A high-temperature acid-responsive degradable microencapsulated acid for acid fracturing and its preparation method

Abstract

This invention discloses a high-temperature acid-responsive degradable microencapsulated acid for acid fracturing and its preparation method, belonging to the technical field of chemical materials for acid fracturing in oil and gas fields. The high-temperature acid-responsive degradable microencapsulated acid for acid fracturing includes an acid core and a microcapsule shell encapsulating the acid core. The microcapsule shell is made of a high-temperature acid-responsive degradable polymer material, which is polymerized from five monomers: acrylamide, tert-butyl methacrylate, 2-acrylamide-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether. The microencapsulated acid of this invention can achieve delayed acid release and shell degradation in high-temperature acid solutions, effectively reducing reservoir damage caused by shell residue while ensuring etching capability. It also exhibits strong temperature and shear resistance, making it suitable for acid fracturing operations in high-temperature / ultra-high-temperature carbonate reservoirs.

Description

Technical Field

[0001] This invention relates to the field of chemical materials technology for acid fracturing in oil and gas fields, and particularly to a high-temperature acid-responsive biodegradable microcapsule acid for acid fracturing and its preparation method. Background Technology

[0002] Acid fracturing (AF) in carbonate reservoirs typically utilizes acid systems such as hydrochloric acid to create acid-etched grooves and enhance fracture conductivity. However, as exploration and development progress to deeper and ultra-deeper formations, reservoir temperatures rise significantly, and conventional acid systems are prone to problems such as excessively rapid acid-rock reactions and short effective ranges. To improve deep acid fracturing efficiency, researchers have begun using high-temperature resistant polymer shells to coat acid cores, creating microencapsulated acid to achieve delayed acid release. Microencapsulated acid reduces acid consumption during its migration through the wellbore and near the fracture, subsequently releasing acid precisely at the distal fracture.

[0003] However, existing research largely emphasizes the temperature resistance and barrier properties of the microencapsulated acid shell, while paying little attention to the degradation challenges of high-temperature polymers. If the shell material is difficult to degrade under reservoir conditions, residual shell material may clog pore throats or accumulate in fractures, causing secondary damage to the reservoir and affecting subsequent flowback and stable oil and gas production. In addition, the shear resistance of the microencapsulated acid needs to be improved to prevent the shell from being sheared and damaged during pumping. Summary of the Invention

[0004] To address the above problems, the present invention aims to provide a high-temperature acid-responsive degradable microencapsulated acid for acid fracturing and its preparation method.

[0005] The technical solution of the present invention is as follows:

[0006] On one hand, a high-temperature acid-responsive biodegradable microcapsule acid for acid pressure is provided, comprising an acid core and a microcapsule shell encapsulating the acid core. The acid core is a solid acid core, and the microcapsule shell is made of a high-temperature acid-responsive biodegradable polymer material. The high-temperature acid-responsive biodegradable polymer material is polymerized from five monomers: acrylamide, tert-butyl methacrylate, 2-acrylamide-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether.

[0007] Preferably, the solid acid core is any one or more of aminosulfonic acid, p-toluenesulfonic acid, citric acid, and lactic acid cured particles.

[0008] Preferably, the molar ratio of acrylamide, tert-butyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether is 4~6:4~6:0.8~1.2:0.8~1.2:0.8~1.2.

[0009] Preferably, the mass ratio of the acid core to the microcapsule shell is 0.5~2:1~2.

[0010] Preferably, the high-temperature acid-responsive biodegradable polymer material is prepared by the following steps:

[0011] S1: Add the five monomers to solvent one, stir to dissolve, and obtain mixture one;

[0012] S2: Nitrogen gas is introduced into the mixture to remove oxygen, then a free radical initiator is added, and the mixture is heated to the target temperature to initiate free radical polymerization;

[0013] S3: Wash and dry the polymerization product to obtain the high-temperature acid-responsive biodegradable polymer material.

[0014] Preferably, in step S2, the free radical initiator is azobisisobutyronitrile (AIBN), and its dosage is 0.5-2% of the total mass of the five monomers; the target temperature is 60-70°C, and the reaction time is 5-7 hours.

[0015] On the other hand, a method for preparing high-temperature acid-responsive degradable microcapsule acid for acid fracturing as described in any one of the above is also provided, wherein the preparation is carried out by a phase separation coating method.

[0016] Preferably, the phase separation coating method includes the following steps:

[0017] S1': Under constant temperature stirring conditions, the high-temperature acid-responsive biodegradable polymer material is dissolved in solvent two to form a shell material solution;

[0018] S2': Add the solid acid core to the shell material solution and disperse to obtain mixture two;

[0019] S3': Add a coagulation inducer to the second mixture, and increase the temperature and stirring speed during the addition process to induce the high-temperature acid-responsive biodegradable polymer material to precipitate and form a film on the surface of the solid acid core;

[0020] S4': Add a desiccant, then filter, wash with a non-solvent and dry to obtain the acid-sensitive high-temperature acid-responsive biodegradable microcapsule acid for acid pressure.

[0021] Preferably, in step S3', the coagulation inducer is polydimethylsiloxane, with a kinematic viscosity of 500~5000 cSt, and its dosage is 1~5% of the mass of the high-temperature acid-responsive biodegradable polymer material; during the dropwise addition process, the temperature is increased from 50℃ to 70~90℃, and the stirring speed is increased from 400rpm to 500~700rpm.

[0022] Preferably, in step S4', the de-adhesive detergent is dichloromethane, and its volume is equal to that of solvent two.

[0023] The beneficial effects of this invention are:

[0024] The microcapsule shell of the present invention has acid-responsive degradation characteristics, and can achieve self-degradation of the shell material under high temperature acid conditions, reducing secondary damage to the reservoir caused by shell material residue; the microcapsule acid has three functions: temperature and shear resistance, delayed acid release and shell material degradability, and is suitable for acid fracturing construction in high temperature / ultra-high temperature carbonate reservoirs. Attached Figure Description

[0025] 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, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram showing the degradation rate test results of polymer shell material in water and hydrochloric acid at different temperatures;

[0027] Figure 2 This is a schematic diagram of the microscopic morphology of the microencapsulated acid.

[0028] Figure 3 The microencapsulated acid was dissolved in 20 wt% hydrochloric acid at 90 °C for 170 seconds. -1 A schematic diagram of the release rate test results under isothermal shear;

[0029] Figure 4 This is a schematic diagram showing the release rate test results of microencapsulated acid in 20wt% hydrochloric acid at different temperatures;

[0030] Figure 5 This is a schematic diagram showing the degradation rate test results of microencapsulated acid in 20wt% hydrochloric acid at different temperatures. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise indicated, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0032] On one hand, the present invention provides a high-temperature acid-responsive biodegradable microcapsule acid for acid pressure, comprising an acid core and a microcapsule shell encapsulating the acid core. The acid core is a solid acid core, and the microcapsule shell is made of a high-temperature acid-responsive biodegradable polymer material. The high-temperature acid-responsive biodegradable polymer material is polymerized from five monomers: acrylamide, tert-butyl methacrylate, 2-acrylamide-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether.

[0033] In this invention, a material polymerized from five monomers—acrylamide (AM), tert-butyl methacrylate (TBMA), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), diallylamine (DAA), and allyl polyoxyethylene ether (APEG)—is used as the microcapsule shell. AM serves as the backbone monomer; AMS serves as the temperature-resistant monomer, and APEG serves as the toughening monomer. Both provide the polymer with temperature-resistant functional groups and flexible segments, thereby improving its temperature and shear resistance and preventing the shell from being sheared and damaged during pumping due to insufficient temperature resistance or excessive brittleness. DAA and TBMA form acid-responsive sites on the main chain and side chain, respectively. The high temperature and high acid concentration in the reservoir during acid fracturing can promote the secondary amine matrix protonation of DAA and the ester hydrolysis of TBMA, thereby improving the polymer's hydrophilicity and promoting the degradation of the shell material.

[0034] In one specific embodiment, the solid acid core is any one or more of aminosulfonic acid, p-toluenesulfonic acid, citric acid, and lactic acid cured particles. Optionally, the particle size of the solid acid core is 10-30 micrometers.

[0035] It should be noted that the above-mentioned solid acid core is only a preferred type of solid acid core of the present invention, and other solid acid cores suitable for acid fracturing in the prior art can also be applied to the present invention.

[0036] In one specific embodiment, the molar ratio of acrylamide, tert-butyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether is 4~6:4~6:0.8~1.2:0.8~1.2:0.8~1.2.

[0037] In one specific embodiment, the mass ratio of the acid core to the microcapsule shell is 0.5~2:1~2.

[0038] In one specific embodiment, the high-temperature acid-responsive biodegradable polymer material is prepared by the following steps:

[0039] S1: Add the five monomers to solvent one, stir to dissolve, and obtain mixture one.

[0040] In one specific embodiment, solvent one is anhydrous ethanol. It should be noted that the purpose of solvent one is to dissolve the five monomers; besides the solvent preferred in this embodiment, other solvents in the prior art capable of achieving this purpose are also applicable to this invention.

[0041] S2: Nitrogen gas is introduced into the mixture to remove oxygen, then a free radical initiator is added, and the mixture is heated to the target temperature to initiate free radical polymerization;

[0042] In one specific embodiment, the free radical initiator is azobisisobutyronitrile (AIBN), and its dosage is 0.5-2% of the total mass of the five monomers; the target temperature is 60-70°C, and the reaction time is 5-7 hours.

[0043] It should be noted that the function of the free radical initiator is to initiate the polymerization reaction of the monomers. The free radical initiator in this embodiment is only one preferred agent of the present invention, and other free radical initiators in the prior art that can achieve this purpose can also be applied to the present invention. Different free radical initiators will result in different initiation temperatures and reaction times. Those skilled in the art can select the corresponding reaction temperature and reaction time according to different types of free radical initiators.

[0044] S3: Wash and dry the polymerization product to obtain the high-temperature acid-responsive biodegradable polymer material.

[0045] It should be noted that the above embodiments are only a preferred preparation method of the high-temperature acid-responsive biodegradable polymer material of the present invention. Those skilled in the art can prepare the high-temperature acid-responsive biodegradable polymer material of the present invention by other preparation methods according to the five monomers and the principle of polymerization reaction described in the present invention.

[0046] On the other hand, the present invention also provides a method for preparing the high-temperature acid-responsive degradable microcapsule acid for acid fracturing as described in any one of the above claims, wherein the preparation is carried out by a phase separation coating method.

[0047] In this invention, a phase separation coating method is used to prepare the high-temperature acid-responsive biodegradable microcapsule acid for acid fracturing. The resulting microcapsule acid has a dense, smooth shell and high sphericity. Although existing technologies such as spray drying can also prepare microcapsule acids, which involves dissolving the shell material, spraying it onto the surface of a solid acid, the sphericity and density of the microcapsules produced by this method are difficult to control. Furthermore, the shell material of this invention is a high-temperature acid-responsive biodegradable polymer material, which is difficult to atomize, resulting in poor coating effect.

[0048] In one specific embodiment, the phase separation coating method includes the following steps:

[0049] S1': Under constant temperature stirring conditions, the high-temperature acid-responsive biodegradable polymer material is dissolved in solvent two to form a shell material solution;

[0050] S2': Add the solid acid core to the shell material solution and disperse to obtain mixture two;

[0051] S3': Add a coagulation inducer to the second mixture, and increase the temperature and stirring speed during the addition process to induce the high-temperature acid-responsive biodegradable polymer material to precipitate and form a film on the surface of the solid acid core;

[0052] S4': Add a desiccant, then filter, wash with a non-solvent and dry to obtain the acid-sensitive high-temperature acid-responsive biodegradable microcapsule acid for acid pressure.

[0053] In a specific embodiment, in step S3', the coagulation inducer is polydimethylsiloxane with a kinematic viscosity of 500~5000 cSt, and its dosage is 1~5% of the mass of the high-temperature acid-responsive biodegradable polymer material; during the dropwise addition process, the temperature is increased from 50°C to 70~90°C, and the stirring speed is increased from 400 rpm to 500~700 rpm.

[0054] In one specific embodiment, in step S4', the de-adhesion detergent is dichloromethane, and its volume is equal to that of the solvent two. It should be noted that the purpose of the de-adhesion detergent is to wash away the coagulation inducer. The agent in this embodiment is only a preferred agent of the present invention, and other de-adhesion detergents in the prior art that can achieve this purpose are also applicable to the present invention.

[0055] In a specific embodiment, in step S1', the second solvent is anhydrous ethanol; in step S4', the non-solvent used for washing is n-heptane. It should be noted that the purpose of the second solvent is to dissolve the high-temperature acid-responsive biodegradable polymer material, and the purpose of the non-solvent is to wash the microcapsule acid after film formation and to prevent it from dissolving the film-forming high-temperature acid-responsive biodegradable polymer material. The reagent in this embodiment is only a preferred reagent of the present invention; other reagents in the prior art that can achieve this purpose are also applicable to the present invention.

[0056] Example 1

[0057] A high-temperature acid-responsive biodegradable microencapsulated acid for acid fracturing is prepared by the following steps:

[0058] (1) Preparation of high-temperature acid-responsive biodegradable polymer materials

[0059] Under stirring conditions of 300 rpm, 0.6 mol of acrylamide (AM), 0.6 mol of tert-butyl methacrylate (TBMA), and 0.1 mol of 2... were added to 300 mL of anhydrous ethanol. Acrylamide 2 Five monomers, namely methyl propanesulfonic acid (AMPS), 0.1 mol diallylamine (DAA), and 0.1 mol allyl polyoxyethylene ether (APEG), were mixed and dissolved. After deoxygenation by purging with nitrogen for 20 min, 1 wt% AIBN of the total monomer mass was added. The mixture was heated to 65 °C to initiate free radical polymerization. After reacting for 6 h, the polymer was obtained, which is the high-temperature acid-responsive biodegradable polymer material.

[0060] (2) Preparation of high-temperature acid-responsive biodegradable microcapsule acid

[0061] Under constant temperature stirring conditions of 50℃ and 400rpm, 5g of the polymer shell material obtained in step S1 was dissolved in 300mL of anhydrous ethanol to form a shell material solution. 5g of p-toluenesulfonic acid was added and dispersed. A coagulation inducing agent (polydimethylsiloxane) equivalent to 3wt% of the shell material mass was added dropwise to the shell material solution. At the same time, the temperature was raised to 80℃ and the stirring speed was increased to 600rpm to induce the shell material to precipitate and form a film on the acid core surface. The time for increasing the temperature and speed was controlled to be 15min. After the dropwise addition was completed, a debinding detergent (dichloromethane) with a volume equal to that of the anhydrous ethanol solvent was added. Then, the mixture was filtered, washed with a non-solvent (n-heptane), and dried to obtain the high-temperature acid-responsive biodegradable microcapsule acid.

[0062] Example 2

[0063] Unlike Example 1, in step (1) of this example, the amount of diallylamine used is 0.08 mol and the amount of acrylamide used is 0.4 mol.

[0064] Example 3

[0065] Unlike Example 1, in step (2) of this example, the amount of p-toluenesulfonic acid used is 10g.

[0066] Example 4

[0067] Unlike Example 1, the amount of tert-butyl methacrylate used in step (1) of this example is 0.4 mol.

[0068] Example 5

[0069] Unlike Example 1, in step (2) of this example, the temperature is raised to 90°C during the addition of the coagulation inducing agent.

[0070] Example 6

[0071] Unlike Example 1, the reaction time of the polymer shell in step (2) of this example is 7 hours.

[0072] Comparative Example 1

[0073] Unlike Example 1, in step (1) of this comparative example, tert-butyl methacrylate monomer was omitted, and only 0.6 mol acrylamide (AM) and 0.1 mol 2 Acrylamide 2 The four monomers are methylpropanesulfonic acid (AMPS), 0.1 mol diallylamine (DAA), and 0.1 mol allyl polyoxyethylene ether (APEG).

[0074] Comparative Example 2

[0075] Unlike Example 1, in step (1) of this comparative example, the tert-butyl methacrylate monomer and diallylamine monomer were replaced with 1-tert-butyl-1-propylene monomer and 1,5-hexadiene monomer, which have similar molecular weights and structures.

[0076] Comparative Example 3

[0077] Unlike Example 1, step (1) of this comparative example omits 2. Acrylamide 2 The monomers are methylpropanesulfonic acid and allyl polyoxyethylene ether, with only 0.6 mol acrylamide (AM), 0.6 mol tert-butyl methacrylate (TBMA) and 0.1 mol diallylamine (DAA) added.

[0078] Test Example 1

[0079] High-temperature acid-responsive degradation tests were conducted on the polymer shells prepared in step (1) of each embodiment and comparative example. Equal amounts of polymer shells were weighed and placed in different solution environments (water, 20wt% hydrochloric acid), and heated in an oil bath at different temperatures (25℃, 60℃, 90℃, 150℃, 180℃) for 4 hours. The shells were then filtered, washed, and dried. The remaining mass of the dissolved polymer was weighed, and the degradation rate of the polymer shells under different working conditions was measured. The test results are as follows: Figure 1 As shown.

[0080] from Figure 1It can be seen that in the water system, the overall degradation rate of each shell material is low, and it only shows a slow upward trend with increasing temperature. In particular, the shell materials of Examples 1 and 2 maintain a low degradation rate even at 180°C, indicating that they have good structural stability in neutral or weakly polar environments. This is mainly due to the introduction of TBMA hydrophobic groups in the shell materials, which makes it difficult for polymer chains to undergo significant hydration and breakage under water conditions, thereby effectively inhibiting premature degradation under non-target conditions. Since the shell material of Example 3 was prepared using the same method as the shell material of Example 1, no additional tests were performed on the shell material of Example 3.

[0081] In contrast, in 20 wt% hydrochloric acid, the degradation behavior of the shell material in this embodiment exhibited a significant synergistic response between temperature and acid concentration. As the temperature increased from 25°C to 180°C, the degradation rate of the shell material in Example 1 rapidly increased and eventually approached complete degradation, indicating that a high-temperature, high-acid environment can effectively trigger the acid-responsive structural units within the shell material. The mechanism lies in the protonation of the secondary amine group of DAA under acidic conditions, while the ester bonds of the TBMA side chain undergo acid-catalyzed hydrolysis, leading to increased polymer hydrophilicity, chain segment relaxation, and eventual disintegration. Example 2, due to a reduced AM content, showed a slightly lower overall hydrophilicity and hydrolysis sensitivity; therefore, under the same conditions, its degradation rate was slightly lower than that of Example 1, but it still maintained a clear acid-responsive degradation characteristic.

[0082] Comparative Example 1, having completely omitted the hydrophobic monomer TBMA, exhibits extremely high hydrophilicity in its shell material, showing a high degradation rate in both water and hydrochloric acid. However, it lacks selective responsiveness, which is detrimental to delayed controlled release during acid fracturing operations. While Comparative Example 2 possesses strong hydrophobicity, its degradation rate in hydrochloric acid is significantly lower than that of the embodiments of this invention due to the lack of acid-responsive groups, making it difficult to achieve effective shell breaking under reservoir conditions. Due to the absence of temperature-resistant monomers, the degradation rate of Comparative Example 3 increases sharply after the temperature exceeds 90°C. This may cause the shell material to degrade directly near the fracture in ultra-deep reservoirs, preventing the acid core from being transported to the far end of the fracture and thus reducing its effectiveness.

[0083] The above results fully demonstrate that the present invention achieves high-temperature acid-responsive degradation performance of the shell material by synergistic design of hydrophobic monomers and acid-responsive monomers, which is "stable in clear water and controllable in high-temperature acid".

[0084] Test Example 2

[0085] The morphology of the microencapsulated acids prepared in each embodiment and comparative example was observed. Equal amounts of microencapsulated acids were weighed, thoroughly dried, and then sputter-coated with gold. Their microstructure was observed using a scanning electron microscope (accelerating voltage set to 10 kV, probe current to 10 μA, and working distance adjusted to 6-8 mm). The test results are as follows: Figure 2 As shown.

[0086] from Figure 2 It can be seen that the microcapsules obtained in Examples 1 and 2 are generally spherical with a smooth and continuous surface. The shell is dense and has no obvious through-holes, indicating that the phase separation-induced film formation process is stable and the shell material can be uniformly precipitated on the acid core surface to form a complete coating structure. This continuous and dense shell is beneficial to improving the mechanical stability of the microcapsules during acid mixing, shearing, and transport processes. In Example 3, while keeping the shell material formulation unchanged, the amount of acid core was increased, resulting in a significant increase in the core-shell ratio. From its morphology, it can be observed that the microcapsules still maintain a complete spherical structure, but slight undulations and uneven thickness are observed on the local surface, indicating that under high core-shell ratio conditions, the spreading tension of the shell material on the acid core surface and the film thickness distribution change. Although no overall rupture or collapse occurred, this uneven structure is easily sheared by high-speed fluid during transport, resulting in premature release. This provides a process window for adjusting the core-shell ratio.

[0087] The microcapsule shell of Comparative Example 1 exhibited obvious damage, collapse, and even hollow structures. Numerous pores and tear marks were visible inside the shell, indicating that the highly hydrophilic shell material failed to effectively achieve phase separation during the encapsulation process, resulting in disordered film structure and encapsulation failure. Although Comparative Example 2 could form a spherical structure, its surface was rough and textured, reflecting strong rigidity of its shell material segments and insufficient interfacial rearrangement ability, which is not conducive to forming a uniform and dense protective layer. The shell material of Comparative Example 3 lacked toughness monomers, exhibiting excessive rigidity and insufficient toughness. Consequently, the shell material could not expand and adhere to the core material surface, weakening the integrity of the encapsulation.

[0088] A comprehensive comparison shows that, through the synergistic optimization of shell material composition and preparation parameters, the embodiments of the present invention have achieved a microcapsule acid structure with complete morphology, dense smoothness, and high sphericity, laying the foundation for its mechanical stability and delayed release under acid pressure conditions.

[0089] Test Example 3

[0090] The temperature and shear resistance properties of the microencapsulated acids prepared in each embodiment and comparative example were tested. The release rate of the microencapsulated acids under high temperature and high shear rate conditions was tested online using the shear stirring module of a high-temperature, high-pressure reactor: equal amounts of microencapsulated acid were weighed and placed in a 20wt% hydrochloric acid solution environment, and subjected to a reaction at 90°C for 170 seconds. -1 Under isothermal shearing conditions, continuous shearing was performed for 2 hours to simulate the microencapsulated acid pumping conditions during acid fracturing. During the test, the online sampling module was activated every 30 minutes to take samples. The hydrogen ion concentration in the obtained liquid samples was determined using acid-base titration, and the concentration of hydrogen ions provided by hydrochloric acid was subtracted to obtain the internal acid concentration released from the microcapsules. The release rate was calculated, and a release rate curve was plotted. This test simulates the temperature and shear resistance of microencapsulated acid carried by high-concentration hydrochloric acid within the wellbore during the shearing process. The test results are as follows: Figure 3 As shown.

[0091] from Figure 3 It can be seen that the shear release rate of Examples 1 and 2 did not exceed 10% after continuous high-temperature shearing for 2 hours, and their shells can effectively suppress the release caused by high-speed shearing during pumping, ensuring that the acid is released inside the reservoir. Although the coating effect of Example 3 was not as good as that of Examples 1 and 2 due to the excessive acid core content, which reduced the overall strength of the shell, its shear release rate was still less than 20%.

[0092] Because the shell material of Comparative Example 1 was too hydrophilic, phase separation failed, resulting in an incomplete structure that could not protect the core material at all, leading to a shear release rate close to 90%. The microencapsulated acid shell of Comparative Example 2 had high sphericity and rigidity, and retained both toughening and heat-resistant monomers, ensuring temperature and shear resistance; therefore, its shear release rate was also below 10%. However, due to the removal of toughening and heat-resistant monomers, the temperature and shear resistance of the shell in Comparative Example 3 was significantly reduced, with a shear release rate as high as 58.1%. This means that most of the acid core was sheared and released before entering the reservoir, resulting in a waste of the acid core material.

[0093] Comprehensive analysis shows that by introducing heat-resistant and toughness-resistant monomers, the present invention effectively improves the temperature and shear resistance of the microcapsule shell, effectively suppressing the premature release of the acid core due to high-temperature shear during pumping, and ensuring the targeted release of the acid core.

[0094] Test Example 4

[0095] The controlled-release performance of the microencapsulated acids prepared in each embodiment and comparative example in hydrochloric acid at different temperatures was tested. An online high-temperature, high-pressure reactor with online sampling capability was used to test the microcapsule release rate: equal amounts of microencapsulated acid were weighed and placed in a 20wt% hydrochloric acid solution, and heated at different temperatures (25℃, 60℃, 90℃, 150℃, 180℃) for 4 hours. During the test, the online sampling module was activated every 30 minutes to take samples. The hydrogen ion concentration in the obtained liquid samples was determined by acid-base titration, and the hydrogen ion concentration provided by hydrochloric acid was subtracted to obtain the internal acid concentration released from the microcapsules. The release rate was calculated, and a release rate curve was plotted. This test simulates the controlled-release performance of the microencapsulated acid in the reservoir. The test results are as follows: Figure 4 As shown.

[0096] from Figure 4It can be seen that under low temperature or low acid concentration conditions, the microencapsulated acid release rate of the embodiments of the present invention is slow, the release curve is flat, and the release rate remains at a low level within 4 hours, indicating that the shell material has a good shielding effect under non-target operating conditions. This characteristic is beneficial to the safe transportation of microencapsulated acid in the wellbore and near-wellbore zone, avoiding premature consumption of acid. As the temperature rises above 90℃, especially under the conditions of 150-180℃ and 20wt% hydrochloric acid concentration, the release rate of microencapsulated acid of the embodiments of the present invention is significantly accelerated, the release curve shows a typical accelerated characteristic, and the final release rate is significantly improved. This change is consistent with... Figure 1 The rapid degradation behavior of the shell material under high temperature and high acid conditions is consistent with that of the shell material, indicating that shell material degradation is the dominant factor controlling acid release. In Example 1, due to the reasonable ratio of acid-responsive groups to hydrophilic and hydrophobic structures in the shell material, rapid and sufficient release can be achieved under high temperature and high acid conditions; in Example 2, due to slightly lower hydrophilicity, the release rate is slightly slower but still has good responsiveness; in Example 3, due to slightly reduced shell density, a higher cumulative release rate is observed under the same conditions.

[0097] Comparative Example 1 showed that the microencapsulated acid exhibited rapid release at various temperatures and acid concentrations, lacking a significant delay phase, further confirming that its shell material did not block the acid core, leading to the direct dissolution of the acid core. Comparative Example 2, on the other hand, showed insufficient release under high temperature and high acid conditions, making it difficult to meet the acid etching requirements within the reservoir and resulting in waste of acid core material. Comparative Example 3 showed a significant increase in release rate after the temperature exceeded 90℃, indicating that its temperature resistance was insufficient to meet the requirements of ultra-deep acid fracturing operations.

[0098] Comprehensive analysis shows that by constructing a high-temperature acid-responsive biodegradable shell material, the present invention enables microencapsulated acid to achieve controllable behavior of "stable at low temperatures and low acid levels, and rapid release at high temperatures and high acid levels," which can effectively meet the dual requirements of delayed release and degradation in ultra-deep carbonate rock acid fracturing construction.

[0099] Test Example 5

[0100] The degradation rate of the microencapsulated acids prepared in each embodiment and comparative example was tested in hydrochloric acid at different temperatures. After the release rate test in Test Example 4, the reactor was opened and cleaned to collect the degraded polymer shell solution. After filtration, washing, and drying, the residual mass of the shell was weighed and divided by the initial mass of the microencapsulated acid sample to calculate the degradation rate. A degradation rate curve was plotted, and the test results are shown below. Figure 5 As shown.

[0101] from Figure 5 It can be seen that as the temperature increases, the overall degradation rate of the microcapsule acid in the embodiments of the present invention increases significantly, rising rapidly above 90°C, and approaching or reaching complete degradation at 150~180°C, indicating that the microcapsules of the present invention can be effectively degraded under high temperature and high acid conditions.

[0102] Comparative Example 1 exhibited a high degradation rate even at lower temperatures, further demonstrating the failure of the encapsulation and its inability to achieve delayed protection. Comparative Example 2, under the same conditions, showed a significantly lower overall degradation rate than the embodiments of this invention, especially with substantial shell residue remaining in the medium-to-high temperature range, indicating a lack of effective acid-responsive degradation performance in its shell. Comparative Example 3 lacked heat-resistant monomers in its shell, resulting in a sharp increase in degradation rate above 90°C, indicating insufficient heat resistance to meet the demands of ultra-deep acid fracturing operations. In contrast, this invention, by introducing acid-responsive structural units into the shell and regulating the hydrophobic-hydrophilic balance, enables the microcapsules to further degrade with increasing temperature and acidity after acid release, achieving an effective connection between the release and degradation processes.

[0103] The above results show that the microencapsulated acid of the present invention not only has good delayed release performance, but also can achieve a high overall degradation rate under the target working conditions, taking into account both construction effect and reservoir protection, and has significant advantages in engineering applications.

[0104] It should be noted that the above embodiments are only some embodiments of the present invention. Changing the type of reagent, the temperature, time, dosage and other parameters of the preparation method can also produce the high-temperature acid-responsive biodegradable microcapsule acid for acid pressure as described in the present invention. The above test examples are only test results of some embodiments of the present invention. The microcapsule acids prepared in other embodiments of the present invention have similar properties and all have the three functions of temperature and shear resistance, delayed acid release and biodegradable shell material.

[0105] The above description is merely a representative embodiment of the present invention and is not intended to limit the present invention in any way. Any embodiment made by those skilled in the art without departing from the scope of the present invention and utilizing the disclosed technical content is an equivalent embodiment of the present invention. Any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A high-temperature acid-responsive biodegradable microencapsulated acid for acid fracturing, characterized in that, The device includes an acid core and a microcapsule shell encapsulating the acid core. The mass ratio of the acid core to the microcapsule shell is 0.5~2:1~2. The acid core is a solid acid core. The microcapsule shell is made of a high-temperature acid-responsive biodegradable polymer material. The high-temperature acid-responsive biodegradable polymer material is polymerized from five monomers: acrylamide, tert-butyl methacrylate, 2-acrylamide-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether. The molar ratio of acrylamide, tert-butyl methacrylate, 2-acrylamide-2-methylpropanesulfonic acid, diallylamine, and allyl polyoxyethylene ether is 4~6:4~6:0.8~1.2:0.8~1.2:0.8~1.

2.

2. The high-temperature acid-responsive biodegradable microencapsulated acid for acid fracturing according to claim 1, characterized in that, The solid acid core is any one or more of aminosulfonic acid, p-toluenesulfonic acid, citric acid, and lactic acid cured particles.

3. The high-temperature acid-responsive biodegradable microencapsulated acid for acid fracturing according to claim 1, characterized in that, The high-temperature acid-responsive biodegradable polymer material is prepared by the following steps: S1: Add the five monomers to solvent one, stir to dissolve, and obtain mixture one; S2: Nitrogen gas is introduced into the mixture to remove oxygen, then a free radical initiator is added, and the mixture is heated to the target temperature to initiate free radical polymerization; S3: Wash and dry the polymerization product to obtain the high-temperature acid-responsive biodegradable polymer material.

4. The high-temperature acid-responsive biodegradable microencapsulated acid for acid fracturing according to claim 3, characterized in that, In step S2, the free radical initiator is azobisisobutyronitrile (AIBN), and its dosage is 0.5-2% of the total mass of the five monomers; the target temperature is 60-70℃, and the reaction time is 5-7h.

5. The method for preparing high-temperature acid-responsive degradable microencapsulated acid for acid fracturing as described in any one of claims 1-4, characterized in that, The preparation was carried out using a phase separation coating method.

6. The method for preparing high-temperature acid-responsive degradable microencapsulated acid for acid fracturing according to claim 5, characterized in that, The phase separation coating method includes the following steps: S1': Under constant temperature stirring conditions, the high-temperature acid-responsive biodegradable polymer material is dissolved in solvent two to form a shell material solution; S2': Add the solid acid core to the shell material solution and disperse to obtain mixture two; S3': Add a coagulation inducer to the second mixture, and increase the temperature and stirring speed during the addition process to induce the high-temperature acid-responsive biodegradable polymer material to precipitate and form a film on the surface of the solid acid core; S4': Add a desiccant, then filter, wash with a non-solvent and dry to obtain the acid-sensitive high-temperature acid-responsive biodegradable microcapsule acid for acid pressure.

7. The method for preparing high-temperature acid-responsive degradable microencapsulated acid for acid fracturing according to claim 6, characterized in that, In step S3', the coagulation inducer is polydimethylsiloxane, with a kinematic viscosity of 500~5000 cSt, and its dosage is 1~5% of the mass of the high-temperature acid-responsive biodegradable polymer material; during the dropwise addition process, the temperature is increased from 50℃ to 70~90℃, and the stirring speed is increased from 400rpm to 500~700rpm.

8. The method for preparing high-temperature acid-responsive degradable microencapsulated acid for acid fracturing according to claim 6, characterized in that, In step S4', the de-adhesive detergent is dichloromethane, and its volume is equal to that of solvent two.

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