Drilling mud additive and drilling mud composition

Abstract

Provided is a drilling mud additive that can provide a drilling mud composition that exhibits an excellent suppression of mud loss at high temperatures and an excellent fluidity and mud cake formation at high temperatures. Also provided is a drilling mud composition that uses this additive.　The present invention provides a drilling mud additive that contains a vinyl alcohol-based polymer that has a vinyl alcohol unit and a vinyl ester unit. This vinyl alcohol-based polymer has a saponification degree of at least 70 mol% and a viscosity-average degree of polymerization of 1000-5000. When a 0.1 mass% aqueous solution of the vinyl alcohol-based polymer is filtered across a membrane filter having a pore diameter of 0.45 μm, the amount of filtered water in 10 minutes is equal to or less than 100 g.

Description

Additive for Drilling Mud and Drilling Mud Composition

[0001] The present invention relates to an additive for drilling mud containing a polyvinyl alcohol-based polymer, and a drilling mud composition using the additive for drilling mud.

[0002] Drilling mud is a slurry that circulates in a wellbore during drilling of oil wells, gas wells, geothermal wells, etc. For example, it plays roles such as transporting drilled cuttings and the like to the surface, protecting the wellbore wall by forming a mud wall, preventing the collapse of the formation, and cooling and lubricating the wellbore bit and drill pipe.

[0003] Such drilling mud is required to have appropriate fluidity (viscosity), form a thin and hard mud wall, have temperature stability, etc. Also, during drilling, a phenomenon (mud loss) occurs where the moisture contained in the drilling mud dissipates into cavities and cracks in the formation, which may cause the collapse of the wellbore wall and the like. Therefore, it is necessary to suppress mud loss. For mud loss suppression, usually, polymers such as carboxymethyl cellulose (CMC), 2-acrylamido-2-methylpropanesulfonic acid (AMPS), etc. are added to the drilling mud. In Prior Art Document 1, an example of using a polyvinyl alcohol-based polymer as an additive for drilling mud for mud loss suppression at high temperatures is disclosed.

[0004] Japanese Patent Application Laid-Open No. 2015 - 196733

[0005] However, there has been a demand for the development of an additive for drilling mud that can obtain a drilling mud composition excellent in mud loss suppression at high temperatures, mud wall formation at high temperatures, and fluidity, and a drilling mud composition using the additive.

[0006] The present invention has been made in view of such circumstances, and provides an additive for drilling mud that can obtain a drilling mud composition excellent in mud loss suppression at high temperatures, mud wall formation at high temperatures, and fluidity, and a drilling mud composition using the additive.

[0007] As a result of diligent research to solve the above problems, the present inventors have found that the above problems can be solved when a polyvinyl alcohol-based polymer has a specific range of saponification degree and viscosity-average degree of polymerization, and when an aqueous solution of the polyvinyl alcohol-based polymer is filtered under predetermined conditions, the amount of filtered water is below a specific value. This led to the completion of the present invention.

[0008] The following inventions are provided: [1] A drilling slurry additive containing a vinyl alcohol polymer having vinyl alcohol units and vinyl ester units, wherein the vinyl alcohol polymer has a degree of saponification of 70 mol% or more and a viscosity-average degree of polymerization of 1000 to 5000, and the amount of filtered water after 10 minutes when a 0.1% by mass aqueous solution of the vinyl alcohol polymer is filtered through a membrane filter with a pore size of 0.45 μm is 100 g or less. [2] The drilling slurry additive according to claim 1, wherein the degree of saponification of the vinyl alcohol polymer is 70 mol% to 99 mol%. [3] The drilling slurry additive according to [1] or [2], wherein the vinyl alcohol polymer has polyfunctional monomer units. [4] The drilling slurry additive according to any one of [1] to [3], wherein the polyfunctional monomer has a structure derived from an isocyanurate having two or more allyl groups. [5] The drilling slurry additive according to any one of [1] to [4], wherein the content of the polyfunctional monomer units is 0.001 mol% to 1.0 mol%, when the total of the vinyl alcohol units and the vinyl ester units is 100 mol%. [6] The drilling slurry additive according to any one of [1] to [5], wherein, when the drilling slurry composition contains 0.1 to 5 parts by mass of the vinyl alcohol polymer, 1 to 10 parts by mass of bentonite, 1 to 5 parts by mass of ribonite, and 0.1 to 5 parts by mass of carboxymethylcellulose per 100 parts by mass of water in the drilling slurry, the amount of water removed when pressurized at 100 psi for 30 minutes at 140°C using an API standard pressure filtration tester is 15 mL or less. [7] A drilling slurry composition containing the drilling slurry additive according to any one of [1] to [6]. [8] The drilling slurry composition according to [7], comprising 0.01 to 5 parts by mass of the vinyl alcohol polymer per 100 parts by mass of water in the drilling slurry.

[0009] The present invention provides an additive for drilling slurry that can be used to obtain a drilling slurry composition that suppresses mud loss at high temperatures, forms mud walls at high temperatures, and has excellent fluidity, and a drilling slurry composition using the additive.

[0010] Embodiments of the present invention will now be described. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature can constitute an invention independently. In addition, any element not specified in the claims in the embodiments below is an optional element and can be omitted. Any number of zeros (for example, one or two) may be added to the end of the numerical values ​​disclosed in the following description. For example, one or two zeros may be added after "1.4" to make it "1.40" or "1.400".

[0011] An additive for drilling slurry according to one embodiment of the present invention is suitable as an additive for drilling slurry used in drilling oil wells, gas wells, geothermal wells, etc. The additive for drilling slurry of this embodiment is a vinyl alcohol-based polymer having vinyl alcohol units and vinyl ester units, and contains a vinyl alcohol-based polymer whose degree of saponification, viscosity-average degree of polymerization, and amount of filtered water of a 0.1% by mass aqueous solution have been adjusted. The additive for drilling slurry may further contain a thickener such as carboxymethylcellulose, a dispersant such as lignin sulfonate, a lubricant such as a poly(meth)acrylic acid polymer or copolymer, etc.

[0012] 1. Polyvinyl alcohol polymer The polyvinyl alcohol polymer (hereinafter also referred to as "PVA") used as an additive for drilling mud according to one embodiment of the present invention is a vinyl alcohol polymer having vinyl alcohol units and vinyl ester units, having a degree of saponification of 70 mol% or more, a viscosity-average degree of polymerization of 1000 to 5000, and having a filtered water volume of 100 g or less in 10 minutes when a 0.1 mass% aqueous solution is filtered through a membrane filter with a pore size of 0.45 μm.

[0013] The PVA is a polymer obtained by saponifying a polymer obtained by polymerizing raw material monomers containing vinyl ester monomers. Preferably, the PVA is obtained by saponifying a copolymer of saponified vinyl ester monomers and other monomers (hereinafter also referred to as "intermediate copolymer"). Some of the structural units (vinyl ester units) derived from the vinyl ester monomers contained in the homopolymer or intermediate copolymer undergo hydrolysis of the ester groups during saponification to become structural units (vinyl alcohol units) having hydroxyl groups.

[0014] Examples of vinyl ester monomers include vinyl acetate, vinyl propionate, vinyl valerate, vinyl caprate, vinyl laurate, vinyl stearate, vinyl benzoate, vinyl pivalate, etc., and these may be used individually or in combination of two or more. Preferably, the vinyl ester monomer contains vinyl acetate, and more preferably, the vinyl ester monomer is vinyl acetate.

[0015] Furthermore, other monomers copolymerizable with vinyl ester monomers may include polyfunctional monomers. These polyfunctional monomers are not particularly limited, and any compound having two or more polymerizable unsaturated bonds in its molecule can be used. Examples include divinyl ethers such as ethanediol divinyl ether, propanediol divinyl ether, butanediol divinyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, polyethylene glycol divinyl ether, propylene glycol divinyl ether, and polypropylene glycol divinyl ether; divinyl sulfonic acid compounds; and the like.

[0016] Furthermore, polyfunctional monomers copolymerizable with vinyl ester monomers include diene compounds such as pentadiene, hexadiene, heptadiene, octadiene, nonadiene, and decadiene; diallyl ether compounds such as glycerin diallyl ether, diethylene glycol diallyl ether, ethylene glycol diallyl ether, triethylene glycol diallyl ether, polyethylene glycol diallyl ether, trimethylolpropane diallyl ether, and pentaerythritol diallyl ether; trialyl ether compounds such as glycerin triallyl ether, trimethylolpropane triallyl ether, and pentaerythritol triallyl ether; and pentaerythritol Other examples include tetraallyl ether compounds such as litol tetraallyl ether; monomers containing allyl ester groups such as diallyl phthalate, diallyl maleate, diallyl itaconate, diallyl terephthalate, and diallyl adipate; monomers containing allylamino groups such as diallylamine, diallylmethylamine, and other diallylamine compounds; monomers containing allylammonium groups such as diallyldimethylammonium chloride and other diallylammonium salts; and polyfunctional monomers containing two or more allyl groups such as diallyl isocyanurate derivatives, triallyl isocyanurate, 1,3-diallylurea, triallyl phosphate, and diallyl disulfide.

[0017] Furthermore, polyfunctional monomers copolymerizable with vinyl ester monomers include ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, triethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, glycerin di(meth)acrylate, glycerin tri(meth)acrylate, pentaerythritol tri(meth)acrylate, trimethylolpropane tri(meth)acrylate, and ditrimethylol Other examples include monomers containing (meth)acrylic acid such as tetrapropane(meth)acrylate, pentaerythritol(meth)acrylate, dipentaerythritol(hexa(meth)acrylate), and isocyanuric acid(tri(meth)acrylate); monomers containing (meth)acrylamide such as N,N'-methylenebis(meth)acrylamide and N,N'-ethylenebis(meth)acrylamide; aromatic monomers such as divinylbenzene and trivinylbenzene; monomers containing a glycidyl group such as allyl glycidyl ether and glycidyl(meth)acrylate; and so on.

[0018] Among these polyfunctional monomers, allyl cyanurate derivatives having a carbonyl group or amide group in the molecule and two or more allyl groups are preferred from the viewpoint of reactivity with vinyl ester monomers. Specifically, triallyl isocyanurate or allyl methacrylate is preferred, and diallyl isocyanurate derivatives or triallyl isocyanurate are particularly preferred from the viewpoint of resistance to decomposition in saponification reactions.

[0019] The diallyl isocyanurate derivative is preferably a compound represented by the following formula (1) (hereinafter also referred to as "compound A").

[0020]

[0021] In formula (1) above, R is a linear, branched, or cyclic alkyl group having 1 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 1 to 18, more preferably 1 to 16, and even more preferably 1 to 14. Specifically, the number of carbon atoms in R is, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and may be within the range of any two of the values ​​exemplified here. Specific examples of linear alkyl group R include methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, and the like, with methyl group or tetradecyl group being preferred.

[0022] Compound A, represented by formula (1) above, may be used alone or in combination of two or more. Preferably, compound A comprises one or more compounds in which R is selected from a methyl group and a tetradecyl group, and more preferably, compound A is a compound in which R is a methyl group or a tetradecyl group.

[0023] The polyvinyl alcohol polymer contains, when the total amount of vinyl alcohol units and vinyl ester units contained in the polyvinyl alcohol polymer is set to 100 mol%, it preferably contains 0.001 to 1.0 mol%, more preferably 0.01 to 0.5 mol%, and even more preferably 0.03 to 0.3 mol% of polyfunctional monomer units. The content of polyfunctional monomer units in the polyvinyl alcohol polymer (the degree of modification of the polyvinyl alcohol polymer) is specifically, for example, 0.001, 0.01, 0.03, 0.04, 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, and 1.0 mol%, and may also be within the range of any two of the values ​​exemplified here.

[0024] The content of polyfunctional monomer units in polyvinyl alcohol polymers (the degree of modification of polyvinyl alcohol polymers) can be calculated, for example, based on the results of NMR measurements or trace nitrogen quantification. A calculation method based on NMR measurements using diallylmethyl isocyanurate and diallyltetradecyl isocyanurate is specifically described in the examples below.

[0025] The polyvinyl alcohol polymer contains, more preferably, 95 to 100 mol%, and even more preferably, 99 to 100 mol%, of structural units derived from vinyl ester monomers (vinyl alcohol units and vinyl ester units) and polyfunctional monomers in total, per 100 mol% of the constituent units contained in the polyvinyl alcohol polymer. Specifically, the total content of structural units derived from vinyl ester monomers and structural units derived from polyfunctional monomers per 100 mol% of the constituent units contained in the polyvinyl alcohol polymer is, for example, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, 99.9, and 100 mol%, and may be within the range of any two of the values ​​exemplified here.

[0026] The intermediate copolymer may be further copolymerized with other monomers copolymerizable with vinyl ester monomers and polyfunctional monomers, to the extent that the effects of the present invention are not impaired. Other monomers include, for example, α-olefin monomers such as ethylene and propylene; alkyl (meth)acrylate 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, propylene) unsaturated carboxylic acids. Examples include ester monomers (such as glycidyl ether); anhydrides of unsaturated carboxylic acids such as maleic anhydride; salts of unsaturated carboxylic acids with sodium, potassium, ammonium, etc.; glycidyl group-containing monomers such as allyl glycidyl ether and glycidyl (meth)acrylate; sulfonic acid group-containing monomers or salts thereof such as 2-acrylamido-2-methylpropanesulfonic acid; phosphate group-containing monomers such as acid phosphooxyethyl methacrylate and acid phosphooxypropyl methacrylate; alkyl vinyl ether monomers; and so on.

[0027] The viscosity-average degree of polymerization of the polyvinyl alcohol polymer is preferably 1000 to 5000. If the viscosity-average degree of polymerization is too low, the mud loss suppression effect may not be sufficient. If the viscosity-average degree of polymerization is too high, the viscosity of the aqueous solution may become high. Also, if the viscosity-average degree of polymerization is too high, the viscosity of the excavation mud slurry will become high, which may worsen operability, such as requiring more power from the pump during slurry transfer and increasing pressure loss. Specifically, the viscosity-average degree of polymerization can be, for example, 1000, 1200, 1400, 1600, 1800, 2000, 2200, 2400, 2600, 2800, 3000, 3200, 3400, 3600, 3800, 4000, 4500, or 5000, and may be within the range of any two of the values ​​exemplified here.

[0028] The "viscosity-average degree of polymerization" is calculated from the intrinsic viscosity [η] (g / dL) measured at 30°C using an Ostwald viscometer with deionized water as the solvent, in accordance with JIS K 6726:1994 "3.7 Average degree of polymerization," using the following formula (A): log(P) = 1.613 × log([η] × 10 ４ / 8.29) ... (A) Here, P represents the viscosity-average degree of polymerization.

[0029] The degree of saponification of the polyvinyl alcohol polymer is preferably 70 mol% or more, and may be, for example, 70 mol% to 99.9 mol%, or 70 mol% to 99 mol%. When this range is satisfied, the hydrophobicity of the remaining ester groups and the action of intermolecular hydrogen bonding of the polyvinyl alcohol polymer are within a favorable range, and the sludge loss suppression effect is sufficient. Specifically, the degree of saponification may be, for example, 70, 75, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.9 mol%, and may be within the range of any two of the values ​​exemplified here.

[0030] The "degree of saponification" can be calculated by measuring it in accordance with JIS K 6726:1994 "3.5 Degree of Saponification".

[0031] The polyvinyl alcohol polymer is characterized by a filtration volume of 10 minutes of a 0.1% by mass aqueous solution of the polyvinyl alcohol polymer through a membrane filter with a pore size of 0.45 μm at room temperature (e.g., 25°C), preferably 100 g or less, more preferably 70 g or less, and even more preferably 2 g to 20 g. When the polyvinyl alcohol polymer is used as an additive for drilling slurry, if the amount of filtered water is 100 g or less, the polyvinyl alcohol polymer will not easily flow out of the drilling slurry, and mud loss will be suppressed. The amount of filtered water is specifically, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 80, 90, and 100 g, and may be within the range of any two of the values ​​exemplified here. The amount of filtered water can be adjusted, for example, by the degree of polymerization, degree of saponification, and amount of modification of the polyvinyl alcohol polymer, and by the types of vinyl ester monomers and polyfunctional monomers.

[0032] <Method for producing polyvinyl alcohol polymers> The method for producing polyvinyl alcohol polymers is not particularly limited, but for example, it may include a polymerization step and a saponification step.

[0033] In the polymerization step, the raw material monomers containing vinyl ester monomers are polymerized to obtain a homopolymer. Alternatively, the raw material monomers containing vinyl ester monomers and other monomers may be polymerized to obtain an intermediate copolymer. The polymerization method for the homopolymer or intermediate copolymer is not particularly limited, but known polymerization methods such as solution polymerization, suspension polymerization, and bulk polymerization can be used. Solution polymerization in alcohol is preferred because it is easy to operate and the same solvent as the saponification step can be used. Methanol is particularly preferred as the alcohol.

[0034] The amount of alcohol added may be, for example, 30.0 to 80.0 parts by mass, preferably 40.0 to 70.0 parts by mass, per 100 parts by mass of vinyl ester monomer.

[0035] Furthermore, if the raw material monomer contains a polyfunctional monomer, the amount of polyfunctional monomer added may be, for example, 0.05 to 0.40 parts by mass, preferably 0.10 to 0.25 parts by mass, per 100 parts by mass of vinyl ester monomer.

[0036] Furthermore, the amount of vinyl ester monomer added relative to 100% by mass of the total amount of raw material monomers added 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.

[0037] Furthermore, the conversion rate of the vinyl ester monomer may be, for example, 40.0 to 70.0%, and preferably 50.0 to 60.0%.

[0038] Furthermore, polymerization initiators can be used in polymerization reactions. Polymerization initiators are not particularly limited, but for example, 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; peroxycarbonate 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 can be used alone or in combination.

[0039] In the saponification process, the intermediate copolymer is saponified to obtain a saponified product. The method for saponifying the intermediate copolymer is not particularly limited, but for example, a simple and preferred method is to dissolve the intermediate copolymer in alcohol and saponify it with an alkali (e.g., sodium hydroxide). The alcohol used is not particularly limited, but examples include methanol, ethanol, and butanol. Among these, methanol is preferred because the solvent can be easily recovered and reused, thus reducing manufacturing costs. The saponification process may include a neutralization step with acetic acid or the like. In one example, when alkali is added to a methanol solution of the intermediate copolymer, the paste becomes gel-like as saponification progresses. This gel-like substance (which may also be a block of gel-like substance) is crushed with a cutter, neutralized with acetic acid, washed, and dried (solvent removal step described later) to obtain granular particles. These granular particles can be crushed (crushing step and classification step described later) to obtain a powder.

[0040] A method for producing a polyvinyl alcohol-based polymer may further include a solvent removal step. In the solvent removal step, the solvent can be removed, for example, by heating and drying.

[0041] When a drilling slurry composition is prepared containing 0.1 to 5 parts by mass of the polyvinyl alcohol polymer, 1 to 10 parts by mass of bentonite, 1 to 5 parts by mass of ribonite, and 0.1 to 5 parts by mass of carboxymethylcellulose (CMC) per 100 parts by mass of water in the drilling slurry (more specifically, for example, when a drilling slurry composition contains 0.2 parts by mass of the vinyl alcohol polymer, 4 parts by mass of bentonite, 2 parts by mass of ribonite, and 0.2 parts by mass of CMC per 100 parts by mass of water in the drilling slurry), it is desirable that the amount of water removed when pressurized at 100 psi for 30 minutes at 140°C using an API standard pressure filtration tester is 15 mL or less. The smaller the amount of water removed, the greater the effect of suppressing mud loss. The amount of dewatering is specifically, for example, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mL, and may be within the range of any two of the values ​​exemplified here.

[0042] The polyvinyl alcohol-based polymer contains 0.1 to 5 parts by mass of the vinyl alcohol-based polymer, 1 to 10 parts by mass of bentonite, 1 to 5 parts by mass of ribonite, and 0.1 to 5 parts by mass of carboxymethyl cellulose (CMC) with respect to 100 parts by mass of the moisture of the drilling mud (more specifically, for example, when the drilling mud composition contains 0.2 parts by mass of the vinyl alcohol-based polymer, 4 parts by mass of bentonite, 2 parts by mass of ribonite, and 0.2 parts by mass of CMC with respect to 100 parts by mass of the moisture of the drilling mud), it is preferable that the thickness of the mud wall remaining on the filter paper after the mud test at 140 °C is 3.0 mm or less. If the mud wall is too thick, the clearance between the drill string and the mud wall becomes small, and there is a risk of drilling obstacles such as detention. The mud wall is more preferably 1.2 to 3.0 mm, specifically, for example, 0, 0.1, 0.5, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0 mm, and it may be within the range between any two of the values exemplified here.

[0043] The polyvinyl alcohol-based polymer contains 0.1 to 5 parts by mass of the vinyl alcohol-based polymer, 1 to 10 parts by mass of bentonite, 1 to 5 parts by mass of ribonite, and 0.1 to 5 parts by mass of carboxymethyl cellulose (CMC) with respect to 100 parts by mass of the moisture of the drilling mud (more specifically, for example, when the drilling mud composition contains 0.2 parts by mass of the vinyl alcohol-based polymer, 4 parts by mass of bentonite, 2 parts by mass of ribonite, and 0.2 parts by mass of CMC with respect to 100 parts by mass of the moisture of the drilling mud), it is preferable that the funnel viscosity is 30 to 60 s / qt. If the funnel viscosity is too high or too low, the fluidity of the drilling mud composition deteriorates, and the operability in transporting drilling cuttings deteriorates. The funnel viscosity is more preferably 40 to 60 s / qt, specifically, for example, 30, 35, 40, 45, 50, 55, 60 s / qt, and it may be within the range between any two of the values exemplified here.

[0044] 2. Drilling Mud Composition The drilling mud composition according to one embodiment of the present invention can contain the above polyvinyl alcohol-based polymer. The drilling mud composition is, for example, a composition mainly composed of water and mud, and containing the above polyvinyl alcohol-based polymer. The drilling mud composition according to one embodiment may contain the above additive for drilling mud. The drilling mud composition is, for example, a composition containing water, mud, and the above additive for drilling mud.

[0045] With respect to 100 parts by mass of the water in the drilling mud composition, the addition amount of the polyvinyl alcohol-based polymer of the present invention is preferably 0.01 to 5 parts by mass, more preferably 0.05 to 1 part by mass. Specifically, the addition amount is, for example, 0.01, 0.05, 0.10, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.30, 0.35, 0.40, 0.50, 1.0, 2.0, 3.0, 4.0, 5.0 parts by mass, and it may be within the range between any two of the values exemplified here.

[0046] The drilling mud composition may contain other components as long as the effects of the present invention are not impaired.

[0047] <Method for Producing Drilling Mud Composition>The drilling mud composition according to one embodiment of the present invention can be adjusted, for example, by adding an additive for drilling mud and optionally other components based on a water-clay suspension in which mud is dispersed and suspended in water. The method for adding the polyvinyl alcohol-based polymer or the additive for drilling mud to the drilling mud composition is not particularly limited. The additive for drilling mud of the present invention may be added to the mud as an aqueous solution, or may be added as pellets or powder.

[0048] <Mud>Examples of the mud include bentonite, ribonite, attapulgite, sericite, hydrous magnesium silicate, etc. Among them, bentonite or ribonite is preferred. For the drilling mud composition, one type of mud may be used, or two or more types of mud may be mixed and used.

[0049] The above-mentioned mud is preferably in an amount of 1 to 30 parts by mass, and more preferably 2 to 20 parts by mass, per 100 parts by mass of water in the drilling slurry composition.

[0050] <Other Components> Other components added to the drilling slurry composition are not particularly limited, but include, for example, carboxymethylcellulose; poly(meth)acrylic acid polymers or copolymers; zeolites; dispersants; inorganic salts; electrolytes; various organic salts of lignin sulfonates, tannins, and lignitides; and water-soluble polymers such as polyethylene glycol (PEG) and polyethylene oxide (PEO).

[0051] Furthermore, the drilling slurry composition of the present invention may contain, as appropriate, known additives such as lubricants, emulsion resins, surfactants, air-entraining agents (AE agents), defoaming agents, shrinkage reducing agents, curing accelerators, curing retarders, and various types of fibers (rock wool, glass fibers, carbon fibers, cellulose fibers, pulp fibers, and various synthetic resin fibers), to the extent that the objectives of the present invention are not impaired, depending on the application.

[0052] <Slurry Test> The drilling slurry composition according to the present invention preferably has a dewatering amount of 15 mL or less when pressurized at 100 psi for 30 minutes at 140°C using an API standard pressure filtration tester. The smaller the dewatering amount, the greater the effect of suppressing mud loss. Specifically, the dewatering amount may be, for example, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, or 15 mL, and may be within the range of any two of the values ​​exemplified here.

[0053] The drilling slurry composition according to the present invention preferably has a slurry wall thickness of 3.0 mm or less remaining on the filter paper after the slurry test at 140°C described above. If the slurry wall is too thick, the clearance between the drill string and the slurry wall will be reduced, which may cause drilling problems such as jamming. The slurry wall is more preferably 1.2 to 3.0 mm thick, specifically, for example, 0, 0.1, 0.5, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, and 3.0 mm, and may be within the range of any two of the values ​​exemplified here.

[0054] The drilling slurry composition according to the present invention preferably has a funnel viscosity of 30 to 60 s / qt. If the funnel viscosity is too high or too low, the fluidity of the drilling slurry composition will be poor, resulting in poor handling when transporting drilling debris. The funnel viscosity is more preferably 40 to 60 s / qt, specifically, for example, 30, 35, 40, 45, 50, 55, 60 s / qt, and may be within the range of any two of the values ​​exemplified herein.

[0055] The above funnel viscosity can be calculated using an API standard funnel-type viscometer as the time required for a specified amount of mud to flow down.

[0056] The above-mentioned dewatering amount, mud wall thickness, and funnel viscosity can be adjusted by adjusting the amount of filtered water in the aqueous solution of the polyvinyl alcohol polymer, depending on the degree of polymerization, degree of saponification, amount of modification, and type of vinyl ester monomer and polyfunctional monomer of the polyvinyl alcohol polymer, and further by adjusting the amount of polyvinyl alcohol polymer added to the drilling mud.

[0057] 3. Use The above polyvinyl alcohol polymer or drilling slurry additive is used to add to drilling slurry used when drilling oil wells, gas wells, geothermal wells, etc. In addition, the drilling slurry composition containing the above polyvinyl alcohol polymer or drilling slurry additive is used as drilling slurry for drilling oil wells, gas wells, geothermal wells, etc.

[0058] The present invention will be described in more detail below with reference to examples. These are all illustrative examples and do not limit the scope of the present invention.

[0059] <Preparation of Polyvinyl Alcohol-Based Polymers> [Examples 1 and 2] In a polymerization vessel equipped with a reflux condenser, dropping funnel, and stirrer, 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 67 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 8 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate-based polymer (intermediate copolymer) was obtained.

[0060] A methanol solution of sodium hydroxide (calculated as 0.008 moles of sodium hydroxide relative to the vinyl acetate-derived structural units) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol-based polymer with a degree of saponification of 88 mol%.

[0061] The dried polyvinyl alcohol-based polymer was pulverized using an ACM pulverizer (manufactured by Hosokawa Micron Corporation) and then classified.

[0062] [Example 3] In a polymerization vessel equipped with a reflux condenser, dropping funnel, and stirrer, 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 67 parts by mass of methanol as a solvent, and 0.004 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 2 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0063] A methanol solution of sodium hydroxide (calculated as 0.005 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 80 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0064] [Example 4] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1, except that the degree of saponification of the polyvinyl alcohol polymer was changed to 99 mol% using a methanol solution of the vinyl acetate polymer from Example 1. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was then pulverized and classified.

[0065] [Example 5] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1, except that the degree of saponification of the polyvinyl alcohol polymer was changed to 99.9 mol% using a methanol solution of the vinyl acetate polymer from Example 1. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was then pulverized and classified.

[0066] [Example 6] In a polymerization vessel equipped with a reflux condenser, dropping funnel, and stirrer, 100 parts by mass of vinyl acetate, 0.33 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 150 parts by mass of methanol as a solvent, and 0.01 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 8 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0067] A methanol solution of sodium hydroxide (calculated as 0.008 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0068] [Example 7] In a polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 33.3 parts by mass of methanol as a solvent, and 0.005 parts by mass of di-n-propyl peroxydicarbonate (perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 7 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0069] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0070] [Example 8] In a polymerization vessel equipped with a reflux condenser, dropping funnel, and stirrer, 100 parts by mass of vinyl acetate, 0.09 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., 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 Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 5 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0071] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0072] [Example 9] In a polymerization vessel equipped with a reflux condenser, dropping funnel, and stirrer, 100 parts by mass of vinyl acetate, 0.22 parts by mass of diallylmethyl isocyanurate (MeDAIC, manufactured by Shikoku Chemicals Co., 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 Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 8 hours with stirring under a nitrogen atmosphere (conversion rate of vinyl acetate: 57%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0073] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0074] [Example 10] In a polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 0.37 parts by mass of diallyltetradecyl isocyanurate (LDAIC, manufactured by Shikoku Chemicals Co., Ltd.), 42.9 parts by mass of methanol as a solvent, and 0.01 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 4 hours with stirring under a nitrogen atmosphere (conversion rate of vinyl acetate: 57%). After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0075] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0076] [Comparative Example 1] In a polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 150 parts by mass of methanol as a solvent, and 0.01 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 8 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0077] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0078] [Comparative Example 2] In a polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 0.16 parts by mass of triallyl isocyanurate (TAIC, manufactured by Shinryo Co., Ltd.), 11.1 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (Perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 8 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0079] A methanol solution of sodium hydroxide (calculated as 0.007 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with a degree of saponification of 88 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0080] [Comparative Example 3] A polyvinyl alcohol polymer was obtained in the same manner as in Example 1, except that the methanol solution of the vinyl acetate polymer from Example 1 was used and the degree of saponification of the polyvinyl alcohol polymer was changed to 40 mol%. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was then pulverized and classified.

[0081] [Comparative Example 4] In a polymerization vessel equipped with a reflux condenser, a dropping funnel, and a stirrer, 100 parts by mass of vinyl acetate, 5 parts by mass of methanol as a solvent, and 0.003 parts by mass of di-n-propyl peroxydicarbonate (perloyl NPP, manufactured by Nippon Oil & Fats Co., Ltd.) as a polymerization initiator were charged, and polymerization was carried out below the boiling point for 4 hours while stirring under a nitrogen atmosphere. After the reaction was stopped, unreacted vinyl acetate was removed from the polymerization system. A methanol solution of the vinyl acetate polymer (intermediate copolymer) was obtained.

[0082] A methanol solution of sodium hydroxide (calculated as 0.004 moles of sodium hydroxide relative to the structural units derived from vinyl acetate) was added to the methanol solution of the vinyl acetate polymer obtained above, and a saponification reaction was carried out at 45°C for 45 minutes. The resulting reaction solution was heated and dried to obtain a polyvinyl alcohol polymer with 80 mol% saponification. The polyvinyl alcohol polymer obtained by drying in the same manner as in Example 1 was pulverized and classified.

[0083] <Measurement and Evaluation> The physical properties and characteristics of the polyvinyl alcohol-based polymers (PVAs) obtained in each example and comparative example were measured. The results are shown in Table 1.

[0084]

[0085] [Viscosity-average degree of polymerization] For the PVAs of the examples and comparative examples obtained above, the viscosity-average degree of polymerization was calculated from the intrinsic viscosity [η] (g / dL) measured at 30°C using an Ostwald viscometer with deionized water as the 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)

[0086] [Degree of Saponification] The degree of saponification of the PVAs in the examples and comparative examples obtained above was measured in accordance with JIS K 6726:1994 "3.5 Degree of Saponification". Specifically, the degree of saponification 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.

[0087] [Amount of Degradation] For the PVA obtained above in Examples 1 to 8 and Comparative Examples 1 to 3 using triallyl isocyanurate, the amount of degradation was calculated using the trace total nitrogen analyzer "TN-2100H" (manufactured by Nitto Seiko Analytech Co., Ltd.) (for detailed methods, please refer to International Publication No. 2023 / 238674).

[0088] Furthermore, for the PVA in Example 9 using diallylmethyl isocyanurate and Example 10 using diallyltetradecyl isocyanurate, the amount of modification (mol%) was calculated by the following method. The amount of modification was calculated using fully saponified PVA. １ The composition was determined by 1H-NMR analysis. Fully saponified PVA was prepared by the following procedure: After removing unreacted vinyl acetate from the polymerization system, the methanol solution of the 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. Then, 20 mL of 10% NaOH methanol solution was added and reacted at 40°C for 1 hour. Subsequently, 3 mL of acetic acid was added for neutralization, the mixture was filtered, and the residue, which mainly consisted 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 NMR (ECX-400, JEOL Ltd.) under the conditions of a measurement temperature of 80°C and 128 cumulative measurements. １¹H-NMR spectra were obtained. (Diallylmethylisocyanurate) From the obtained spectra, the integral value of the peak originating from the methyl group on the nitrogen atom (3.2–3.4 ppm) was taken as a, and the integral value of the peak originating from the methylene group not adjacent to the hydroxyl group of the vinyl alcohol unit and the methylene group not adjacent to the ester group of the vinyl ester monomer unit (1.3–2 ppm) was taken as b. The amount of modification of PVA (copolymerization amount of triallylmethylisocyanurate in PVA) X (mol%) was calculated from the following formula (B). (Diallyltetradecylisocyanurate) From the obtained spectrum, the integral value of the peak (0.7–0.9 ppm) originating from the methyl group at the end of the tetradecyl group on the nitrogen atom was taken as a, and the integral value of the peak (1.3–2 ppm) originating from the methylene group not adjacent to the hydroxyl group of the vinyl alcohol unit and the methylene group not adjacent to the ester group of the vinyl ester monomer unit was taken as b. The copolymer amount X (mol%) of diallyltetradecylisocyanurate in PVA was calculated from the following formula (B): X = ((1 / 3)a / (b / 2)) × 100 ... (B)

[0089] [Filtration Volume] An aqueous PVA solution adjusted to a concentration of 0.1% by mass was filtered using a 0.45 μm filter (manufactured by ADVANTEC, pore size: 0.45 μm, diameter 47 mm) with an aspirator under reduced pressure for 10 minutes. The volume of the filtrate was defined as the filtration volume. For the PVA in Examples 1 to 10, the filtration volume was 100 g or less.

[0090] [Mud Test] <Preparation of Mud> To 100 parts by mass of water, 4 parts by mass of bentonite (Tergel E, manufactured by Ternite), 2 parts by mass of ribonite, 0.2 parts by mass of carboxymethylcellulose (CMC) (Terpolymer H, manufactured by Ternite), and the powder of the polyvinyl alcohol polymer of the present invention were added and stirred at room temperature (e.g., 25°C) for 5 hours. Then, the mixture was left to stand for 18 hours to allow the bentonite and ribonite to swell sufficiently to obtain a mud slurry. The amount of polyvinyl alcohol polymer added was 0.05 parts by mass in Example 2 and 0.2 parts by mass in the other Examples 1, 3-10, and Comparative Examples 1-4, per 100 parts by mass of water. The properties of the obtained mud slurry were measured by the following method. The results are shown in Table 1.

[0091] <Dewatering Amount, Mud Wall Thickness> Dewatering amount was measured as the amount of water filtered out of mud water heated to 140°C under a differential pressure of 100 psi for 30 minutes, according to the method described in "Appendix H" of "API 10" (American Institute Specification 10). 4A filter paper was used for filtration. The thickness of the solid component remaining on the filter paper after filtration was defined as the mud wall thickness.

[0092] <Funnel Viscosity> 1500 ml of muddy water was placed in an API standard funnel-type viscometer, and the time required for 946 ml of the mud to flow through was measured.

Claims

1. An additive for drilling slurry containing a vinyl alcohol polymer having vinyl alcohol units and vinyl ester units, wherein the vinyl alcohol polymer has a degree of saponification of 70 mol% or more and a viscosity-average degree of polymerization of 1000 to 5000, and the amount of filtered water after 10 minutes when a 0.1% by mass aqueous solution of the vinyl alcohol polymer is filtered through a membrane filter with a pore size of 0.45 μm is 100 g or less.

2. The drilling slurry additive according to claim 1, wherein the degree of saponification of the vinyl alcohol-based polymer is 70 mol% to 99 mol%.

3. The additive for drilling slurry according to claim 1, wherein the vinyl alcohol-based polymer has polyfunctional monomer units.

4. The drilling slurry additive according to claim 1, wherein the polyfunctional monomer has a structure derived from an isocyanurate having two or more allyl groups.

5. The drilling slurry additive according to claim 1, wherein the content of the polyfunctional monomer units is 0.001 mol% to 1.0 mol%, when the total of the vinyl alcohol units and the vinyl ester units is 100 mol%.

6. The drilling slurry additive according to claim 1, wherein, when the drilling slurry composition contains 0.1 to 5 parts by mass of the vinyl alcohol polymer, 1 to 10 parts by mass of bentonite, 1 to 5 parts by mass of ribonite, and 0.1 to 5 parts by mass of carboxymethylcellulose per 100 parts by mass of water in the drilling slurry, the amount of water removed when pressurized at 100 psi for 30 minutes at 140°C using an API standard pressure filtration tester is 15 mL or less.

7. A drilling slurry composition comprising the drilling slurry additive according to any one of claims 1 to 6.

8. The drilling slurry composition according to claim 7, comprising 0.01 to 5 parts by mass of the vinyl alcohol-based polymer per 100 parts by mass of water in the drilling slurry.

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