A plugging agent for shale and its preparation method and application

Abstract

This invention belongs to the field of drilling engineering plugging agent preparation technology, and particularly relates to a plugging agent for shale, its preparation method and application. The plugging agent preparation method includes (1) mixing modified silica with a first monomer mixture and carrying out a first reaction to obtain a first intermediate product; (2) adding a second monomer mixture to the first intermediate product and carrying out a second reaction to obtain a core-shell polymer; wherein, the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(10-40); (3) mixing the core-shell polymer with sodium methoxide and carrying out an alcoholysis reaction. This invention achieves synergistic effect of plugging and chemical stability by constructing a multifunctional shell rich in cations, amide groups, hydroxyl groups and ester groups on the surface of modified silica. The dual mechanism of electrostatic attraction and hydrogen bonding enables the plugging agent to form a strong adsorption and difficult desorption firm plugging layer on the shale surface, resulting in good plugging effect.

Description

Technical Field

[0001] This invention belongs to the field of drilling engineering plugging agent preparation technology, and particularly relates to a plugging agent for shale, its preparation method and application. Background Technology

[0002] Rock spalling and wellbore collapse caused by water expansion in shale formations are among the core technical challenges hindering safe drilling in shale gas. Shale formations are characterized by well-developed micro- and nano-pores, bedding planes, and structural fractures. During drilling operations, under the combined effects of bottomhole pressure differential and capillary effects, the water phase of drilling fluid easily intrudes into the deeper reservoir along the pore and fracture network, further expanding the fractures and pores and exacerbating wellbore instability. Furthermore, exemplified by the Wufeng-Longmaxi Formation shale in the Changning area of ​​the Sichuan Basin, this shale is rich in the weakly expansive clay mineral illite, which undergoes strong hydration upon contact with water, rapidly altering the shale's mechanical properties. The key to solving wellbore instability lies in effectively sealing off the water intrusion pathways near the wellbore while simultaneously mitigating the hydration effect of illite on the wellbore surface.

[0003] Silica is considered an ideal plugging material due to its controllable nanoparticle size and high hardness. However, its high hardness and poor deformation ability make it difficult to form a tight seal on the wellbore surface. Modifying the silica surface with silane coupling agents to create active sites and introducing functional monomers through free radical copolymerization to form a composite structure with SiO2 as the core and polymer as the shell can combine the rigid support of inorganic nanoparticles with the flexible adhesion of polymers. However, the plugging effect of this type of plugging agent is still limited and unsatisfactory. Achieving both plugging and stabilization of shale remains a research challenge. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a sealing agent for shale, its preparation method, and its application.

[0005] In a first aspect, the present invention provides a method for preparing a plugging agent for shale, comprising the following steps:

[0006] (1) Modified silica is mixed with a first monomer mixture and subjected to a first reaction to obtain a first intermediate product; the first monomer mixture includes vinyl acetate, acrylamide and cationic monomers;

[0007] (2) A second monomer mixture is added to the first intermediate product to carry out a second reaction to obtain a core-shell polymer with modified silica as the core; wherein, the second monomer mixture includes vinyl acetate, acrylamide and cationic monomer; the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(10-40).

[0008] (3) The core-shell polymer is mixed with sodium methoxide and subjected to alcoholysis reaction.

[0009] As an optional implementation, the alcoholysis reaction is carried out at a temperature of 45°C-55°C for 3-5 hours.

[0010] As an optional implementation, the alcoholysis reaction is further followed by a step of adjusting the pH to 6.0-8.0.

[0011] As an optional implementation, the mass ratio of the core-shell polymer to the sodium methoxide is 10:(0.1-0.5).

[0012] As an optional implementation, the mass ratio of vinyl acetate, acrylamide and cationic monomer in the total mass of the first monomer mixture and the second monomer mixture is (1-2):(5-7):(1-3).

[0013] As an optional implementation, the vinyl acetate content in the first monomer mixture is 80%-100% based on the total mass of vinyl acetate in the first monomer mixture and the second monomer mixture;

[0014] As an optional implementation, based on the total mass of acrylamide in the first monomer mixture and the second monomer mixture, the mass content of acrylamide in the first monomer mixture is 10%-30%;

[0015] As an optional implementation, the cationic monomer content in the first monomer mixture is 20%-40% based on the total mass of the cationic monomers in the first monomer mixture and the second monomer mixture;

[0016] As an optional implementation, the cationic monomer includes at least one of methacryloyloxyethyltrimethylammonium chloride, dimethyl diallyl ammonium chloride, acryloyloxyethyltrimethylammonium chloride, and methacrylamidopropyltrimethylammonium chloride.

[0017] As an optional implementation, the temperature of the first reaction is 55℃-65℃, and the time is 20min-40min;

[0018] As an optional implementation, the temperature of the second reaction is 55℃-65℃, and the time is 1h-3h.

[0019] As an optional implementation, the modified silica includes silica modified with a silane coupling agent.

[0020] As an optional implementation, the silane coupling agent includes KH-570.

[0021] Secondly, the present invention provides a sealing agent for shale, which is prepared by the above-described preparation method.

[0022] Secondly, the present invention provides the application of the above-mentioned plugging agent in oil drilling fluid.

[0023] The technical solution provided by the embodiments of the present invention has the following advantages compared with the prior art:

[0024] 1. The method for preparing a shale plugging agent provided by the present invention includes (1) mixing modified silica with a first monomer mixture and carrying out a first reaction to obtain a first intermediate product; the first monomer mixture includes vinyl acetate, acrylamide and cationic monomer; (2) adding a second monomer mixture to the first intermediate product and carrying out a second reaction to obtain a core-shell polymer with modified silica as the core; wherein the second monomer mixture includes vinyl acetate, acrylamide and cationic monomer; the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(10-40); (3) mixing the core-shell polymer with sodium methoxide and carrying out an alcoholysis reaction. This invention achieves a synergistic effect of plugging and chemical stabilization by constructing a multifunctional shell rich in cations, amides, hydroxyl groups, and esters on the surface of modified silica. The dual mechanism of electrostatic attraction and hydrogen bonding enables the plugging agent to form a strong, difficult-to-desorb, and robust plugging layer on the shale surface. The modification with hydrophobic ester groups effectively blocks the intrusion of the aqueous phase and inhibits shale hydration. The combination of a rigid core and a flexible shell balances the mechanical strength and sealing integrity of the plugging layer. Furthermore, the presence of hydroxyl and amide groups within the plugging agent facilitates the formation of a hydrogen bond network within the molecule, enhancing polymer strength and further improving the plugging effect. Attached Figure Description

[0025] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of the sealing agent prepared in Example 1 of the present invention;

[0028] Figure 2 The infrared spectrum of the sealing agent prepared in Example 1 of this invention;

[0029] Figure 3 The image shows the proton nuclear magnetic resonance spectrum of the blocking agent prepared in Example 1 of this invention.

[0030] Figure 4 An electron microscope image of the sealing agent prepared in Example 1 of this invention;

[0031] Figure 5 This is a morphological image of the sealing agent prepared in Example 1 of the present invention after pressure transmission test. Detailed Implementation

[0032] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0033] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0034] In a first aspect, embodiments of the present invention provide a method for preparing a plugging agent for shale, comprising the following steps:

[0035] (1) Modified silica is mixed with a first monomer mixture and subjected to a first reaction to obtain a first intermediate product; the first monomer mixture includes vinyl acetate, acrylamide and cationic monomers;

[0036] (2) A second monomer mixture is added to the first intermediate product to carry out a second reaction to obtain a core-shell polymer with modified silica as the core; wherein, the second monomer mixture includes vinyl acetate, acrylamide and cationic monomer; the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(10-40).

[0037] (3) The core-shell polymer is mixed with sodium methoxide and subjected to alcoholysis reaction.

[0038] This invention achieves a synergistic effect of plugging and chemical stabilization by constructing a multifunctional shell rich in cations, amides, hydroxyl groups, and esters on the surface of modified silica. The dual mechanism of electrostatic attraction and hydrogen bonding enables the plugging agent to form a strong, difficult-to-desorb, and robust plugging layer on the shale surface. The modification with hydrophobic ester groups effectively blocks the intrusion of the aqueous phase and inhibits shale hydration. The combination of a rigid core and a flexible shell balances the mechanical strength and sealing integrity of the plugging layer. Furthermore, the presence of hydroxyl and amide groups within the plugging agent facilitates the formation of a hydrogen bond network within the molecule, enhancing polymer strength and further improving the plugging effect.

[0039] This invention uses modified silica as a core to provide rigid support, endowing the plugging agent with excellent compressive strength and pore support capabilities, which helps the plugging agent maintain structural integrity in complex environments. A polymer shell is formed on the surface of the modified silica through polymerization. This shell provides good flexibility to the plugging agent, and under downhole stress conditions, it partially deforms to form a dense and highly efficient sealing layer, significantly improving the sealing effect. Vinyl acetate is introduced into the polymer shell and undergoes alcoholysis, retaining both ester and hydroxyl groups. On the one hand, the retained ester groups give the plugging agent surface a certain degree of hydrophobicity, effectively reducing the penetration of water molecules into the shale and weakening the hydration effect of the shale in the wellbore. On the other hand, the hydroxyl groups can form hydrogen bonds with the shale surface, improving the adsorption of the plugging agent on the shale surface, enhancing the adhesion of the sealing layer to the wellbore, and ensuring the sealing effect of the plugging agent. Introducing amide groups into the core-shell polymer can synergistically enhance the hydrogen bond network with the shale surface, further improving the adsorption force between the plugging agent and the shale, enhancing the plugging effect, and mitigating wellbore instability. Introducing cationic monomers into the shell layer allows the plugging agent to carry a positive charge, which then electrostatically attracts the negatively charged minerals on the shale surface, further enhancing the adsorption force between the plugging agent and the shale. At the same time, it can neutralize some of the negative charge, inhibit shale hydration, and improve wellbore stability.

[0040] This invention regulates the mass ratio of total monomer content to modified silica, which is beneficial for the polymer to form a uniform coating layer on the core surface and for maintaining a suitable particle size. It achieves an optimal balance between rigidity and flexibility, further improving the compressive strength and shale adsorption of the plugging agent, thereby enhancing the plugging effect and improving wellbore instability.

[0041] As an optional implementation, the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(15-35), which is more conducive to improving the plugging effect of the plugging agent. Optionally, the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(20-30), which is beneficial for controlling the thickness of the core and shell layers, and is more conducive for the plugging agent to take into account both rigid support and flexible deformation, resulting in a better plugging effect.

[0042] For example, the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:10, 100:15, 100:20, 100:25, 100:30, 100:35, 100:40, etc.

[0043] As an optional implementation, the alcoholysis reaction is carried out at a temperature of 45°C-55°C for 3-5 hours. For example, the alcoholysis reaction temperature is 45°C, 48°C, 50°C, 52°C, 55°C, etc., and the reaction time is 3 hours, 4 hours, 5 hours, etc.

[0044] As an optional implementation, the alcoholysis reaction is followed by a step of adjusting the pH to 6.0-8.0. Optionally, glacial acetic acid is used to adjust the pH.

[0045] The present invention regulates the temperature and pH value of the alcoholysis reaction to facilitate a smoother reaction.

[0046] As an optional implementation, the mass ratio of the core-shell polymer to the sodium methoxide is 10:(0.1-0.5). This invention regulates the mass ratio of the core-shell polymer to sodium methoxide to achieve an optimal balance between the ester groups and hydroxyl groups in the core-shell polymer, thereby optimizing the overall performance of the plugging agent. Exemplary examples include mass ratios of 10:0.1, 10:0.2, 10:0.3, 10:0.4, and 10:0.5.

[0047] The present invention regulates parameters such as the mass ratio of the core-shell polymer to the sodium methoxide in the alcoholysis reaction, which is beneficial to achieving a balance between hydrophilicity and hydrophobicity of the plugging agent and the number of functional groups, thereby improving the plugging effect of the plugging agent and alleviating wellbore instability.

[0048] As an optional embodiment, the mass ratio of vinyl acetate, acrylamide, and cationic monomer in the total mass of the first monomer mixture and the second monomer mixture is (1-2):(5-7):(1-3). Exemplarily, the mass ratio of vinyl acetate, acrylamide, and cationic monomer is 1:5:1, 1:5:3, 1:7:1, 1:7:3, 2:5:1, 2:5:3, 2:7:1, 2:7:3, etc. As an optional embodiment, the mass content of vinyl acetate in the first monomer mixture is 80%-100% based on the total mass of vinyl acetate in the first monomer mixture and the second monomer mixture. Exemplarily, the mass content of vinyl acetate in the first monomer mixture is 80%, 85%, 90%, 95%, 100%, etc., based on the total mass of vinyl acetate in the first monomer mixture and the second monomer mixture.

[0049] As an optional implementation, the acrylamide content in the first monomer mixture is 10%-30% based on the total mass of acrylamide in the first monomer mixture and the second monomer mixture. For example, the acrylamide content in the first monomer mixture is 10%, 15%, 20%, 25%, 30%, etc., based on the total mass of acrylamide in the first monomer mixture and the second monomer mixture.

[0050] As an optional implementation, the cationic monomer content in the first monomer mixture is 20%-40% based on the total mass of the cationic monomers in the first monomer mixture and the second monomer mixture. For example, the cationic monomer content in the first monomer mixture is 20%, 25%, 30%, 35%, 40%, etc., based on the total mass of the cationic monomers in the first monomer mixture and the second monomer mixture.

[0051] As an optional implementation, the cationic monomer includes at least one of methacryloyloxyethyltrimethylammonium chloride, dimethyl diallyl ammonium chloride, acryloyloxyethyltrimethylammonium chloride, and methacrylamidopropyltrimethylammonium chloride.

[0052] As an optional implementation, the temperature of the first reaction is 55℃-65℃, and the time is 20min-40min. Optionally, the first reaction may further include the step of adding an initiator, which may include, but is not limited to, azobisisobutyronitrile (AIBN).

[0053] As an optional implementation, the temperature of the second reaction is 55℃-65℃, and the time is 1h-3h. Optionally, the second monomer mixture and the initiator solution are added dropwise to the first intermediate product simultaneously. After the addition is completed, the mixture is kept at 55℃-65℃ for 1h-3h. This holding stage corresponds to the second reaction, allowing the monomers to be fully converted. After the reaction is completed, the product is precipitated, washed, and dried to obtain the core-shell polymer.

[0054] In this invention, the modified silica is first reacted with a first monomer mixture, and then a second monomer mixture is added. This facilitates the controllable and uniform growth of the polymer on the surface of the modified silica, forming a uniform shell, avoiding the formation of homopolymers, and improving the dispersion uniformity of each functional group in the product.

[0055] As an optional implementation, the modified silica includes silica modified with a silane coupling agent.

[0056] As an optional implementation, the silane coupling agent includes KH-570, which can better connect silica to the shell polymer.

[0057] As an optional implementation method, the modification method for modified silica includes: dispersing nano-silica with a diameter of 100nm-800nm ​​in ethanol, ultrasonicating to form a uniform dispersion, and preheating and stirring at 75℃-85℃; mixing a silane coupling agent, ethanol, and water, adjusting the pH to 4-5 with acetic acid to obtain a hydrolysate; uniformly adding the hydrolysate dropwise to the silica-containing dispersion, stirring and refluxing at 75℃-85℃ for 4-8 hours; after the reaction, cooling, centrifuging, washing, and drying to obtain modified silica. Optionally, the particle size of the modified silica is approximately 110-810nm.

[0058] Optionally, the mass ratio of nano-silica to KH-570 is (1-2):1.

[0059] The present invention has a short process and mild conditions in the preparation of the sealing agent, and the particle size of the obtained sealing agent can be controlled, making it suitable for micro and nanopores of different sizes.

[0060] Secondly, the present invention provides a sealing agent for shale, which is prepared by the above-described preparation method.

[0061] Thirdly, the present invention provides the application of the above-mentioned plugging agent in oil drilling fluids, especially in mudstone formations prone to wellbore instability.

[0062] The raw materials used in the following examples and comparative examples are all commercially available.

[0063] Example 1

[0064] This embodiment provides a method for preparing a plugging agent for shale, including the following steps:

[0065] (1) 5.0 g of nano-SiO2 with an average diameter of approximately 300 nm was dispersed in 100 mL of anhydrous ethanol and sonicated for 30 min to form a uniform dispersion. The dispersion was placed in a three-necked flask, preheated at 80 °C and stirred to obtain a SiO2 dispersion for later use. 2.5 g of silane coupling agent KH-570 (methacryloyloxypropyltrimethoxysilane), 10.0 mL of anhydrous ethanol and 1.0 mL of deionized water were mixed, and the pH was adjusted to 4 with acetic acid. The mixture was stirred and hydrolyzed for 30 min to obtain a hydrolysate for later use. The hydrolysate was added dropwise to the preheated SiO2 dispersion at a uniform rate over 1 h. After the addition was complete, the mixture was stirred and refluxed at 80 °C for 6 h. After the reaction was completed, the mixture was cooled to room temperature, centrifuged, and washed three times each with anhydrous ethanol and acetone. The product was vacuum dried at 60 °C for 12 h to obtain K-SiO2 powder with C=C double bonds grafted onto its surface.

[0066] (2) Dissolve 0.16 g of azobisisobutyramidine hydrochloride (AIBA) in 12 mL of water, mix well to obtain 12 mL of initiator solution, and set aside. Mix 1 g of acrylamide (AM), 0.4 g of methacryloyloxyethyltrimethylammonium chloride (DMC), and 1.6 g of vinyl acetate (VAc) to form the first monomer mixture, and set aside. Mix 3.8 g of acrylamide (AM), 1.2 g of methacryloyloxyethyltrimethylammonium chloride (DMC), and 0 g of vinyl acetate (VAc) to form the second monomer mixture, and set aside.

[0067] 2.0 g of the above K-SiO2 powder was mixed with 200 mL of water, ultrasonically dispersed, and transferred to a four-necked flask equipped with a stirrer, condenser, and nitrogen delivery tube. The first monomer mixture was added to the flask, stirred to dissolve, forming the initial reaction solution. Nitrogen gas was then purged for 30 min, the temperature was raised to 60 °C, and 4 mL of initiator solution was added. The polymerization reaction was carried out for 30 min to obtain the first intermediate product. Then, the above second monomer mixture was mixed with 50 mL of water and added dropwise to the first intermediate product, with 8 mL of initiator solution added simultaneously over a period of 2 h. After the addition was complete, the reaction was maintained at 60 °C for 2 h. After the reaction was completed, the mixture was cooled, and the product solution was poured into 800 mL of acetone to precipitate. The precipitate was filtered, washed three times with acetone, and dried under vacuum at 50 °C to constant weight to obtain a white powdery nanopolymer, i.e., a core-shell polymer.

[0068] (3) Take 10g of the above core-shell polymer and 150mL of anhydrous methanol, stir at 50℃ until completely dissolved to obtain a polymer solution. Dissolve 0.1g of sodium methoxide in 20mL of anhydrous methanol to prepare a catalyst solution. Slowly add the catalyst solution to the polymer solution and react at 50℃ for 4h. After the reaction is complete, cool to room temperature, add glacial acetic acid to neutralize the pH to 7.0, pour the reaction solution into 500mL of acetone to precipitate, filter, and wash three times with acetone. The washed product is dried under vacuum at 50℃ to constant weight to obtain the final nano-blocking agent N-ADV-1, the structural schematic diagram of which is shown in the figure. Figure 1 .

[0069] Figure 2 This is the infrared spectrum of the plugging agent in this embodiment. Figure 2 It can be seen that it is 1448cm -1 and 1411cm -1 The peak at this location is a characteristic peak of cationic quaternary ammonium salts, indicating that CN in the shell... + The existence of; 2936cm -1 The peak at 3201 cm⁻¹ indicates the stretching vibration of aliphatic CH bonds, suggesting the presence of carbon chains in the shell. -1 and 3432cm -1 The peak at that point is a broad peak formed by the superposition of hydroxyl and amino groups. Figure 3This is the 1H NMR spectrum of the plugging agent in this embodiment. Figure 3 It can be seen that 1.07ppm, 1.08ppm, and 1.10ppm are overlapping peaks of multiple methyl groups in the polymer; 1.57ppm and 1.66ppm are peaks of methylene groups in the main chain; 2.11ppm is the peak of acetyl methyl groups in vinyl acetate, indicating successful copolymerization of vinyl acetate; 3.10ppm and 3.16ppm are peaks of quaternary ammonium salt methyl groups in DMC, indicating successful copolymerization of DMC; and 3.55ppm and 3.56ppm are overlapping peaks of adjacent methylene groups of ester groups, representing the presence of the KH570 alkane chain. Figure 4 These are electron microscope images of the sealing agent in this embodiment. Figure 4 The nano-blocking agent was observed to have a spherical structure, with the lighter-colored regions forming the shell and the darker-colored regions forming the core, exhibiting a core-shell structure. Figure 4 Based on the provided particles, the overall diameter of the spherical structure is approximately 600-700 nm, and the core layer is approximately 300-400 nm. These characterization results demonstrate that this embodiment successfully prepared a nano-blocking agent with a core-shell structure.

[0070] Example 2

[0071] This embodiment provides a method for preparing a sealing agent for shale formation, which is basically the same as that in Example 1, except that the mass of modified silica is adjusted to 4g.

[0072] Example 3

[0073] This embodiment provides a method for preparing a plugging agent for shale formation, which is basically the same as that in Example 1. The main difference is that the mass of acrylamide in the first monomer mixture is adjusted to 1.17g, the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 0.3g, and the mass of vinyl acetate is adjusted to 1.2g; and the mass of acrylamide in the second monomer mixture is adjusted to 4.43g and the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 0.9g.

[0074] Example 4

[0075] This embodiment provides a method for preparing a plugging agent for shale formation, which is basically the same as that in Example 1. The main difference is that the mass of acrylamide in the first monomer mixture is adjusted to 0.91g, the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 0.3g, and the mass of vinyl acetate is adjusted to 2.4g; the mass of acrylamide in the second monomer mixture is adjusted to 3.49g, and the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 0.9g.

[0076] Example 5

[0077] This embodiment provides a method for preparing a plugging agent for shale formation, which is basically the same as that in Example 1. The main difference is that the mass of acrylamide in the first monomer mixture is adjusted to 0.91g, the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 0.6g, and the mass of vinyl acetate is adjusted to 1.2g; the mass of acrylamide in the second monomer mixture is adjusted to 3.49g, and the mass of methacryloyloxyethyltrimethylammonium chloride is adjusted to 1.8g.

[0078] Comparative Example 1

[0079] This comparative example provides a method for preparing a plugging agent for shale, which is basically the same as that in Example 1. The main difference is that the modified silica is removed when preparing the first intermediate product, and the first monomer mixture and the second monomer mixture are used as raw materials to prepare the plugging agent according to the method in Example 1.

[0080] Comparative Example 2

[0081] This comparative example provides a method for preparing a plugging agent for shale, which is basically the same as Example 1, except that step (3) is removed and a core-shell polymer is used as the plugging agent.

[0082] Comparative Example 3

[0083] This comparative example provides a method for preparing a plugging agent for shale, which is basically the same as that in Example 1. The main difference is that the total amount of the first monomer mixture and the second monomer mixture is adjusted to 25g, and the mass ratio of AM, DMC and VAc is the same as in Example 1.

[0084] Comparative Example 4

[0085] This comparative example provides a method for preparing a plugging agent for shale, using nano-silica (average diameter 300 nm) as the plugging agent.

[0086] Test Example 1

[0087] This test example provides the performance test of the plugging agents prepared by each embodiment and comparative example on the inhibition of clay mineral hydration in shale. The plugging agents provided by each embodiment and comparative example are used as test samples, and the test methods are as follows.

[0088] (1) Test method for linear expansion rate: The linear expansion rate was tested using a shale expansion tester according to SY / T 5613-2016. The sample to be tested was mixed with water to prepare a 2wt% test solution. 10g of bentonite powder was pressed into a core slice under 10MPa and placed in a test cup. The test solution was added, and the thickness change of the core slice was recorded at room temperature for 16h. Ultrapure water was used as a blank group for testing. The thickness change rate was used to characterize the linear expansion rate. The linear expansion rate is a parameter characterizing the hydration expansion of shale clay minerals. The lower the linear expansion rate, the smaller the hydration expansion. The test results are shown in Table 1.

[0089] (2) Test method for rolling recovery rate: The rolling recovery rate was tested according to SY / T 5613-2016. The sample to be tested was mixed with water to prepare a 2wt% test solution. 50g of 80-100 mesh shale cuttings (from the Longmaxi Formation in the Sichuan Basin) was added to 300mL of the test solution and rolled in a roller furnace at 100℃ for 16h. After removal, the cuttings were passed through a 100-mesh sieve, dried and weighed, and the recovery rate was calculated. Ultrapure water was used as a blank group for testing. The rolling recovery rate is a parameter characterizing the ability of the plugging agent to inhibit shale hydration, dispersion and disintegration. The higher the rolling recovery rate, the better the effect of the plugging agent in preventing water molecules from entering the shale through adsorption, coating or blocking, and the more conducive it is to maintaining wellbore stability. Conversely, the rolling recovery rate indicates poor inhibition performance. The test results are shown in Table 1.

[0090] (3) Test method for Zeta potential: Shale powder (from Longmaxi Formation in Sichuan Basin), plugging agent and water were mixed to prepare the test solution. The content of shale powder in the test solution was 0.1 wt% and the content of plugging agent was 1 wt%. The Zeta potential of the particle surface in the test solution was measured using a Zeta potential analyzer. Ultrapure water was used as a blank group for the test. The test results are shown in Table 1.

[0091] Table 1 Test Results

[0092]

[0093] As can be seen from the test results in Table 1, compared with Comparative Example 1 (using conventional polymers as plugging agents), Comparative Example 2 (using core-shell polymers as plugging agents), Comparative Example 3 (using excessive polymers), and Comparative Example 4 (using nano-silica as plugging agents), the plugging agent provided in this embodiment of the invention exhibits a lower linear expansion rate and a higher rolling recovery rate when applied to shale. This indicates that the plugging agent can effectively inhibit the hydration expansion and dispersion of clay minerals such as bentonite, which is beneficial for maintaining the integrity of the wellbore structure. Furthermore, compared with Comparative Example 4 and the blank group, the zeta potential of the plugging agent provided in this embodiment of the invention increases, indicating that the plugging agent neutralizes the surface charge in the clay minerals and has a tendency to inhibit hydration dispersion.

[0094] Test Example 2

[0095] This test example provides performance tests of the plugging agents prepared in each embodiment and comparative example for shale plugging. The test methods are as follows, and the test results are shown in Table 2.

[0096] (1) Pressure transmission test method: The sealing agent was mixed with water to prepare a 2wt% test solution, and the sealing performance of different sealing agents was tested using a shale sealing performance evaluation device. Specifically, a natural shale core (collected from the Wufeng-Longmaxi Formation in the southwest of the Sichuan Basin) was placed into a core holder, the test solution was injected into the upstream chamber, and a pressure of 2 MPa was applied; the downstream chamber was at 1 MPa, and a pressure sensor was connected. The time for the upstream and downstream pressures to reach equilibrium was recorded. The longer the time, the better the sealing effect.

[0097] Figure 5 In Example 1, after the pressure transmission test, the dried shale was observed using a field emission scanning electron microscope to examine its surface morphology and the distribution of the sealing agent, thereby illustrating the sealing effect of the sealing agent. Figure 5 It can be observed that nanoscale spherical particles are uniformly and densely attached to the shale surface and pore entrances, forming a continuous sealing film, which directly confirms that the sealing agent provided in Example 1 has a good sealing ability for nanoscale pores or cracks.

[0098] (2) Shale strength testing method: Mix the plugging agent with water to prepare a 3wt% test solution. Use a triaxial pressure testing machine to test the shale plugging performance of different plugging agents: Select standard shale cores (Ø25mm×100mm) with adjacent sampling locations and similar properties, and divide them into two parts. One part is directly tested for compressive strength using a pressure testing machine (referred to as control group P1), and the other part is placed in an aging tank containing the test solution. Roll aging is performed to simulate the downhole environment. Then, the change in compressive strength is tested (referred to as experimental group P2). The strength loss is calculated according to the formula strength loss η=(P1-P2) / P1×100%.

[0099] (3) Test method for filtration loss of sand bed: Prepare a base slurry with a concentration of 5wt% by mixing 5g of bentonite and water, and then add 3wt% of sealing agent to the base slurry to prepare the base slurry to be tested. In the filtration loss meter, put in 5g of 100-120 mesh quartz sand, compact it, pour in 200mL of the base slurry to be tested, apply a pressure of 1-2 MPa, and record the volume of liquid filtered out within 30min. The volume of liquid filtered out is the filtration loss. The smaller the filtration loss, the better the sealing effect.

[0100] All the above tests were conducted using ultrapure water as the blank group.

[0101] Table 2 Test Results

[0102]

[0103] As can be seen from the test results in Table 2, compared with Comparative Example 1 (using conventional polymers as plugging agents), Comparative Example 2 (using core-shell polymers as plugging agents), Comparative Example 3 (using excessive polymers), and Comparative Example 4 (using nano-silica as plugging agents), the plugging agent prepared in the embodiments of the present invention has a longer equilibrium time, indicating that the plugging agent of the present invention can effectively block pores under pressure differential. The plugging agent prepared in the embodiments of the present invention has better strength and less strength loss, indicating that the plugging agent of the present invention can effectively block pores under pressure differential while suppressing shale strength loss. The plugging agent prepared in the embodiments of the present invention has less filtration loss, indicating that the plugging agent can form a dense, low-permeability plugging layer inside the shale and has a good plugging effect.

[0104] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0105] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for the preparation of a plugging agent for shale, characterized in that, Includes the following steps: (1) Modified silica is mixed with a first monomer mixture and subjected to a first reaction to obtain a first intermediate product; the first monomer mixture includes vinyl acetate, acrylamide and cationic monomers; (2) A second monomer mixture is added to the first intermediate product to carry out a second reaction to obtain a core-shell polymer with modified silica as the core; wherein, the second monomer mixture includes vinyl acetate, acrylamide and cationic monomer; the ratio of the total mass of the first monomer mixture and the second monomer mixture to the mass of the modified silica is 100:(10-40). (3) The core-shell polymer is mixed with sodium methoxide and subjected to alcoholysis reaction; wherein the mass ratio of the core-shell polymer to the sodium methoxide is 10:(0.1-0.5).

2. The production method according to claim 1, characterized by, The alcoholysis reaction is carried out at a temperature of 45℃-55℃ for 3-5 hours. And / or, the alcoholysis reaction is followed by a step of adjusting the pH to 6.0-8.

0.

3. The preparation method according to claim 1, characterized in that, In the total mass of the first monomer mixture and the second monomer mixture, the mass ratio of vinyl acetate, acrylamide and cationic monomer is (1-2):(5-7):(1-3).

4. The production method according to any one of claims 1 to 3, characterized by, Based on the total mass of vinyl acetate in the first monomer mixture and the second monomer mixture, the mass content of vinyl acetate in the first monomer mixture is 80%-100%, but not 100%. And / or, based on the total mass of acrylamide in the first monomer mixture and the second monomer mixture, the mass content of acrylamide in the first monomer mixture is 10%-30%; And / or, based on the total mass of the cationic monomers in the first monomer mixture and the second monomer mixture, the mass content of the cationic monomers in the first monomer mixture is 20%-40%; And / or, the cationic monomer includes at least one of methacryloyloxyethyltrimethylammonium chloride, dimethyl diallyl ammonium chloride, acryloyloxyethyltrimethylammonium chloride, and methacrylamidopropyltrimethylammonium chloride.

5. The production method according to any one of claims 1 to 3, characterized by, The temperature of the first reaction is 55℃-65℃, and the time is 20min-40min; And / or, the temperature of the second reaction is 55℃-65℃, and the time is 1h-3h.

6. The production method according to any one of claims 1 to 3, characterized by, The modified silica includes silica modified with silane coupling agents.

7. The production method according to claim 6, wherein The silane coupling agent includes KH-570.

8. A plugging agent for shale, characterized by, It is prepared by the preparation method described in any one of claims 1-7.

9. The application of the plugging agent according to claim 8 in oil drilling fluid.

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