid loss reduction performance. If the average particle size of the polyvinyl alcohol polymer in the 0.4% by mass aqueous solution is 120 nm or less, little gel-like deposits are generated during production. Specific examples of the average particle size include 50.0, 55.0, 55.5, 56.0, 56.5, 57.0, 57.5, 58.0, 58.5, 59.0, 59.5, 60.0, 60.5, 61.0, 61.5, 62.0, 62.5, 63.0, 63.5, 64.0, 64.5, 65.0, 65.5, 66.0, 66.5, 67.0, 67.5, 68.0, 68.5, 69.0, 69.5, 70.0, 80.0, 90.0, 100.0, 110.0, and 120.0 nm, and may be within a range between any two of the values ​​exemplified here. The particle size distribution can be adjusted, for example, by adding an alcohol such as methanol in the polymerization step or by the amount of copolymerization of compound 3-A.

[0129] The polyvinyl alcohol polymer preferably has a proportion of particles having a size of 400 nm or more of 10% or less, more preferably 5% or less, in a particle size distribution obtained by subjecting a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer to dynamic light scattering measurement at 25° C. Specific examples of the proportion of particles having a size of 400 nm or more include 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10%, and may be within a range between any two of the values ​​exemplified here.

[0130] When a 10% by mass aqueous solution of the polyvinyl alcohol polymer is filtered through a 100 mesh (0.154 mm mesh) filter and the resulting aqueous solution is measured, the slope of a logarithmic plot of the storage modulus G' (30°C) in the angular frequency range of 1.0 to 10.0 rad / s (slope of G') is preferably 1.2 to 1.5. Furthermore, the maximum value of the angular frequency-dependent loss tangent (tan δ) is preferably 10 to 100, more preferably greater than 10.0, for example, 11.0 to 50.0. Furthermore, the minimum value of the angular frequency-dependent loss tangent (tan δ) is preferably 3.0 to 8.0, more preferably 3.5 to 5.0. Viscoelastic properties can be adjusted, for example, by the amount of copolymerization or degree of polymerization of compound 3-A.

[0131] The viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol polymer at 20°C is preferably 30 to 80 mPa·s, more preferably 35 to 70 mPa·s, from the viewpoints of mixability, reduction in pump pressure during transfer, suppression of pressure loss, etc. The viscosity is measured at a shear rate of 10 s in accordance with the description of the rotational viscometer method in JIS K6726:1994 and JIS K 7117:1999. －１The viscosity can be calculated as follows. Specifically, the viscosity may be, for example, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, or 80 mPa·s, and may be within a range between any two of the values ​​exemplified here. Various methods can be used when mixing a polyvinyl alcohol-based polymer with cement. For example, cement, a polyvinyl alcohol-based polymer, and optionally other additives may be dry-blended, and the resulting mixture may be stirred and mixed with water to obtain a slurry, which may be transported to an oil well. In this case, if the viscosity of the aqueous solution, and therefore the viscosity of the polyvinyl alcohol-based polymer, is low, the stirring power is small, making mixing easier and reducing pressure loss. Furthermore, the polyvinyl alcohol-based polymer may be first mixed with water and then mixed with cement. Even in such cases, a low viscosity aqueous solution of the polyvinyl alcohol-based polymer may be required from the standpoint of mixability (stirring power) and suppression of air entrapment. From the standpoint of such mixing, a polyvinyl alcohol-based polymer satisfying the above range is preferred.

[0132] The viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol polymer at 20°C is preferably 300 to 30,000 mPa s, and more preferably 1,000 to 20,000 mPa s, from the viewpoint of the effect of reducing fluid loss at high temperatures. Specific examples of the viscosity include 300, 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000, 18000, 19000, 20000, 21000, 22000, 23000, 24000, 25000, 26000, 27000, 28000, 29000, and 30000 mPa·s, and may be within a range between any two of the values ​​exemplified here. Deep oil wells have high temperatures, and there is a demand for the effect of reducing fluid loss at higher temperatures. Cement slurries containing polyvinyl alcohol-based polymers tend to have a higher fluid loss reduction effect when the viscosity is high, and a polyvinyl alcohol-based polymer (oil well cement additive) that can increase the viscosity of cement compositions is desired. Therefore, for example, a high viscosity when dissolved in water, i.e., a viscosity of 300 mPa·s, is a preferred corresponding characteristic. On the other hand, if the viscosity of the cement slurry is too high, the pressure loss during slurry transfer will be too high, so a viscosity of 30,000 mPa·s or less is a preferred corresponding characteristic.

[0133] The polyvinyl alcohol-based polymer has a 300 mesh (0.045 mm mesh) filter penetration rate of preferably 95% by mass or more, more preferably 99 to 100% by mass, of a 1.0% by mass aqueous solution of the polyvinyl alcohol-based polymer, calculated as solid content. It is preferable that the polyvinyl alcohol-based polymer does not contain excessively large gel particles, from the viewpoint of the strength of the cement after hardening and the production of the polyvinyl alcohol-based polymer.

[0134] <Applications> The polyvinyl alcohol-based polymer can be used as an additive for oil well cement, and an additive for oil well cement according to one embodiment of the present invention comprises the polyvinyl alcohol-based polymer. Such an additive for oil well cement is suitable as an additive for oil well cement used in cementing oil wells, gas wells, steam wells for geothermal power generation, etc.

[0135] Cementing, which is performed when drilling a well, is the process of injecting cement into the gap between the drilled well and a steel pipe inserted into it. A widely used cementing method involves mixing cement and various additives in a dry state, then pumping the mixture into a slurry using high-pressure water. The oil well cement additive of the present invention reduces the loss of water content from the cement slurry during cementing (i.e., reduces fluid loss), making it possible to maintain the fluidity of the cement slurry. If fluid loss is large, the fluidity of the cement slurry is lost, making it difficult to perform sufficient cementing.

[0136] For details about fluid loss, see the explanation in 1-1 above.

[0137] The fluid loss of the polyvinyl alcohol polymer measured by this test method under conditions of 0.5% bwoc and 80°C is, for example, 50 cc or less, preferably 46 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 20 cc or more. Furthermore, the fluid loss under conditions of 0.8% bwoc and 110°C (high temperature conditions) is, for example, 90 cc or less, preferably 60 cc or less. The lower limit is not particularly limited, but may be, for example, 0 cc or more or 30 cc or more.

[0138] 3-2. Method for Producing Polyvinyl Alcohol-Based Polymer Powder The method for producing a polyvinyl alcohol-based polymer powder according to one embodiment of the present invention can include a pulverization step. In the pulverization step, the polyvinyl alcohol-based polymer obtained by the above-described method for producing a polyvinyl alcohol-based polymer (e.g., heat-dried) is pulverized. The pulverization method is not particularly limited, and known pulverization techniques can be used. Suitable pulverization devices include rotary blade pulverizers, turbo mill pulverizers, turbo disc mill pulverizers, turbo cutter pulverizers, jet mill pulverizers, impact pulverizers, hammer pulverizers, and vibration pulverizers. For example, an ACM pulverizer can be used as the pulverization device.

[0139] The method for producing powder may include a classification step of classifying the powder after pulverization. The classification method is not particularly limited, and may include a method using a sieve or a method utilizing the difference in the settling velocity or movement distance of particles in a fluid such as air or water. Pulverization devices such as ACM pulverizers are also equipped with a classification function using an air flow classifier, and classification may be performed using such a device, for example.

[0140] In the particle size distribution of the pulverized polyvinyl alcohol polymer powder, the content of particles with a particle size of less than 75 μm (under 75 μm sieve) is preferably 30% by mass or less, more preferably 8% by mass or less. Furthermore, in the particle size distribution, the content of particles with a particle size of 500 μm or more (over 500 μm sieve) is preferably 10% by mass or less, more preferably 0.05% by mass or less. If the polyvinyl alcohol polymer powder contains a large number of excessively large particles, the strength of the hardened cement will decrease. The particle size distribution can be measured by sieving using sieves with 75 μm and 500 μm openings in accordance with JIS Z 8815:1994.

[0141] 3-3. Method for producing cement slurry A method for producing a cement slurry according to one embodiment of the present invention includes a cement mixing step. The cement mixing step includes a cement mixing step in which the polyvinyl alcohol-based polymer obtained by the method for producing a polyvinyl alcohol-based polymer described above is mixed with a cement raw material to obtain the cement slurry. The polyvinyl alcohol-based polymer to be mixed may be a powder of a pulverized (and further classified) polyvinyl alcohol-based polymer.

[0142] The cement slurry may contain the polyvinyl alcohol-based polymer. The cement slurry is, for example, a composition containing the polyvinyl alcohol-based polymer, cement, and water.

[0143] The cement slurry may contain other additives as needed. The other additives may be mixed together with the polyvinyl alcohol-based polymer in the cement mixing step, or may be mixed before or after mixing the polyvinyl alcohol-based polymer.

[0144] The method for adding the polyvinyl alcohol-based polymer to the cement slurry is not particularly limited, and conventional methods such as a method of previously mixing the polyvinyl alcohol-based polymer with dry cement, a method of mixing the polyvinyl alcohol-based polymer when forming a cement slurry, or a method of mixing the polyvinyl alcohol-based polymer with previously mixed water and then mixing the polyvinyl alcohol-based polymer with cement can be used.

[0145] The content of the polyvinyl alcohol polymer in the cement slurry is, for example, 0.01 to 30% bwoc, preferably 0.05 to 10% bwoc, and more preferably 0.1 to 5% bwoc. The term "by weight of cement" (bwoc) refers to the weight of the additive in dry form added to the cement composition based on the cement solids content only.

[0146] The present invention will be described in more detail below with reference to examples, but these examples are merely illustrative and are not intended to limit the scope of the present invention.

[0147] <<First Aspect>> <Preparation of Polyvinyl Alcohol-Based Polymer> [Example 1-1] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and the mixture was stirred under a nitrogen stream at the boiling point for 8 hours to polymerize (conversion of vinyl acetate: 56.7%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate-based polymer (intermediate copolymer) was obtained.

[0148] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out for 45 minutes at 45° C. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a degree of saponification of 90.7 mol%.

[0149] The polyvinyl alcohol polymer obtained after drying was pulverized using an ACM Pulverizer (manufactured by Hosokawa Micron Corporation) and classified.

[0150] [Example 1-2] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1-1, except that the methanol solution of the vinyl acetate polymer of Example 1-1 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 94.7 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1-1 was pulverized and classified.

[0151] [Example 1-3] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1-1, except that the methanol solution of the vinyl acetate polymer of Example 1-1 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 99.1 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1-1 was pulverized and classified.

[0152] [Example 1-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 4 hours with stirring under a nitrogen stream (vinyl acetate conversion: 54.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0153] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 90.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0154] [Example 1-5] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1-4, except that the methanol solution of the vinyl acetate polymer of Example 1-4 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 95.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0155] Comparative Example 1-1 The polymerization step, saponification step, solvent removal step, pulverization step, and classification step were carried out in the same manner as in Example 1, except that polymerization was carried out without adding a comonomer, and the amount of methanol added and the conversion rate of vinyl acetate were changed.

[0156] Comparative Example 1-2 A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS (abbreviated as "TAIC" in the tables), manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 8 hours with stirring under a nitrogen stream (vinyl acetate conversion: 56.0%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0157] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0158] [Comparative Example 1-3] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.002 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 42.4%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0159] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0160] [Comparative Example 1-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.33 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 150 parts by mass of methanol as a solvent, and 0.07 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (conversion of vinyl acetate: 69.1%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0161] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 80.8 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0162] [Comparative Example 1-5] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.006 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 8 hours with stirring under a nitrogen stream (vinyl acetate conversion: 65.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0163] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 89.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 1-1.

[0164] <Measurement and Evaluation> The physical properties and characteristics of the polyvinyl alcohol polymer (PVA) obtained in each Example and Comparative Example were measured. The results are shown in Table 1-1. The measurement results of the fluid loss of the cement slurry when no PVA was added are shown as Reference Example 1-1.

[0165]

[0166] [Viscosity-average degree of polymerization] The viscosity-average degree of polymerization of the PVAs obtained in the Examples and Comparative Examples was calculated from the intrinsic viscosity [η] (g / dL) measured at 30°C using an Ostwald viscometer in ion-exchanged water as a solvent in accordance with JIS K 6726:1994 "3.7 Average degree of polymerization" using the following formula (A). P represents the viscosity-average degree of polymerization. log(P)=1.613×log([η]×10 ４ / 8.29) ...(A)

[0167] [Saponification Degree] The saponification degree of the PVA obtained in the Examples and Comparative Examples was measured in accordance with JIS K 6726:1994 "3.5 Saponification Degree." That is, the saponification degree was determined by back titration using N / 10 sulfuric acid as specified in JIS K 8951:2006 and N / 10 sodium hydroxide solution as specified in JIS K 8576:2019.

[0168] [Viscosity of 4% aqueous solution] The PVAs obtained in the Examples and Comparative Examples were measured at a shear rate of 10 s in accordance with JIS K 6726:1994 "4.2 Viscosity measurement" and the rotational viscometer method of JIS K 7117:1999. －１ The viscosity of an aqueous solution adjusted to 4% by mass was measured.

[0169] [Amount of copolymerization] For the PVAs obtained above according to Examples 1-1 to 1-5 and Comparative Example 1-5, the amount of copolymerization (mol %) of diallyl methyl isocyanurate was calculated by the following method. １ The H-NMR analysis was performed. Fully saponified PVA was prepared by the following procedure. After removing unreacted vinyl acetate from the polymerization system, a methanol solution of a vinyl acetate polymer (intermediate copolymer) was dried in a dryer. 12 g of the dried sample and 388 g of methanol were added to a flask and dissolved at 40°C for 1 hour. 20 mL of a 10% NaOH methanol solution was then added, and the mixture was allowed to react at 40°C for 1 hour. 3 mL of acetic acid was then added for neutralization, filtered, and the residue, mainly composed of PVA, was washed by Soxhlet extraction using methanol to obtain fully saponified PVA. The obtained fully saponified PVA was dried in a dryer at 90°C for 1 hour. The dried PVA was dissolved in heavy water and analyzed using an NMR (ECX-400, manufactured by JEOL Ltd.) at a measurement temperature of 80°C and an accumulation number of 128. １ A H-NMR spectrum was obtained. From the obtained spectrum, the integral value of the peak (3.2 to 3.4 ppm) derived from the methyl group on the nitrogen atom was defined as a, and the integral value of the peak (1.3 to 2 ppm) derived from the methylene group to which the hydroxyl group of the hydroxyl group-containing vinyl unit is not adjacent and the methylene group to which the ester group of the vinyl ester monomer unit is not adjacent was defined as b. The copolymerization amount X (mol %) of diallylmethyl isocyanurate in the PVA was calculated from the following formula (B): X = ((1 / 3)a / (b / 2)) × 100 (B)

[0170] For the PVAs according to Comparative Examples 1-2 to 1-4 using triallyl isocyanurate, the copolymerization amount was calculated using a trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytech Co., Ltd.) (for detailed methods, see WO 2023 / 238674).

[0171] [Particle Size (Dynamic Light Scattering Measurement)] Dynamic light scattering measurements were performed on the PVAs obtained in the Examples and Comparative Examples to calculate the average particle size and the percentage of particles with a particle size of 400 nm or more. A 0.4% by mass aqueous solution of PVA was prepared and filtered through a 300 mesh (0.045 mm mesh) filter. The filtrate was placed in a quartz cell and measured using a dynamic light scattering measurement device (ELS-Z2, manufactured by Otsuka Electronics Co., Ltd.). The measurement conditions were 25°C, solvent: water, solvent refractive index: 1.33, and solvent viscosity: 0.89 (cP). From the measurement results, the average particle size of the PVA was obtained by cumulant analysis. Furthermore, from the measurement results, the percentage of particles with a particle size of 400 nm or more was calculated based on the scattered light intensity.

[0172] [300 Mesh Filter Passage Rate] The passage rate of the PVA obtained in the Examples and Comparative Examples through a 300 mesh filter was measured. PVA was dissolved in water at 25°C to obtain an aqueous solution with a concentration of 1.0% by mass. 100 mL of the resulting PVA aqueous solution was filtered through a 300 mesh filter (opening size: 0.045 mm), and the mass of the PVA remaining on the filter was measured. The proportion of PVA that passed through the filter was calculated from the measured mass of the PVA residue.

[0173] [Viscoelasticity Measurement] For the PVAs obtained in the Examples and Comparative Examples, the slope of the storage modulus G' and the maximum and minimum values ​​of the loss tangent (tan δ) were determined using the following method. A 10% by mass or greater aqueous solution of PVA was prepared and filtered through a 100-mesh (0.154 mm mesh) filter. The concentration of the filtered aqueous solution was measured, and if the concentration was 10% by mass or greater, pure water was added to adjust the concentration to 10.0% by mass. The dynamic viscoelasticity of the prepared solution was measured using a rheometer (Anton Paar MCR). A cone plate CP50-2 (diameter: 50 mm, angle: 2°) was used. The measurement conditions were a temperature of 30°C, a shear strain (oscillation) of 1 to 5% (logarithmic slope), and an angular frequency of 628.3 to 0.06 rad / s (linear slope), and the storage modulus G' and loss modulus G'' were obtained. The slope of the storage modulus G' was calculated as the slope when the storage modulus G' (30°C) in the angular frequency range of 1.0 to 10.0 rad / s was plotted logarithmically. The maximum and minimum values ​​of the angular frequency-dependent loss tangent (tan δ) were calculated as the maximum and minimum values ​​of the loss tangent (tan δ) calculated by (loss modulus G'' / storage modulus G').

[0174] [Particle size distribution of powder] The pulverized PVA obtained in the examples and comparative examples was sieved using sieves with 75 μm and 500 μm openings in accordance with JIS Z 8815:1994 to measure the particle size distribution.

[0175] Fluid Loss The fluid loss reduction effect of PVA was measured according to the fluid loss evaluation method of the American Petroleum Institute (API) Standard 10B-2 (April 2013). The specific measurement procedure is as follows: Class G oil well cement was blended with the amounts of PVA and 0.4% bwoc of a set retarder (CR-270, manufactured by Flotek Industries) shown in the table, and then mixed with water according to the procedure described in the American Petroleum Institute (API) Standard 10B-2 (April 2013) to obtain a cement slurry with a water content of 30% by mass. The obtained cement slurry was placed in a fluid loss evaluation tester (Model 7120, Chandler Engineering), and a test was carried out at the temperature shown in the table under a pressure of 1000 psi according to the procedure described in American Petroleum Institute (API) Standard 10B-2 (April 2013), to calculate the amount of fluid loss.

[0176] As shown in Table 1-1, the evaluation was carried out at temperatures ranging from 20°C to 110°C and with PVA amounts ranging from 0.25 to 0.8% bwoc. In Table 1-1, cases in which fluid loss measurement was not carried out are indicated by "-".

[0177] Examples 1-1 to 1-5 had low aqueous solution viscosities and small fluid losses at both 80°C and 110°C. On the other hand, Comparative Examples 1-1 and 1-5 had large fluid losses already at 80°C. Comparative Examples 1-2 and 1-3 did not have sufficiently low aqueous solution viscosities. Comparative Example 1-4 had large fluid losses at 110°C.

[0178] <<Second Aspect>> <Preparation of Polyvinyl Alcohol-Based Polymer> [Example 2-1] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.008 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and the mixture was stirred under a nitrogen stream at the boiling point for 4 hours to polymerize (conversion of vinyl acetate: 55.1%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate-based polymer (intermediate copolymer) was obtained.

[0179] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out for 45 minutes at 45° C. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a degree of saponification of 90.7 mol%.

[0180] The polyvinyl alcohol polymer obtained after drying was pulverized using an ACM Pulverizer (manufactured by Hosokawa Micron Corporation) and classified.

[0181] [Example 2-2] A polyvinyl alcohol polymer was obtained in the same manner as in Example 2-1, except that the methanol solution of the vinyl acetate polymer of Example 2-1 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 94.7 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 2-1 was pulverized and classified.

[0182] [Example 2-3] A polyvinyl alcohol polymer was obtained in the same manner as in Example 2-1, except that the methanol solution of the vinyl acetate polymer of Example 2-1 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 99.1 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 2-1 was pulverized and classified.

[0183] [Example 2-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.009 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 3.4 hours with stirring under a nitrogen stream (vinyl acetate conversion: 35.6%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0184] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 90.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0185] [Example 2-5] A polyvinyl alcohol polymer was obtained in the same manner as in Example 2-4, except that the methanol solution of the vinyl acetate polymer of Example 2-4 was used and the saponification degree of the polyvinyl alcohol polymer was changed to 95.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized in the same manner as in Example 2-1.

[0186] Comparative Example 2-1 The polymerization step, saponification step, solvent removal step, pulverization step, and classification step were carried out in the same manner as in Example 2-1, except that polymerization was carried out without adding a comonomer, and the amount of methanol added and the conversion rate of vinyl acetate were changed.

[0187] Comparative Example 2-2 A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS (abbreviated as "TAIC" in the tables), manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (conversion of vinyl acetate: 56.0%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0188] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a vinyl alcohol polymer with a saponification degree of 88.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0189] [Comparative Example 2-3] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.002 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 42.4%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0190] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0191] [Comparative Example 2-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.33 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 150 parts by mass of methanol as a solvent, and 0.013 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 69.1%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0192] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.006 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 80.8 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0193] [Comparative Example 2-5] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 8 hours with stirring under a nitrogen stream (vinyl acetate conversion: 44.5%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0194] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 92 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0195] Comparative Example 2-6: A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.08 parts by mass of 1,5-hexadiene (Tokyo Chemical Industry Co., Ltd.), 21.2 parts by mass of methanol as a solvent, and 0.03 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 49.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0196] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 2-1.

[0197] <Measurement and Evaluation> The physical properties and characteristics of the polyvinyl alcohol polymer (PVA) obtained in each Example and Comparative Example were measured. The results are shown in Table 2-1. The measurement results of the fluid loss of the cement slurry when no PVA was added are shown as Reference Example 2-1.

[0198]

[0199] The viscosity average degree of polymerization, the degree of saponification, and the viscosity of a 4% aqueous solution were measured and calculated in the same manner as in the first aspect.

[0200] [Amount of copolymerization] For the PVAs obtained above according to Examples 2-1 to 2-5 and Comparative Example 2-5, the amount of copolymerization (mol %) of diallyl tetradecyl isocyanurate was calculated by the following method. １ The hydroxyl group content was determined by H-NMR analysis. Fully saponified PVA was prepared by the following procedure. After removing unreacted vinyl acetate from the polymerization system, a methanol solution of a vinyl acetate polymer (intermediate copolymer) was dried in a dryer. 12 g of the dried sample and 388 g of methanol were added to a flask and dissolved at 40°C for 1 hour. 20 mL of a 10% NaOH methanol solution was then added, and the mixture was allowed to react at 40°C for 1 hour. 3 mL of acetic acid was then added for neutralization, filtered, and the residue, primarily composed of PVA, was washed by Soxhlet extraction with methanol to obtain fully saponified PVA. The resulting fully saponified PVA was dried in a dryer at 90°C for 1 hour. The dried PVA was dissolved in heavy water, and a 1H-NMR spectrum was obtained using an NMR (ECX-400, manufactured by JEOL Ltd.) at a measurement temperature of 80°C and an accumulation count of 128. From the obtained spectrum, the integral value of the peak (0.7 to 0.9 ppm) derived from the methyl group at the end of the tetradecyl group on the nitrogen atom was defined as a, and the integral value of the peak (1.3 to 2 ppm) derived from the methylene group to which the hydroxyl group of the hydroxyl group-containing vinyl unit is not adjacent and the methylene group to which the ester group of the vinyl ester monomer unit is not adjacent was defined as b, and the copolymerization amount X (mol %) of diallyltetradecyl isocyanurate in the PVA was calculated from the following formula (B): X = ((1 / 3)a / (b / 2)) × 100 ... (B)

[0201] For the PVAs according to Comparative Examples 2-2 to 2-4 using triallyl isocyanurate, the copolymerization amount was calculated using a trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytech Co., Ltd.) (for detailed methods, see WO 2023 / 238674).

[0202] For the PVA according to Comparative Example 2-6 using 1,5-hexadiene, the copolymerization amount of each monomer was calculated based on the reactivity ratio.

[0203] The measurement and calculation of [particle size (dynamic light scattering measurement)], [300 mesh filter penetration rate], and [powder particle size distribution] were carried out in the same manner as in the first aspect.

[0204] [Fluid Loss] The fluid loss reduction effect of PVA was measured in the same manner as in the first aspect, according to the fluid loss evaluation method of the American Petroleum Institute (API) standard 10B-2 (April 2013).

[0205] As shown in Table 2-1, the evaluation was carried out at temperatures ranging from 20°C to 110°C and with PVA amounts ranging from 0.25 to 0.8% bwoc. In Table 2-1, cases where fluid loss was not measured are indicated by "-".

[0206] Examples 2-1 to 2-5 had high aqueous solution viscosities and small fluid losses at both 80°C and 110°C. On the other hand, Comparative Examples 2-1 and 2-5 to 2-6 had low aqueous solution viscosities and large fluid losses already at 80°C. Comparative Examples 2-2 and 2-3 had low aqueous solution viscosities. Comparative Example 2-4 had low aqueous solution viscosity and large fluid loss at 110°C.

[0207] <<Third Aspect>> <Preparation of Polyvinyl Alcohol-Based Polymer> [Example 3-1] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and the mixture was stirred under a nitrogen stream at the boiling point for 8 hours to polymerize (conversion of vinyl acetate: 56.7%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate-based polymer (intermediate copolymer) was obtained.

[0208] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out for 45 minutes at 45° C. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a degree of saponification of 90.7 mol%.

[0209] The polyvinyl alcohol polymer obtained after drying was pulverized using an ACM Pulverizer (manufactured by Hosokawa Micron Corporation) and classified.

[0210] Example 3-2: A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 4 hours with stirring under a nitrogen stream (vinyl acetate conversion: 54.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0211] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 90.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0212] Example 3-3: A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.008 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 4 hours with stirring under a nitrogen stream (conversion of vinyl acetate: 55.1%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0213] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out for 45 minutes at 45° C. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a degree of saponification of 90.7 mol%.

[0214] The polyvinyl alcohol polymer obtained after drying was pulverized using an ACM Pulverizer (manufactured by Hosokawa Micron Corporation) and classified.

[0215] [Example 3-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.009 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 3.4 hours with stirring under a nitrogen stream (vinyl acetate conversion: 35.6%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0216] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 90.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0217] [Comparative Example 3-1] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (conversion of vinyl acetate: 56.0%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0218] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0219] Comparative Example 3-2 A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAICROS (abbreviated as "TAIC" in the tables), manufactured by EVONIC Corporation), 66.7 parts by mass of methanol as a solvent, and 0.002 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 42.4%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0220] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.1 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0221] [Comparative Example 3-3] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.33 parts by mass of triallyl isocyanurate (TAICROS, manufactured by EVONIC Corporation), 150 parts by mass of methanol as a solvent, and 0.013 parts by mass of di-n-propyl peroxydicarbonate (PERROYL NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (conversion of vinyl acetate: 69.1%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0222] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.006 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 80.8 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0223] [Comparative Example 3-4] A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallyl methyl isocyanurate (MeDAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.006 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 8 hours with stirring under a nitrogen stream (vinyl acetate conversion: 65.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0224] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 89.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0225] Comparative Example 3-5: A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyl tetradecyl isocyanurate (L-DAIC, manufactured by Shikoku Chemical Industries, Ltd.), 66.7 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 8 hours with stirring under a nitrogen stream (vinyl acetate conversion: 44.5%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0226] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 90 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 92.0 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0227] Comparative Example 3-6: A polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer was charged with 100 parts by mass of vinyl acetate, 0.08 parts by mass of 1,5-hexadiene (Tokyo Chemical Industry Co., Ltd.), 21.2 parts by mass of methanol as a solvent, and 0.03 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, NOF Corporation) as a polymerization initiator, and polymerization was carried out at the boiling point for 5 hours with stirring under a nitrogen stream (vinyl acetate conversion: 49.3%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of a vinyl acetate polymer (intermediate copolymer) was obtained.

[0228] To the methanol solution of the vinyl acetate polymer obtained above, a methanol solution of sodium hydroxide (0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was dried by heating to obtain a polyvinyl alcohol polymer with a saponification degree of 88.2 mol%. The polyvinyl alcohol polymer obtained by drying was pulverized and classified in the same manner as in Example 3-1.

[0229] <Measurements and Evaluations> The physical properties and characteristics of the polyvinyl alcohol polymers (PVA) obtained in each of the Examples and Comparative Examples were measured, and the results are shown in Table 3-1.

[0230]

[0231] The viscosity average degree of polymerization, the degree of saponification, and the viscosity of a 4% aqueous solution were measured and calculated in the same manner as in the first aspect.

[0232] [Amount of Copolymerization] For the PVAs obtained above in Examples 3-1 to 3-4 and Comparative Examples 3-4 to 3-5, the amounts of copolymerization (mol %) of diallyl methyl isocyanurate and diallyl tetradecyl isocyanurate were calculated using the following method. The amount of copolymerization was calculated by 1H-NMR analysis of the fully saponified PVA. The fully saponified PVA was prepared using the following procedure. After removing unreacted vinyl acetate from the polymerization system, a methanol solution of a vinyl acetate polymer (intermediate copolymer) was dried to solidification in a dryer. 12 g of the dried sample and 388 g of methanol were added to a flask and dissolved at 40°C for 1 hour. 20 mL of a 10% NaOH methanol solution was then added, and the mixture was allowed to react at 40°C for 1 hour. 3 mL of acetic acid was then added for neutralization, filtered, and the residue, primarily composed of PVA, was washed by Soxhlet extraction with methanol to obtain a fully saponified PVA. The resulting fully saponified PVA was dried in a dryer at 90°C for 1 hour. The dried PVA was dissolved in heavy water, and a 1H-NMR spectrum was obtained using an NMR (ECX-400, manufactured by JEOL Ltd.) at a measurement temperature of 80°C and an accumulation count of 128. (Diallylmethyl isocyanurate) From the obtained spectrum, the integral value of the peak (3.2 to 3.4 ppm) derived from the methyl group on the nitrogen atom was defined as a, and the integral value of the peak (1.3 to 2 ppm) derived from the methylene group to which the hydroxyl group of the hydroxyl group-containing vinyl unit is not adjacent and the methylene group to which the ester group of the vinyl ester monomer unit is not adjacent was defined as b, and the copolymerization amount X (mol %) of diallylmethyl isocyanurate in the PVA was calculated from the following formula (B): (Diallyltetradecyl isocyanurate) From the obtained spectrum, the integral value of the peak (0.7 to 0.9 ppm) derived from the methyl group at the end of the tetradecyl group on the nitrogen atom was defined as a, and the integral value of the peak (1.3 to 2 ppm) derived from the methylene group to which the hydroxyl group of the hydroxyl group-containing vinyl unit is not adjacent and the methylene group to which the ester group of the vinyl ester monomer unit is not adjacent was defined as b, and the copolymerization amount X (mol %) of diallyltetradecyl isocyanurate in the PVA was calculated from the following formula (B): X = ((1 / 3)a / (b / 2)) × 100 ... (B)

[0233] For the PVAs according to Comparative Examples 3-1 to 3-3 using triallyl isocyanurate, the copolymerization amount was calculated using a trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytech Co., Ltd.) (for detailed methods, see WO 2023 / 238674).

[0234] For the PVA according to Comparative Example 3-6 using 1,5-hexadiene, the copolymerization amount of each monomer was calculated based on the reactivity ratio.

[0235] The measurements and calculations of [particle size (dynamic light scattering measurement)], [300 mesh filter penetration rate], [viscoelasticity measurement], and [powder particle size distribution] were carried out in the same manner as in the first aspect. When no measurement was carried out, this is indicated by "-".

[0236] [Fluid Loss] The fluid loss reduction effect of PVA was measured in the same manner as in the first aspect, according to the fluid loss evaluation method of the American Petroleum Institute (API) standard 10B-2 (April 2013).

[0237] As shown in Table 3-1, the evaluation was carried out at temperatures ranging from 20°C to 110°C and with PVA amounts ranging from 0.25 to 0.8% bwoc. In Table 3-1, cases where fluid loss was not measured are indicated by "-".

[0238] [Adhesion] In each Example and Comparative Example, after polymerization under the respective conditions, the polymerization liquid was recovered and the polymerization process was repeated four times without cleaning the polymerization vessel (the saponification process was carried out by transferring to a separate vessel). The state of adhesion of gel-like matter to the polymerization vessel was visually confirmed. The state of adhesion of gel-like matter was evaluated according to the following criteria. Note that gel-like matter means a substance that is insoluble in methanol and does not dissolve even when boiled for four hours or more while maintaining its gel shape. ○: Adhesion of gel-like matter to the polymerization vessel was not clearly observed ×: Adhesion of gel-like matter to the polymerization vessel was clearly observed

[0239] In Examples 3-1 to 3-4, fluid loss was small at both 80°C and 110°C, and gel-like deposits were suppressed. On the other hand, in Comparative Examples 3-1 and 3-2, adhesion of gel-like material to the polymerization vessel was clearly observed. In Comparative Example 3-3, adhesion of gel-like material to the polymerization vessel was clearly observed. Furthermore, in Comparative Example 3-3, fluid loss was small at 80°C, but fluid loss was large at 110°C, and it cannot be said that a sufficient fluid loss reduction effect was achieved under high temperature conditions. In Comparative Examples 3-4 to 3-6, fluid loss was already large at 80°C.

Claims

1. A saponified copolymer of a vinyl ester monomer and a raw material monomer containing compound 1-A represented by the following formula (1-1): In the formula (1-1), R is a linear, branched, or cyclic alkyl group having 1 to 7 carbon atoms, and the viscosity-average degree of polymerization is 2,000 to 5,000.

2. A saponified copolymer of a vinyl ester monomer and a raw material monomer containing compound 2-A represented by the following formula (2-1): In the formula (2-1), R is a linear, branched, or cyclic alkyl group having 8 to 18 carbon atoms, and the viscosity-average degree of polymerization is 1,800 to 5,000.

3. The polyvinyl alcohol polymer according to claim 1 or 2, having a degree of saponification of 80.0 to 99.9 mol %.

4. The polyvinyl alcohol-based polymer according to claim 1 or 2, which contains 0.001 to 1.0 mol % of structural units derived from compound 2-A relative to 100 mol % of structural units derived from vinyl ester-based monomers contained in the polyvinyl alcohol-based polymer.

5. The polyvinyl alcohol polymer according to claim 1 or 2, wherein the vinyl ester monomer includes vinyl acetate.

6. The polyvinyl alcohol-based polymer according to claim 1 or 2, wherein the average particle size calculated by cumulant analysis of the particle size distribution obtained by subjecting a 0.4 mass % aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C is 50.0 to 120.0 nm.

7. The polyvinyl alcohol-based polymer according to claim 1 or 2, wherein in a particle size distribution obtained by subjecting a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer to dynamic light scattering measurement at a temperature of 25°C, the proportion of particles having a size of 400 nm or more is 10% or less.

8. The polyvinyl alcohol-based polymer according to claim 1, wherein, when a 10% by mass aqueous solution of the polyvinyl alcohol-based polymer is filtered through a 100 mesh (0.154 mm mesh) filter and the aqueous solution is measured, the slope of a double logarithmic plot of the storage modulus G' (30°C) in the angular frequency range of 1.0 to 10.0 rad / s is 1.2 to 1.5, the maximum value of the angular frequency-dependent loss tangent (tan δ) is 10.0 to 100.0, and the minimum value of the angular frequency-dependent loss tangent (tan δ) is 3.0 to 8.

0.

9. The polyvinyl alcohol-based polymer according to claim 1, wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer has a viscosity of 30 to 80 mPa·s at 20°C.

10. The polyvinyl alcohol-based polymer according to claim 2, wherein a 4% by mass aqueous solution of the polyvinyl alcohol-based polymer has a viscosity of 300 to 30,000 mPa·s at 20°C.

11. A powder of the polyvinyl alcohol polymer according to claim 1 or 2, wherein in the particle size distribution, the content of particles having a particle size of less than 75 μm is 30% by mass or less, and the content of particles having a particle size of 500 μm or more is 10% by mass or less.

12. An additive for oil well cement, comprising the polyvinyl alcohol polymer according to claim 1 or 2.

13. A cement slurry containing the polyvinyl alcohol polymer according to claim 1 or 2.

14. A method for producing a polyvinyl alcohol-based polymer, comprising: a polymerization step of polymerizing raw material monomers including a vinyl ester-based monomer and a compound 3-A represented by the following formula (3-1) to obtain a copolymer; and a saponification step of saponifying the copolymer to obtain a polyvinyl alcohol-based polymer, In the formula (3-1), R represents a linear, branched, or cyclic alkyl group, and the polyvinyl alcohol polymer has a viscosity-average degree of polymerization of 2,000 to 5,000.

15. The method according to claim 14, wherein the degree of saponification of the polyvinyl alcohol polymer is 80.0 to 99.9 mol %.

16. The production method according to claim 14, wherein the polyvinyl alcohol polymer contains 0.001 to 1.0 mol % of structural units derived from compound 3-A relative to 100 mol % of structural units derived from vinyl ester monomers.

17. The method of claim 14, wherein the vinyl ester monomer comprises vinyl acetate.

18. The manufacturing method according to claim 14, wherein the average particle size calculated by cumulant analysis of the particle size distribution obtained by performing dynamic light scattering measurement at 25°C on a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer is 50.0 to 120.0 nm.

19. The manufacturing method described in claim 14, wherein the proportion of particles of 400 nm or more is 10% or less in the particle size distribution obtained by performing dynamic light scattering measurement on a 0.4 mass% aqueous solution of the polyvinyl alcohol-based polymer at a temperature of 25°C.

20. The method of claim 14, wherein R is a linear, branched, or cyclic alkyl group having 1 to 18 carbon atoms.

21. The method according to claim 14, wherein the viscosity of a 4% by mass aqueous solution of the polyvinyl alcohol polymer is 30 to 80 mPa·s.

22. A method for producing a powder of a polyvinyl alcohol-based polymer, comprising a pulverization step of pulverizing a polyvinyl alcohol-based polymer obtained by the production method according to any one of claims 14 to 21 to obtain the powder, wherein the powder has a particle size distribution in which the content of particles having a particle size of less than 75 μm is 30 mass % or less and the content of particles having a particle size of 500 μm or more is 10 mass % or less.

23. A method for producing a cement slurry, comprising a step of mixing a polyvinyl alcohol-based polymer obtained by the production method according to any one of claims 14 to 21 with a cement raw material to obtain the cement slurry.

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