Temperature-pH Dual-Response Smart Fracturing Proppant and Its Preparation Method

Abstract

This invention belongs to the field of oil and gas field development technology and discloses a temperature-pH dual-responsive smart fracturing proppant and its preparation method. The proppant adopts a three-layer core-shell structure, including a high-strength ceramic or quartz sand core, a polyvinyl alcohol crosslinking transition layer reinforced with nano-silica, and a poly(N-isopropylacrylamide-co-acrylic acid) copolymer smart response layer. In the low-temperature proppant carrying stage, the smart response layer hydrates and swells to form a gel layer, giving the proppant excellent suspension performance and friction reduction ability, realizing water fracturing. In the high-temperature reservoir environment, the phase transformation of the temperature-sensitive segment and the protonation of the pH-sensitive segment cause the smart response layer to shrink and detach rapidly, realizing autonomous gel breaking and aiding in fracturing. The nano-reinforced transition layer significantly improves the anti-fracture performance, reducing the fracture rate to 6% to 11% and increasing the long-term conductivity by 35% to 58%.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development technology, specifically relating to a temperature-pH dual-responsive smart fracturing proppant and its preparation method. Background Technology

[0002] Fracturing technology is a key technique for improving the efficiency of tight oil and gas reservoir development. Fracturing proppant plays a crucial role in supporting fractures and maintaining flow channels. In traditional fracturing operations, proppant relies on high-viscosity fracturing fluid for carrying and transporting, which leads to complex and costly fracturing fluid formulation. Furthermore, incomplete gel breaking of the fracturing fluid can damage the reservoir.

[0003] Chinese patent application CN113122220A discloses a variable viscosity fracturing fluid and its preparation method. This technology improves fracturing performance by adding a variable viscosity drag reducer and a surface-modified ceramic particle proppant to the fracturing fluid. The surface-modified ceramic particles in this patent improve suspension performance by grafting a polymer coating onto the surface of sintered ceramic particles. The preparation method includes immersing the ceramic particles in a solution containing multiple monomers to carry out a surface polymerization reaction.

[0004] However, existing technologies still have the following shortcomings: First, fracturing fluid formulation still requires the addition of various additives, including proppant carriers, drag reducers, and crosslinking agents, making the formulation complex and costly. Second, the proppant's proppant-carrying capacity mainly depends on the viscosity adjustment of the applied fracturing fluid; the proppant itself lacks environmental adaptability and cannot autonomously adjust its functional state according to temperature and pH changes. Third, the fragments generated by the collision of proppant particles during high-speed proppant transport can easily clog fractures and reduce conductivity; existing surface polymer coatings offer limited improvement in the proppant's anti-fracture performance. Fourth, the residue after fracturing fluid debonding damages the reservoir and affects oil and gas production; existing proppant surface coatings lack autonomous debonding and decoction mechanisms.

[0005] Therefore, there is an urgent need to develop a smart responsive fracturing proppant that can provide excellent suspension performance during the low-temperature proppant carrying stage, autonomously break down and detach in high-temperature reservoir environments, and at the same time have high resistance to fracturing. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention provides a temperature-pH dual-responsive intelligent fracturing proppant and its preparation method. This proppant achieves intelligent conversion of proppant carrying, propping, and gel breaking functions through a three-layer core-shell structure design and a temperature-pH dual-responsive mechanism. It can be used for fracturing operations with clean water without the need to add proppant carrying agents and drag reducing agents, significantly reducing configuration complexity and reservoir damage.

[0007] To achieve the above objectives, the first aspect of the present invention provides a temperature-pH dual-responsive smart fracturing proppant, comprising the following components by mass parts:

[0008] The core layer consists of 100 parts, composed of high-strength ceramsite or quartz sand. The high-strength ceramsite has a compressive strength of 52-86 MPa and a bulk density of 1.45-1.72. The particle size is 0.3-0.9 mm; the Mohs hardness of the quartz sand is 7-7.5, and the density is 2.60-2.66. The particle size is 0.25-0.85 mm;

[0009] The reinforcing transition layer consists of 8-18 parts, comprising a polyvinyl alcohol crosslinked network reinforced with nano-silica, wherein the nano-silica has a particle size of 8-25 nm and a specific surface area of ​​180-320 nm. The mass fraction of the polyvinyl alcohol in the reinforcing transition layer is 12%–28%; the degree of alcoholysis of the polyvinyl alcohol is 87%–99%, the degree of polymerization is 1700–2400, and the degree of crosslinking is 18%–35%.

[0010] The smart response layer comprises 3-12 parts, consisting of a poly(N-isopropylacrylamide-co-acrylic acid) copolymer, wherein the molar ratio of N-isopropylacrylamide units to acrylic acid units in the copolymer is (65-82):(18-35), and the number average molecular weight of the copolymer is [missing information]. - The low critical solution temperature is 28-35℃, and the pH response range is 4.5-6.8.

[0011] Furthermore, in the enhanced transition layer, nano-silica interacts with the hydroxyl groups of polyvinyl alcohol through hydrogen bonds and achieves covalent bonding through a silane coupling agent, wherein the silane coupling agent is selected from... - Glycidyl etheroxypropyltrimethoxysilane, -aminopropyltriethoxysilane or -Methacryloxypropyltrimethoxysilane, used in an amount of 2% to 8% of the mass of nano-silica.

[0012] Furthermore, the polyvinyl alcohol crosslinking network uses glutaraldehyde or maleic anhydride as a crosslinking agent, with the amount of crosslinking agent being 5% to 15% of the mass of polyvinyl alcohol. The crosslinking reaction is carried out under acidic conditions of pH 2-4, at a reaction temperature of 55-75℃, and for a reaction time of 3-6 hours.

[0013] Furthermore, in the intelligent response layer, the poly(N-isopropylacrylamide-co-acrylic acid) copolymer is prepared by free radical polymerization, with ammonium persulfate or azobisisobutyronitrile as the initiator, and the amount used is 0.3% to 1.2% of the total monomer mass. The polymerization reaction is carried out in... Perform at 60-75℃ for 4-8 hours under protection.

[0014] Furthermore, the high-strength ceramsite is prepared by mixing bauxite, kaolin and binder in a mass ratio of (50-70):(20-35):(5-15) and then sintering at 1180-1280℃ for 1.5-3 hours. After sintering, the main crystalline phases of the ceramsite are mullite and corundum, with a mullite content of 45% to 68%.

[0015] A second aspect of this invention provides a method for preparing the above-mentioned temperature-pH dual-responsive smart fracturing proppant, comprising the following steps:

[0016] Step 1, Core Layer Preparation: Wash high-strength ceramsite or quartz sand sequentially with deionized water and anhydrous ethanol, then dry at 100-120℃ for 2-4 hours to obtain a clean core; for ceramsite cores, pulverize bauxite and kaolin to a particle size of less than 50 mm. m, mixed evenly with binder, granulated with water, dried at 100-110℃ to a moisture content of 2%-5%, then sintered at 1180-1280℃ for 1.5-3h, cooled and sieved to obtain ceramsite cores with a particle size of 0.3-0.9mm;

[0017] Step 2, Construction of the enhanced transition layer: Disperse nano-silica in anhydrous ethanol, add silane coupling agent, stir and react at 70-85℃ for 2-4 hours, centrifuge, wash and dry to obtain surface-modified nano-silica; dissolve polyvinyl alcohol in hot water at 90-98℃ to prepare an aqueous solution with a mass fraction of 5%-12%, cool to room temperature and add surface-modified nano-silica, ultrasonically disperse for 30-60 minutes; immerse the clean core in the above solution, soak at 30-45℃ for 6-12 hours, remove and pre-dry at 60-70℃; immerse the pre-dried coating core in an acidic solution containing a crosslinking agent, adjust the pH to 2-4, crosslink at 55-75℃ for 3-6 hours, remove, wash and dry at 80-100℃ for 2-4 hours to obtain a support intermediate with an enhanced transition layer;

[0018] Step 3, Construction of the Smart Response Layer: Dissolve N-isopropylacrylamide and acrylic acid in a molar ratio of (65-82):(18-35) in deionized water, with a total monomer mass fraction of 15%–25%. Add an initiator, and... Polymerize at 60-75℃ for 4-8 hours under protection to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution; add the proppant intermediate obtained in step 2 to the above copolymer solution, impregnate and coat at 25-35℃ for 8-16 hours, raise the temperature to 40-50℃ and continue the reaction for 2-4 hours to graft and anchor the copolymer on the surface of the reinforcing transition layer, remove and vacuum dry at 30-40℃ for 12-24 hours to obtain a temperature-pH dual-responsive smart fracturing proppant.

[0019] Furthermore, in step one, the binder is selected from bentonite, sodium carboxymethyl cellulose, or polyacrylamide, and the amount used is 5% to 15% of the dry basis material mass.

[0020] Furthermore, in step two, the dispersion of nano-silica is carried out using an ultrasonic dispersion device with an ultrasonic power of 400-800W and a frequency of 20-40kHz.

[0021] Furthermore, in step three, after the polymerization reaction is completed, an antioxidant with a mass fraction of 0.5% to 2% is added to the reaction system. The antioxidant is selected from tert-butylhydroquinone, 2,6-di-tert-butyl-4-methylphenol, or propyl gallate.

[0022] The beneficial effects of this invention are as follows:

[0023] First, this invention achieves multifunctional integration of the proppant through a three-layer core-shell structure design. The high-strength core provides excellent compressive strength, ensuring that the proppant maintains its crack conductivity under high closure stress. The nano-silica-reinforced polyvinyl alcohol crosslinked layer serves as a reinforcing transition layer. On the one hand, it significantly improves the proppant's anti-fracture performance through the reinforcing effect of nanofillers. Experiments show that the breakage rate is reduced from 15%–22% of traditional proppants to 6%–11%, effectively solving the problem of particle blockage. On the other hand, this layer provides an anchoring basis for the smart responsive layer, enhancing the coating bonding strength through interfacial chemical bonding. The outer poly(N-isopropylacrylamide-co-acrylic acid copolymer) smart responsive layer endows the proppant with environmental adaptability.

[0024] Secondly, this invention introduces a temperature-pH dual-response mechanism, enabling intelligent functional switching at different stages of fracturing. During the surface solution preparation and cryogenic proppant transport stages, the temperature is below the low critical solution temperature of the intelligent response layer. The poly(N-isopropylacrylamide) segments are in a hydrated, extended state, while the acrylic acid segments exist as deprotonated carboxylate ions at neutral pH. The synergistic effect of electrostatic repulsion and hydration leads to the coating fully swelling to form a gel layer with a thickness of 80-150 mm. The gel layer imparts excellent suspension properties to the proppant. Clear water proppant carrying experiments show a 3.5-5.2-fold increase in suspension time. Simultaneously, the lubricating effect of the gel layer reduces the friction coefficient by 45%-62%, enabling clear water fracturing without the need for proppant-carrying agents or drag-reducing agents. When the proppant enters the high-temperature reservoir environment with the fracturing fluid, the temperature rises above the low critical solution temperature. The poly(N-isopropylacrylamide) segments undergo a hydrophobic phase transition and shrinkage. Simultaneously, the acidic reservoir environment protonates the acrylic acid segments, converting carboxylate groups to carboxylic acids, eliminating electrostatic repulsion. This dual effect leads to rapid collapse and depolymerization of the smart response layer, reducing its thickness to 10-25 μm. It gradually sheds its shell, achieving autonomous gel breaking and drainage functions, reducing the turbidity of the return liquid by 70% to 85%, and effectively reducing reservoir damage.

[0025] Third, this invention adopts a green chemical preparation route, using water as the main medium throughout the process and avoiding the use of organic solvents, thus meeting environmental protection requirements. Both the reinforcing transition layer and the smart response layer are constructed using an aqueous solution impregnation coating process, which is simple, easy to industrialize, and reduces production costs by 18%–28% compared to traditional modified proppant agents.

[0026] Fourth, this invention significantly improves the mechanical properties of the proppant through the interfacial reinforcement mechanism of nano-silica. Nano-silica is uniformly dispersed in the polyvinyl alcohol matrix. Through the hydrogen bonding interaction between silanol and polyvinyl alcohol hydroxyl groups and the covalent bonding of the silane coupling agent, a strong interfacial bond is formed at the coating-core interface, increasing the shear strength by 2.1-3.5 times. Simultaneously, the rigid particle toughening mechanism of the nanofiller effectively prevents crack propagation, greatly enhancing the proppant's resistance to impact breakage during high-speed transport, and increasing its long-term conductivity under 52 MPa closure stress by 35%–58%.

[0027] Fifth, this invention represents a technological leap from passive to active intelligence in fracturing proppant. Traditional proppants have a single function, providing only support. The intelligent proppant of this invention can autonomously adjust its functional state according to changes in temperature and pH. In the low-temperature proppant carrying stage, it actively provides suspension and drag reduction functions, and in the high-temperature reservoir stage, it actively breaks down the gel and removes the crust to assist in drainage. This intelligent response characteristic makes fracturing operations simpler and more efficient, providing a new technical solution for the efficient development of unconventional oil and gas reservoirs. Detailed Implementation

[0028] The technical solution of the present invention will be clearly and completely described below through specific embodiments. Experimental methods not specifying specific conditions in the embodiments are generally performed under conventional conditions or according to the manufacturer's recommendations. All reagents used are analytical grade or chemically pure and purchased from common domestic and international reagent suppliers.

[0029] Example 1

[0030] Preparation of temperature-pH dual-responsive smart fracturing proppant:

[0031] Step 1, Kernel layer preparation:

[0032] High-strength ceramsite was selected as the core material. Bauxite and kaolin were separately crushed to a particle size of less than 50 mm. The raw materials were prepared by mixing m at a mass ratio of 60:30, adding 10 parts of bentonite as a binder, and then adding an 8% (w / w) aqueous solution for granulation. The granulated material was dried at 105℃ to a moisture content of 3%, and sieved to obtain green granules with a particle size of 0.8-1.2 mm. The green granules were placed in a high-temperature furnace and sintered at 1230℃ with a heating rate of 5℃ / min for 2 hours. After cooling to room temperature in the furnace, the ceramsite core with a particle size of 0.5-0.7 mm was obtained by sieving. Tests showed that the compressive strength of the ceramsite core was 68 MPa and the bulk density was 1.58. The main crystalline phases are mullite and corundum, with mullite content of 56%.

[0033] The core of the ceramic pellet was washed sequentially with deionized water and anhydrous ethanol, and then dried at 110℃ for 3 hours to obtain a clean core for later use.

[0034] Step 2, Enhance the construction of the transition layer:

[0035] 8 Particle size 15nm, specific surface area 250 Nano-silica dispersed in 200 Add 5% by mass of nano-silica to anhydrous ethanol. - Glycidyl etheroxypropyltrimethoxysilane was stirred at 75°C for 3 hours, centrifuged, washed three times with anhydrous ethanol, and vacuum dried at 80°C for 2 hours to obtain surface-modified nano-silica.

[0036] Polyvinyl alcohol 25 with a degree of polymerization of 2000 and a degree of alcoholysis of 93% was used. Dissolve in 250 Dissolve completely in 95°C hot water by stirring, then cool to room temperature to obtain a 10% (w / w) aqueous solution of polyvinyl alcohol. Add 2.5 g of the solution to this solution. Surface-modified nano-silica was dispersed for 45 minutes using an ultrasonic dispersion device with a power of 600W and a frequency of 25kHz to obtain a uniform composite coating liquid.

[0037] 100 The cleaned ceramic particle cores were immersed in the aforementioned composite coating solution at 35°C for 10 hours, then removed and preliminarily dried at 65°C for 4 hours. The preliminarily dried coating cores were then immersed in an acidic solution containing glutaraldehyde (glutaraldehyde concentration 2.5%). The pH was adjusted to 3 with hydrochloric acid, and the crosslinking reaction was carried out at 65℃ for 4 hours. After removal, it was washed with deionized water until neutral and dried at 90℃ for 3 hours to obtain a support intermediate with a reinforcing transition layer. The thickness of the reinforcing transition layer was 35-45 mm. m, nano-silica is uniformly dispersed in the polyvinyl alcohol matrix.

[0038] Step 3, Building the Intelligent Response Layer:

[0039] N-Isopropylacrylamide 25.8 (0.228 mol) and 5.2 mol of acrylic acid. (0.072 mol) Dissolves in 150 mL of water. In deionized water, the monomer molar ratio is 76:24, and the total monomer mass fraction is 20.7%. Ammonium persulfate (0.6% of the total monomer mass) is added to the solution, and then... After deoxygenation for 30 min under protective conditions, polymerization was carried out at 68℃ for 6 h. After the reaction was complete, 1% (w / w) of tert-butylhydroquinone was added as an antioxidant to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution. Gel permeation chromatography showed that the copolymer's number-average molecular weight was [value missing]. The molecular weight distribution index was 1.8. The lower critical solution temperature was determined to be 32℃ by differential scanning calorimetry.

[0040] 80 The proppant intermediate obtained in step two was added to the above copolymer solution and impregnated at 30°C for 12 hours. The temperature was then raised to 45°C and the reaction continued for 3 hours, allowing the copolymer to be grafted and anchored onto the surface of the reinforcing transition layer via hydrogen and covalent bonds. After removal, it was vacuum dried at 35°C for 18 hours to obtain a temperature-pH dual-responsive smart fracturing proppant. Tests showed that the thickness of the smart response layer was 5-8 mm. m.

[0041] Example 2

[0042] Preparation of temperature-pH dual-responsive smart fracturing proppant:

[0043] Step 1, Kernel layer preparation:

[0044] Quartz sand was selected as the core material. The desired hardness is 7.2 on the Mohs scale and the density is 2.63. Natural quartz sand was screened to obtain particles with a diameter of 0.4-0.6 mm. The particles were then washed sequentially with deionized water, dilute hydrochloric acid, deionized water, and anhydrous ethanol, and dried at 115℃ for 3 hours to obtain clean quartz sand cores for later use.

[0045] Step 2, Enhance the construction of the transition layer:

[0046] 12 Particle size 10nm, specific surface area 290 Nano-silica dispersed in 250 Add 3% by mass of nano-silica to anhydrous ethanol. -Aminopropyltriethoxysilane was stirred at 80°C for 2.5 h, centrifuged, washed three times with anhydrous ethanol, and dried under vacuum at 85°C for 2 h to obtain surface-modified nano-silica.

[0047] Polyvinyl alcohol 22 with a degree of polymerization of 1900 and a degree of alcoholysis of 96% was used. Dissolved in 220 Dissolve completely in 92°C hot water by stirring, then cool to room temperature to obtain a 10% (w / w) aqueous solution of polyvinyl alcohol. Add 3.3... Surface-modified nano-silica was dispersed for 40 minutes using an ultrasonic dispersion device with a power of 700W and a frequency of 30kHz to obtain a uniform composite coating liquid.

[0048] 100 The cleaned quartz sand cores were immersed in the aforementioned composite coating solution at 40°C for 8 hours, then removed and preliminarily dried at 70°C for 3.5 hours. The preliminarily dried coating cores were then immersed in an acidic solution containing maleic anhydride at a concentration of 3.0%. The pH was adjusted to 2.5 with hydrochloric acid, and the crosslinking reaction was carried out at 70°C for 5 hours. After removal, it was washed with deionized water until neutral, and dried at 95°C for 2.5 hours to obtain a support intermediate with an enhanced transition layer.

[0049] Step 3, Building the Intelligent Response Layer:

[0050] N-Isopropylacrylamide 28.5 (0.252 mol) and 6.5% acrylic acid (0.090 mol) Dissolves in 170 °C In deionized water, the monomer molar ratio is 74:26, and the total monomer mass fraction is 20.6%. Azobisisobutyronitrile (AIBN) of 0.8% (total monomer mass) is added to the solution, and then... After deoxygenation for 30 min under protective conditions, polymerization was carried out at 72℃ for 5 h. After the reaction was complete, 1.5% (w / w) of 2,6-di-tert-butyl-4-methylphenol was added as an antioxidant to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution. Gel permeation chromatography showed that the copolymer's number-average molecular weight was [missing value]. Differential scanning calorimetry was used to determine the lower critical solution temperature at 30℃.

[0051] 75 The proppant intermediate obtained in step two was added to the above copolymer solution and impregnated at 28°C for 14 hours, then the temperature was raised to 48°C and the reaction continued for 2.5 hours. After removal, it was vacuum dried at 38°C for 20 hours to obtain a temperature-pH dual-responsive smart fracturing proppant.

[0052] Example 3

[0053] Preparation of temperature-pH dual-responsive smart fracturing proppant:

[0054] Step 1, Kernel layer preparation:

[0055] The ceramsite core was prepared using the same method as in Example 1, but the sintering temperature was adjusted to 1260℃ and the holding time was 2.5 h. Tests showed that the ceramsite core had a compressive strength of 75 MPa and a bulk density of 1.65. .

[0056] Step 2, Enhance the construction of the transition layer:

[0057] 15 Particle size 20nm, specific surface area 210 Nano-silica dispersed in 300 Add 7% by mass of nano-silica to anhydrous ethanol. 1-Methacryloxypropyltrimethoxysilane was stirred at 78°C for 3.5 h, centrifuged, washed three times with anhydrous ethanol, and dried under vacuum at 82°C for 2.5 h to obtain surface-modified nano-silica.

[0058] Polyvinyl alcohol 28 with a degree of polymerization of 2200 and a degree of alcoholysis of 89% was used. Dissolve in 250 Dissolve completely in 96°C hot water by stirring, then cool to room temperature to obtain an 11.2% (w / w) polyvinyl alcohol aqueous solution. Add 4.2... Surface-modified nano-silica was dispersed for 50 minutes using an ultrasonic dispersion device with a power of 500W and a frequency of 35kHz to obtain a uniform composite coating liquid.

[0059] 100 The cleaned ceramic particle cores were immersed in the aforementioned composite coating solution at 42°C for 9 hours, then removed and preliminarily dried at 68°C for 3.8 hours. The preliminarily dried coating cores were then immersed in an acidic solution containing glutaraldehyde (glutaraldehyde concentration 3.2%). The pH was adjusted to 2.8 with hydrochloric acid, and the crosslinking reaction was carried out at 62°C for 4.5 h. After removal, it was washed with deionized water until neutral, and dried at 92°C for 3.2 h to obtain a support intermediate with an enhanced transition layer.

[0060] Step 3, Building the Intelligent Response Layer:

[0061] N-Isopropylacrylamide 30.2 (0.267 mol) and acrylic acid 4.8 (0.067 mol) Dissolves in 180 In deionized water, the monomer molar ratio is 80:20, and the total monomer mass fraction is 19.4%. Ammonium persulfate, accounting for 1.0% of the total monomer mass, is added to the solution, and the solution is then purged with... After deoxygenation for 30 min under protective conditions, polymerization was carried out at 70℃ for 5.5 h. After the reaction was complete, 0.8% (w / w) of propyl gallate was added as an antioxidant to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution. Gel permeation chromatography showed that the copolymer's number-average molecular weight was [value missing]. The differential scanning calorimetry method determined the lower critical solution temperature to be 34℃.

[0062] 85 The proppant intermediate obtained in step two was added to the above copolymer solution and impregnated at 32°C for 10 hours, then the temperature was raised to 46°C and the reaction continued for 3.5 hours. After removal, it was vacuum dried at 36°C for 16 hours to obtain a temperature-pH dual-responsive smart fracturing proppant.

[0063] Example 4

[0064] Preparation of temperature-pH dual-responsive smart fracturing proppant:

[0065] Step 1, Kernel layer preparation:

[0066] The ceramsite core was prepared using the same method as in Example 1, but the mass ratio of bauxite, kaolin, and bentonite was adjusted to 55:32:13, the sintering temperature was 1200℃, and the holding time was 2.8 h. Tests showed that the ceramsite core had a compressive strength of 61 MPa and a bulk density of 1.52. .

[0067] Step 2, Enhance the construction of the transition layer:

[0068] The method was basically the same as in Example 1, but the amount of nano silica was adjusted to 18% by mass in the reinforcing transition layer, the amount of silane coupling agent was adjusted to 6% by mass of nano silica, and the crosslinking reaction time was adjusted to 3.5 h.

[0069] Step 3, Building the Intelligent Response Layer:

[0070] N-Isopropylacrylamide 27.6 (0.244 mol) and acrylic acid 7.4 (0.103 mol) Dissolves in 175 In deionized water, the monomer molar ratio is 70:30, and the total monomer mass fraction is 20.0%. Ammonium persulfate (0.5% of the total monomer mass) is added to the solution, and the solution is then purged with... After deoxygenation for 30 min under protective conditions, polymerization was carried out at 66℃ for 6.5 h. After the reaction was complete, 1.2% (w / w) of tert-butylhydroquinone was added as an antioxidant to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution. Gel permeation chromatography showed that the copolymer's number-average molecular weight was [missing value]. The differential scanning calorimetry method determined the lower critical solution temperature to be 29℃.

[0071] 82 The proppant intermediate obtained in step two was added to the above copolymer solution and impregnated at 27°C for 15 hours, then the temperature was raised to 47°C and the reaction continued for 2.8 hours. After removal, it was vacuum dried at 34°C for 22 hours to obtain a temperature-pH dual-responsive smart fracturing proppant.

[0072] Comparative Example 1

[0073] Conventional ceramsite proppant: Ceramsite cores were prepared using the same method as in Example 1, without any surface modification treatment, and were used directly as fracturing proppant.

[0074] Comparative Example 2

[0075] Single-layer polyvinyl alcohol coating support: The ceramic core was prepared using the same method as in Example 1, but only the reinforcing transition layer was constructed in step two, without constructing the smart response layer.

[0076] Comparative Example 3

[0077] Non-nano-reinforced bilayer coating support: The ceramic core was prepared using the same method as in Example 1. In step two, no nano-silica was added, and only a pure polyvinyl alcohol crosslinked network was used as a transition layer. Then, the smart response layer was constructed according to step three of Example 1.

[0078] The proppant prepared in Examples 1-4 and Comparative Examples 1-3 were subjected to systematic performance tests. The test methods were in accordance with the petroleum and natural gas industry standards SY / T 5108-2014 "Performance Test Method of Proppants for Hydraulic Fracturing and Gravel Packing Operations" and SY / T 6302-2009 "Recommended Method for Evaluating the Short-Term Conductivity of Fracturing Proppant Packing Layers". The test results are shown in Tables 1 and 2.

[0079] Table 1 Basic performance parameters of proppant

[0080] Table 2 Propionette Smart Response Performance Parameters

[0081] The data in Tables 1 and 2 show that:

[0082] The temperature-pH dual-responsive smart fracturing proppants prepared in Examples 1-4 exhibited a breakage rate of 6.2%–9.3% under a closure stress of 52 MPa, significantly lower than the 18.5% of the conventional ceramsite proppant in Comparative Example 1, and also superior to the 11.2% of the single-layer coated proppant in Comparative Example 2 and the 13.6% of the double-layer coated proppant without nano-reinforcement in Comparative Example 3. This indicates that the polyvinyl alcohol crosslinking layer reinforced with nano-silica effectively improves the proppant's resistance to breakage and solves the problem of fragments clogging the fractures caused by impact breakage.

[0083] Regarding smart response performance, the proppant in Examples 1-4 forms a thickness of 88-108 at 25°C. The hydration layer (m) imparts excellent suspension properties to the proppant, with a suspension time of 172-208 min, which is 4.1-5.0 times that of the conventional proppant in Example 1. Friction reduction reaches 48%-58%, achieving both clear water sand carrying and drag reduction functions. When the temperature rises to 80℃, the intelligent response layer rapidly shrinks to a thickness of 15-22 m. The gel breaking time was 3.8-4.6 h, and the turbidity of the return liquid decreased to 38-48 NTU, significantly lower than the 165 NTU of Comparative Example 1. Comparative Example 2, although possessing an enhanced transition layer, lacked a smart response layer, resulting in a hydration layer thickness of only 28 m. The suspension performance and drag reduction effect are limited, and the breaking time exceeds 24 hours, which cannot meet the requirements for rapid breaking. Although Comparative Example 3 has a double-layer structure, it lacks nano-reinforcement, and its breakage rate and flow conductivity improvement effect are not as good as those of Examples 1-4.

[0084] Long-term conductivity tests showed that the proppants in Examples 1-4 maintained a conductivity retention rate of 82%–88% after 90 days of long-term conductivity testing under a closure stress of 52 MPa, representing a 38%–51% increase in conductivity compared to Comparative Example 1. This is because the smart response layer desquamated and removed residues at high temperatures, while the enhanced transition layer provided high anti-fracture performance, ensuring the long-term support effect of the proppant.

[0085] Mechanistic analysis shows that the smart responsive behavior of the proppant in this invention originates from the synergistic effect of the thermosensitive phase transition of the poly(N-isopropylacrylamide) segment and the pH-responsive protonation of the acrylic acid segment. Under low temperature and neutral pH conditions, the amide groups of the poly(N-isopropylacrylamide) segment form hydrogen bonds with water molecules and are in a hydrated extended state; the acrylic acid segment deprotonates to form carboxylate ions. This generates electrostatic repulsion; the two effects work synergistically to cause the polymer chain to swell sufficiently. When the temperature rises above the lower critical solution temperature, hydrogen bonding weakens, and the poly-N-isopropylacrylamide segments hydrophobically shrink; simultaneously, the acidic reservoir environment causes the carboxylate group to protonate into carboxylic acid. Electrostatic repulsion disappears; the dual effect leads to rapid collapse of the polymer chains, reducing their thickness and causing them to gradually detach from the surface of the reinforcing transition layer. Fourier transform infrared spectroscopy confirmed the structural changes of the smart response layer under different temperature and pH conditions, differential scanning calorimetry determined the low critical solution temperature range, and dynamic light scattering tests revealed the swelling-shrinkage transition process of the polymer chains.

[0086] The reinforcement mechanism of nano-silica in the reinforcing transition layer mainly includes: the high specific surface area and high surface energy of the nanofiller enable strong interfacial interactions with the polyvinyl alcohol matrix, forming a tight interfacial bond through hydrogen bonding between silanol and polyvinyl alcohol hydroxyl groups and covalent bonding mediated by silane coupling agents; nano-silica, as rigid particles, exerts a toughening effect by preventing crack propagation and stress transfer; and the uniform dispersion of the nanofiller ensures the overall improvement of the mechanical properties of the reinforcing transition layer. Transmission electron microscopy observation shows that nano-silica exhibits a good dispersion state in the polyvinyl alcohol matrix, with a particle spacing of 50-120 nm, forming an effective reinforcing network.

[0087] In summary, this invention has successfully developed a smart fracturing proppant by introducing a three-layer core-shell structure design and a temperature-pH dual response mechanism. This proppant can provide suspension and drag reduction functions during the low-temperature proppant carrying stage, enabling water fracturing. It can also autonomously break down and detach from the shell in high-temperature reservoir environments, reducing reservoir damage. At the same time, it has excellent anti-fracture performance and long-term conductivity, providing a new technical solution for the efficient development of unconventional oil and gas reservoirs.

Claims

1. A temperature-pH dual-responsive smart fracturing proppant, characterized in that, By mass parts, it includes the following components: The core layer consists of 100 parts, composed of high-strength ceramsite or quartz sand. The high-strength ceramsite has a compressive strength of 52-86 MPa and a bulk density of 1.45-1.

72. The particle size is 0.3-0.9 mm; the Mohs hardness of the quartz sand is 7-7.5, and the density is 2.60-2.

66. The particle size is 0.25-0.85 mm; The reinforcing transition layer consists of 8-18 parts, comprising a polyvinyl alcohol crosslinked network reinforced with nano-silica, wherein the nano-silica has a particle size of 8-25 nm and a specific surface area of ​​180-320 nm. The mass fraction of the polyvinyl alcohol in the reinforcing transition layer is 12%–28%; the degree of alcoholysis of the polyvinyl alcohol is 87%–99%, the degree of polymerization is 1700–2400, and the degree of crosslinking is 18%–35%. The smart response layer comprises 3-12 parts, consisting of a poly(N-isopropylacrylamide-co-acrylic acid) copolymer, wherein the molar ratio of N-isopropylacrylamide units to acrylic acid units in the copolymer is (65-82):(18-35), and the number average molecular weight of the copolymer is [missing information]. - The low critical solution temperature is 28-35℃, and the pH response range is 4.5-6.

8.

2. The temperature-pH dual-responsive smart fracturing proppant according to claim 1, characterized in that, In the enhanced transition layer, nano-silica interacts with the hydroxyl groups of polyvinyl alcohol through hydrogen bonds and achieves covalent bonding through a silane coupling agent, wherein the silane coupling agent is selected from... - Glycidyl etheroxypropyltrimethoxysilane, -aminopropyltriethoxysilane or -Methacryloxypropyltrimethoxysilane, used in an amount of 2% to 8% of the mass of nano-silica.

3. The temperature-pH dual-responsive smart fracturing proppant according to claim 1 or 2, characterized in that, The polyvinyl alcohol crosslinking network uses glutaraldehyde or maleic anhydride as a crosslinking agent, with the amount of crosslinking agent being 5% to 15% of the mass of polyvinyl alcohol. The crosslinking reaction is carried out under acidic conditions of pH 2-4, at a reaction temperature of 55-75℃, and for a reaction time of 3-6 hours.

4. The temperature-pH dual-responsive smart fracturing proppant according to any one of claims 1-3, characterized in that, In the intelligent response layer, the poly(N-isopropylacrylamide-co-acrylic acid) copolymer is prepared by free radical polymerization, with ammonium persulfate or azobisisobutyronitrile as the initiator, and the amount used is 0.3% to 1.2% of the total monomer mass. The polymerization reaction is carried out in... Perform at 60-75℃ for 4-8 hours under protection.

5. The temperature-pH dual-responsive smart fracturing proppant according to any one of claims 1-4, characterized in that, The high-strength ceramsite is prepared by mixing bauxite, kaolin and binder in a mass ratio of (50-70):(20-35):(5-15) and then sintering at 1180-1280℃ for 1.5-3 hours. After sintering, the main crystalline phases of the ceramsite are mullite and corundum, with a mullite content of 45% to 68%.

6. The preparation method of the temperature-pH dual-responsive smart fracturing proppant according to any one of claims 1-5, characterized in that, Includes the following steps: Step 1, Core Layer Preparation: Wash high-strength ceramsite or quartz sand sequentially with deionized water and anhydrous ethanol, then dry at 100-120℃ for 2-4 hours to obtain a clean core; for ceramsite cores, pulverize bauxite and kaolin to a particle size of less than 50 mm. m, mixed evenly with binder, granulated with water, dried at 100-110℃ to a moisture content of 2%-5%, then sintered at 1180-1280℃ for 1.5-3h, cooled and sieved to obtain ceramsite cores with a particle size of 0.3-0.9mm; Step 2, Construction of the enhanced transition layer: Disperse nano-silica in anhydrous ethanol, add silane coupling agent, stir and react at 70-85℃ for 2-4 hours, centrifuge, wash and dry to obtain surface-modified nano-silica; dissolve polyvinyl alcohol in hot water at 90-98℃ to prepare an aqueous solution with a mass fraction of 5%-12%, cool to room temperature and add surface-modified nano-silica, ultrasonically disperse for 30-60 minutes; immerse the clean core in the above solution, soak at 30-45℃ for 6-12 hours, remove and pre-dry at 60-70℃; immerse the pre-dried coating core in an acidic solution containing a crosslinking agent, adjust the pH to 2-4, crosslink at 55-75℃ for 3-6 hours, remove, wash and dry at 80-100℃ for 2-4 hours to obtain a support intermediate with an enhanced transition layer; Step 3, Construction of the Smart Response Layer: Dissolve N-isopropylacrylamide and acrylic acid in a molar ratio of (65-82):(18-35) in deionized water, with a total monomer mass fraction of 15%–25%. Add an initiator, and... Polymerize at 60-75℃ for 4-8 hours under protection to obtain a poly(N-isopropylacrylamide-co-acrylic acid) copolymer solution; add the proppant intermediate obtained in step 2 to the above copolymer solution, impregnate and coat at 25-35℃ for 8-16 hours, raise the temperature to 40-50℃ and continue the reaction for 2-4 hours to graft and anchor the copolymer on the surface of the reinforcing transition layer, remove and vacuum dry at 30-40℃ for 12-24 hours to obtain a temperature-pH dual-responsive smart fracturing proppant.

7. The preparation method according to claim 6, characterized in that, In step one, the binder is selected from bentonite, sodium carboxymethyl cellulose or polyacrylamide, and the amount used is 5% to 15% of the dry basis material mass.

8. The preparation method according to claim 6 or 7, characterized in that, In step two, the nano-silica is dispersed using an ultrasonic dispersion device with an ultrasonic power of 400-800W and a frequency of 20-40kHz.

9. The preparation method according to any one of claims 6-8, characterized in that, In step two, the silane coupling agent is selected from... - Glycidyl etheroxypropyltrimethoxysilane, -aminopropyltriethoxysilane or - Methacryloxypropyltrimethoxysilane, used in an amount of 2% to 8% of the mass of nano-silica; the crosslinking agent is selected from glutaraldehyde or maleic anhydride, used in an amount of 5% to 15% of the mass of polyvinyl alcohol.

10. The preparation method according to any one of claims 6-9, characterized in that, In step three, after the polymerization reaction is completed, an antioxidant with a mass fraction of 0.5% to 2% is added to the reaction system. The antioxidant is selected from tert-butylhydroquinone, 2,6-di-tert-butyl-4-methylphenol, or propyl gallate.

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