Polyvinyl alcohol polymer

Abstract

This polyvinyl alcohol-based polymer is obtained through saponification of a copolymer of a vinyl ester monomer and a trifunctional monomer, and is characterized in that the average degree of polymerization of the polyvinyl alcohol-based polymer as measured using a method described in JIS K 6726:1994 is 3000-6000, and the average reactive number of unsaturated sites of the trifunctional monomer is 1.7-2.3.

Description

[Technical Field]

[0001] The present invention relates to a polyvinyl alcohol polymer, its use, and a production method. [Background technology]

[0002] Polymers having a polyvinyl alcohol (PVA) skeleton (hereinafter, in this specification, these are collectively referred to as "polyvinyl alcohol-based polymers," "vinyl alcohol-based polymers," or simply "PVA") are known as hydrophilic synthetic resins, and various applications that take advantage of these properties are being developed.

[0003] One such application is as an additive to oil well cement, which is used in cementing oil, gas, and geothermal steam wells. Oil well cement is mixed with water and other additives to form a slurry, which fills the gap between the steel pipe (casing) and the wellbore to secure and protect it. Therefore, a highly fluid cement slurry is desirable for ease of filling. The loss of water from the cement slurry due to high pressure during injection and underground heat is commonly referred to as "fluid loss." Fluid loss reduces the fluidity of the cement slurry, resulting in poor cementing and poor hardening after hardening. For this reason, fluid loss reducing agents are typically added to oil well cement slurries.

[0004] It has been proposed to use polyvinyl alcohol polymers as the main component of such fluid loss reducing agents, and for example, Patent Documents 1 and 2 describe PVA used as a fluid loss reducing agent.

[0005] However, in recent years, as shale gas wells, in particular, have been drilled deeper, the pressure and temperature conditions have become more severe. Conventional PVA-containing fluid loss reduction agents, as described above, have not yet achieved the fluid loss reduction performance required for cement slurries injected under such severe conditions of high temperature and pressure. Specifically, PVA-based fluid loss reduction agents dissolve at high temperatures and, under even higher pressures, leach into the wellbore, resulting in reduced fluid loss reduction. Therefore, PVA-based fluid loss reduction agents must be resistant to dissolution at high temperatures. Furthermore, increasing the amount of conventional fluid loss reduction agents added cannot solve the problem of increased viscosity of the cement slurry, resulting in reduced fluidity and increased costs.

[0006] To address the above-mentioned problems, Patent Document 3 discloses an additive for oil well cement, which contains a vinyl alcohol polymer that is a saponified copolymer of a vinyl ester monomer and a polyfunctional monomer, and has a saponification degree of 70 to 95 mol% and a viscosity-average polymerization degree of 1,000 to 10,000.

[0007] Patent Document 4 discloses a polyvinyl alcohol polymer obtained by saponifying a homopolymer made of a vinyl ester monomer, a copolymer of a vinyl ester monomer and a monofunctional monomer other than a vinyl ester, a copolymer of a vinyl ester monomer and a polyfunctional monomer, or a copolymer of a vinyl ester monomer, a monofunctional monomer other than a vinyl ester, and a polyfunctional monomer, wherein the particle size at 50% cumulative frequency in the particle size distribution of a 0.4% by mass aqueous solution of the polyvinyl alcohol polymer is measured by dynamic light scattering at a temperature of 25°C is 50 nm or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2007 / 146348 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-196733 [Patent Document 3] International Publication No. 2019 / 163490 [Patent Document 4] International Publication No. 2022 / 024792 Summary of the Invention [Problem to be solved by the invention]

[0009] However, it has become known that in conventional polymerization reactions of polyfunctional monomers and vinyl ester monomers, crosslinked products are formed during polymerization, resulting in the generation of deposits that are insoluble in solvents within the system. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention can provide the following.

[0011] A polyvinyl alcohol-based polymer obtained by saponifying a copolymer of a vinyl ester monomer and a trifunctional monomer, the average degree of polymerization of the polyvinyl alcohol polymer measured by the method described in JIS K 6726:1994 is 3000 to 6000; The average number of unsaturated sites in the trifunctional monomer is 1.7 to 2.3. A polyvinyl alcohol polymer characterized by:

[0012] In one embodiment, the amount of polymerizable unsaturated moieties relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol polymer may be 0.05 to 0.30 mol %. In another embodiment, the amount of trifunctional monomer units relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol polymer may be 0.05 to 0.30 mol %.

[0013] In one embodiment, the viscosity of a 4% aqueous solution of the polyvinyl alcohol polymer may be 40 to 200 mPa·s, and the degree of saponification may be 75 to 99 mol %.

[0014] In one embodiment, the pass rate of a 1.0% by mass aqueous solution of the polyvinyl alcohol-based polymer through a filter with a 45 μm pore size may be 95% by mass or more, calculated as solids, and the pass rate of a 1.0% by mass aqueous solution of the polyvinyl alcohol-based polymer through a membrane filter with a pore size of 0.45 μm may be 5% by mass or less, calculated as solids. In another embodiment, the particle size of the polyvinyl alcohol-based polymer may be 20% by mass or less of 75 μm or less, and 10% by mass or less of 500 μm or more.

[0015] In another embodiment, an additive for oil well cement containing the polyvinyl alcohol-based polymer can be provided. In another embodiment, a method for producing the polyvinyl alcohol-based polymer can be provided. These embodiments may be combined in any combination as long as they are not inconsistent. [Effects of the Invention]

[0016] The polyvinyl alcohol polymer according to the present invention can suppress the formation of solvent-insoluble deposits during the manufacturing process, and also exhibits an excellent effect of reducing fluid loss even under severe high-temperature or high-pressure environments. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, embodiments for carrying out the present invention will be described. Note that the embodiments described below are merely examples of embodiments of the present invention, and are not intended to narrow the scope of the present invention. Numerical ranges in this specification include the upper and lower limits unless otherwise specified. The term "polymer" in this specification follows the definition of polymer by the International Union of Pure and Applied Chemistry (IUPAC) Commission on Macromolecular Nomenclature, namely, "a polymer molecule is a molecule with a large relative molecular mass and has a structure composed of multiple repetitions of units derived, substantially or conceptually, from molecules with a small relative molecular mass."

[0018] <Chemical structure of polyvinyl alcohol polymer> The polyvinyl alcohol polymer according to the present invention is a polymer obtained by saponifying a copolymer of a vinyl ester monomer and a trifunctional monomer, or a copolymer of a vinyl ester monomer, a monofunctional monomer other than a vinyl ester, and a trifunctional monomer. The polyvinyl alcohol polymer according to the present invention has a high-order polymer structure different from conventional PVA, and is therefore able to maintain a swollen state at high temperatures. The presence of such a polymer structure can be confirmed by the fact that the average degree of polymerization measured by the method described in JIS K 6726:1994 is 3000 to 6000, and the average number of reaction sites of the unsaturated moieties of the trifunctional monomer is 1.7 to 2.3, as described below.

[0019] <Raw material components of polyvinyl alcohol polymer> Examples of the vinyl ester monomer include vinyl acetate, vinyl propionate, vinyl butyrate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, and vinyl pivalate, and mixtures thereof may also be used. From the viewpoint of ease of polymerization, vinyl acetate is preferred.

[0020] As a trifunctional monomer copolymerizable with a vinyl ester monomer, a compound having three polymerizable unsaturated bonds in the molecule can be used. Examples of such compounds include: triallyl ether compounds such as glycerin triallyl ether, trimethylolpropane triallyl ether, and pentaerythritol triallyl ether; compounds containing three allylamino groups such as triallylamine; compounds containing three allyl groups such as triallyl isocyanurate and triallyl phosphate; compounds having three (meth)acrylic acids such as glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, and isocyanuric acid tri(meth)acrylate; and trifunctional aromatic monomers such as trivinylbenzene.

[0021] Furthermore, examples of monofunctional monomers copolymerizable with vinyl ester monomers (i.e., monofunctional monomers other than vinyl esters) include the following compounds: 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; alkyl (methyl, ethyl, propyl, etc.) ester monomers of unsaturated carboxylic acids; anhydrides of unsaturated carboxylic acids such as maleic anhydride; sodium, potassium, ammonium, etc. salts of unsaturated carboxylic acids; sulfonic acid group-containing monomers or salts thereof such as 2-acrylamido-2-methylpropanesulfonic acid; and alkyl vinyl ether monomers.

[0022] From the viewpoints of reactivity with vinyl ester monomers and resistance to decomposition in saponification reactions, compounds having a carbonyl group or an amide group in the molecule are preferred as monomers copolymerizable with vinyl ester monomers, and compounds that are resistant to alkali decomposition are even more preferred. In a preferred embodiment, PVA is obtained by saponifying a copolymer of a vinyl ester monomer and a trifunctional monomer, and a crosslinked structure is formed from the three functional groups, thereby providing resistance to dissolution at high temperatures. As such trifunctional monomers, compounds having a ring structure are preferred, compounds having a heterocyclic structure are more preferred, and triallyl isocyanurate (TAIC) is particularly preferred.

[0023] <Average degree of polymerization> The polyvinyl alcohol polymer may have an average degree of polymerization of 3000 to 6000, preferably 3000 to 5500, more preferably 3000 to 5000, as measured by the method described in JIS K 6726:1994 "3.7 Average degree of polymerization."

[0024] <Calculation method for average number of responses> The average reaction number of the unsaturated sites of the trifunctional monomer in the polyvinyl alcohol polymer is in the range of 1.7 to 2.3, preferably 1.7 to 2.2, and more preferably 1.7 to 2.1. The average reaction number is determined by the following formula: 1 H-NMR and 13 For example, when NMR is used, the amount of comonomer modification and the amount of residual unsaturation are calculated by the following procedure, and the average reaction number can be derived therefrom.

[0025] <Copolymerization amount> When a vinyl ester monomer is copolymerized with a monomer other than the vinyl ester, the copolymerization amount is preferably 0.001 to 1.0 mol %, more preferably 0.005 to 0.5 mol %, and even more preferably 0.01 to 0.2 mol %, of structural units derived from monomers other than the vinyl ester relative to 100 mol % of structural units derived from vinyl alcohol units in the vinyl alcohol polymer. Adjusting the copolymerization amount within this range improves the dissolution resistance at high temperatures and prevents excessive crosslinking of the vinyl alcohol polymer, which is also preferred from the viewpoint of production.

[0026] The amount of polymerizable unsaturated moieties relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol polymer is preferably 0.05 to 0.30 mol%, more preferably 0.10 to 0.30 mol%, and the amount of trifunctional monomer units relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol polymer is preferably 0.05 to 0.30 mol%, more preferably 0.10 to 0.30 mol%.

[0027] <Amount of comonomer modification> The copolymerization amount (comonomer modification amount) of trifunctional monomers in PVA was measured in heavy water or heavy dimethyl sulfoxide solvent. 1 H-NMR and 13It can be calculated using C-NMR and a trace total nitrogen analyzer. For example, when using the trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytech Co., Ltd.), it can be calculated using the following procedure.

[0028] A sample of vinyl alcohol polymer is collected on a quartz board, which is then set in the auto boat controller "ABC-210" (manufactured by Nitto Seiko Analytech Co., Ltd.) and automatically inserted into an electric furnace where it is burned in an argon / oxygen stream. The NO gas generated during this process is measured with a chemiluminescence detector. A calibration curve is prepared in advance using a standard solution (N-pyridine / toluene), and the nitrogen concentration is calculated from this calibration curve.

[0029] Example of measurement conditions Reaction tube: Double tube for ABC Electric furnace temperature Inlet Temp: 800℃, Outlet Temp: 900℃ Gas flow rate: Ar: 300 mL / min, O2: 300 mL / min, Ozone: 300 mL / min Sample amount: approx. 9 to 15 mg

[0030] <Calculation of remaining unsaturation> Polyvinyl alcohol polymer was dissolved in heavy water and measured using NMR (JEOL "ECX-400") at a measurement temperature of 80°C and an accumulation count of 1024. 1 The H-NMR spectrum is obtained to identify the structure.

[0031] The amount of unsaturation (mol %) in the polyvinyl alcohol polymer can be calculated from the integral value of the peak derived from unsaturation, based on the integral value of the peaks of the methylene groups in the main chain derived from the vinyl alcohol units of the polyvinyl alcohol polymer and the methylene groups in the main chain derived from the vinyl acetate units (1.8 to 2.5 ppm). 1In the H-NMR spectrum, if the integral value of the methylene groups in the main chain derived from the vinyl alcohol unit and the methylene groups in the main chain derived from the vinyl acetate unit is b, and the integral value derived from the unsaturated bond derived from the trifunctional monomer containing three unsaturated bonds is a (for example, in the case of triallyl isocyanurate, the peak at around 6.3 ppm derived from the -CH= of the allyl group), taking into account the number of protons on the carbon (2 for the methylene group, 1 for the allyl group), the residual unsaturation amount X (mol %) is X = {a / (b / 2+a)}*100 This can be calculated as follows.

[0032] <Calculation of the average number of reactions at unsaturated sites of trifunctional monomers> The average number of unsaturated moieties reacted in the trifunctional monomer can be calculated from the amount of modification of the comonomer in the vinyl alcohol polymer and the amount of residual unsaturation. Specifically, the average number of unsaturated moieties reacted, Z, is Z = -{2 / (comonomer modification amount * 2)}*(residual unsaturation amount) + 3 It can be calculated as follows.

[0033] <Saponification degree> In this specification, the saponification degree of PVA can be measured by the method described in Japanese Industrial Standards JIS K6726:1994, "3.5 Saponification degree." The saponification degree of the present polyvinyl alcohol polymer is preferably 75 to 99 mol%, more preferably 77 to 97 mol%, and even more preferably 79 to 95 mol%.

[0034] <Particle size distribution> The particle size distribution of the PVA particles can be quantified by performing dynamic light scattering measurement on a 0.4% by mass aqueous solution of PVA (dilute aqueous solution) at a temperature of 25°C. The particle size at 50% cumulative frequency in the particle size distribution of the PVA particles is preferably 50 nm or more, and more preferably in the range of 70 to 1000 nm. The cumulative frequency is determined from the scattering intensity distribution frequency obtained by dynamic light scattering measurement.

[0035] The average particle size of PVA in a dilute aqueous solution can be set depending on the application, and is preferably 60 to 2000 nm, more preferably 70 to 1500 nm, for example. In this specification, the average particle size of PVA in a dilute aqueous solution is determined by performing dynamic light scattering measurement on a 0.4 mass % aqueous solution of PVA at 25°C, and then subjecting the particle size distribution to cumulant analysis.

[0036] When used as an additive for oil well cement, a PVA average particle size of 60 nm or more in a dilute aqueous solution is preferable because it prevents the PVA from leaking out of the cement slurry and improves fluid loss reduction. A PVA average particle size of 2000 nm or less in a dilute aqueous solution is preferable from the standpoint of productivity.

[0037] From the viewpoint of the strength of the cement after hardening and the manufacturing process of PVA, it is preferable that the PVA particles do not contain excessively large gel particles. More specifically, the pass rate of a 1.0 mass % aqueous solution of PVA through a 300 mesh (opening 0.045 mm) filter is preferably 95 mass % or more, and more preferably in the range of 97 to 100 mass %, calculated as solid content.

[0038] The 300 mesh filter penetration rate can be calculated using the following procedure. Dried PVA is dissolved in water at 25°C to obtain an aqueous solution with a concentration of 1.0 mass%. 100 mL of the resulting PVA aqueous solution is filtered through a 300 mesh filter (opening size 0.045 mm), and the mass of PVA remaining on the filter is measured. The percentage of PVA that passed through the filter is calculated from the measured mass of the PVA residue.

[0039] From the viewpoint of dissolution resistance, it is preferable that the PVA particles have a particle size distribution that makes it difficult for them to pass through a membrane filter with a pore size of 0.45 μm. More specifically, the passage rate of a 1.0 mass % aqueous solution of PVA through a membrane filter with a pore size of 0.45 μm is preferably 10 mass % or less, and more preferably 0 to 5 mass %, calculated as solid content.

[0040] The penetration rate of a 0.45 μm membrane filter can be calculated using the following procedure: A PVA aqueous solution adjusted to a concentration of 1.0% by mass is filtered under reduced pressure (10 mmHg) for 10 minutes using a 0.45 μm filter (manufactured by ADVANTEC, Material: Mixed Cellulose ester, pore size: 0.45 μm, diameter: 47 mm). The penetration rate is calculated from the amount of solids in the filtrate.

[0041] The particle size of the present polyvinyl alcohol polymer is preferably 20% by mass or less, more preferably 18% by mass or less, of 75 μm or less (under a 75 μm sieve). Also, it is preferably 10% by mass or less, more preferably 5% by mass or less, of 500 μm or more (over a 500 μm sieve). In a more preferred embodiment, the particle size may be 20% by mass or less of 75 μm or less and 10% by mass or less of 500 μm or more.

[0042] <Viscosity> In this specification, the viscosity of the polyvinyl alcohol polymer can be measured by the method described in JIS K 6726:1994, "4.2 Viscosity Measurement." From the viewpoint of improving productivity, the viscosity is preferably in the range of 40 to 200 mPa s, and more preferably in the range of 50 to 160 mPa s.

[0043] <Manufacturing method> In an embodiment of the present invention, the vinyl ester monomer or its copolymer can be polymerized in two stages as described below. Such two-stage polymerization reduces the amount of insoluble deposits remaining in the system. Specifically, an initiator is first added to a mixture of a vinyl ester monomer and a trifunctional monomer, and a first polymerization reaction is carried out in an organic solvent until a first polymerization rate is reached. The first polymerization rate may be, for example, in the range of 5 to 60%, and preferably in the range of 10 to 55%.

[0044] After the first polymerization reaction is completed, an organic solvent (which may be the same organic solvent or a different organic solvent) is added separately from the first polymerization reaction, and the second polymerization reaction is carried out until a second polymerization rate is reached. The second polymerization rate is higher than the first polymerization rate, and may be, for example, in the range of 10 to 65%, preferably in the range of 15 to 60%. Alcohol is preferably used as the organic solvent. Examples of alcohol that can be used include methanol, ethanol, and butanol, with methanol being preferred. The concentration of the polymer in the organic solvent solution can be set as desired, and may be, for example, 10% by mass or more and 80% by mass or less.

[0045] After the second polymerization reaction is completed, the unreacted vinyl ester monomer is discharged from the polymerization system, and the resulting polymer (e.g., polyvinyl ester) solution is saponified by any method to prepare PVA. An example of a saponification method is to add an alkali catalyst to an alcohol solution of the polymer. An example of the saponification procedure is described below.

[0046] Examples of alcohols that can be used as a solvent for the polymer include methanol, ethanol, and butanol, and methanol is preferred. The concentration of the polymer in the alcohol solution can be set as desired, and may be, for example, 10% by mass or more and 80% by mass or less.

[0047] Next, an alkali catalyst is added to the solution to carry out a saponification reaction. Examples of alkali catalysts include hydroxides and alcoholates of alkali metals such as sodium hydroxide, potassium hydroxide, sodium methylate, sodium ethylate, and potassium methylate. Among these, sodium hydroxide is preferred. The amount of alkali catalyst added is not particularly limited, but is preferably 1.0 to 100.0 millimolar equivalents relative to the polymer, and more preferably 5.0 to 30.0 millimolar equivalents. The reaction temperature during saponification is not particularly limited, but is preferably 10 to 70°C, and more preferably 30 to 55°C. The reaction time is also not particularly limited, and may be, for example, 20 minutes or more to 2 hours.

[0048] The degree of saponification can be appropriately adjusted depending on the application of the PVA, and may be, for example, 72 to 99 mol%, preferably 75 to 99 mol%. After the saponification reaction, a washing step to remove impurities such as sodium acetate and a drying step may be carried out as necessary.

[0049] <Oil well cement additive> In one embodiment, an additive for oil well cement containing the above-mentioned PVA can be provided, which is suitable for use in cementing oil wells, gas wells, steam wells for geothermal power generation, and the like.

[0050] Cementing, which is carried out when drilling a well, involves injecting cement into the gap between the drilled well and the steel pipe inserted into it. A widely used cementing method involves mixing cement and various additives, such as fluid loss additives, in a dry state, then turning the mixture into a slurry using high-pressure water and pumping it into the well.

[0051] Using PVA as a fluid loss reducing agent 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 the fluid loss is large, the fluidity of the cement slurry will be lost, making it difficult to perform sufficient cementing.

[0052] Fluid loss evaluation is one of the evaluation items for oil well cements 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.

[0053] <Oil well cement composition> In one embodiment, an oil well cement composition containing an oil well cement and the above-mentioned additive for oil well cement can be provided. The above-mentioned oil well cement may be any cement used for cementing oil wells, gas wells, steam wells for geothermal power generation, etc., and is not particularly limited.

[0054] In the above composition, the content of the additive for oil well cement is preferably 0.01 to 10% bwoc, more preferably 0.05 to 5% bwoc. By setting such a range, fluid loss can be effectively reduced. Note that "bwoc" (by weight of cement) means based on the weight of cement, and refers to the weight of the additive in a dry state added to the cement composition based only on the solid content of the cement.

[0055] <Oil well cement slurry> In one embodiment, an oil well cement slurry containing an oil well cement, the above-mentioned additive for oil well cement, and water can also be provided. The water content of the above cement slurry is preferably 20 to 40% by mass.

[0056] The method of incorporating the additive for oil well cement into the cement slurry is not particularly limited. For example, there are methods such as mixing the composition containing the oil well cement and the additive with water after preparing the composition, and methods of mixing the oil well cement, the additive, and water without preparing the composition.

Example

[0057] Hereinafter, the present invention will be described in more detail based on examples. Note that the examples described below show an example of a representative embodiment of the present invention, and the present invention is not limited to the following examples. [[ID=u22]]

[0058] <Preparation of PVA> [Example 1] Into a polymerization kettle equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 70.2 parts by mass of methanol (described as "initial methanol" in the table), 0.16 parts by mass of triallyl isocyanurate (TAIC) as a trifunctional monomer, and 5.0×10 -6 parts by mass of Peroyl NPP (manufactured by NOF Corporation) as an initiator were charged, and polymerization was carried out under boiling point while stirring in a nitrogen atmosphere. When the polymerization rate reached 30%, 26.1 parts by mass of methanol (described as "subsequent added methanol" in the table) was added, and polymerization was carried out until the polymerization rate reached 44%. Next, the unreacted vinyl acetate monomer was removed outside the polymerization system to obtain a methanol solution of polyvinyl acetate-TAIC copolymer.

[0059] A methanol solution of sodium hydroxide was added to the obtained methanol solution of vinyl acetate-TAIC copolymer (0.008 mol% of sodium hydroxide with respect to the copolymer). Then, a saponification reaction was carried out at 45 °C for 45 minutes to obtain PVA with a saponification degree of 91.3 mol%.

[0060] Also, during the experiment, it was visually confirmed whether there were any deposits insoluble in the system remaining. When no deposits were visually confirmed, it was evaluated as "○" (Good), when some deposits were seen, it was evaluated as "△" (Poor), and when many deposits were seen, it was evaluated as "×" (NG).

[0061] <000024 June [Examples 2 to 6, Comparative Examples 1 to 4] Except that the charged amounts were changed as shown in Table 1 below, PVA of Examples 2 to 6 and Comparative Examples 1 to 4 was obtained by the same procedure as in Example 1. Note that Comparative Examples 2 and 3 reproduced Example 3 and Example 5 of Patent Document 3, respectively. Also, Comparative Example 4 reproduced Example 5 of Patent Document 4. That is, in the comparative examples, no additional methanol was added.

[0062] [Measurement of Physical Properties of PVA] [Copolymerization Amount, Unsaturation Amount] Calculated using a trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytic Co., Ltd.) according to the method described above.

[0063] [Saponification degree] The degree of saponification was measured and calculated in accordance with the method described in Japanese Industrial Standard JIS K6726:1994 "3.5 Degree of saponification."

[0064] [Average degree of polymerization] Measurements were made and calculated in accordance with Japanese Industrial Standard JIS K6726:1994, "3.7 Average Degree of Polymerization." However, before injecting the approximately 1% by mass PVA aqueous solution prepared for measuring the average degree of polymerization into the Ostwald counter, it was filtered through a 300 mesh (0.045 mm opening) filter. The PVA concentration used to calculate the average degree of polymerization was the concentration value after filtration.

[0065] [viscosity] The compositions according to the Examples and Comparative Examples prepared as described above were adjusted to a concentration of 4% by mass to form aqueous solutions, which were used as samples for viscosity measurement. Using these samples, measurements were made and calculated in accordance with Japanese Industrial Standard JIS K6726:1994, "4.2 Viscosity Measurement."

[0066] [Fluid loss measurement] The fluid loss reduction effect of PVA was measured in accordance with the fluid loss evaluation method of the American Petroleum Institute (API) standard 10B-2 (April 2013). The specific measurement procedure is shown below.

[0067] Class G oil well cement was blended with the amounts of PVA and 0.4% bwoc (Flotek Industries "CR-270") set retarder shown in the table and mixed with water according to the procedure specified in API Standard 10B-2 (April 2013) to obtain a cement slurry with a water content of 30% by mass. The resulting cement slurry was loaded into a Chandler Engineering Model 7120 fluid loss tester and tested at the temperatures specified in the table under a pressure of 1000 psi, according to the procedure specified in API Standard 10B-2 (April 2013). Fluid loss was calculated.

[0068] The evaluation was carried out at temperatures ranging from 40°C to 140°C and with PVA addition amounts ranging from 0.4 to 1.2% bwoc, as shown in Table 2. In the table, "-" indicates that fluid loss measurements were not performed.

[0069] [Table 1]

[0070] [Table 2]

[0071] The results of the examples confirmed that the additive for oil well cement of the present invention has good fluid loss reduction performance, and can be produced without generating solvent-insoluble deposits or excessively increasing the viscosity in the system. On the other hand, it was confirmed that the comparative examples were lacking in one of the performances.

[0072] (Appendix 1) A polyvinyl alcohol-based polymer obtained by saponifying a copolymer of a vinyl ester monomer and a trifunctional monomer, the average degree of polymerization of the polyvinyl alcohol polymer measured by the method described in JIS K 6726:1994 is 3000 to 6000; The average number of unsaturated sites in the trifunctional monomer is 1.7 to 2.3. A polyvinyl alcohol polymer characterized by:

[0073] (Appendix 2) 2. The polyvinyl alcohol-based polymer according to claim 1, wherein the amount of polymerizable unsaturated moieties relative to the total of vinyl alcohol units and vinyl acetate units of the polyvinyl alcohol-based polymer is 0.05 to 0.30 mol %.

[0074] (Appendix 3) 3. The polyvinyl alcohol-based polymer according to claim 1, wherein the amount of trifunctional monomer units relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol-based polymer is 0.05 to 0.30 mol %.

[0075] (Appendix 4) 4. The polyvinyl alcohol-based polymer according to any one of claims 1 to 3, wherein a 4% aqueous solution of the polyvinyl alcohol-based polymer has a viscosity of 40 to 200 mPa·s.

[0076] (Appendix 5) 5. The polyvinyl alcohol-based polymer according to any one of claims 1 to 4, wherein the polyvinyl alcohol-based polymer has a degree of saponification of 75 to 99 mol %.

[0077] (Appendix 6) The polyvinyl alcohol-based polymer according to any one of Appendices 1 to 5, wherein a 1.0 mass % aqueous solution of the polyvinyl alcohol-based polymer passes through a filter with a 45 μm mesh size at a rate of 95 mass % or more, calculated as solid content, and a 1.0 mass % aqueous solution of the polyvinyl alcohol-based polymer passes through a membrane filter with a pore size of 0.45 μm at a rate of 5 mass % or less, calculated as solid content.

[0078] (Appendix 7) 7. The polyvinyl alcohol-based polymer according to any one of Appendices 1 to 6, wherein the particle size of the polyvinyl alcohol-based polymer is 20% by mass or less of 75 μm or less and 10% by mass or less of 500 μm or more.

[0079] (Appendix 8) An additive for oil well cement, comprising the polyvinyl alcohol polymer according to any one of Appendices 1 to 7.

[0080] (Appendix 9) A method for producing a polyvinyl alcohol-based polymer, comprising: adding an initiator to a mixture of a vinyl ester monomer and a trifunctional monomer, and carrying out a first polymerization reaction in an organic solvent until a first polymerization rate is reached; After the first polymerization reaction, an organic solvent is added, and a second polymerization reaction is carried out until a second polymerization rate higher than the first polymerization rate is reached; a step of adding a solution of an alkali catalyst in an organic solvent to a solution of the copolymer of the vinyl ester monomer and the trifunctional monomer obtained after the second polymerization reaction, and carrying out a saponification reaction to obtain a polyvinyl alcohol-based polymer having an average degree of polymerization of 3000 to 6000 as measured by the method described in JIS K 6726:1994 and an average number of reaction unsaturated sites of the trifunctional monomer of 1.7 to 2.3; A manufacturing method comprising:

Claims

1. A method for producing a polyvinyl alcohol-based polymer, comprising: adding an initiator to a mixture of a vinyl ester monomer and a trifunctional monomer, and carrying out a first polymerization reaction in an organic solvent until a first polymerization rate is reached; After the first polymerization reaction, an organic solvent is added, and a second polymerization reaction is carried out until a second polymerization rate higher than the first polymerization rate is reached; a step of adding a solution of an alkali catalyst in an organic solvent to a solution of the copolymer of the vinyl ester monomer and the trifunctional monomer obtained after the second polymerization reaction, and carrying out a saponification reaction to obtain a polyvinyl alcohol-based polymer having an average degree of polymerization of 3000 to 6000 as measured by the method described in JIS K 6726:1994 and an average number of reaction unsaturated sites of the trifunctional monomer of 1.7 to 2.3; A manufacturing method comprising:

2. 2. The method for producing a polyvinyl alcohol-based polymer according to claim 1, wherein the amount of polymerizable unsaturated moieties relative to the total of vinyl alcohol units and vinyl acetate units in the polyvinyl alcohol-based polymer is 0.05 to 0.30 mol %.

3. 3. The method for producing a polyvinyl alcohol-based polymer according to claim 1, wherein the amount of the trifunctional monomer unit relative to the total of the vinyl alcohol unit and the vinyl acetate unit in the polyvinyl alcohol-based polymer is 0.05 to 0.30 mol %.

4. 3. The method for producing a polyvinyl alcohol-based polymer according to claim 1, wherein a 4% aqueous solution of the polyvinyl alcohol-based polymer has a viscosity of 40 to 200 mPa.s.

5. 3. The method for producing a polyvinyl alcohol-based polymer according to claim 1, wherein the polyvinyl alcohol-based polymer has a degree of saponification of 75 to 99 mol %.

6. 3. The method for producing a polyvinyl alcohol-based polymer according to claim 1 or 2, wherein a 1.0 mass% aqueous solution of the polyvinyl alcohol-based polymer passes through a filter with a 45 μm mesh size at a rate of 95 mass% or more, calculated as solids, and a 1.0 mass% aqueous solution of the polyvinyl alcohol-based polymer passes through a membrane filter with a pore size of 0.45 μm at a rate of 5 mass% or less, calculated as solids.

7. 3. The method for producing a polyvinyl alcohol-based polymer according to claim 1, wherein the particle size of the polyvinyl alcohol-based polymer is 20% by mass or less of 75 μm or less and 10% by mass or less of 500 μm or more.

8. 3. A method for producing an additive for oil well cement, comprising the polyvinyl alcohol polymer obtained by the production method according to claim 1.

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