Clay stabilizer, method of making and water-based working fluid

Abstract

The application provides a clay stabilizer, a preparation method and a water-based working fluid. The clay stabilizer comprises a quaternary ammonium chitosan zwitterionic polymer, the quaternary ammonium chitosan zwitterionic polymer comprises a chitosan backbone, a carboxyl substituent grafted on the chitosan backbone and a quaternary ammonium salt substituent grafted on the chitosan backbone, the degree of substitution of the carboxyl substituent is 60-80%, the mass percentage of the quaternary ammonium salt substituent in the quaternary ammonium chitosan zwitterionic polymer is 30-60%, and the polydispersity coefficient of the quaternary ammonium chitosan zwitterionic polymer is 2.2-2.5. The clay stabilizer can simultaneously inhibit the hydration and expansion of clay, control the coordinated control of the dispersion, migration and deposition behavior of fine particles under fluid scouring, realize the dynamic migration regulation of clay fine particles, and reduce the entry of clay fine particles into deep narrow pore throats and the induced plugging.

Description

Technical Field

[0001] This invention relates to the field of organic additives for oil and gas fields, specifically to a clay stabilizer, its preparation method, and a water-based working fluid. Background Technology

[0002] Existing technologies typically employ inorganic salts, organic cationic polymers, and chitosan quaternary ammonium salts as clay stabilizers for clay stabilization treatment. Inorganic salt clay stabilizers primarily inhibit clay hydration and swelling through ion exchange and compression-diffusion double-layer electrostatics, exhibiting some short-term stabilization effects. However, their effectiveness is easily affected by ion concentration, flowback, and reservoir hydration environment, resulting in insufficient long-term stability. Organic cationic polymers and chitosan quaternary ammonium salt clay stabilizers enhance clay surface stability through cation adsorption, but may lead to charge overcompensation or disordered flocculation, making it difficult to control the transport of fine clay particles.

[0003] Therefore, developing a clay stabilizer that can effectively regulate the dynamic migration of clay particles, thereby reducing the problem of clay particles entering deep, narrow pore throats and causing blockage, has become the key to solving the development of water-sensitive conglomerate reservoirs. Summary of the Invention

[0004] This invention provides a clay stabilizer that, through the amphoteric charge regulation capabilities of appropriate amounts of carboxyl substituents and quaternary ammonium salt substituents, can suppress clay hydration and swelling while simultaneously coordinating the control of fine particle dispersion, migration, and deposition behavior during reservoir flow. This solves the technical problem of dynamic transport regulation of clay fine particles, reduces the entry of clay fine particles into deep, narrow pore throats and the resulting blockage, and effectively overcomes the defects of existing technologies.

[0005] The present invention also provides a method for preparing a clay stabilizer, which involves introducing quaternary ammonium cations in steps while retaining carboxyl anions to form a zwitterionic polymer of quaternized chitosan with adsorption, bridging and steric hindrance functions. Under fluid scouring, the clay is transformed from a disordered transport state to a moderately adsorbed and controlled aggregated cluster state, thereby achieving the regulation of the dynamic transport of clay fine particles.

[0006] This invention also provides a water-based working fluid, in which a clay stabilizer is dispersed in an aqueous phase to obtain a water-based working fluid. When the water-based working fluid is applied to conglomerate reservoirs, it effectively reduces clay hydration swelling, clay spalling, clay fine particle migration, and pore throat blockage, which is beneficial to improving the permeability recovery rate and single-well productivity of conglomerate reservoirs.

[0007] On one hand, the present invention provides a clay stabilizer comprising a quaternized chitosan zwitterionic polymer, wherein the quaternized chitosan zwitterionic polymer comprises a chitosan backbone, carboxyl substituents grafted onto the chitosan backbone, and quaternary ammonium salt substituents grafted onto the chitosan backbone, wherein the degree of substitution of the carboxyl substituents is 60-80%, the quaternary ammonium salt substituents account for 30-60% of the mass percentage of the quaternized chitosan zwitterionic polymer, and the polydispersity index of the quaternized chitosan zwitterionic polymer is 2.2-2.5.

[0008] According to one embodiment of the present invention, the number-average molecular weight of the quaternized chitosan zwitterionic polymer is (1-3)×10⁻⁶. 4 The weight-average molecular weight is (2-6)×10 4 .

[0009] According to one embodiment of the present invention, the structural unit of the quaternary ammonium salt substituent is -R2-N. + (R3)3X - Wherein, R2 is a C3 alkylene group containing a hydroxyl group, and R3 is a C1-C2 alkylene group. 12 Alkyl group, where X is Cl, Br, or I;

[0010] And / or, the structural unit of the carboxyl substituent is -R1-COOM; wherein R1 is a C1-C3 alkylene group and M is H, Na, K or NH4.

[0011] In another aspect, the present invention provides a method for preparing the aforementioned clay stabilizer, comprising the following steps:

[0012] A chitosan-based raw material containing carboxyl groups is dispersed in a first solvent to obtain a chitosan-based raw material dispersion; wherein, the first solvent includes an alcohol solvent and water;

[0013] The cationic etherifying agent used to form the quaternary ammonium salt substituent is added to water to obtain a cationic etherifying agent solution;

[0014] The cationic etherifying agent solution was added to the chitosan raw material dispersion in multiple portions to carry out a quaternization reaction. After separation and purification, a clay stabilizer was obtained.

[0015] In the step of adding the cationic etherifying agent solution to the chitosan raw material dispersion each time, the addition rate of the cationic etherifying agent solution is 2.5-8.5 mL / h;

[0016] The time interval T between each two adjacent steps of adding the cationic etherifying agent solution to the chitosan raw material dispersion is 2h≤T≤5h.

[0017] According to one embodiment of the present invention, the carboxyl-containing chitosan raw material includes at least one of carboxymethyl chitosan, carboxymethyl chitosan salt, carboxyethyl chitosan, and carboxyethyl chitosan salt;

[0018] And / or, the cationic etherifying agent comprises at least one of 2,3-epoxypropyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, and N-(2,3-epoxypropyl)-N,N-dimethyldodecylammonium chloride;

[0019] And / or, the alcohol solvent includes water-soluble alcohols;

[0020] And / or, the volume ratio of the alcohol solvent to water is (1-5):1;

[0021] And / or, the mass ratio of the carboxyl-containing chitosan raw material to the cationic etherifying agent in the cationic etherifying agent solution is 1:(2-6);

[0022] And / or, the mass concentration of the carboxyl-containing chitosan raw material in the chitosan raw material dispersion is 0.03-0.1 g / mL;

[0023] And / or, the mass concentration of the cationic etherifying agent in the cationic etherifying agent solution is 0.33-0.77 g / mL.

[0024] According to one embodiment of the present invention, in each two adjacent steps of adding the cationic etherifying agent solution to the chitosan raw material dispersion, the volume ratio of the cationic etherifying agent solution added in the previous step to that added in the next step is 1:(0.5-2).

[0025] And / or, in the step of adding the cationic etherifying agent solution to the chitosan raw material dispersion for the last time, after the last addition of the cationic etherifying agent solution, maintain the quaternization reaction treatment for 2-5 hours before performing the separation and purification;

[0026] And / or, the temperature of the quaternization reaction is 60-90°C.

[0027] In another aspect, the present invention provides a water-based working fluid, the water-based working fluid comprising an aqueous phase and a clay stabilizer, wherein the clay stabilizer comprises the clay stabilizer described above, or a clay stabilizer prepared by the clay stabilizer preparation method described above.

[0028] According to one embodiment of the present invention, the clay stabilizer accounts for 0.1-3% of the mass percentage of the water-based working fluid;

[0029] And / or, the aqueous phase in the water-based working fluid includes at least one of water, oilfield produced water, and brine, wherein the oilfield produced water has a mineralization of 8000-40000 mg / L and a pH of 6.0-8.0, and the brine has a mineralization of 8000-40000 mg / L and a pH of 6.0-8.0.

[0030] According to one embodiment of the present invention, the water-based working fluid further includes potassium formate, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitor.

[0031] According to one embodiment of the present invention, in the water-based working solution, the mass percentage of potassium formate is 5-60%, the mass percentage of hydroxyethyl cellulose is 0.2-1.0%, the mass percentage of hydroxypropyl starch is 0.5-2.0%, and the mass percentage of corrosion inhibitor is 0.1-0.5%.

[0032] The implementation of this invention has at least the following beneficial effects:

[0033] The clay stabilizer provided by this invention comprises a quaternized chitosan zwitterionic polymer, which uses a chitosan backbone with carboxyl and quaternary ammonium salt substituents grafted onto the backbone. The carboxyl substituents form multi-point hydrogen bonds with the clay surface, reducing the ability of water molecules to penetrate the crystal layers and thus inhibiting hydration swelling. The quaternary ammonium salt substituents capture clay through electrostatic adsorption and, under fluid scouring, form aggregated clusters through bridging, reducing the entry of clay particles into deep pores and throats and preventing blockage. By controlling the degree of substitution of carboxyl substituents to 60-80%, the mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer to 30-60%, and the polydispersity index of the quaternized chitosan zwitterionic polymer to 2.2-2.5, the clay stability rate is made greater than 90%. Furthermore, the quaternized chitosan zwitterionic polymer is first adsorbed onto the clay surface, and then, during fluid flow, through molecular chain bridging, zwitterionic charge regulation, and steric hindrance, the clay fine particles gradually aggregate to form a stable cluster dynamic coating. This achieves comprehensive control over clay hydration swelling, fine particle migration, and pore throat blockage, meeting the requirements of conglomerate reservoir protection for long-term anti-swelling and migration regulation. Attached Figure Description

[0034] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0035] Figure 1 This is a schematic diagram of the dynamic coating mechanism of clay particles by the clay stabilizer of the present invention.

[0036] Figure 2Images showing the clay stabilizer dispersions prepared according to Comparative Example 1 and Example 1 of the present invention; wherein, Figure 2 Image 'a' in Comparative Example 1 shows a clay stabilizer dispersion prepared with a mass percentage of 3%. Figure 2 Image b in Example 1 of this invention shows a clay stabilizer dispersion with a mass percentage of 3%. Figure 2 Image c in Example 1 of this invention is a clay stabilizer dispersion with a mass percentage of 5%.

[0037] Figure 3 The concentration-clay stability curves of the clay stabilizer dispersions prepared according to Example 1 and Comparative Example 1 of the present invention.

[0038] Figure 4 This is a comparison diagram of the dynamic transport of the blank control test solution in the microchannel, serving as a control example.

[0039] Figure 5 Comparison of the dynamic transport of the test solution prepared with clay stabilizer for Comparative Example 1 in the microchannel.

[0040] Figure 6 This is a comparison diagram of the dynamic transport of the test liquid prepared with the clay stabilizer of Example 1 of the present invention in the microchannel. Detailed Implementation

[0041] To enable those skilled in the art to better understand the solutions of this invention, the following provides a more detailed description of this application. The specific embodiments listed below are merely descriptions of the principles and features of this invention; the examples are only for explaining the invention and are not intended to limit its scope. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this invention.

[0042] To effectively regulate the dynamic transport of clay particles, the inventors studied quaternized chitosan polymers, which consist of a chitosan backbone and quaternary ammonium salt substituents grafted onto the backbone. These substituents can adsorb onto the clay surface, increasing surface stability and inhibiting hydration. However, excessively high density of quaternary ammonium salt substituents can lead to charge overcompensation or disordered flocculation, making it difficult to simultaneously achieve clay surface stability, interlayer swelling prevention, and fine particle dynamic transport regulation. Therefore, the inventors proposed the clay stabilizer of this invention.

[0043] This invention provides a clay stabilizer comprising a quaternized chitosan zwitterionic polymer. The quaternized chitosan zwitterionic polymer comprises a chitosan backbone, carboxyl substituents grafted onto the chitosan backbone, and quaternary ammonium salt substituents grafted onto the chitosan backbone. The degree of substitution of the carboxyl substituents is 60-80%, the quaternary ammonium salt substituents account for 30-60% of the mass percentage of the quaternized chitosan zwitterionic polymer, and the polydispersity index of the quaternized chitosan zwitterionic polymer is 2.2-2.5.

[0044] This invention starts with the complex hydration environment and fine-grained transport characteristics of conglomerate reservoirs. First, it analyzes and identifies the limitations of conventional quaternized chitosan polymers, finding that they cannot achieve controlled agglomeration of clay particles under dynamic flow conditions. Then, by introducing zwitterions (i.e., carboxyl and quaternary ammonium salt substituents grafted onto the chitosan backbone), and controlling the degree of substitution of carboxyl substituents to 60-80%, the mass percentage of quaternary ammonium salt substituents in the zwitterion polymer to 30-60%, and the polydispersity index of the zwitterion polymer to 2.2-2.5, the amphoteric charge regulation capabilities of the appropriate amounts of carboxyl and quaternary ammonium salt substituents are utilized. This not only achieves a clay stability rate greater than 90%, but also allows the clay to first adsorb onto the clay surface. Subsequently, during fluid flow, through molecular chain bridging, amphoteric charge regulation, and steric hindrance, the clay particles gradually agglomerate to form a stable, dynamically coated cluster. This solves the problem of clay stabilizers simultaneously meeting the multiple requirements of clay surface stability, interlayer swelling prevention, and controlled agglomeration of clay fine particles.

[0045] Furthermore, the polydispersity index can reflect the molecular weight distribution and, when controlled within the aforementioned range, indicates a moderate molecular weight distribution. The clay stabilizer system contains molecules with different chain lengths. Components with lower molecular weights can rapidly diffuse into clay particles, quickly inhibiting the hydration and swelling of clay particles, while components with higher molecular weights are responsible for long-term stabilization and bridging. This is beneficial for the quaternized chitosan zwitterionic polymer to exert the synergistic effect of rapid short-chain adsorption and strong long-chain bridging.

[0046] Compared to quaternized chitosan polymers, the clay stabilizer in this invention comprises a quaternized chitosan zwitterionic polymer. It uses the chitosan backbone as a framework, grafting carboxyl substituents and quaternary ammonium salt substituents onto the chitosan backbone. It does not simply pursue a high content or high grafting rate of quaternary ammonium salt substituents, but rather, through the synergistic regulation of cations and anions, it enables clay in oilfield produced water or simulated oilfield produced water to transform from a disordered transport state into a moderately adsorbed and controlled agglomerated cluster state. This not only reduces clay hydration swelling but also solves the technical problem of dynamic transport control of fine clay particles, reducing the entry of fine clay particles into deep, narrow pores and throats and preventing blockage.

[0047] Figure 1This is a schematic diagram illustrating the dynamic coating mechanism of clay particles by the clay stabilizer of the present invention. Figure 1 As shown, the clay stabilizer provided by this invention adsorbs onto the clay surface through electrostatic interaction between the quaternary ammonium salt substituents and the negative charge on the clay surface. Subsequently, the carboxyl groups in the clay stabilizer form hydrogen bonds with the clay surface, enhancing the stability of the adsorption layer. Then, molecular chain fragments containing quaternary ammonium cations in the clay stabilizer enter the interior or edge regions of the clay crystal layer, reducing the ability of water molecules to penetrate the crystal layer and encapsulating the clay. Subsequently, under fluid scouring, the clay already encapsulated by the clay stabilizer continues to capture surrounding clay particles, forming stable aggregates through molecular chain bridging. These aggregates remain relatively stable under the influence of steric hindrance and amphoteric charge regulation, reducing the infiltration of fine clay particles into deep, narrow pores and throats.

[0048] Furthermore, in the dynamic coating process of clay particles by the clay stabilizer of the present invention, the clay stabilizer first adsorbs onto the surface of the clay particles, and then, during fluid flow, through molecular chain bridging, amphoteric charge regulation, and steric hindrance, the dispersed fine particles gradually aggregate to form stable clusters. This dynamic coating process is similar to a snowball effect; as the fluid flows, the clay particles are coated layer by layer, and the cluster size gradually increases and tends to stabilize. The present invention figuratively refers to this as a "snowball" type dynamic coating.

[0049] For example, the degree of substitution of the carboxyl substituent can be a range of 60%, 65%, 70%, 75%, 80%, or any two of these.

[0050] For example, the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer can be in the range of 30%, 30.8%, 35%, 40%, 45%, 50%, 55%, 58.7%, 60%, or any two of these.

[0051] For example, the polydispersity index of the above-mentioned quaternized chitosan zwitterionic polymer can be in the range of 2.2, 2.3, 2.4, 2.5 or any two of them.

[0052] It should be noted that the quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer are grafted onto the chitosan backbone by chemical bonds, and the carboxyl substituents are grafted onto the chitosan backbone by chemical bonds.

[0053] Furthermore, the degree of substitution of carboxyl substituents in quaternized chitosan zwitterionic polymers refers to the average number of carboxyl substituents attached to the structural units of the quaternized chitosan zwitterionic polymer. That is, the percentage of the total number of carboxyl substituents to the total number of repeating structural units in the quaternized chitosan zwitterionic polymer. The degree of substitution of carboxyl substituents can be tested using potentiometric titration, colloidal titration, elemental analysis, and nuclear magnetic resonance spectroscopy.

[0054] Furthermore, the mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer can be tested using potentiometric titration, colloidal titration, elemental analysis, and nuclear magnetic resonance spectroscopy. Specifically, the elemental analysis method for determining the mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer is described in the performance testing section on the test method for quaternary ammonium salt grafting rate (mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer).

[0055] In this embodiment of the invention, the repeating structural unit of the quaternized chitosan zwitterionic polymer is [-GlcN(R4)(OR5)(CH2OR6)-]. n Wherein, GlcN is glucosamine, R4, R5, and R6 are each independently selected from hydrogen, quaternary ammonium salt substituents, or carboxyl substituents, and n is the total number of repeating structural units. Furthermore, at least one of R4, R5, and R6 is selected from a quaternary ammonium salt substituent and at least one is selected from a carboxyl substituent.

[0056] For example, the repeating structural unit of the quaternized chitosan zwitterionic polymer is [-GlcN(R4)(OR5)(CH2OR6)-]. n Wherein, GlcN is glucosamine, R4 is a quaternary ammonium salt substituent grafting site, R5 is a carboxyl substituent grafting site, R6 is a carboxyl substituent grafting site, and n is the total number of repeating structural units.

[0057] It should be noted that the present invention does not limit the specific substitution positions of carboxyl substituents and quaternary ammonium salt substituents on the chitosan backbone. Any substitution structure that can simultaneously provide carboxyl anions and quaternary ammonium cations and achieve a dynamic coating effect on clay can be used in the present invention.

[0058] It should be noted that the polydispersity index of quaternized chitosan zwitterionic polymers characterizes the breadth of their molecular weight distribution. Specifically, the polydispersity index of quaternized chitosan zwitterionic polymers is the ratio of their weight-average molecular weight to their number-average molecular weight.

[0059] Furthermore, the number-average molecular weight, weight-average molecular weight, and polydispersity index of quaternized chitosan zwitterionic polymers can be tested using gel permeation chromatography or viscometry. Specifically, for the gel permeation chromatography test methods of the number-average molecular weight and weight-average molecular weight of quaternized chitosan zwitterionic polymers, please refer to the test methods for number-average molecular weight and weight-average molecular weight in the performance testing section.

[0060] In some embodiments, the structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -R2-N. + (R3)3X - Wherein, R2 is a C3 alkylene group containing a hydroxyl group, and R3 is a C1-C2 alkylene group. 12 Alkyl group, where X is Cl, Br, or I. Quaternary ammonium salt substituents can be introduced and grafted from C3 alkylene groups containing hydroxyl groups, giving them stable positively charged adsorption sites while retaining some hydrophilicity and steric flexibility. R3 is selected from C1-C... 12 Alkyl groups can adjust the degree of hydrophobic shielding and charge exposure. X being Cl, Br, or I is beneficial for maintaining ionic balance in the quaternization reaction and keeping the system solubility.

[0061] For example, the structural unit -R2-N of the above-mentioned quaternary ammonium salt substituent + (R3)3X - R3 in the equation can be C1 alkyl, C2 alkyl, C3 alkyl, C4 alkyl, C5 alkyl, C6 alkyl, C7 alkyl, C8 alkyl, C9 alkyl, C 10 Alkyl, C 11 Alkyl, C 12 The range of alkyl groups or any two thereof.

[0062] In some embodiments, the structural unit of the carboxyl substituent in the quaternized chitosan zwitterionic polymer is -R1-COOM; wherein R1 is a C1-C3 alkylene group, and M is H, Na, K, or NH4. The C1-C3 alkylene group of the carboxyl substituent can form carboxymethyl, carboxyethyl, or carboxypropyl branches, facilitating their grafting onto the chitosan backbone. The presence of H, Na, K, or NH4 allows for the regulation of the carboxyl group's free or salt-like state, enabling the carboxyl substituent to provide stable anionic sites in the aqueous phase and enhancing electrostatic repulsion and hydration layer construction capabilities.

[0063] In some embodiments, the number-average molecular weight of the quaternized chitosan zwitterionic polymer is (1-3)×10⁻⁶. 4 The weight-average molecular weight is (2-6)×10 4Number-average molecular weight reflects the average length of the molecular chains in the quaternized chitosan zwitterionic polymer. Controlling it within the aforementioned range ensures good solubility and diffusion in aqueous solutions. Weight-average molecular weight reflects the contribution of larger molecular weight components in the quaternized chitosan zwitterionic polymer. These long-chain molecules can form bridges between multiple clay particles, enhancing inter-particle connectivity and reducing particle dispersion and migration. Controlling it within the aforementioned range ensures sufficient adsorption, coating, and bridging capabilities.

[0064] For example, the number-average molecular weight of the above-mentioned quaternized chitosan zwitterionic polymer can be 1×10⁻⁶. 4 1.3×10 4 1.4×10 4 1.5×10 4 1.6×10 4 1.7×10 4 1.8×10 4 2×10 4 2.5×10 4 3×10 4 or a range consisting of any two of them.

[0065] For example, the weight-average molecular weight of the above-mentioned quaternized chitosan zwitterionic polymer can be 2 × 10⁻⁶. 4 2.9×10 4 3×10 4 3.2×10 4 3.4×10 4 3.6×10 4 3.8×10 4 4×10 4 4.2×10 4 4.5×10 4 5×10 4 5.5×10 4 6×10 4 or a range consisting of any two of them.

[0066] This invention provides a method for preparing a clay stabilizer, comprising the following steps:

[0067] A chitosan-based raw material containing carboxyl groups is dispersed in a first solvent to obtain a chitosan-based raw material dispersion; wherein, the first solvent includes an alcohol solvent and water;

[0068] A cationic etherifying agent used to form quaternary ammonium salt substituents is added to water to obtain a cationic etherifying agent solution;

[0069] The cationic etherifying agent solution was added to the chitosan raw material dispersion in multiple portions to carry out the quaternization reaction. After separation and purification, the clay stabilizer was obtained.

[0070] In each step of adding the cationic etherifying agent solution to the chitosan raw material dispersion, the addition rate of the cationic etherifying agent solution is 2.5-8.5 mL / h.

[0071] The time interval T between each two adjacent steps of adding the cationic etherifying agent solution to the chitosan raw material dispersion is 2h≤T≤5h.

[0072] In the method system described in this embodiment of the invention, a chitosan raw material dispersion and a cationic etherifying agent solution are first prepared. Then, the cationic etherifying agent solution is added to the chitosan raw material dispersion in multiple portions to carry out a quaternization reaction.

[0073] Each addition of the cationic etherifying agent solution allows it to gradually diffuse within the system and uniformly contact the reaction sites of the carboxyl-containing chitosan raw material. This avoids excessive local quaternization, interchain crosslinking, or micellar precipitation caused by large-scale addition, which could affect the solubility of the clay stabilizer and its controlled adsorption on the clay surface. The addition rate of the cationic etherifying agent solution is controlled at 2.5-8.5 mL / h to maintain a low instantaneous concentration in the system, promoting a stable quaternization reaction and preventing uneven grafting, increased side reactions, and a narrow molecular weight distribution due to excessively high local concentrations. The interval between two consecutive additions of the cationic etherifying agent solution is controlled at 2-5 h to allow for sufficient diffusion, adsorption, and reaction of the added cationic etherifying agent. This ensures the transformation and uniform distribution of the previously formed quaternization intermediate structure, and allows the quaternization intermediate structure to fully extend, exposing more reaction sites. Subsequent additions then improve the uniformity and controllability of the quaternary ammonium salt substituents in the product.

[0074] Furthermore, by repeatedly adding the cationic etherifying agent solution, controlling the addition rate, and controlling the interval between two consecutive additions, the cationic etherifying agent in the system can gradually undergo quaternization with the sites on the carboxyl-containing chitosan raw materials. This achieves synergistic control over the degree of carboxyl substitution, quaternary ammonium salt substituents, and molecular weight distribution, resulting in moderate carboxyl substitution and quaternary ammonium salt substituent content in the clay stabilizer (60-80% carboxyl substitution, 30-60% quaternary ammonium salt substituents by mass percentage in the quaternized chitosan zwitterionic polymer) and moderate polydispersity index of the quaternized chitosan zwitterionic polymer (2.2-2.5). This suppresses clay particle swelling and migration, reduces the probability of pore throat blockage, maintains stable seepage channels, and meets the requirements of long-term anti-swelling and migration control for the protection of highly water-sensitive reservoirs.

[0075] For example, in each step of adding a cationic etherifying agent solution to the chitosan raw material dispersion, the addition rate of the cationic etherifying agent solution can be a range of 2.5 mL / h, 3 mL / h, 4 mL / h, 5 mL / h, 6 mL / h, 7 mL / h, 8 mL / h, 8.3 mL / h, 8.5 mL / h, or any combination thereof.

[0076] For example, in each step of adding the cationic etherifying agent solution to the chitosan raw material dispersion, the interval T of the cationic etherifying agent solution can be a range of 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h or any combination thereof.

[0077] It can be understood that the time interval T between each two adjacent steps of adding cationic etherifying agent solution to the chitosan raw material dispersion specifically refers to the following: each time cationic etherifying agent solution is added to the chitosan raw material dispersion, after the addition of cationic etherifying agent solution is completed (i.e., after all cationic etherifying agent solution has been added to the chitosan raw material dispersion), another time interval T is elapsed before the next step of adding cationic etherifying agent solution to the chitosan raw material dispersion is performed, and so on, until all cationic etherifying agent solution has been added to the chitosan raw material dispersion.

[0078] It should be noted that carboxyl-containing chitosan raw materials include the chitosan backbone and the carboxyl substituents grafted onto the chitosan backbone. The degree of substitution of carboxyl substituents in carboxyl-containing chitosan raw materials refers to the average number of carboxyl substituents attached to each structural unit. That is, the percentage of the total number of carboxyl substituents relative to the total number of zwitterionic polymer structural units of the quaternized chitosan.

[0079] It is understandable that during the quaternization reaction of carboxyl-containing chitosan raw materials and cationic etherifying agents, the carboxyl substituents in the carboxyl-containing chitosan raw materials are not destroyed. Therefore, the degree of substitution of the carboxyl substituents in the carboxyl-containing chitosan raw materials is the same as the degree of substitution of the carboxyl substituents in the quaternized chitosan zwitterionic polymer.

[0080] In this embodiment of the invention, the carboxyl-containing chitosan raw material includes chitosan derivatives, which include at least one of carboxymethyl chitosan, carboxymethyl chitosan salt, carboxyethyl chitosan, and carboxyethyl chitosan salt. These substances all possess good hydrophilic dispersibility and reactivity, and can provide appropriate carboxyl substituents. They can also serve as polymer backbones for introducing quaternary ammonium salt substituents, which is beneficial for controlling the content of carboxyl substituents and quaternary ammonium salt substituents in clay stabilizers.

[0081] In some embodiments, the cationic etherifying agent includes at least one of 2,3-epoxypropyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, and N-(2,3-epoxypropyl)-N,N-dimethyldodecylammonium chloride. These substances can all undergo quaternization reactions with carboxyl-containing chitosan raw materials and can also provide appropriate quaternary ammonium salt substituents, which is beneficial for controlling the quaternary ammonium salt substituent content in clay stabilizers.

[0082] In some embodiments, the alcohol solvent includes a water-soluble alcohol. The water-soluble alcohol achieves its miscibility with water. Exemplarily, the water-soluble alcohol may include at least one selected from methanol, ethanol, diethanol, isopropanol, and n-propanol.

[0083] It should be understood that the above examples are for illustrative purposes only and are not limiting. The chitosan raw material containing carboxyl groups can also be other chitosan raw materials containing carboxyl groups that have the same technical effect. The cationic etherifying agent can also be other cationic etherifying agents that can be used to form quaternary ammonium salt substituents. The water-soluble alcohol can be selected according to the actual process requirements.

[0084] In this embodiment of the invention, the volume ratio of alcohol solvent to water is (1-5):1, for example, it can be a range of 1:1, 2:1, 3:1, 4:1, 5:1 or any two of these. By controlling the volume ratio of alcohol solvent to water, the dispersion state of carboxyl-containing chitosan raw material in the first solvent is maintained, mass transfer is improved, and it is beneficial to its quaternization reaction with the cationic etherifying agent.

[0085] In this embodiment of the invention, the mass ratio of the carboxyl-containing chitosan raw material to the cationic etherifying agent in the cationic etherifying agent solution is 1:(2-6), for example, it can be a range of 1:2, 1:3, 1:4, 1:5, 1:6, or any combination thereof. By adjusting the mass ratio of the carboxyl-containing chitosan raw material to the cationic etherifying agent, a suitable grafting balance is formed between the carboxyl substituents and quaternary ammonium salt substituents in the clay stabilizer, which is beneficial to improving the clay stabilization rate of the clay stabilizer.

[0086] It should be noted that the mass ratio of the carboxyl-containing chitosan raw material to the cationic etherifying agent in the cationic etherifying agent solution refers to the ratio of the mass of the carboxyl-containing chitosan raw material to the total mass of the cationic etherifying agent in the multiple additions of the cationic etherifying agent solution to the chitosan raw material dispersion.

[0087] In some embodiments, the mass concentration of carboxyl-containing chitosan raw material in the chitosan raw material dispersion is 0.03-0.1 g / mL, for example, it can be a range of 0.03 g / mL, 0.05 g / mL, 0.06 g / mL, 0.08 g / mL, 0.1 g / mL, or any combination thereof. By controlling the mass concentration of the chitosan raw material, it is fully dispersed, ensuring a sufficient effective collision probability during the quaternization reaction, while avoiding local agglomeration caused by excessive concentration.

[0088] In some embodiments, the mass concentration of the cationic etherifying agent in the cationic etherifying agent solution is 0.33-0.77 g / mL, for example, it can be a range of 0.33 g / mL, 0.4 g / mL, 0.5 g / mL, 0.6 g / mL, 0.77 g / mL, or any combination thereof. By controlling the mass concentration of the cationic etherifying agent, not only is it sufficiently dispersed, but it also helps to control the quaternization reaction rate and avoids an increase in side reactions due to excessively high concentrations. Furthermore, within the above range, the reaction rate, feeding stability, and side reaction suppression are all considered, which is beneficial for improving the clay stabilization rate of the clay stabilizer.

[0089] In some embodiments, in each adjacent step of adding a cationic etherifying agent solution to the chitosan raw material dispersion, the volume ratio of the cationic etherifying agent solution added in the previous step to that added in the next step is 1:(0.5-2), for example, it can be a range of 1:0.5, 1:1, 1:1.5, 1:2 or any two of them.

[0090] By controlling the volume ratio of the cationic etherifying agent solution added in the previous and subsequent additions, the reaction load after each addition of the cationic etherifying agent can be kept relatively balanced, and the effective concentration of the cationic etherifying agent in the entire system can be maintained within a relatively stable range. This avoids uneven substitution or sudden viscosity changes caused by a single addition. Furthermore, within this range, the molecular weight, quaternary ammonium salt substituent content, and polydispersity index of the clay stabilizer can be adjusted to meet the needs of different application scenarios.

[0091] It should be noted that in each step of adding the cationic etherifying agent solution to the chitosan raw material dispersion, the cationic etherifying agent solution (the one added previously) is added first. After the addition is completed, after an interval T, the next step of adding the cationic etherifying agent solution (the one added later) to the chitosan raw material dispersion is performed.

[0092] In some embodiments, during the final addition of the cationic etherifying agent solution to the chitosan raw material dispersion, the quaternization reaction is maintained for 2-5 hours after the final addition of the cationic etherifying agent solution, followed by separation and purification. Maintaining the quaternization reaction for 2-5 hours after the final addition of the cationic etherifying agent solution facilitates the continued quaternization of unreacted cationic etherifying agent, improving the uniformity of quaternary ammonium salt substituent introduction.

[0093] For example, the duration of the quaternization reaction can be a range of 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h or any combination thereof.

[0094] In some embodiments, the temperature of the quaternization reaction is 60-90°C, for example, a range of 60°C, 70°C, 80°C, 90°C or any combination thereof.

[0095] Generally, if the quaternization reaction temperature is too low (<60℃), the reaction rate is slow and the grafting efficiency is low; if the quaternization reaction temperature is too high (>90℃), the reaction is intensified, and side reactions and degradation of carboxyl-containing chitosan raw materials increase. Within the above range, a balance is struck between reaction activation and reaction efficiency to ensure that the obtained quaternized chitosan zwitterionic polymer has dual action sites of both carboxyl and quaternary ammonium salt substituents, and that the quaternary ammonium salt substituents are evenly distributed, achieving controllable molecular weight, which is beneficial for improving the clay stabilization rate of the clay stabilizer.

[0096] This invention provides a water-based working fluid, which includes an aqueous phase and a clay stabilizer. The clay stabilizer includes the aforementioned clay stabilizer or a clay stabilizer prepared by the aforementioned clay stabilizer preparation method.

[0097] Applying clay stabilizers to water-based working fluids and then to highly water-sensitive conglomerate reservoirs, the synergistic effect of carboxyl and quaternary ammonium salt substituents in the clay stabilizers induces the transformation of dispersed clay from a disordered migration state to a stable aggregated state under pore-throat flow conditions in these reservoirs. This effectively reduces clay hydration swelling, clay spalling, fine clay particle migration, and pore-throat blockage.

[0098] It should be noted that a strong water-sensitive conglomerate reservoir refers to a conglomerate reservoir containing water-sensitive clay minerals that is prone to clay hydration and swelling, particle exfoliation, fine-grained migration, and pore throat blockage under the influence of external water-based fluids.

[0099] In some embodiments, the water-based working fluid can be at least one of drilling fluid, completion fluid, workover fluid, fracturing fluid, water injection fluid, well washing fluid, sand flushing fluid, or reservoir protection fluid. These water-based working fluids can effectively inhibit clay swelling and control fine particle migration in different operating scenarios, giving the clay stabilizer broad compatibility.

[0100] In some embodiments, a highly water-sensitive reservoir is a region containing clay. The clay includes water-sensitive clay. Applying a water-based working fluid to a conglomerate reservoir containing water-sensitive clay results in superior reservoir protection.

[0101] In some embodiments, the clay includes at least one selected from illite-montmorillonite mixed layers, montmorillonite, illite, chlorite, and kaolinite. The water-based working fluid of the present invention can inhibit clay swelling, control fine particle migration, and protect reservoir permeability for all of these substances.

[0102] In some embodiments, the clay stabilizer accounts for 0.1-3% of the mass percentage of the water-based working fluid. For example, the mass percentage can be 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, or any combination thereof. By controlling the mass percentage of the clay stabilizer in the water-based working fluid, a sufficiently effective concentration of the quaternized chitosan zwitterionic polymer can be formed in the liquid phase, allowing it to adsorb, coat, and charge-regulate with the clay particles. Furthermore, within the aforementioned range, a balance is struck between stabilizing the clay particles and maintaining the fluid's rheological properties, thereby better reducing hydration swelling, particle shedding, and pore throat blockage.

[0103] In some embodiments, the aqueous phase of the water-based working fluid includes at least one of water, produced water, and brine. The produced water has a salinity of 8000-40000 mg / L and a pH of 6.0-8.0, while the brine also has a salinity of 8000-40000 mg / L and a pH of 6.0-8.0. Setting the salinity and pH of the produced water and the brine within these ranges ensures that the aqueous phase closely approximates the target reservoir's hydrochemical environment, reducing abnormal clay hydration and ion impacts caused by deviations in salinity or pH.

[0104] For example, the salinity of the produced water from the oilfield can be a range of 8000 mg / L, 10000 mg / L, 15000 mg / L, 20000 mg / L, 25000 mg / L, 30000 mg / L, 35000 mg / L, 40000 mg / L, or any two of these.

[0105] For example, the pH of the produced water from the oilfield can be a range of 6.0, 6.3, 6.5, 6.8, 7, 7.5, 8, or any two of these.

[0106] For example, the mineralization of the above-mentioned brine can be a range of 8000 mg / L, 10000 mg / L, 15000 mg / L, 20000 mg / L, 25000 mg / L, 30000 mg / L, 35000 mg / L, 40000 mg / L or any two of these.

[0107] For example, the pH of the above-mentioned saline solution can be a range of 6.0, 6.3, 6.5, 6.8, 7, 7.5, 8 or any two of them.

[0108] In some embodiments, the water-based working fluid also includes potassium formate, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitors. Adding potassium formate, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitors to the water-based working fluid, and using it as a potassium formate-based solids-free workover fluid, achieves the functions of inhibiting clay particle expansion, increasing viscosity, and reducing filtration loss. This complementary function helps protect the reservoir, prevent stuck pipe, reduce leakage, and extend equipment life.

[0109] It should be noted that the solids-free workover fluid does not contain insoluble solid particles such as bentonite, calcium carbonate, or barite. Instead, it uses a potassium formate aqueous solution as the continuous liquid phase. Based on this, clay stabilizers, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitors are added. Utilizing their synergistic effects, it achieves a comprehensive effect of inhibiting clay hydration and swelling, controlling fine particle migration, low filtration loss, and low corrosion. This potassium formate-based solids-free workover fluid can reduce the blockage of reservoir pore throats by exogenous solid particles and is suitable for workover operations in highly water-sensitive conglomerate reservoirs.

[0110] In some embodiments, the water-based working fluid contains 5-60% potassium formate, 0.2-1.0% hydroxyethyl cellulose, 0.5-2.0% hydroxypropyl starch, and 0.1-0.5% corrosion inhibitor by mass. By optimizing the amounts of potassium formate, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitor in the water-based working fluid, it is ensured that the water-based working fluid has suitable apparent viscosity, plastic viscosity, and dynamic shear force, lower API filtration loss and corrosion rate, and better clay stability and core permeability recovery rate.

[0111] For example, the mass percentage of potassium formate in the above-mentioned water-based working solution can be 5%, 10%, 20%, 30%, 40%, 50%, 54%, 55%, 60%, or any combination thereof.

[0112] For example, the mass percentage of hydroxyethyl cellulose in the above-mentioned water-based working solution can be 0.2%, 0.5%, 0.8%, 1%, or any combination thereof.

[0113] For example, the mass percentage of hydroxypropyl starch in the above-mentioned water-based working fluid can be 0.5%, 1%, 1.5%, 2%, or any combination thereof.

[0114] For example, the mass percentage of corrosion inhibitor in the above-mentioned water-based working fluid can be 0.1%, 0.3%, 0.5%, or any combination thereof.

[0115] It should be noted that the aforementioned water-based working fluid, as a potassium formate-based solids-free workover fluid, is only one type of water-based working fluid of this invention. The core function of the clay stabilizer provided by this invention is to inhibit clay hydration swelling, clay spalling, and disperse the migration of fine clay particles. Therefore, the clay stabilizer can be used in water-based working fluids that come into contact with the reservoir, such as drilling fluids, completion fluids, fracturing fluids, water injection fluids, well washing fluids, sand flushing fluids, and reservoir protection fluids. Other treatment agents in different water-based working fluids can be adjusted according to the operational purpose without affecting the dynamic coating effect of the clay stabilizer of this invention on clay.

[0116] The present invention will be further described below through specific embodiments.

[0117] Example 1

[0118] Example 1 provides a clay stabilizer, which is a quaternized chitosan zwitterionic polymer. The quaternized chitosan zwitterionic polymer comprises a chitosan backbone, carboxyl substituents grafted onto the chitosan backbone, and quaternary ammonium salt substituents grafted onto the chitosan backbone. The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N. + (CH3)3Cl - The structural unit of the carboxyl substituent is -CH2COOH.

[0119] The method for preparing the clay stabilizer provided in Example 1 includes the following steps:

[0120] S1. Disperse 0.5 g of carboxymethyl chitosan in 10 mL of the first solvent to obtain a chitosan raw material dispersion. The number-average molecular weight of the carboxymethyl chitosan is 1.6 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 75%; the first solvent consists of ethanol and water in a volume ratio of 2:1.

[0121] S2. Dissolve 2.3g of 2,3-epoxypropyltrimethylammonium chloride in 5mL of water to obtain a cationic etherifying agent solution.

[0122] S3. Heat the chitosan raw material dispersion to 80℃, then add the cationic etherifying agent solution to the chitosan raw material dispersion at a stirring speed of 300 rpm at a rate of 5 mL / h for the first time. After the first addition is completed, maintain the temperature for 4 h. Then, add the cationic etherifying agent solution to the chitosan raw material dispersion for the second time at a rate of 5 mL / h. After the second addition is completed, maintain the temperature for 4 h to obtain the reaction mixture.

[0123] The volume ratio of the first added cationic etherifying agent solution to the second added cationic etherifying agent solution is 1:1.

[0124] S4. Cool the reaction mixture to room temperature (20℃), add 50 mL of ethanol for precipitation. Then filter and wash the resulting solid product three times with ethanol. The washed solid product is then vacuum dried at 65℃ for 10 h to obtain the clay stabilizer.

[0125] Example 2

[0126] This embodiment 2 is basically the same as embodiment 1, except that:

[0127] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3Cl - The structural unit of the carboxyl substituent is -CH2CH2COOH.

[0128] In step S1, carboxymethyl chitosan is replaced with carboxyethyl chitosan, and the number average molecular weight of carboxyethyl chitosan is 1.68 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 75%.

[0129] Example 3

[0130] This embodiment 3 is basically the same as embodiment 1, except that:

[0131] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3Cl - The structural unit of the carboxyl substituent is -CH2COONa.

[0132] In step S1, carboxymethyl chitosan is replaced with sodium carboxymethyl chitosan, the number-average molecular weight of which is 1.73 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 75%.

[0133] Example 4

[0134] This embodiment 4 is basically the same as embodiment 1, except that:

[0135] In step S2, 2,3-epoxypropyltrimethylammonium chloride is replaced with 3-chloro-2-hydroxypropyltrimethylammonium chloride.

[0136] Example 5

[0137] This embodiment 5 is basically the same as embodiment 1, except that:

[0138] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)2(C 12 H 25 )Cl - The structural unit of the carboxyl substituent is -CH2COOH.

[0139] In step S2, 2,3-epoxypropyltrimethylammonium chloride is replaced with N-(2,3-epoxypropyl)-N,N-dimethyldodecylammonium chloride.

[0140] Example 6

[0141] This embodiment 6 is basically the same as embodiment 1, except that:

[0142] In step S2, the amount of 2,3-epoxypropyltrimethylammonium chloride used is 1.0 g.

[0143] Example 7

[0144] This embodiment 7 is basically the same as embodiment 1, except that:

[0145] In step S2, the amount of 2,3-epoxypropyltrimethylammonium chloride used is 1.5g.

[0146] Example 8

[0147] This embodiment 8 is basically the same as embodiment 1, except that:

[0148] In step S2, the amount of 2,3-epoxypropyltrimethylammonium chloride used is 3.0 g.

[0149] Example 9

[0150] This embodiment 9 is basically the same as embodiment 1, except that:

[0151] In step S1, the carboxymethyl chitosan comes from different sources, and its number-average molecular weight is 1.52 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 60%.

[0152] Example 10

[0153] This embodiment 10 is basically the same as embodiment 1, except that:

[0154] In step S1, the performance parameters of carboxymethyl chitosan differed; the number-average molecular weight of carboxymethyl chitosan was 1.62 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 80%.

[0155] Example 11

[0156] This embodiment 11 is basically the same as embodiment 1, except that:

[0157] In step S3, the chitosan raw material dispersion is heated to 60°C. After the first addition is completed, the mixture is kept at this temperature for 5 hours. After the second addition is completed, the mixture is kept at this temperature for 5 hours.

[0158] Example 12

[0159] This embodiment 12 is basically the same as embodiment 1, except that:

[0160] In step S3, the chitosan raw material dispersion is heated to 90°C. After the first addition is completed, the mixture is kept at this temperature for 2 hours. After the second addition is completed, the mixture is kept at this temperature for 2 hours.

[0161] Example 13

[0162] This embodiment 13 is basically the same as embodiment 1, except that:

[0163] In step S3, a cationic etherifying agent solution is added to the chitosan raw material dispersion for the first time at a rate of 8.3 mL / h. A cationic etherifying agent solution is then added to the chitosan raw material dispersion for the second time at a rate of 8.3 mL / h.

[0164] Example 14

[0165] This embodiment 14 is basically the same as embodiment 1, except that:

[0166] In step S3, a cationic etherifying agent solution is added to the chitosan raw material dispersion for the first time at a rate of 2.5 mL / h. A cationic etherifying agent solution is then added to the chitosan raw material dispersion for the second time at a rate of 2.5 mL / h.

[0167] Example 15

[0168] This embodiment 15 is basically the same as embodiment 1, except that:

[0169] In step S1, the volume ratio of ethanol to water in the first solvent is 5:1.

[0170] Example 16

[0171] This embodiment 16 is basically the same as embodiment 1, except that:

[0172] In step S1, 0.5 g of carboxymethyl chitosan is dispersed in 15 mL of the first solvent to obtain a chitosan raw material dispersion. At this point, the mass concentration of carboxymethyl chitosan in the chitosan derivative dispersion is 0.03 g / mL.

[0173] In step S2, 2.3 g of 2,3-epoxypropyltrimethylammonium chloride is dissolved in 3 mL of water to obtain a cationic etherifying agent solution. At this point, the mass concentration of 2,3-epoxypropyltrimethylammonium chloride in the cationic etherifying agent solution is 0.77 g / mL.

[0174] Example 17

[0175] This embodiment 17 is basically the same as embodiment 1, except that:

[0176] In step S1, 0.5 g of carboxymethyl chitosan is dispersed in 5 mL of the first solvent to obtain a chitosan raw material dispersion. At this point, the mass concentration of carboxymethyl chitosan in the chitosan derivative dispersion is 0.1 g / mL.

[0177] In step S2, 2.3 g of 2,3-epoxypropyltrimethylammonium chloride is dissolved in 7 mL of water to obtain a cationic etherifying agent solution. At this point, the mass concentration of 2,3-epoxypropyltrimethylammonium chloride in the cationic etherifying agent solution is 0.33 g / mL.

[0178] Example 18

[0179] This embodiment 18 is basically the same as embodiment 1, except that step S3 is different.

[0180] Step S3 specifically involves heating the chitosan raw material dispersion to 80°C. Then, while stirring at 300 rpm, a cationic etherifying agent solution is added to the chitosan raw material dispersion at a rate of 3.3 mL / h for the first time. After the first addition, the mixture is kept at this temperature for 2.7 h. Then, a cationic etherifying agent solution is added a second time at a rate of 3.3 mL / h. After the second addition, the mixture is kept at this temperature for 2.7 h to obtain the reaction mixture.

[0181] The volume ratio of the first, second, and third cationic etherifying agent solutions added is 1:1:1.

[0182] Example 19

[0183] This embodiment 19 is basically the same as embodiment 1, except that step S4 is different.

[0184] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3Br - The structural unit of the carboxyl substituent is -CH2COOH.

[0185] In step S4, the reaction mixture was cooled to room temperature (20°C), and 50 mL of ethanol was added for precipitation. The resulting solid product was then dispersed in 20 mL of deionized water, and 1.5 g of sodium bromide was added for anion exchange treatment for 6 h under conditions of stirring at 300 rpm and in the dark, to form an exchange solution. The exchange solution was then transferred to a dialysis bag with a molecular weight cutoff of 3500 Da, using deionized water as the dialysate (the dialysate was placed outside the dialysis bag, and the volume of dialysate was 20 times the volume of the exchange solution). Dialysis was performed for 48 h, with the dialysate replaced every 8 h. After dialysis, the retained solution in the dialysis bag was collected. 50 mL of ethanol was then added to the retained solution for precipitation. The mixture was then filtered, and the resulting solid product was washed three times with ethanol. The washed solid product was then vacuum dried at 65°C for 10 h to obtain the clay stabilizer.

[0186] Example 20

[0187] This embodiment 20 is basically the same as embodiment 1, except that:

[0188] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3I - The structural unit of the carboxyl substituent is -CH2COOH.

[0189] In step S4, the reaction mixture was cooled to room temperature (20°C), and 50 mL of ethanol was added for precipitation. The resulting solid product was then dispersed in 20 mL of deionized water, followed by anion exchange treatment with 2.5 g of potassium iodide for 6 h under stirring at 300 rpm and in the dark, to form an exchange solution. The exchange solution was then transferred to a dialysis bag with a molecular weight cutoff of 3500 Da, using deionized water as the dialysate (the dialysate was placed outside the dialysis bag, and the volume of dialysate was 20 times the volume of the exchange solution). Dialysis was performed for 48 h, with the dialysate replaced every 8 h. After dialysis, the retained solution in the dialysis bag was collected. 50 mL of ethanol was then added to the retained solution for precipitation. The mixture was then filtered, and the resulting solid product was washed three times with ethanol. The washed solid product was then vacuum dried at 65°C for 10 h to obtain the clay stabilizer.

[0190] Example 21

[0191] This embodiment 21 is basically the same as embodiment 1, except that:

[0192] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3Cl - The structural unit of the carboxyl substituent is -CH2COOK.

[0193] In step S1, carboxymethyl chitosan is replaced with potassium carboxymethyl chitosan, which has a number-average molecular weight of 1.82 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 75%.

[0194] Example 22

[0195] This embodiment 22 is basically the same as embodiment 1, except that:

[0196] The structural unit of the quaternary ammonium salt substituent in the quaternized chitosan zwitterionic polymer is -CH2CH(OH)CH2N + (CH3)3Cl - The structural unit of the carboxyl substituent is -CH2COONH4.

[0197] In step S4, the reaction mixture was cooled to room temperature (20°C), and 50 mL of ethanol was added for precipitation. The resulting solid product was then dispersed in 20 mL of deionized water. Subsequently, under stirring at 300 rpm, 10% ammonia solution was added to adjust the pH to 7.5, and the mixture was stirred for 2 hours to form an ammonium salt solution. The ammonium salt solution was then transferred to a dialysis bag with a molecular weight cutoff of 3500 Da. Deionized water was used as the dialysate (the dialysate was placed outside the dialysis bag, and the volume of dialysate was 20 times the volume of the ammonium salt solution). Dialysis was performed for 24 hours, with the dialysate replaced every 8 hours. After dialysis, the retained solution in the dialysis bag was collected. 50 mL of ethanol was then added to the retained solution for precipitation. The mixture was then filtered, and the resulting solid product was washed three times with ethanol. The washed solid product was then vacuum-dried at 40°C for 12 hours to obtain the clay stabilizer.

[0198] Comparative Example 1

[0199] Comparative Example 1 is basically the same as Example 1, except that:

[0200] In step S1, carboxymethyl chitosan is replaced with chitosan, and the number average molecular weight of chitosan is 1.6 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 0%.

[0201] Comparative Example 2

[0202] Comparative Example 2 is basically the same as Example 1, except that:

[0203] In step S1, the performance parameters of carboxymethyl chitosan differed; the number-average molecular weight of carboxymethyl chitosan was 1.49 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 50%.

[0204] Comparative Example 3

[0205] Comparative Example 3 is basically the same as Example 1, except that:

[0206] In step S1, the performance parameters of carboxymethyl chitosan differed; the number-average molecular weight of carboxymethyl chitosan was 1.67 × 10⁻⁶. 4 The degree of carboxymethyl substitution is 90%.

[0207] Comparative Example 4

[0208] Comparative Example 4 is basically the same as Example 1, except that step S3 is different.

[0209] Step S3 specifically involves heating the chitosan raw material dispersion to 80°C. Then, while stirring at 300 rpm, a cationic etherifying agent solution is added to the chitosan raw material dispersion at a rate of 5 mL / h, and the mixture is kept at this temperature for 8 hours to obtain the reaction mixture.

[0210] Comparative Example 5

[0211] Comparative Example 5 is basically the same as Example 1, except that step S3 is different.

[0212] Step S3 specifically involves heating the chitosan raw material dispersion to 80°C. Then, while stirring at 300 rpm, a cationic etherifying agent solution is added to the chitosan raw material dispersion at a rate of 5 mL / h for the first time. After the first addition, the mixture is kept at this temperature for 1 hour. Then, a cationic etherifying agent solution is added to the chitosan raw material dispersion a second time at a rate of 5 mL / h. After the second addition, the mixture is kept at this temperature for 7 hours to obtain the reaction mixture.

[0213] Comparative Example 6

[0214] Comparative Example 6 is basically the same as Example 1, except that step S3 is different.

[0215] Step S3 specifically involves heating the chitosan raw material dispersion to 80°C. Then, while stirring at 300 rpm, a cationic etherifying agent solution is added to the chitosan raw material dispersion at a rate of 5 mL / h for the first time. After the first addition, the mixture is kept at this temperature for 6 hours. Then, a cationic etherifying agent solution is added to the chitosan raw material dispersion a second time at a rate of 5 mL / h. After the second addition, the mixture is kept at this temperature for 2 hours to obtain the reaction mixture.

[0216] Comparative Example 7

[0217] Comparative Example 7 is basically the same as Example 1, except that:

[0218] In step S3, a cationic etherifying agent solution is added to the chitosan raw material dispersion for the first time at a rate of 1 mL / h. A cationic etherifying agent solution is then added to the chitosan raw material dispersion for the second time at a rate of 1 mL / h.

[0219] Comparative Example 8

[0220] Comparative Example 8 is basically the same as Example 1, except that:

[0221] In step S3, a cationic etherifying agent solution is added to the chitosan raw material dispersion for the first time at a rate of 15 mL / h. A cationic etherifying agent solution is then added to the chitosan raw material dispersion for the second time at a rate of 15 mL / h.

[0222] Comparative Example 9

[0223] Comparative Example 9 is basically the same as Example 1, except that:

[0224] In step S1, the first solvent is replaced with water.

[0225] The degree of substitution of carboxyl substituents, the grafting rate of quaternary ammonium salt substituents (mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymers of Examples 1-18 and Comparative Examples 1-9), number-average molecular weight, weight-average molecular weight, and polydispersity index are shown in Table 1.

[0226] Application Example 1

[0227] Application Example 1 provides a water-based working fluid. The water-based working fluid is a clay stabilizer dispersion, which includes simulated oilfield produced water and a clay stabilizer.

[0228] The salinity of the produced water from the simulated oilfield was 8535 mg / L. The produced water from the simulated oilfield included Na... + K + Mg 2+ Ca 2 + Cl - ,Br - Mix with water to simulate Na produced from oil fields. + The mass concentration was 2876 mg / L, K + The mass concentration was 46 mg / L, Mg 2 + The mass concentration was 14 mg / L, Ca 2+ The mass concentration was 378 mg / L, Cl - The mass concentration was 5211 mg / L, Br - The mass concentration is 10 mg / L, and the pH is 6.88. The mass percentage of clay stabilizer in the clay stabilizer dispersion is 2%.

[0229] The method for preparing the clay stabilizer dispersion provided in Application Example 1 includes the following steps: adding clay stabilizer to simulated oilfield produced water at a stirring rate of 300 rpm and stirring for 1 hour to obtain the clay stabilizer dispersion.

[0230] Application Example 2

[0231] Application Example 2 provides a water-based working fluid. The water-based working fluid is a potassium formate-based solids-free workover fluid, which includes simulated oilfield produced water, potassium formate, clay stabilizer, hydroxyethyl cellulose Natrosol 210HHX, hydroxypropyl starch HPS-1, and imidazoline polyoxyethylene ether BXH-101.

[0232] The salinity of the produced water from the simulated oilfield was 8535 mg / L. The produced water from the simulated oilfield included Na... + K + Mg 2+ Ca 2 + Cl - ,Br - Mix with water to simulate Na produced from oil fields. + The mass concentration was 2876 mg / L, K + The mass concentration was 46 mg / L, Mg 2 + The mass concentration was 14 mg / L, Ca 2+ The mass concentration was 378 mg / L, Cl - The mass concentration was 5211 mg / L, Br - The mass concentration was 10 mg / L, and the pH was 6.88.

[0233] The well workover fluid contains 54% potassium formate, 1% clay stabilizer, 0.5% Natrosol 210HHX hydroxyethyl cellulose, 1% hydroxypropyl starch HPS-1, and 0.3% imidazoline polyoxyethylene ether.

[0234] The method for preparing the workover fluid provided in this application example 2 includes the following steps: adding potassium formate, clay stabilizer, hydroxyethyl cellulose Natrosol 210HHX, hydroxypropyl starch HPS-1 and imidazoline polyoxyethylene ether to simulated oilfield produced water at a stirring rate of 300 rpm, and stirring for 1 h to obtain the workover fluid.

[0235] Performance testing

[0236] 1. Using the method described in Application Example 1, the clay stabilizers prepared in Example 1 and Comparative Example 1 were dispersed in simulated oilfield produced water, respectively, to prepare clay stabilizer dispersions with a mass percentage of 3%. Simultaneously, the clay stabilizer prepared in Example 1 was also prepared into a clay stabilizer dispersion with a mass percentage of 5%, and the dispersion state of the clay stabilizer dispersions was observed. The test results are shown below. Figure 2 .

[0237] Figure 2 Images showing the clay stabilizer dispersions prepared according to Comparative Example 1 and Example 1 of the present invention; wherein, Figure 2 Image 'a' in Comparative Example 1 shows a clay stabilizer dispersion prepared with a mass percentage of 3%. Figure 2Image b in Example 1 of this invention shows a clay stabilizer dispersion with a mass percentage of 3%. Figure 2 Image c in Example 1 of this invention is a clay stabilizer dispersion with a mass percentage of 5%.

[0238] like Figure 2 As shown in Figure a, the clay stabilizer dispersion of Comparative Example 1, with a mass percentage of 3%, was turbid, indicating insufficient dispersion. Figure 2 As shown in b, the clay stabilizer dispersion of Example 1, with a mass percentage of 3%, is clear and uniformly dispersed. Figure 2 As shown in Figure c, the clay stabilizer dispersion of Example 1, with a mass percentage of 5%, remained clear and uniformly dispersed. The above test results indicate that the clay stabilizer provided by this invention has good compatibility with oilfield produced water.

[0239] 2. The quaternary ammonium salt grafting rate, number-average molecular weight, weight-average molecular weight, and clay stability of the clay stabilizers prepared in Examples 1-22 and Comparative Examples 1-9 were tested. The test results are shown in Table 1.

[0240] (1) Test method for quaternary ammonium salt grafting rate: The C, H and N elements in the clay stabilizer, the carboxyl-containing chitosan raw material used to prepare the clay stabilizer, and the cationic etherifying agent used to prepare the clay stabilizer were quantitatively determined using an elemental analyzer (Elementar vario MICROcube). The elemental composition of the clay stabilizer was compared with that of the cationic etherifying agent. If the nitrogen-carbon ratio (mass of N element / mass of C element) of the clay stabilizer was higher than that of the carboxyl-containing chitosan raw material (mass of N element / mass of C element), it indicated that the carboxyl-containing chitosan raw material had achieved quaternary ammonium salt grafting. Subsequently, the quaternary ammonium salt grafting rate was calculated based on the law of conservation of mass, that is, the mass percentage of quaternary ammonium salt substituents in the zwitterionic polymer of quaternized chitosan.

[0241] The conditions for quantitative determination were: combustion temperature of 950°C and detection limit of 0.01 wt%.

[0242] The determination of hydrogen (H) is mainly used for mass balance verification and auxiliary qualitative judgment, and is not involved in the specific calculation of the quaternary ammonium salt grafting rate. When the sum of the mass fractions of carbon (C), hydrogen (H), and nitrogen (N) is 100 ± 0.5%, it indicates that the N and C test results are reliable and can be used to calculate the quaternary ammonium salt grafting rate. When the sum of the mass fractions of C, H, and N is not 100 ± 0.5%, it indicates that the N and C test results are incorrect, and the test conditions should be checked and the test repeated.

[0243] The formula for calculating the quaternary ammonium salt grafting rate is:

[0244]

[0245] Where x is the quaternary ammonium salt grafting rate of the clay stabilizer, in units of %; M3 is the mass fraction ratio of N to C in the clay stabilizer, in units of %; M2 is the mass fraction ratio of N to C in the carboxyl-containing chitosan raw material, in units of %; and M1 is the mass fraction ratio of N to C in the cationic etherifying agent, in units of %.

[0246] (2) Test methods for number-average molecular weight and weight-average molecular weight: The clay stabilizer was dispersed in a neutral aqueous phase (sodium nitrate aqueous solution with a molar concentration of 0.1 mol / L) to prepare a dispersion with a mass concentration of 2.0 mg / mL. The dispersion was then filtered through a 0.22 μm filter membrane, and the filtrate was collected. The filtrate was analyzed using an Agilent 1260 gel permeation chromatograph to obtain a chromatogram. The weight-average molecular weight and number-average molecular weight of the clay stabilizer were output according to the calibration curve, and the polydispersity index (PDI) was calculated.

[0247] Method for establishing calibration curves: Monodisperse polyethylene glycol (PEG) was used as a standard with molecular weights of 200 Da, 400 Da, 1000 Da, 2000 Da, 5000 Da, and 10000 Da. PEG standard solutions with a mass concentration of 2.0 mg / mL were prepared using a neutral aqueous phase (0.1 mol / L sodium nitrate aqueous solution). The PEG standard solutions were analyzed using an Agilent 1260 gel permeation chromatograph. Linear fitting was performed with retention time as the x-axis and log(molecular weight) as the y-axis to obtain the calibration curve.

[0248] The formula for calculating the polydispersity index is:

[0249]

[0250] Wherein, PDI is the polydispersity index of the clay stabilizer; M w M is the weight-average molecular weight of the clay stabilizer. n This represents the number-average molecular weight of the clay stabilizer.

[0251] (3) The clay stability rate was tested by centrifugation method according to SY / T5970-2016 "Evaluation Method of Clay Stabilizer for Fracturing, Acidizing and Water Injection in Oil and Gas Fields".

[0252] Test method for clay stability: Following the method in Application Example 1, clay stabilizer was dispersed in simulated oilfield produced water to prepare a 2% (w / w) clay stabilizer dispersion. Then, industrial-grade sodium bentonite (200 mesh, purchased from Bohai Drilling Engineering Co., Ltd., China National Petroleum Corporation) was added to the clay stabilizer dispersion to prepare the clay dispersion test solution. The sodium bentonite content in the clay dispersion test solution was 1% (w / w). The clay dispersion test solution was then shaken at 150 r / min for 30 min, followed by a standing treatment for 120 min to allow the sodium bentonite to fully hydrate and expand. The settled clay dispersion test solution was then centrifuged, and the volume of the expanded sodium bentonite was measured.

[0253] Meanwhile, deionized water and kerosene were used as control samples. Deionized water and industrial-grade kerosene (purchased from Beijing Yue Rui Hua Qiang Technology Co., Ltd.) were used to replace the clay stabilizer dispersion to prepare clay dispersion test solutions, and the volume of sodium-based bentonite after expansion was measured. The clay stability rate was calculated, and a higher clay stability rate indicates better anti-swelling properties of the clay stabilizer.

[0254] The formula for calculating the stability rate of clay is:

[0255]

[0256] Where B is the clay stability rate (%), V1 is the volume of bentonite after expansion in the clay dispersion test solution (mL), V2 is the volume of bentonite after expansion in deionized water (mL), and V0 is the volume of bentonite after expansion in kerosene (mL).

[0257] Table 1

[0258]

[0259] As shown in Table 1, compared with the comparative examples, the clay stabilizer provided by this invention has a carboxyl substitution degree of 60-80% in the quaternized chitosan zwitterionic polymer, a quaternary ammonium salt grafting rate (the mass percentage of quaternary ammonium salt substituents in the quaternized chitosan zwitterionic polymer), and a polydispersity index of 2.2-2.5. When the clay stabilizers of Examples 1-18 were prepared into clay stabilizer dispersions with a mass percentage of 2% using simulated oilfield produced water, the clay stability rate was greater than 90%. This indicates that by controlling the carboxyl substitution degree, quaternary ammonium salt grafting rate, and polydispersity index of the quaternized chitosan zwitterionic polymer, this invention enables the clay stabilizer to possess good aqueous dispersibility, clay surface adsorption capacity, and bridging coating capacity, thereby exhibiting superior anti-swelling performance. The quaternized chitosan zwitterionic polymer of Example 1 has a carboxyl substitution degree of 75%, a quaternary ammonium salt grafting rate of 45.04%, and a clay stability rate of 97.49%, exhibiting the best overall performance.

[0260] Furthermore, the number-average molecular weight of the quaternized chitosan zwitterionic polymer provided by this invention is (1.30-1.72)×10⁻⁶. 4 The weight-average molecular weight is (2.98-4.17)×10⁻⁶. 4 This indicates that the clay stabilizer contains molecular segments of different chain lengths, which is beneficial for the clay stabilizer to exert a synergistic effect of rapid adsorption of short chains and strong bridging of long chains, providing a molecular structural basis for achieving a synergistic anti-swelling effect of rapid adsorption and long-term stability.

[0261] Compared to Example 1, Comparative Example 1 uses chitosan without carboxyl substituents, making the clay stabilizer a cationic quaternized chitosan. Although this clay stabilizer can electrostatically adsorb onto clay through quaternary ammonium cations, it lacks the multi-point hydrogen bonding, amphoteric charge regulation, and hydration layer stabilization provided by carboxyl substituents. This makes it prone to disordered flocculation due to insufficient adsorption layer stability or excessive cationic interaction, thus reducing the clay stability. The degree of carboxyl substitution of the quaternized chitosan zwitterionic polymer in Comparative Example 2 is lower than the range of carboxyl substitution found in the quaternized chitosan zwitterionic polymer provided by this invention. Although the quaternary ammonium salt grafting rate is high, there are insufficient carboxyl anion sites, weakening the synergistic regulatory ability of carboxyl anions and quaternary ammonium cations. This results in insufficient hydrogen bond coating and hydration swelling inhibition on the clay surface, leading to a decrease in clay stability. The degree of carboxyl substitution of the quaternized chitosan zwitterionic polymer in Comparative Example 3 is higher than the range of carboxyl substitution of the quaternized chitosan zwitterionic polymer provided by this invention. Excessive carboxyl substituents enhance the negative charge repulsion of the molecular chain and cause excessive extension, reducing the effective contact between the cationic etherifying agent and the reactive site, resulting in a lower grafting rate of quaternary ammonium salt. At the same time, it leads to a wider molecular weight distribution, an increased polydispersity index, and difficulty in forming a dense and stable adsorption coating layer, resulting in a decrease in clay stability.

[0262] Compared to Example 1, in the preparation method of the clay stabilizer in Comparative Example 4, a cationic etherifying agent solution is added to the chitosan raw material dispersion in one step. The cationic etherifying agent is prone to causing local over-quaternization, inter-chain association, or even local cross-linking in the system. The degree of chitosan molecular linking is relatively low, resulting in a wider molecular weight distribution, an increased polydispersity index, and a significant decrease in clay stability.

[0263] Compared to Example 1, in the preparation method of the clay stabilizer in Comparative Example 5, a cationic etherifying agent solution was added to the chitosan raw material dispersion in two separate additions. However, the interval between the two additions of the cationic etherifying agent solution was too short. The cationic etherifying agent added in the first addition did not have sufficient time to diffuse, adsorb, and react before the next addition, resulting in uneven utilization of reaction sites, poor uniformity of product molecular structure, and decreased anti-swelling performance. In the preparation method of the clay stabilizer in Comparative Example 6, a cationic etherifying agent solution was added to the chitosan raw material dispersion in two separate additions. However, the interval between the two additions of the cationic etherifying agent solution was too long. After the first addition of the cationic etherifying agent solution, the reaction system underwent a long period of heat treatment, which made the already formed quaternized intermediate structure prone to chain segment rearrangement, molecular chain association, or local aggregation, thus burying or shielding the reactive sites. When a cationic etherifying agent solution is added a second time, the effective contact between the cationic etherifying agent and the reaction sites of the chitosan molecular chain decreases, leading to a reduction in the quaternary ammonium salt grafting rate, a broadening of the product molecular weight distribution, and an increase in the polydispersity index. This makes it difficult to form a molecular structure that combines rapid adsorption and effective bridging and coating, thus resulting in a decrease in clay stability. This indicates that the time interval between two adjacent additions of the cationic etherifying agent solution to the chitosan raw material dispersion must be within the range provided by this invention to balance reaction efficiency, grafting uniformity, and product molecular weight distribution.

[0264] Compared to Example 1, in the preparation method of the clay stabilizer in Comparative Example 7, the addition rate of the cationic etherifying agent solution was too low, resulting in a significant extension of the feeding process and the overall reaction time. During the long reaction period, the proportion of hydrolysis or side reactions of the cationic etherifying agent in the aqueous phase increased. Simultaneously, the chitosan molecular chains may undergo segment rearrangement, association, or partial degradation, reducing the effective reaction efficiency between the cationic etherifying agent and the chitosan molecular chain reaction sites. This leads to a wider molecular weight distribution of the product, the quaternary ammonium salt grafting rate approaching its lower limit, and a decrease in clay stability. In the preparation method of the clay stabilizer in Comparative Example 8, the addition rate of the cationic etherifying agent solution was too fast, which also caused excessively high local concentrations and uneven reactions, resulting in an increased polydispersity index of the product. This made it difficult to simultaneously achieve rapid adsorption of short chains and bridging and coating of long chains, leading to a decrease in clay stability. This demonstrates that the addition rate of the cationic etherifying agent solution must be within the range provided by this invention to balance reaction efficiency, grafting uniformity, and product molecular weight distribution.

[0265] Compared to Example 1, in the preparation method of the clay stabilizer in Comparative Example 9, water is used to disperse the carboxyl-containing chitosan raw material. The carboxyl-containing chitosan raw material is prone to swelling, local viscosity increase or insufficient dispersion and mass transfer in a single aqueous phase, which affects the effective contact between the cationic etherifying agent and the reaction sites on the chitosan raw material, resulting in a quaternary ammonium salt grafting rate of less than 30%, thereby causing a decrease in the clay stability.

[0266] Furthermore, comparing Examples 1, 6, 7, 16, and 17, it can be seen that when the quaternary ammonium salt grafting rate of the quaternized chitosan zwitterionic polymer is close to the lower or upper limit of the range defined in this invention, the clay stability rate is lower than that of Example 1. This indicates that a higher quaternary ammonium salt grafting rate in the clay stabilizer is not necessarily better, nor can the best anti-swelling effect be obtained simply by increasing the number of cationic sites. Therefore, only by controlling the degree of carboxyl substitution and the quaternary ammonium salt grafting rate of the quaternized chitosan zwitterionic polymer within the range defined in this invention can the clay stabilizer simultaneously exert electrostatic adsorption, hydrogen bond coating, zwitterionic charge regulation, and bridging coating effects, thereby achieving synergistic control over clay hydration swelling and fine particle dynamic transport.

[0267] 3. Using the method described in Example 1, the clay stabilizers prepared in Example 1 and Comparative Example 1 were dispersed in simulated oilfield produced water, respectively, to prepare clay stabilizer dispersions with a mass percentage of 0.2-2.0%. The clay stability rate was tested using the aforementioned clay stability rate test method. The test results are shown below. Figure 3 .

[0268] Figure 3 The concentration-clay stability rate curves of the clay stabilizer dispersions prepared according to Examples 1 and 1 of the present invention are shown. Figure 3 As shown, the clay stabilization rate gradually increases with the increase of the mass percentage of clay stabilizer in the clay stabilizer dispersion. This is mainly because increasing the concentration of clay stabilizer can more effectively inhibit the hydration and swelling of clay.

[0269] When the mass percentage of the clay stabilizer is 0.2-0.4%, the clay stabilization rates of both Example 1 and Comparative Example 1 are less than 80%, failing to effectively inhibit clay hydration swelling. When the mass percentage of the clay stabilizer is greater than 0.4%, the clay stabilization rate of Example 1 is significantly better than that of Comparative Example 1. The above test results indicate that the clay stabilizer provided by this invention has superior clay swelling inhibition ability, facilitates the formation of a protective layer on the clay surface, and effectively reduces clay hydration swelling.

[0270] 4. Using the method described in Example 1, the clay stabilizers of Example 1 and Comparative Example 1 were dispersed in simulated oilfield produced water to prepare test solutions with a mass percentage of 2% clay stabilizer dispersion. Simulated oilfield produced water was used as a blank control test solution. A pretreated microchannel model was used to simulate the reservoir, and the blank control test solution, the test solution of Comparative Example 1, and the test solution of Example 1 were injected into the pretreated microchannel model for microchannel dynamic transport tests. The test results are shown below. Figure 4 , Figure 5 , Figure 6 .

[0271] Pretreatment method for microchannel model: Prepare NaCl aqueous solution with a mineralization of 15000 mg / L, and add industrial grade sodium bentonite (sodium bentonite with a mesh size of 200 mesh, purchased from Bohai Drilling Engineering Co., Ltd. of China National Petroleum Corporation) to prepare clay pretreatment suspension, in which the mass percentage of sodium bentonite in clay pretreatment suspension is 20%.

[0272] A NaCl aqueous solution was injected into a transparent microchannel model for presaturation to expel gas from the microchannels. Subsequently, a clay pretreatment suspension was injected into the presaturated microchannel model until the pores and throat regions were filled, and clay particles (sodium bentonite) were observed uniformly distributed on the pore and throat surfaces under a microscope. After injection, the microchannel model was incubated at 80°C for 1 hour. During this time, the water in the microchannel model evaporated, and the clay particles adhered to the pore and throat surfaces, resulting in a microchannel model with attached clay particles. This clay-particle-attached microchannel model simulates the initial state of water-sensitive clay particles on the reservoir pore and throat surfaces. The clay-particle-attached microchannel model was then cooled to room temperature (25°C). Simulated oilfield produced water was then used to flush the microchannel model to remove unstable, loosely attached clay particles until no large particle agglomerates were discharged from the outlet fluid, thus obtaining the pretreated microchannel model.

[0273] Testing method for dynamic transport in microchannels: The test solution was injected into a pretreated microchannel model using a micro-injection pump. During the injection process, the adhesion, peeling, migration, aggregation, and blockage of clay particles in the microchannel model were recorded at 0 min, 10 min, 20 min, 30 min, 40 min, and 60 min using a microscopic digital acquisition device. The migration trajectory, aggregation morphology, and pore throat blockage of the clay particles in the microchannel model were analyzed using ImageJ image analysis software.

[0274] Injection method of the test solution: First, start the injection of the test solution at a flow rate of 5 μL / min to ensure stable propagation of the test solution in the injection line and to remove air bubbles in the injection line. After the injection of the test solution is stable, adjust the flow rate to 1 μL / min and maintain a constant flow rate to inject into the pretreated microchannel model, so that the test solution continuously and smoothly enters the network of the microchannel model.

[0275] Figure 4 This is a comparison diagram of the dynamic transport of the blank control test solution in the microchannel, serving as a control example. Figure 5 Comparison of the dynamic transport of the test solution prepared with clay stabilizer for Comparative Example 1 in the microchannel. Figure 6 This is a comparison diagram of the dynamic transport of the test liquid prepared with the clay stabilizer of Example 1 of the present invention in the microchannel.

[0276] like Figure 4 As shown, under the flushing of the blank control test solution, clay particles pre-attached to the pores and throat surfaces of the microchannel model are easily detached, dispersed, and migrated with the flow. Some fine particles enter the narrow throat of the microchannel and cause local blockage. Figure 5 As shown, under the washing action of the test solution prepared with the clay stabilizer in Comparative Example 1, the peeling and migration of clay particles pre-attached to the pore and throat surfaces of the microchannel model were reduced. However, under continuous washing action, fine particle migration and unstable aggregation still occurred, indicating that a single cationic clay stabilizer is difficult to effectively control the dynamic transport behavior of clay fine particles. Figure 6 As shown, under the flushing of the test liquid prepared with the clay stabilizer in Example 1, the adhesion stability of the clay particles pre-attached to the pores and throat surfaces of the microchannel model is significantly enhanced, the migration of dispersed fine particles with the flow is reduced, and relatively stable agglomerates are gradually formed, thus reducing the degree of pore and throat blockage in the microchannel.

[0277] In the microchannel dynamic transport testing method of this invention, clay particles are artificially introduced into the pores and throats of the microchannel model to simulate the dynamic environment of dispersed particles in formation fluids. The test results show that the clay stabilizer provided by this invention can not only reduce clay hydration swelling, but also promote the formation of controlled agglomerates of dispersed clay through dynamic coating and bridging effects. That is, it achieves regulation of the dynamic transport and controlled agglomeration of dispersed clay, thereby reducing the transport capacity of dispersed clay and pore-throat blockage.

[0278] The clay stabilizer provided by this invention has a suitable quaternary ammonium salt grafting rate and molecular weight distribution, and exhibits good effects in terms of compatibility with oilfield produced water, clay stability, and microchannel dynamic transport regulation. Compared with the control example and Comparative Example 1, the clay stabilizer prepared in Example 1 can adsorb, coat, and bridge clay under fluid scouring, transforming the clay from a disordered dispersed transport state to a stable aggregated cluster state. The above test results show that the clay stabilizer provided by this invention can simultaneously inhibit clay hydration swelling and control fine-grained aggregates, making it suitable for the long-term anti-swelling and transport regulation requirements of clay in the protection of highly water-sensitive conglomerate reservoirs.

[0279] 5. Using the method of Application Example 2, the clay stabilizers prepared in Example 1 and Comparative Example 1 were applied to potassium formate-based solids-free workover fluids to prepare the corresponding potassium formate-based solids-free workover fluids. The rheological properties, API filtration loss, clay stability, corrosion inhibition properties, and core permeability repair properties of the potassium formate-based solids-free workover fluids were tested, and the test results are shown in Table 2.

[0280] (1) Test method for rheological properties: According to SY / T5970-2016 and GB / T 29170-2012 "Laboratory testing of drilling fluids for oil and gas industry". A ZNN-D6 six-speed rotary viscometer was used to measure the viscometer readings of the workover fluid at rotation speeds of 600 r / min and 300 r / min, and the apparent viscosity, plastic viscosity and dynamic shear force were calculated.

[0281] The formula for calculating apparent viscosity is:

[0282]

[0283] Where AV is the apparent viscosity, measured in mPa·s; θ 600 The readings are from the viscometer at a rotation speed of 600 r / min.

[0284] The formula for calculating plastic viscosity is:

[0285]

[0286] Where PV is the plastic viscosity, measured in mPa·s; θ 600 The reading is the viscometer reading at a rotation speed of 600 r / min; θ 300 The readings are from the viscometer at a rotation speed of 300 r / min.

[0287] The formula for calculating dynamic shear force is:

[0288]

[0289] Where YP is the dynamic shear force, in Pa; PV is the plastic viscosity, in mPa·s; θ300 The readings are from the viscometer at a rotation speed of 300 r / min.

[0290] (2) Test method for filtration loss reduction performance: According to SY / T 5241-1991 "Evaluation procedure for filtration loss reduction agents for water-based drilling fluids". Place 150 mL of workover fluid in the mud cup of the medium-pressure filtration loss tester, install filter paper with a pore size of 11 μm and a sealing ring, and measure the volume of filtrate collected in 7.5 min under a pressure difference of 0.7 MPa, and calculate the API filtration loss.

[0291] The formula for calculating API filtration loss is:

[0292]

[0293] Wherein, FLAPI is the API filtration loss, in mL; V 7.5 The volume of filtrate collected in 7.5 minutes is expressed in mL.

[0294] (3) Test method for clay stability rate: The above test method for clay stability rate is adopted, and the clay stabilizer dispersion is replaced with well workover fluid to test the clay stability rate.

[0295] (4) Test method for corrosion inhibition performance: The static corrosion pad method in SY / T 5405-2019 "Test Methods and Evaluation Indicators for Corrosion Inhibitors for Acidizing" was used. N80 steel pads were used as test materials. Before testing, the steel pads were degreased, cleaned with ethanol, dried, and weighed to obtain pretreated steel pads. The pretreated steel pads were completely immersed in the workover fluid, avoiding contact with the container wall or bottom. At room temperature (20℃), the static corrosion treatment was carried out for 5 days. Then, the N80 steel pads were taken out, rinsed with deionized water, cleaned with ethanol, dried, and weighed to obtain corrosion-treated steel pads. The corrosion rate was calculated.

[0296] The formula for calculating the corrosion rate is:

[0297]

[0298] Where X is the corrosion rate in mm / a; W1 is the mass of the pretreated steel strip in g; W2 is the mass of the corrosion-treated steel strip in g; 87600 is a unit conversion constant; and A is the surface area of ​​the pretreated steel strip in cm². 2 T represents the static corrosion treatment time, in hours (h); D represents the density of the pretreated steel strips, in g / cm³. 3 .

[0299] (5) Test method for core permeability recovery performance: in accordance with SY / T 6540-2021 "Indoor evaluation method for damage to oil reservoir by drilling fluid and completion fluid".

[0300] Natural core samples from the target reservoir were selected, washed, and dried to determine their length, diameter, and basic pore-permeability parameters. The cores were then evacuated and submerged in simulated oilfield produced water under vacuum. Vacuum treatment continued for 30 minutes, followed by a 15 MPa pressure and settling for 12 hours to ensure complete saturation of the core pores with simulated produced water. At a simulated reservoir temperature of 100°C, the cores were forward-displaced using simulated oilfield produced water. After pressure and flow rate stabilized, the initial permeability of the cores was measured. Subsequently, a potassium formate-based cement-free workover fluid was injected in reverse at 100°C and a flow rate of 2 mL / min to contaminate the cores for 2 hours, resulting in a contaminated core. This contaminated core was then forward-displaced using simulated oilfield produced water at a simulated reservoir temperature of 100°C. After pressure and flow rate stabilized, the permeability of the contaminated core after flowback was measured.

[0301] The formula for calculating the permeability recovery rate is:

[0302]

[0303] Where RV is the core permeability recovery rate, in %; and K1 is the initial core permeability, in 10⁻⁶. -3 μm 2 K2 represents the permeability of the contaminated core after backflow, in units of 10⁻⁶. -3 μm 2 .

[0304] Table 2

[0305]

[0306] As shown in Table 2, compared with the comparative example, the potassium formate-based solids-free workover fluid prepared with the clay stabilizer in Example 1 of this invention exhibits good rheological properties and filtration loss control capabilities, while maintaining a high clay stability rate and core permeability recovery rate. Furthermore, the test results for rheological properties, filtration loss control capabilities, clay stability rate, and core permeability recovery rate of the potassium formate-based solids-free workover fluid prepared with the clay stabilizer in Examples 2-18 of this invention are basically consistent with the side-test results for the potassium formate-based solids-free workover fluid prepared with the clay stabilizer in Example 1, and are all significantly better than the side-test results for the potassium formate-based solids-free workover fluid prepared with the clay stabilizer in Comparative Example 1. The above test results indicate that the clay stabilizer provided by this invention, when applied to potassium formate-based solids-free workover fluid, can achieve a comprehensive effect of inhibiting clay hydration swelling, controlling fine particle migration, low filtration loss, and low corrosion, making it suitable for workover operations in highly water-sensitive conglomerate reservoirs.

[0307] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A clay stabilizer, characterized in that, The clay stabilizer comprises a quaternized chitosan zwitterionic polymer, wherein the quaternized chitosan zwitterionic polymer comprises a chitosan backbone, carboxyl substituents grafted onto the chitosan backbone, and quaternary ammonium salt substituents grafted onto the chitosan backbone, wherein the degree of substitution of the carboxyl substituents is 60-80%, and the quaternary ammonium salt substituents account for 30-60% of the mass percentage of the quaternized chitosan zwitterionic polymer; The number-average molecular weight of the quaternized chitosan zwitterionic polymer is (1-3)×10⁻⁶. 4 The weight-average molecular weight is (2-6)×10 4 The polydispersity index of the quaternized chitosan zwitterionic polymer is 2.2-2.5; The structural unit of the quaternary ammonium salt substituent is -R2-N. + (R3)3X - Wherein, R2 is a C3 alkylene group containing a hydroxyl group, and R3 is a C1-C2 alkylene group. 12 Alkyl group, where X is Cl, Br, or I; The structural unit of the carboxyl substituent is -R1-COOM; wherein R1 is a C1-C3 alkylene group and M is H, Na, K or NH4.

2. A method for preparing the clay stabilizer according to claim 1, characterized in that, Includes the following steps: A chitosan-based raw material containing carboxyl groups is dispersed in a first solvent to obtain a chitosan-based raw material dispersion; wherein, the first solvent includes an alcohol solvent and water; The cationic etherifying agent used to form the quaternary ammonium salt substituent is added to water to obtain a cationic etherifying agent solution; The cationic etherifying agent solution was added to the chitosan raw material dispersion in multiple portions to carry out a quaternization reaction. After separation and purification, a clay stabilizer was obtained. In the step of adding the cationic etherifying agent solution to the chitosan raw material dispersion each time, the addition rate of the cationic etherifying agent solution is 2.5-8.5 mL / h; The time interval T between each two adjacent steps of adding the cationic etherifying agent solution to the chitosan raw material dispersion is 2h≤T≤5h.

3. The method for preparing the clay stabilizer according to claim 2, characterized in that, The carboxyl-containing chitosan raw materials include at least one of carboxymethyl chitosan, carboxymethyl chitosan salt, carboxyethyl chitosan, and carboxyethyl chitosan salt; And / or, the cationic etherifying agent comprises at least one of 2,3-epoxypropyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, and N-(2,3-epoxypropyl)-N,N-dimethyldodecylammonium chloride; And / or, the alcohol solvent includes water-soluble alcohols; And / or, the volume ratio of the alcohol solvent to water is (1-5):1; And / or, the mass ratio of the carboxyl-containing chitosan raw material to the cationic etherifying agent in the cationic etherifying agent solution is 1:(2-6); And / or, the mass concentration of the carboxyl-containing chitosan raw material in the chitosan raw material dispersion is 0.03-0.1 g / mL; And / or, the mass concentration of the cationic etherifying agent in the cationic etherifying agent solution is 0.33-0.77 g / mL.

4. The method for preparing the clay stabilizer according to claim 2 or 3, characterized in that, In each of the two adjacent steps of adding the cationic etherifying agent solution to the chitosan raw material dispersion, the volume ratio of the cationic etherifying agent solution added in the previous step to that added in the next step is 1:(0.5-2). And / or, in the step of adding the cationic etherifying agent solution to the chitosan raw material dispersion for the last time, after the last addition of the cationic etherifying agent solution, maintain the quaternization reaction treatment for 2-5 hours before performing the separation and purification; And / or, the temperature of the quaternization reaction is 60-90°C.

5. A water-based working fluid, characterized in that, The water-based working fluid comprises an aqueous phase and a clay stabilizer, wherein the clay stabilizer comprises the clay stabilizer according to claim 1, or a clay stabilizer prepared by the method of preparing the clay stabilizer according to any one of claims 2-4.

6. The water-based working fluid according to claim 5, characterized in that, The clay stabilizer accounts for 0.1-3% of the mass of the water-based working fluid; And / or, the aqueous phase in the water-based working fluid includes at least one of water, oilfield produced water, and brine, wherein the oilfield produced water has a mineralization of 8000-40000 mg / L and a pH of 6.0-8.0, and the brine has a mineralization of 8000-40000 mg / L and a pH of 6.0-8.

0.

7. The water-based working fluid according to claim 5 or 6, characterized in that, The water-based working fluid also includes potassium formate, hydroxyethyl cellulose, hydroxypropyl starch, and corrosion inhibitors.

8. The water-based working fluid according to claim 7, characterized in that, In the water-based working solution, the mass percentage of potassium formate is 5-60%, the mass percentage of hydroxyethyl cellulose is 0.2-1.0%, the mass percentage of hydroxypropyl starch is 0.5-2.0%, and the mass percentage of corrosion inhibitor is 0.1-0.5%.

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