Chitosan graft copolymer, preparation method therefor, and drilling fluid

Abstract

The present invention relates to the field of oilfield chemical drilling fluid treatment agents, and disclosed are a chitosan graft copolymer, a preparation method therefor, and a drilling fluid. The chitosan graft copolymer has a structure as shown in formula (I), the number average molecular weight is 200,000-600,000, m is 1,200-3,500, and X and Z are each independently H, -R1COOM1 or a polymer segment; R1 is an alkylene group of C1-C5, and M1 is H, Na or K; the polymer segment comprises a structural unit A and a structural unit B, the structural unit A comprising at least two -COOM2, M2 being H, Na or K, the structural unit B comprising a cyclic lactam structure, and Y being H or a quaternary ammonium salt structure having an ester group. The chitosan graft copolymer provided by the present invention has good wall-fixing performance and filtration loss reduction performance, thereby ensuring well wall stability in easily collapsed formations in shale fracture zones by means of both wall-fixing and reducing filtrate infiltration.

Description

Chitosan graft copolymer, preparation method thereof and drilling fluid

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Chinese patent application 202310962104.X filed on August 1, 2023, the contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to the technical field of oilfield chemical drilling fluid treating agents, in particular to a chitosan graft copolymer and a preparation method thereof, and drilling fluid. Background Art

[0004] In recent years, with the continuous expansion of oil and gas exploration and development, the number of highly active shale, well-developed bedding fractures, and fractured zones, which are prone to collapse, has increased, making wellbore stabilization increasingly difficult. For highly active shale formations, currently available strong inhibitors such as polyetheramino alkyl glycosides, polyetheramines, and polyamines have been able to effectively address this problem. However, for wellbore formations prone to collapse, such as wellbore fractures and fractured zones, drilling fluids not only require strong inhibitory properties, but also have higher requirements for chemical wall fixation and filtrate intrusion reduction.

[0005] In recent years, commonly used inhibitors for wall-settling have primarily included silicates, organosilicones, and polyaluminum. These inhibitors utilize physical or chemical changes in temperature, pH, and salinity upon entering the formation to deposit and plug bedding fractures or form a cemented shell, thereby sealing or consolidating the wellbore. However, these inhibitors have limited effectiveness in preventing wellbore collapse, significantly impact drilling fluid rheology, are susceptible to failure at high temperatures, and pose environmental risks. Furthermore, they offer limited fluid loss reduction, leading to stress changes in the formation caused by filtrate intrusion and subsequent wellbore collapse. To effectively address the challenges of wellbore instability and fluid loss reduction in collapse-prone formations, such as those with developed bedding fractures and fractured zones, the performance of currently used inhibitors is in urgent need of improvement. In addition to enhancing their wall-settling performance, their fluid loss reduction performance must also be enhanced to meet the technical requirements for wellbore stability in such formations. Therefore, it is necessary to develop highly effective wall-settling and fluid loss reduction agents that combine both wall-settling and fluid loss reduction capabilities to meet urgent field technical needs.

[0006] Summary of the Invention

[0007] The present invention aims to overcome the problem of the prior art lacking both good wall-stabilizing and fluid-loss reduction effects by providing a chitosan graft copolymer, a preparation method thereof, and a drilling fluid. The chitosan graft copolymer provided by the present invention has good wall-stabilizing and fluid-loss reduction effects, particularly high-temperature (240°C) wall-stabilizing and fluid-loss reduction performance, and is non-biotoxic and environmentally friendly.

[0008] In order to achieve the above object, the first aspect of the present invention provides a chitosan graft copolymer, wherein the chitosan graft copolymer has a structure represented by formula (I) and a number average molecular weight of 200,000-600,000;

[0009] m is 1200-3500, X and Z are each independently H or -R1COOM1 or a polymer segment, and the content of the polymer segment is 0.15-20wt% based on the total amount of the chitosan graft copolymer; wherein R1 is a C1-C5 alkylene group, M1 is H, Na or K; the polymer segment contains a structural unit A and a structural unit B, the structural unit A contains at least two -COOM2, M2 is H, Na or K, and the structural unit B contains a cyclic lactam structure; and Y is H or a quaternary ammonium salt structure with an ester group.

[0010] Preferably, the mass ratio of -R1COOM1 to the polymer segment is 1:1-1.5, and the mass ratio of the structural unit A to the structural unit B in the polymer segment is 1:0.5-2.

[0011] Preferably, Y is H or

[0012] wherein R2-R4 are each independently H or a C1-C5 alkyl group, preferably H; R5 is a C1-C5 alkylene group; R6-R8 are each independently a C1-C5 alkyl group; T - F - 、Cl - Br - or I - , preferably Cl - .

[0013] Preferably, the structural unit A is a structural unit having a structure represented by formula (1), and the structural unit B is a structural unit having a structure represented by formula (2);

[0014] Among them, R9-R 13 Each is independently H or a C1-C5 alkyl group, preferably H; M2 and M3 are each independently H, Na or K.

[0015] A second aspect of the present invention provides a method for preparing a chitosan graft copolymer, the method comprising the following steps:

[0016] (1) Carboxylating chitosan with a halogenated carboxylic acid represented by formula (II) to obtain a first intermediate product;

[0017] (2) subjecting the first intermediate product to an addition reaction with a quaternary ammonium salt represented by formula (III) to obtain a second intermediate product;

[0018] (3) in the presence of an initiator, subjecting the second intermediate product to a graft copolymerization reaction with a monomer in water; wherein the monomer comprises: a monomer A' and a monomer B', wherein the monomer A' is a monomer having a structure represented by formula (1'), and the monomer B' is a monomer having a structure represented by formula (2');

[0019] Among them, R1-R 13 The definitions are the same as those above; T is F, Cl, Br or I, preferably Cl.

[0020] The third aspect of the present invention provides a chitosan graft copolymer prepared by the method described in the second aspect.

[0021] A fourth aspect of the present invention provides a drilling fluid, which comprises the chitosan graft copolymer described in the first aspect or the third aspect as a fluid loss additive.

[0022] Through the above technical solution, the present invention can achieve the following beneficial effects:

[0023] The chitosan graft copolymer provided by the present invention has good wall-solidifying performance and fluid loss reduction performance in drilling fluid, especially high-temperature (240°C) wall-solidifying performance and fluid loss reduction effect, and has no biological toxicity and is green and environmentally friendly. The chitosan graft copolymer provided by the present invention has a good chemical consolidation effect on the well wall, and has a good wall-solidifying effect on easily collapsed strata such as bedding fractures and broken zones. At the same time, it prevents the filtrate from invading the stratum and causing stress changes, has a good fluid loss reduction effect, and can better solve the problem of well wall instability in easily collapsed strata such as bedding fractures and broken zones on site, and meet the technical requirements for well wall stability in easily collapsed strata such as bedding fractures and broken zones. Moreover, in the preparation process of the chitosan graft copolymer provided by the present invention, the reaction conditions are mild, water is used as the solvent, the reaction energy consumption is low, and there is no discharge of wastewater, waste gas, and waste residue. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 is an infrared spectrum of a sample obtained in Example 1 of the present invention;

[0025] Figures 2(a)-(d) are photos of an unsoaked core column, a core column soaked in water, a core column soaked in Example 1, and a core column soaked in a comparative example, respectively;

[0026] FIG3 is a hydrogen nuclear magnetic resonance spectrum of the sample prepared in Example 4 of the present invention;

[0027] FIG4 is a carbon NMR spectrum of the sample prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0028] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0029] In the present invention, the C1-C5 alkyl group refers to an alkyl group with 1 to 5 carbon atoms; the C1-C5 alkylene group refers to an alkylene group with 1 to 5 carbon atoms.

[0030] In the present invention, unless otherwise specified, the content of each structural unit in the chitosan graft copolymer is calculated based on the feed amount.

[0031] The first aspect of the present invention provides a chitosan graft copolymer, wherein the chitosan graft copolymer has a structure represented by formula (I) and a number average molecular weight of 200,000-600,000;

[0032] m is 1200-3500, X and Z are each independently H or -R1COOM1 or a polymer segment, and the content of the polymer segment is 0.15-20wt% based on the total amount of the chitosan graft copolymer, for example, 0.15wt%, 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt% , 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt%, 20wt% and values ​​between any two groups; wherein, R1 is a C1-C5 alkylene group, M1 is H, Na or K; the polymer segment contains a structural unit A and a structural unit B, the structural unit A contains at least two -COOM2, M2 is H, Na or K, and the structural unit B contains a cyclic lactam structure; Y is H or a quaternary ammonium salt structure with an ester group.

[0033] In the present invention, X and Z are each independently H or -R1COOM1 or a polymeric segment, which means that the types of substituents at the X and Z positions may be the same or different. For example, when X is H, Z may be H or -R1COOM1 or a polymeric segment. This is only a schematic illustration of the present invention.

[0034] The inventors of the present invention have found in their research that by simultaneously introducing structural unit A and structural unit B into the chitosan structure and controlling the content of each structural unit within the above-mentioned range, the obtained chitosan graft copolymer has a good fluid loss reduction effect, especially a high temperature (240°C) fluid loss reduction effect.

[0035] In the present invention, the number average molecular weight of the chitosan graft copolymer is measured by gel chromatography.

[0036] In the present invention, the C1-C5 alkylene group may be linear, branched, or cyclic, for example, -CH2-, -CH2CH2-, or -CH2CH2CH2-.

[0037] In a preferred embodiment of the present invention, Y is H or

[0038] wherein R2-R4 are each independently H or a C1-C5 alkyl group, preferably H; R5 is a C1-C5 alkylene group; R6-R8 are each independently a C1-C5 alkyl group, preferably a methyl group; - F - 、Cl - Br - or I - , preferably Cl - .

[0039] In the present invention, R6 to R8 may be the same or different, but are preferably the same.

[0040] In the present invention, the C1-C5 alkyl group may be linear, branched, or cyclic, for example, -CH3, -CH2CH3, -CH2CH2CH3, -CH(CH3)2, or -C(CH3)3.

[0041] In a preferred embodiment of the present invention, the mass ratio of -R1COOM1 to the polymeric segment is 1:1-1.5, and the mass ratio of the structural unit A to the structural unit B in the polymeric segment is 1:0.5-2.

[0042] In a preferred embodiment of the present invention, the structural unit A is a structural unit having a structure shown in formula (1)

[0043] Among them, R9-R 10 Each is independently H or a C1-C5 alkyl group, preferably H; M3 and M4 are each independently H, Na or K.

[0044] In a preferred embodiment of the present invention, the structural unit B is a structural unit having a structure shown in formula (2)

[0045] Among them, R 11 -R 13 Each is independently H or a C1-C5 alkyl group.

[0046] In the present invention, when R9-R 13When each is H, structural unit A may be a structural unit provided by maleic anhydride, and structural unit B may be a structural unit provided by N-vinylcaprolactam. That is, in one embodiment of the present invention, the polymeric segment in the chitosan graft copolymer structure includes structural units provided by maleic anhydride and N-vinylcaprolactam.

[0047] In a preferred embodiment of the present invention, the polymeric segment further contains at least one of a structural unit C, a structural unit D, a structural unit E, and a structural unit F; wherein the structural unit C has at least one of the structural units having a structure represented by formula (3), the structural unit D is a structural unit having a structure represented by formula (4), the structural unit E is a structural unit having a structure represented by formula (5), and the structural unit F is a structural unit having a structure represented by formula (6);

[0048] Among them, R 14 -R 16 、R 19 -R 21 、R 23 -R 25 、R 28 -R 30 Each independently represents H or a C1-C5 alkyl group, preferably H;

[0049] R 17 is -CO-O;

[0050] R 18 、R 22 、R 26 Each is independently a bond or a C1-C5 alkylene group;

[0051] R 27 is a C1-C5 alkyl group, preferably -CH3, -CH2CH3, -CH(CH3)2, -C(CH3)3;

[0052] M5 and M6 are each independently H, Na or K.

[0053] In the present invention, when R 14 -R 16 H, R 17 -CO-O, R 18 When R is a C4 alkylene group, the structural unit C is a structural unit provided by acryloyloxybutanesulfonic acid. 19 -R 21 H, R 22 When R is a bond, the structural unit D is a structural unit provided by acrylonitrile. 23 -R 25 H, R 26When R is a bond, the structural unit E is a structural unit provided by vinyltrialkoxysilane. 23 -R 30 Each is H, and the structural unit E is a structural unit provided by sulfonated styrene. That is, in a specific embodiment of the present invention, the polymer segment in the chitosan graft copolymer further includes a structural unit provided by at least one of acryloxybutanesulfonic acid, acrylonitrile, sulfonated styrene and vinyltrialkoxysilane.

[0054] In a preferred embodiment of the present invention, the vinyltrialkoxysilane is at least one selected from vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriisopropoxysilane and vinyltri-tert-butoxysilane.

[0055] In a preferred embodiment of the present invention, the mass ratio of chitosan, structural unit A, structural unit B, structural unit C, structural unit D, structural unit E and structural unit F is 1:(1.6-3):(2-4):(2.5-4):(1-2.5):(3-5.5):(4-5), preferably 1:(2-3):(2.5-4):(3-4):(1-2):(3-5):(4-4.5).

[0056] The inventors of the present invention have found in their research that by introducing structural unit C on the basis of the polymer chain segment of the chitosan graft copolymer containing unit A and structural unit B, and controlling the content of each structural unit within the above-mentioned range, the fluid loss reduction effect of the chitosan graft copolymer can be further improved, especially the fluid loss reduction effect at high temperature (240°C).

[0057] Furthermore, on the basis of the structural unit A, structural unit B, and structural unit C contained in the polymer chain segment of the chitosan graft copolymer, the structural unit D is introduced, and the content of each structural unit is controlled within the above range, which can further improve the fluid loss reduction effect of the chitosan graft copolymer, especially the high temperature (240°C) fluid loss reduction effect.

[0058] Furthermore, on the basis of the structural unit A, structural unit B, structural unit C and structural unit D in the polymer chain segment of the chitosan graft copolymer, the structural unit E is introduced, and the content of each structural unit is controlled within the above range. In addition to having a good fluid loss reduction effect, the chitosan graft copolymer also has good wall solidification performance.

[0059] Furthermore, on the basis of the structural unit A, structural unit B, structural unit C, structural unit D and structural unit E contained in the polymer chain segment of the chitosan graft copolymer, the structural unit F is introduced, and the content of each structural unit is controlled within the above range, which can further improve the chitosan graft copolymer's fluid loss reduction effect and wall solidification performance.

[0060] A second aspect of the present invention provides a method for preparing a chitosan graft copolymer. According to a preferred embodiment, the method comprises the following steps:

[0061] (1) alkalizing and gelatinizing a chitosan aqueous solution with an alkaline compound and an alcohol compound, and then subjecting the alkalized gelatinization product to a carboxylation reaction with a halogenated carboxylic acid represented by formula (II) to obtain a first intermediate product;

[0062] (2) subjecting the first intermediate product to an addition reaction with a quaternary ammonium salt represented by formula (III) to obtain a second intermediate product;

[0063] (3) in the presence of an initiator, subjecting the second intermediate product to a graft copolymerization reaction with a monomer in water; wherein the monomer comprises: a monomer A' and a monomer B', wherein the monomer A' is a monomer having a structure represented by formula (1'), and the monomer B' is a monomer having a structure represented by formula (2');

[0064] Among them, R1-R 13 The definitions are the same as those above; T is F, Cl, Br or I, preferably Cl.

[0065] In the present invention, a chitosan aqueous solution is first activated using an alkaline compound and an alcohol compound, and then the activated chitosan is modified using a halogenated carboxylic acid and a quaternary ammonium salt in sequence, and then the modified chitosan is grafted onto monomers A' and B' to obtain the chitosan graft copolymer. The chitosan graft copolymer provided by the present invention has good wall-fixing performance and fluid loss reduction performance in drilling fluid, especially high-temperature (240°C) wall-fixing performance and fluid loss reduction effect, is non-biotoxic, and is environmentally friendly. In addition, during the preparation process of the chitosan graft copolymer provided by the present invention, the reaction conditions are mild, water is used as the solvent, the reaction energy consumption is low, and no wastewater, waste gas, or waste residue is discharged.

[0066] In a preferred embodiment of the present invention, in step (1), chitosan is activated by alkalizing and gelatinizing chitosan using alkaline compounds and alcohol compounds, so that chitosan can be easily subjected to subsequent reactions.

[0067] In the present invention, in order to promote the full progress of the alkalization gelatinization process, the conditions of the alkalization gelatinization include: a temperature of 55-70° C., preferably 56-67° C., a time of 0.5-2 h, and the alkalization gelatinization is carried out under stirring conditions.

[0068] In a preferred embodiment of the present invention, the alkaline compound is selected from sodium hydroxide and / or potassium hydroxide.

[0069] In a preferred embodiment of the present invention, the alcohol compound is selected from at least one of ethanol, glycerol and isopropanol.

[0070] In a preferred embodiment of the present invention, the mass ratio of chitosan, water, alkaline compound and alcohol compound is 1:(10-30):(1-3):(2-5). When the amount of each raw material is within the above range, the chitosan can be fully activated.

[0071] In a preferred embodiment of the present invention, in order to promote the full progress of the carboxylation reaction, the conditions of the carboxylation reaction include: temperature of 70-85° C., time of 3-6 hours, and pH value of 9-11.

[0072] In the present invention, in order to make the pH value of the solution undergoing the carboxylation reaction meet the above conditions, an alkaline pH regulator commonly used in the art can be used for adjustment. Preferably, the alkaline pH regulator is selected from sodium hydroxide or potassium hydroxide.

[0073] In a preferred embodiment of the present invention, in order to ensure that the activated chitosan is fully modified, the mass ratio of chitosan to halogenated carboxylic acid is 1:(3-5), preferably 1:(4-5).

[0074] In the present invention, there is no particular limitation on the type of the halogenated carboxylic acid, as long as it has the structure shown in formula (II). Preferably, the halogenated carboxylic acid is selected from at least one of chloroacetic acid, 3-chloropropionic acid and 4-chlorobutyric acid.

[0075] In a preferred embodiment of the present invention, in order to allow the first intermediate product to fully react with the quaternary ammonium salt to generate the second intermediate product, the conditions of the addition reaction include: a temperature of 105-130° C. and a time of 2-4 hours.

[0076] In a preferred embodiment of the present invention, the mass ratio of chitosan to quaternary ammonium salt is 1:(3-5), preferably 1:(4-5). This preferred solution can ensure that the first intermediate product is fully modified by the quaternary ammonium salt.

[0077] In the present invention, there is no particular limitation on the type of the quaternary ammonium salt, as long as it has the structure shown in formula (III). Preferably, the quaternary ammonium salt is selected from methacryloyloxyethyltrimethylammonium chloride and methacryloyloxyethyltrimethylammonium bromide.

[0078] In a preferred embodiment of the present invention, step (3) comprises: mixing the second intermediate product, monomer A' and monomer B' in water to obtain a mixture; then adding an initiator to the mixture to carry out a graft copolymerization reaction to obtain a reaction product; and then drying, granulating and crushing the reaction product to obtain the chitosan graft copolymer.

[0079] In the present invention, there is no particular limitation on the mixing conditions, as long as the second intermediate product, monomer A' and monomer B' can be fully dissolved. Preferably, the mixing is performed under heating conditions, with the heating temperature being 45-68°C.

[0080] In a preferred embodiment of the present invention, to promote sufficient reaction between the second intermediate product and the monomer, the graft copolymerization reaction conditions include: a temperature of 75-95°C, preferably 78-94°C, a reaction time of 2-8 hours, and a pH value of 8-10. Preferably, the graft copolymerization reaction is carried out under the protection of an inert gas, wherein the inert gas is selected from nitrogen or argon.

[0081] In the present invention, in order to make the pH value of the solution for the graft copolymerization reaction meet the above conditions, an alkaline pH regulator commonly used in the art can be used for adjustment. Preferably, the alkaline pH regulator is selected from sodium hydroxide or potassium hydroxide.

[0082] In a preferred embodiment of the present invention, the monomer A' is a monomer having a structure represented by formula (1'). Preferably, the monomer A' is selected from maleic anhydride.

[0083] In a preferred embodiment of the present invention, the monomer B' is a monomer having a structure represented by formula (2'). Preferably, the monomer B' is selected from N-vinylcaprolactam.

[0084] In the present invention, there is no particular limitation on the type and amount of the initiator, and any of the initiators conventionally used in the art may be used. Preferably, the initiator is selected from at least one of dibenzoyl peroxide, ammonium persulfate, cerium ammonium nitrate, and azobisisobutylamidine hydrochloride.

[0085] In a preferred embodiment of the present invention, the mass ratio of chitosan to initiator is 1:(0.05-0.2). By adopting this preferred solution, the chitosan graft copolymer prepared has good fluid loss reduction performance.

[0086] In a preferred embodiment of the present invention, in order to improve the high temperature (240°C) fluid loss reduction performance and wall solidification performance of the prepared chitosan graft copolymer, in step (3), the monomers subjected to the graft copolymerization reaction further include at least one of monomer C', monomer D', monomer E' and monomer F'; wherein the monomer C' is at least one selected from the monomers having the structure shown in formula (3'), the monomer D' is a monomer having the structure shown in formula (4'), the monomer E' is a monomer having the structure shown in formula (5'), and the monomer F' is a monomer having the structure shown in formula (6');

[0087] Among them, R 14 -R 30The definitions are the same as those described above, and M5 and M6 are H, Na or K.

[0088] In a preferred embodiment of the present invention, the monomer C' is selected from acryloxybutanesulfonic acid; the monomer D' is selected from acrylonitrile; the monomer E' is selected from vinyltrialkoxysilane; and the monomer F' is selected from sulfonated styrene.

[0089] The three alkoxy groups in monomer E' may be the same or different, preferably the same, and all are alkoxy groups having 1 to 4 carbon atoms. In a preferred embodiment of the present invention, the vinyl trialkoxysilane is selected from vinyl trimethoxysilane, vinyl triethoxysilane, vinyl triisopropoxysilane or vinyl tri-tert-butoxysilane.

[0090] In a preferred embodiment of the present invention, the mass ratio of chitosan, monomer A', monomer B', monomer C', monomer D', monomer E', and monomer F' is 1:(1.6-3):(2-4):(2-4):(1-2.5):(3-5.5):(4-5), preferably 1:(2-3):(3-4):(2.5-4)(1-2):(3-5):(4-4.5). This preferred embodiment can improve the wall-building properties and fluid loss reduction properties of the prepared chitosan graft copolymer.

[0091] According to a particularly preferred embodiment of the present invention, the preparation method of the chitosan graft copolymer used as the fluid loss reducer comprises the following steps: (1) adding chitosan (polymerization degree m is 1200-3500) to distilled water, stirring evenly, adding an alkaline compound such as sodium hydroxide and an alcohol compound such as ethanol, stirring evenly, and then performing an alkalization gelatinization reaction, then adding a halogenated carboxylic acid represented by formula (II) such as chloroacetic acid, stirring evenly, adjusting the pH value of the reaction solution to 9-11, and performing a carboxylation reaction for 3-6 hours to obtain an intermediate product A1;

[0092] (2) adding a quaternary ammonium salt represented by formula (III), such as methacryloyloxyethyl trimethylammonium chloride, to the intermediate product A1 to carry out an addition reaction to obtain an intermediate product B1;

[0093] (3) adding the monomers required for the reaction into distilled water and stirring evenly to obtain a monomer aqueous solution; adding the monomer aqueous solution into the intermediate product B1, stirring evenly and adjusting the pH value to 8-10; adding an initiator such as dibenzoyl peroxide initiator under nitrogen protection to carry out graft copolymerization; drying, granulating, and crushing to obtain a chitosan graft copolymer, which can be directly used as a fluid loss reducer for drilling fluid without further separation and purification.

[0094] In the present invention, it should be noted that the chitosan graft copolymer obtained can be directly used as a fluid loss reducer for drilling fluids without further post-treatment after the reaction is completed. However, when performing relevant structural characterization, the chitosan graft copolymer can be subjected to corresponding purification treatment according to the characterization requirements.

[0095] A third aspect of the present invention provides a chitosan graft copolymer prepared by the method described in the second aspect. The chitosan graft copolymer provided by the present invention has good wall-stabilizing and fluid loss reduction properties in drilling fluids, particularly high-temperature (240°C) wall-stabilizing and fluid loss reduction properties, and is non-biotoxic and environmentally friendly.

[0096] A fourth aspect of the present invention provides a drilling fluid, which comprises the chitosan graft copolymer described in the first aspect or the third aspect as a fluid loss additive.

[0097] In a preferred embodiment of the present invention, based on the total amount of the drilling fluid, the content of the chitosan graft copolymer is 0.15-20 wt%, preferably 0.5-10 wt%, further 0.5-5 wt%, and even more preferably 0.5-3 wt%.

[0098] In the present invention, there is no special limitation on the other components in the drilling fluid except the chitosan graft copolymer. Those skilled in the art can select drilling fluid components to prepare the drilling fluid according to actual needs, and the present invention does not limit this.

[0099] In the present invention, the method for purifying the chitosan graft copolymer comprises the following steps:

[0100] (1) Accurately weigh 20.00 g of chitosan graft copolymer product and extract it with petroleum ether 1-2 times to remove unreacted methacryloyloxyethyltrimethylammonium chloride, wherein the mass ratio of the crude chitosan graft copolymer product to petroleum ether is 1.0:2-5;

[0101] (2) Chitosan is insoluble in water under alkaline conditions, while chitosan graft copolymer is highly water soluble. Utilizing the difference in water solubility between chitosan graft copolymer and chitosan, chitosan is separated by filtration. The product extracted with petroleum ether is precipitated and washed with distilled water. Chitosan graft copolymer is soluble in water, while chitosan is insoluble in water. Unreacted chitosan is removed by filtration. The mass ratio of the product to distilled water is 1:15-20.

[0102] (3) The product aqueous solution after washing and filtering to remove chitosan is vacuum dried at 40-80°C to remove water to obtain a chitosan graft copolymer concentrate, which is then recrystallized in anhydrous ethanol and ether, and then recrystallized with ethyl acetate and dichloromethane. It is washed with water to remove sodium ions, unreacted monomers and homopolymers, initiator sodium salt and other impurities, and then freeze-dried at -50°C (freeze drying can remove more than 95%-99% of the water, so that the dried product can be stored for a long time without deterioration and can maintain its original structure, that is, a chitosan graft copolymer product is obtained. The purified chitosan graft copolymer product is sealed and stored for subsequent infrared and nuclear magnetic characterization analysis.

[0103] The present invention will be described in detail below through examples. In the following examples and comparative examples, all raw materials involved are commercially available unless otherwise specified.

[0104] Example 1

[0105] (1) 12 g of chitosan (degree of polymerization m is 1500) was added to 200 g of distilled water and stirred evenly. 20 g of sodium hydroxide and 30 g of ethanol were added and stirred evenly. Alkaline gelatinization reaction was carried out at 56° C. for 0.5 h. 42 g of chloroacetic acid was added and stirred evenly. The pH value of the reaction solution was adjusted to 9 with 20 wt % sodium hydroxide aqueous solution. The reaction was carboxylated at 72° C. for 3 h to obtain intermediate product A1.

[0106] (2) 44 g of methacryloyloxyethyl trimethylammonium chloride was added to the intermediate product A1, and the addition reaction was carried out at 108° C. for 2 h to obtain the intermediate product B1;

[0107] (3) 32 g of acryloxybutanesulfonic acid, 22 g of maleic anhydride, 18 g of acrylonitrile, 24 g of N-vinylcaprolactam, 43 g of vinyltrimethoxysilane, and 48 g of sulfonated styrene were added to 300 g of distilled water and stirred uniformly to obtain a monomer aqueous solution; the monomer aqueous solution was added to the intermediate product B1, stirred uniformly, heated to 45° C., and the pH value was adjusted to 8 with a 20 wt % sodium hydroxide aqueous solution. Under nitrogen protection, 0.9 g of dibenzoyl peroxide initiator was added, and graft copolymerization was carried out at 78° C. for 2 h. The chitosan graft copolymer was then dried, granulated, and crushed to obtain a chitosan graft copolymer.

[0108] The product prepared in Example 1 of the present invention was subjected to infrared testing, and the infrared spectrum is shown in FIG1 : at 1596 cm -1 The characteristic absorption peak of chitosan NH is 1216 cm -1 The characteristic absorption peak of C=O bond is 1356cm -1 The characteristic absorption peak of the quaternary ammonium group is 2213 cm -1 is the characteristic absorption peak of carbon-nitrogen triple bond, 1045 cm -1 is the characteristic absorption peak of sulfonic acid group, 1403 cm-1 The characteristic absorption peak of lactam is 1253 cm -1 The characteristic absorption peak of silane is shown in Figure 2, which indicates that carbonyl, quaternary ammonium, cyano, sulfonic acid, lactam and silane groups have been introduced into the chitosan molecular structure.

[0109] Calculation based on the feed amounts (the same applies hereinafter) indicates that, based on the chitosan feed amount, the mass ratio of structural unit A, structural unit B, structural unit C, structural unit D, structural unit E, and structural unit F in the product prepared in Example 1 of the present invention is 22:24:32:18:43:48. The number average molecular weight of the chitosan graft copolymer was measured by gel permeation chromatography and was 278,000.

[0110] Example 2

[0111] (1) 12 g of chitosan (degree of polymerization m = 2000) was added to 240 g of distilled water and stirred evenly. 22 g of potassium hydroxide and 35 g of glycerol were added and stirred evenly. The mixture was alkalized and gelatinized at 58° C. for 1.0 h. 44 g of 3-chloropropionic acid was added and stirred evenly. The pH value of the reaction solution was adjusted to 10 with a 20% potassium hydroxide aqueous solution. The reaction mixture was carboxylated at 75° C. for 4 h to obtain an intermediate product A2.

[0112] (2) Add 46 g of methacryloyloxyethyl trimethylammonium chloride to the intermediate product A2, and carry out addition reaction at 115° C. for 3 h to obtain the intermediate product B2;

[0113] (3) 36 g of acryloxybutanesulfonic acid, 24 g of maleic anhydride, 22 g of acrylonitrile, 28 g of N-vinylcaprolactam, 48 g of vinyltriethoxysilane, and 50 g of sulfonated styrene were added to 350 g of distilled water and stirred uniformly to obtain a monomer aqueous solution; the monomer aqueous solution was added to the intermediate product B2, stirred uniformly, heated to 50° C., and the pH value was adjusted to 9 with a 20% potassium hydroxide aqueous solution. Under helium protection, 1.1 g of ammonium persulfate initiator was added, and graft copolymerization was carried out at 84° C. for 4 h. The mixture was dried, granulated, and crushed to obtain a chitosan graft copolymer.

[0114] Based on the chitosan feed amount, the mass ratio of structural unit A, structural unit B, structural unit C, structural unit D, structural unit E, and structural unit F in the product prepared in Example 2 of the present invention was 12:14:18:11:24:25. The number average molecular weight of the chitosan graft copolymer was 371,000 as measured by gel permeation chromatography.

[0115] Example 3

[0116] (1) 12 g of chitosan (degree of polymerization m = 3000) was added to 280 g of distilled water and stirred evenly. 24 g of sodium hydroxide and 45 g of isopropyl alcohol were added and stirred evenly. The mixture was alkalized and gelatinized at 60° C. for 1.5 h. 50 g of 4-chlorobutyric acid was added and stirred evenly. The pH value of the reaction solution was adjusted to 11 with 20% aqueous sodium hydroxide solution. The reaction mixture was carboxylated at 78° C. for 5 h to obtain intermediate product A3.

[0117] (2) Add 50 g of methacryloyloxyethyl trimethylammonium chloride to the intermediate product A3, and carry out addition reaction at 120° C. for 4 h to obtain the intermediate product B3;

[0118] (3) 42 g of acryloxybutanesulfonic acid, 26 g of maleic anhydride, 24 g of acrylonitrile, 30 g of N-vinylcaprolactam, 52 g of vinyltriisopropoxysilane, and 52 g of sulfonated styrene were added to 400 g of distilled water and stirred uniformly to obtain a monomer aqueous solution; the monomer aqueous solution was added to the intermediate product B3, stirred uniformly, heated to 60° C., and the pH value was adjusted to 10 with a 20% sodium hydroxide aqueous solution. Under nitrogen protection, 1.5 g of ammonium cerium nitrate initiator was added, and graft copolymerization was carried out at 90° C. for 6 h. The chitosan graft copolymer was then dried, granulated, and crushed to obtain a chitosan graft copolymer.

[0119] Based on the chitosan feed amount, the mass ratio of structural unit A, structural unit B, structural unit C, structural unit D, structural unit E, and structural unit F in the product prepared in Example 3 of the present invention was 13:15:21:12:26:26. The number average molecular weight of the chitosan graft copolymer was 556,000 as measured by gel permeation chromatography.

[0120] Example 4

[0121] (1) 12 g of chitosan (degree of polymerization m = 1800) was added to 300 g of distilled water and stirred evenly. 26 g of sodium hydroxide and 50 g of isopropyl alcohol were added and stirred evenly. The mixture was alkalized and gelatinized at 67° C. for 2 h. 54 g of chloroacetic acid was added and stirred evenly. The pH value of the reaction solution was adjusted to 11 with a 20% aqueous sodium hydroxide solution. The mixture was carboxylated at 81° C. for 6 h to obtain an intermediate product A4.

[0122] (2) Add 52 g of methacryloyloxyethyl trimethylammonium chloride to the intermediate product A4 and carry out an addition reaction at 130° C. for 4 h to obtain the intermediate product B4;

[0123] (3) 48 g of acryloxybutanesulfonic acid, 28 g of maleic anhydride, 28 g of acrylonitrile, 32 g of N-vinylcaprolactam, 55 g of vinyltrimethoxysilane, and 54 g of sulfonated styrene were added to 450 g of distilled water and stirred evenly to obtain a monomer aqueous solution; the monomer aqueous solution was added to the intermediate product B4, stirred evenly, heated to 68° C., and the pH value was adjusted to 10 with a 20% sodium hydroxide aqueous solution. Under nitrogen protection, 1.8 g of azobisisobutylamidine hydrochloride initiator was added, and graft copolymerization was carried out at 94° C. for 8 h. The chitosan graft copolymer was dried, granulated, and crushed to obtain a chitosan graft copolymer.

[0124] The crude chitosan graft copolymer product prepared in Example 4 was purified and then subjected to infrared and nuclear magnetic resonance characterization analysis. The purification method of the crude chitosan graft copolymer product included the following steps: (1) accurately weighing 20.00 g of the chitosan graft copolymer product and extracting it with petroleum ether for 1-2 times to remove unreacted methacryloyloxyethyltrimethylammonium chloride, wherein the mass ratio of the crude chitosan graft copolymer product to petroleum ether was 1.0:2-5; (2) chitosan is insoluble in water under alkaline conditions, while the chitosan graft copolymer has good water solubility. The chitosan was filtered and separated by utilizing the difference in water solubility between the chitosan graft copolymer and chitosan. The product extracted with petroleum ether is precipitated and washed with distilled water. The chitosan graft copolymer is soluble in water, while chitosan is insoluble in water. The unreacted chitosan is removed by filtration. The mass ratio of the product to distilled water is 1:15-20. (3) The aqueous solution of the product after washing and filtering to remove chitosan is vacuum dried at 40-80°C to remove water to obtain a chitosan graft copolymer concentrate. The concentrate is then recrystallized in anhydrous ethanol and ether, and then recrystallized with ethyl acetate and dichloromethane. The concentrate is washed to remove sodium ions, unreacted monomers and homopolymers, initiator sodium salt and other impurities, and then freeze-dried at -50°C (freeze-drying can remove more than 95%-99% of the water, so that the dried product can be stored for a long time without deterioration and can maintain its original structure, thus obtaining a chitosan graft copolymer product. The purified chitosan graft copolymer product is sealed and stored for subsequent infrared and nuclear magnetic characterization analysis.

[0125] Example 4 has a similar infrared spectrum to Example 1, indicating that carbonyl groups, quaternary ammonium groups, cyano groups, sulfonic acid groups, lactam groups and silane groups are introduced into the molecular structure of chitosan.

[0126] The chitosan graft copolymer product prepared in Example 4 was subjected to nuclear magnetic resonance testing. The hydrogen spectrum is shown in Figure 3, and the carbon spectrum is shown in Figure 4. Combining Figures 3 and 4, it can be seen that glycosidic rings, hydroxyl groups, ether bonds, amino groups, CN bonds, C-Si bonds, Si-O bonds, etc. are introduced into the chitosan molecular structure.

[0127] Based on the chitosan feed amount, the mass ratio of structural unit A, structural unit B, structural unit C, structural unit D, structural unit E, and structural unit F in the product prepared in Example 4 of the present invention was 28:32:48:28:55:54. The number average molecular weight of the chitosan graft copolymer was 333,000 as measured by gel permeation chromatography.

[0128] Examples 5-12

[0129] The chitosan graft copolymer was prepared according to the method of Example 4, except that in step (3), the monomer components and amounts used in the preparation of the monomer aqueous solution and the number average molecular weight of the chitosan graft copolymer were as shown in Table 1.

[0130] Table 1

[0131] Comparative Example 1

[0132] The chitosan graft copolymer was prepared according to the method of Example 4, except that in step (3), 28 g of maleic anhydride was not added during the preparation of the monomer aqueous solution.

[0133] Comparative Example 2

[0134] The chitosan graft copolymer was prepared according to the method of Example 4, except that in step (3), 32 g of N-vinylcaprolactam was not added during the preparation of the monomer aqueous solution.

[0135] Test Example 1

[0136] The wall-solidifying and cementing properties of the samples prepared in Examples 1-12 and Comparative Examples 1-2 were tested as follows: core columns were soaked in the sample at a mass concentration of 3 wt% at 240° C. for 16 h, and the compressive strength of the core columns was tested after soaking.

[0137] The compressive strength of a core column was tested as follows: a soaked core column was placed on a compressive strength tester. The load was increased along the axis of the core column at a rate of 0.5 MPa per second until the column failed. The compressive strength was calculated as the pressure. In this application, compressive strength refers to uniaxial compressive strength. The test results are shown in Table 2.

[0138] The core column was prepared by the following method: 100 g of API-compliant drilling-grade bentonite was placed in a drying oven and dried at 105°C for 4 h. After being taken out and cooled to room temperature, 25 g of the dried bentonite was weighed, 3 mL of distilled water was added, the bentonite and distilled water were mixed evenly, and the core column was pressed at a pressure of 10 MPa for 5 min to form a core column with a diameter of 2.4 cm and a height of 2.6 cm.

[0139] Figure 2(a) is a photograph of an unsoaked core column. Figure 2(b) is a photograph of a core column soaked in clean water. Figure 2(c) is a photograph of a core column soaked in 3wt% of the sample of Example 1. Figure 2(d) is a photograph of a core column soaked in 3wt% of the sample of Comparative Example 1. As can be seen from Figure 2, the core column soaked in clean water is severely hydrated and has a muddy shape, and the shape of the core column is incomplete; the core column soaked in Example 1 has a complete appearance without cracks, and has a layer of adsorption film on the surface, showing good wall solidification and filtration loss reduction performance; the core column soaked in Comparative Example 1 has many cracks on the appearance and is severely cracked, indicating that the filtrate has seriously invaded. Combined with the compressive strength results in Table 2, it can be seen that the wall solidification effect of Comparative Example 1 is not obvious.

[0140] Table 2

[0141] As shown in Table 2, the chitosan graft copolymer prepared in the embodiment of the present invention exhibits outstanding high temperature (240° C.) wall-solidifying and bonding performance.

[0142] Test Example 2

[0143] The plugging and fluid loss reduction performance of the samples prepared in Examples 1-12 and Comparative Examples 1-2 was tested. Specifically, the effect of a chitosan graft copolymer sample at a mass concentration of 3 wt% on the fluid loss of a 4 wt% sodium bentonite-based slurry was tested. The test conditions were: high-temperature aging at 240°C for 16 hours.

[0144] The preparation method of 4 wt% sodium bentonite-based slurry is as follows: 2 g of anhydrous sodium carbonate and 40 g of sodium bentonite for drilling fluid testing are added to 1 L of water, stirred for 20 minutes, and then cured at room temperature for 24 hours to prepare the slurry.

[0145] According to GB / T 16783.1-2014 "Field Testing of Drilling Fluids in the Petroleum and Natural Gas Industry Part 1: Water-Based Drilling Fluids", the filtration loss of 4 wt% sodium bentonite-based slurry before and after adding 3 wt% of the example and comparative example samples was tested. The test results are shown in Table 3.

[0146] Table 3

[0147] As shown in Table 3, the chitosan graft copolymer samples prepared in the examples of the present invention exhibit outstanding high temperature (240° C.) plugging and fluid loss reduction performance.

[0148] Test Example 3

[0149] The biotoxicity of the chitosan graft copolymer samples prepared in Examples 1-12 of the present invention and the samples prepared in Comparative Examples 1-2 were tested.

[0150] Biotoxicity EC 50The test method of the value is as follows: add the samples of the embodiment and comparative example provided by the present invention into a sodium chloride solution with a mass concentration of 3%, and prepare 0 mg.dm -3 、5000mg.dm -3 、10000mg.dm -3 、25000mg.dm -3 、50000mg.dm -3 、100000mg.dm -3 10 mL of each test sample solution was added and allowed to stand for 60 min. 10 mg of luminescent bacteria T3 powder was added to the above test sample solution in sequence and thoroughly shaken and mixed. The biotoxicity EC of the luminescent bacteria after 15 min of contact with the test sample solution was measured using a 3 wt% sodium chloride solution as a control. 50 Value. EC 50 When the value is ≥30000 mg / L, it is considered to be non-biotoxic and meets the permissible discharge standard. The test results are shown in Table 4.

[0151] Table 4

[0152] It can be seen from the experimental results in Table 4 that the EC 50 Value ≥5.04×10 5 mg / L, far greater than the emission standard of 30000mg / L, non-biological toxicity, green and environmentally friendly.

[0153] From the above examples, it can be seen that the chitosan graft copolymer provided by the present invention has good high temperature (240°C) wall solidification performance and filtration loss reduction performance, which ensures the stability of the well wall in the easily collapsed formation in the shale fracture zone from the two aspects of wall solidification and reducing filtrate intrusion.

[0154] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A chitosan graft copolymer, characterized in that: The chitosan graft copolymer has a structure shown in formula (I) and a number average molecular weight of 200,000-600,000; m is 1200-3500, X and Z are H or -R1COOM1 or polymeric chain segments, and the content of the polymeric chain segments is 0.15-20wt% based on the total amount of the chitosan graft copolymer; wherein R1 is a C1-C5 alkylene group, M1 is H, Na or K; the polymeric chain segments contain structural units A and structural units B, the structural unit A contains at least two -COOM2, M2 is H, Na or K, and the structural unit B contains a cyclic lactam structure; Y is H or a quaternary ammonium salt structure with an ester group.

2. The chitosan graft copolymer according to claim 1, wherein Y is H or wherein R2-R4 are each independently H or C1-C5 alkyl, preferably H; R5 is C1-C5 alkylene; R6-R8 are each independently C1-C5 alkyl; T - F - , Cl - Br - or I - , preferably Cl - .

3. The chitosan graft copolymer according to claim 1 or 2, wherein The mass ratio of -R1COOM1 to the polymer segment is 1:1-1.5, and the mass ratio of the structural unit A to the structural unit B in the polymer segment is 1:0.5-2.

4. The chitosan graft copolymer according to any one of claims 1 to 3, wherein The structural unit A has a structure shown in formula (1), and the structural unit B has a structure shown in formula (2); Among them, R9-R 13 Each is independently H or a C1-C5 alkyl group, preferably H; M3 and M4 are each independently H, Na or K.

5. The chitosan graft copolymer according to any one of claims 1 to 4, wherein The polymer segment further contains at least one of structural unit C, structural unit D, structural unit E and structural unit F; Preferably, the polymer segment further contains a structural unit C, a structural unit D, a structural unit E and a structural unit F; wherein the structural unit C has a structure shown in formula (3), the structural unit D has a structure shown in formula (4), the structural unit E has a structure shown in formula (5), and the structural unit F has a structure shown in formula (6); Among them, R 14 -R 16 , R 19 -R 21 , R 23 -R 25 , R 28 -R 30 Each is independently H or a C1-C5 alkyl group, preferably H; R 17 is -CO-O; R 18 , R 22 , R 26 Each is independently a bond or a C1-C5 alkylene group; R 27 is a C1-C5 alkyl group, preferably -CH3, -CH2CH3, -CH(CH3)2, -C(CH3)3; M5 and M6 are each independently H, Na or K.

6. The chitosan graft copolymer according to claim 5, wherein The mass ratio of the chitosan, structural unit A, structural unit B, structural unit C, structural unit D, structural unit E and structural unit F is 1: (1.6-3): (2-4): (2.5-4): (1-2.5): (3-5.5): (4-5), preferably 1: (2-3): (2.5-4): (3-4): (1-2): (3-5): (4-4.5).

7. A method for preparing a chitosan graft copolymer, characterized in that: The method comprises the following steps: (1) subjecting chitosan to a carboxylation reaction with a halogenated carboxylic acid represented by formula (II) to obtain a first intermediate product; (2) subjecting the first intermediate product to an addition reaction with a quaternary ammonium salt represented by formula (III) to obtain a second intermediate product; (3) in the presence of an initiator, subjecting the second intermediate product to a graft copolymerization reaction with a monomer; wherein the monomer comprises: a monomer A' and a monomer B', wherein the monomer A' has a structure represented by formula (1'), and the monomer B' has a structure represented by formula (2'); Among them, R1-R 13 The definition is the same as that in claim 1 or 2; T is F, Cl, Br or I, preferably Cl.

8. The method according to claim 7, wherein: The monomer further comprises at least one of monomer C', monomer D', monomer E' and monomer F'; wherein the monomer C' has a structure as shown in formula (3'), the monomer D' has a structure as shown in formula (4'), the monomer E' has a structure as shown in formula (5'), and the monomer F' has a structure as shown in formula (6'); Among them, R 14 -R 30 The definition is the same as that in claim 5, and M5 and M6 are H, Na or K.

9. The method according to claim 8, wherein: The mass ratio of chitosan, monomer C', monomer D', monomer E' and monomer F' is 1:(2-4):(1-2.5):(3-5.5):(4-5), preferably 1:(2.5-4):(1-2):(3-5):(4-4.5).

10. The method according to any one of claims 7 to 9, wherein: The mass ratio of chitosan, halogenated carboxylic acid and quaternary ammonium salt is 1:(3-5):(3-5); Preferably, the mass ratio of chitosan, monomer A', and monomer B' is 1:(1.6-3):(2-4), more preferably 1:(2-3):(3-4); Preferably, the polymerization degree of the chitosan is 1200-3500.

11. The method according to any one of claims 7 to 10, wherein: The halogenated carboxylic acid is selected from at least one of chloroacetic acid, 3-chloropropionic acid and 4-chlorobutyric acid; Preferably, the quaternary ammonium salt is selected from methacryloyloxyethyl trimethylammonium chloride; Preferably, the initiator is selected from at least one of dibenzoyl peroxide, ammonium persulfate, cerium ammonium nitrate and azobisisobutylamidine hydrochloride.

12. The method according to any one of claims 7 to 11, wherein: The conditions of the carboxylation reaction include: temperature of 70-85°C, time of 3-6h, pH of 9-11; Preferably, the conditions of the addition reaction include: temperature of 105-130°C and time of 2-4h; Preferably, the conditions of the graft copolymerization reaction include: temperature of 75-95° C., time of 2-8 h, pH value of 8-10, and the graft copolymerization reaction is carried out under the protection of inert gas.

13. The method according to any one of claims 7 to 12, wherein: The method further comprises alkalizing and gelatinizing the chitosan in the presence of an alkaline compound and an alcohol before the carboxylation reaction, wherein the conditions of the alkalizing and gelatinizing reaction include: a temperature of 55-70° C. and a time of 0.5-2 hours; Preferably, the alkaline compound is selected from sodium hydroxide and / or potassium hydroxide; Preferably, the alcohol compound is selected from at least one of ethanol, glycerol and isopropanol.

14. A chitosan graft copolymer obtained by the method according to any one of claims 7 to 13.

15. A drilling fluid, characterized in that: The chitosan graft copolymer according to any one of claims 1 to 6 and 14 is included as a fluid loss reducer.

16. The drilling fluid according to claim 15, wherein: Based on the total amount of the drilling fluid, the content of the chitosan graft copolymer is 0.15-20wt%, preferably 0.5-10wt%.

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