Composite fracturing agent for shale oil reservoir and preparation method of composite fracturing agent

Abstract

The invention discloses a composite fracturing agent for a shale oil reservoir and a preparation method of the composite fracturing agent, and belongs to the technical field of oilfield chemistry. The composite fracturing agent for the shale oil reservoir is prepared from the following components in percentage by weight: 0.3 to 0.6 percent of a composite thickening agent, 0.15 to 0.25 percent of a cross-linking agent, 0.08 to 0.12 percent of a gel breaker, 0.03 to 0.06 percent of a bactericide, 0.15 to 0.25 percent of a surfactant and 0.25 to 0.35 percent of a clay stabilizer, and the balance of water. The composite thickening agent comprises a modified polyacrylamide thickening agent and hydroxypropyl guar gum. According to the composite fracturing agent for the shale oil reservoir, a modified polyacrylamide thickening agent and hydroxypropyl guar gum are compounded to obtain a composite thickening agent, the composite thickening agent is matched with a cross-linking agent, a gel breaker, a bactericide, a surfactant, a clay stabilizer and water according to a specific proportion, the multiple components have a synergistic effect, a thickening system provides viscosity support, the cross-linking system enhances elasticity, and the fracturing effect is good. The gel breaking system controls flowback, the surfactant and the clay stabilizer protect the reservoir, and the bactericide guarantees the stability of the system.

Description

Technical Field

[0001] The present application relates to a composite fracturing fluid for shale oil reservoirs and a preparation method thereof, belonging to the technical field of oilfield chemistry. Background Art

[0002] With the continuous increase in global energy demand and the gradual reduction of conventional oil reserves, shale oil, as a representative of unconventional oil and gas resources, has received extensive attention in its development and utilization. Shale oil reservoirs mainly refer to shale rock formations rich in organic matter. During the long geological history process, through complex sedimentation, compaction, and thermal evolution of organic matter and other effects, a certain amount of shale oil is generated and stored inside the shale, and these shale rock formations become the storage carriers of shale oil, that is, shale oil reservoirs. Due to the particularity of geological conditions, shale oil reservoirs usually have characteristics such as low porosity, low permeability, diverse temperature and pressure conditions, and relatively high environmental protection requirements, which make it difficult for traditional extraction technologies to achieve economically effective development.

[0003] In oilfields, the most widely used synthetic polymer thickener at present is polyacrylamide (HPAM), which has excellent water solubility. Compared with natural plant gum fracturing fluid systems, synthetic polymer fracturing fluids have characteristics such as strong thickening ability, good colloid stability, strong sand suspension ability, and no residue after gel breaking, with the least damage to the oil reservoir. However, the viscosity of the HPAM solution decreases with the increase in salinity and temperature, which limits the application of HPAM in high-temperature and high-salinity oil reservoirs. The currently used fracturing fluid systems have environmental pollution risks, and their performance is unstable under harsh environments such as high temperature, high pressure, and high salinity, making it difficult to meet the increasingly strict environmental protection requirements and the needs of efficient development, affecting the fracturing effect and the long-term stability of the reservoir. Therefore, developing a fracturing fluid suitable for shale oil reservoirs is of great significance for improving oil and gas extraction efficiency, reducing costs, and protecting the environment. Summary of the Invention

[0004] In order to solve the above problems, a composite fracturing fluid for shale oil reservoirs and a preparation method thereof are provided. The composite fracturing fluid for shale oil reservoirs obtains a composite thickener by compounding a modified polyacrylamide thickener and hydroxypropyl guar gum, and is combined with a crosslinking agent, a gel breaker, a bactericide, a surfactant, a clay stabilizer, and water in a specific proportion. Through the synergistic action of multiple components, the thickening system provides viscosity support, the crosslinking system enhances elasticity, the gel breaking system controls backflow, the surfactant and the clay stabilizer protect the reservoir, and the bactericide ensures the stability of the system.

[0005] According to one aspect of the present application, a composite fracturing fluid for shale oil reservoirs is provided. By weight percentage, it includes 0.3 - 0.6% of a composite thickener, 0.15 - 0.25% of a crosslinking agent, 0.08 - 0.12% of a breaker, 0.03 - 0.06% of a bactericide, 0.15 - 0.25% of a surfactant, and 0.25 - 0.35% of a clay stabilizer; the balance is water; the composite thickener includes a modified polyacrylamide thickener and hydroxypropyl guar gum.

[0006] Specifically, the composite thickener is prepared by compounding a modified polyacrylamide thickener and hydroxypropyl guar gum. On the one hand, when hydroxypropyl guar gum is compounded with the modified polyacrylamide, the network structure is enhanced through hydrogen bonding and physical entanglement, improving the rheological properties of the fracturing fluid. On the other hand, the modified polyacrylamide provides high-temperature stability, and hydroxypropyl guar gum provides rapid thickening. Hydroxypropyl guar gum provides an initial and relatively fast thickening effect, while the modified polyacrylamide ensures the viscosity of the fracturing fluid over a long time and under complex working conditions with its stable structure. The two can cooperate with each other in terms of thickening performance, enabling the fracturing fluid to maintain an appropriate viscosity level at different stages and in different environments.

[0007] Optionally, the surfactant includes an anionic gemini surfactant and a fluorocarbon surfactant, and the mass ratio of the anionic gemini surfactant to the fluorocarbon surfactant is (2 - 4):1.

[0008] Specifically, the present application uses a compound of an anionic gemini surfactant and a fluorocarbon surfactant. The anionic gemini surfactant has two hydrophobic chains and two hydrophilic groups, which can effectively reduce the interfacial tension between the fracturing fluid and crude oil and prevent clay swelling and crude oil emulsion blockage. The fluorocarbon surfactant has high chemical stability and can adapt to reservoir environments with high temperature, high pressure, and high salinity. The compounding of the two takes into account emulsion stability and penetration ability, improving the flowback rate of the fracturing fluid.

[0009] Optionally, the clay stabilizer is obtained by grafting a polyethylene glycol monomethyl ether chain segment onto polydimethyldiallylammonium chloride. The specific preparation method includes the following steps: S001 Add polydimethyldiallylammonium chloride to deionized water, stir until completely dissolved, and then add polyethylene glycol monomethyl ether. S002 Under continuous stirring and nitrogen protection, add an initiator and carry out a graft polymerization reaction at 50 - 70 °C for 3 - 6 h. S003 After the reaction is completed, carry out alcohol precipitation washing, filtration, and drying and pulverization to obtain the clay stabilizer.

[0010] Specifically, the cationic polydimethyldiallylammonium chloride main chain is grafted with a nonionic polyethylene glycol monomethyl ether side chain. The cationic groups adsorb negatively charged clay particles, inhibiting their hydration swelling. The polyethylene glycol chain segment has multiple hydroxyl groups and ether bonds, enabling effective grafting, providing steric hindrance to prevent the aggregation and migration of clay particles, increasing hydrophilicity and solubility in water, making it easier to disperse uniformly in the fracturing fluid system, and better playing the role of stabilizing clay. The cationic polydimethyldiallylammonium chloride main chain grafted with a nonionic polyethylene glycol monomethyl ether side chain combines cationic charge density and nonionic hydrophilicity, and has high resistance to high temperature and salt.

[0011] Specifically, the initiator is a potassium persulfate-sodium sulfite redox system, and the mass ratio of sodium persulfate to sodium sulfite is 1:1.

[0012] Optionally, the mass of the polyethylene glycol monomethyl ether is 15% - 20% of the mass of the polydimethyldiallylammonium chloride; the mass of the initiator is 0.5% - 2% of the sum of the masses of the polyethylene glycol monomethyl ether and the polydimethyldiallylammonium chloride; the total concentration of the polymer and the monomer in S01 is 10% - 30%.

[0013] Specifically, this application makes specific limitations on the dosages of the polyethylene glycol monomethyl ether, the polydimethyldiallylammonium chloride, and the initiator to obtain a clay stabilizer with an appropriate grafting rate.

[0014] Optionally, the modified polyacrylamide thickener includes the following steps: S01 Add acrylamide, styrene, and 2-vinylpyridine to deionized water in sequence, stir evenly to obtain a monomer solution; add a buffer to the monomer solution, continue stirring, and adjust the pH value to 6 - 8. S02 Under continuous stirring and nitrogen protection conditions, add the initiator, and carry out a polymerization reaction at 60 - 70 °C for 5 - 7 h. S03 After the reaction is completed, cool, precipitate with alcohol, wash, filter, dry, and pulverize to obtain the modified polyacrylamide thickener.

[0015] Specifically, acrylamide is a common monomer with good thickening properties. The amide groups in its molecules can form hydrogen bonds with water molecules, enabling the polymer to exhibit good water solubility and thickening characteristics in aqueous solutions. The introduction of styrene brings a rigid benzene ring structure to the polymer. The presence of the benzene ring increases the rigidity of the molecular chain, making it less likely for the molecular chain to curl, break, etc. under high-temperature and high-pressure environments, thereby improving the thermal stability and mechanical stability of the entire polymer and helping to maintain the viscosity of the fracturing fluid under the complex working conditions of shale oil reservoirs. On this basis, 2-vinylpyridine is further introduced. The nitrogen-containing heterocyclic structure of 2-vinylpyridine has a unique electron cloud distribution and chemical activity. On the one hand, it can form weak interactions such as hydrogen bonds and electrostatic interactions with functional groups (such as amide groups) on other monomer units, enhancing the mutual connection between molecular chains and enabling the molecular chains to form a more regular and compact structure in the solution, further enhancing the ability to resist deformation and degradation under high temperature and pressure. On the other hand, the presence of the nitrogen-containing heterocyclic structure can also improve the compatibility between the polymer and other additives (such as crosslinking agents, breaker agents, etc.) in the composite fracturing fluid, enabling each component to better cooperate and play a role, and avoiding phenomena such as precipitation and delamination that affect the performance of the fracturing fluid due to incompatibility.

[0016] Specifically, the molar ratio of acrylamide to styrene is 8:2; the molar amount of 2-vinylpyridine is 5% - 10% of the total molar amount of acrylamide and styrene; the buffer is a sodium dihydrogen phosphate - disodium hydrogen phosphate buffer system; the initiator is an ammonium persulfate - sodium bisulfite redox system, and the mass ratio of ammonium persulfate to sodium bisulfite is 1:1.5; Optionally, the crosslinking agent is zirconium oxychloride; the breaker agent is ammonium persulfate and modified nano-titanium dioxide; the bactericide is dodecyl dimethyl benzyl ammonium chloride.

[0017] Specifically, Zr 4+ Forms reversible ionic crosslinking points with carboxyl groups and amide groups on the molecular chains of modified polyacrylamide and hydroxypropyl guar gum, constructing a three-dimensional network structure, which can effectively improve the viscosity, elasticity, and sand suspension ability of the fracturing fluid, and is beneficial to the formation and expansion of vertical fractures.

[0018] Specifically, ammonium persulfate and modified nano-titanium dioxide jointly control the gel-breaking speed: maintaining a high viscosity during the construction stage and quickly gel-breaking after the construction is completed to reduce the damage to the reservoir.

[0019] Optionally, the mass ratio of the modified nano-titanium dioxide to ammonium persulfate is 1:(7 - 10); the modified nano-titanium dioxide is prepared by surface modification of nano-titanium dioxide with cetyltrimethylammonium bromide.

[0020] Specifically, the surface of nano-titanium dioxide is modified with cetyltrimethylammonium bromide, which enhances lipophilicity and dispersibility, and also improves the catalytic activity. Nano-titanium dioxide can accelerate the rate of free radical generation from ammonium persulfate at a lower temperature. The synergistic effect of the two makes the gel-breaking reaction faster and more complete, effectively reducing the viscosity after the completion of the fracturing fluid construction, facilitating flowback, and reducing the damage to the shale oil reservoir.

[0021] Specifically, the preparation method of the modified nano-titanium dioxide includes the following steps: Dissolve cetyltrimethylammonium bromide in water to obtain a cetyltrimethylammonium bromide solution with a concentration of 0.05 - 0.1 mol / L; ultrasonically disperse nano-titanium dioxide in deionized water to obtain a nano-titanium dioxide suspension; add the cetyltrimethylammonium bromide solution to the nano-titanium dioxide suspension and react at 40 - 60 °C for 2 - 4 h to obtain the modified nano-titanium dioxide; wherein, the mass of cetyltrimethylammonium bromide is 2% - 5% of the mass of nano-titanium dioxide.

[0022] Specifically, the particle size of the nano-titanium dioxide is 20 - 50 nm.

[0023] Optionally, the mass ratio of the modified polyacrylamide thickener to the hydroxypropyl guar gum is (1 - 3):1.

[0024] Specifically, the present application limits the mass ratio of the modified polyacrylamide thickener to the hydroxypropyl guar gum to achieve a better fracturing effect.

[0025] Optionally, the anionic gemini surfactant includes sodium bis(dodecylphenyl) ether disulfonate, sodium dioctyl sulfosuccinate, or sodium dodecyl diphenyl ether dicarboxylate; the fluorocarbon surfactant includes sodium perfluorooctanoate or sodium perfluorooctanesulfonate.

[0026] According to another aspect of the present application, there is also provided a preparation method of the above composite fracturing fluid for a shale oil reservoir, including the following steps: (1) Weigh each raw material according to the weight percentage, add the composite thickener to water, and stir evenly until completely dissolved; (2) Then sequentially add the crosslinking agent, gel breaker, bactericide, surfactant, and clay stabilizer, and continuously stir until all raw materials are evenly dispersed to obtain the composite fracturing fluid.

[0027] Specifically, the preparation of the composite fracturing fluid is simple and convenient for popularization and utilization.

[0028] The beneficial effects of the present application include but are not limited to: 1. The composite fracturing fluid for shale oil reservoirs according to the present application is obtained by compounding a modified polyacrylamide thickening agent and hydroxypropyl guar gum to obtain a composite thickening agent, and cooperating with a crosslinking agent, a breaker, a bactericide, a surfactant, a clay stabilizer and water in specific proportions. Through the synergistic action of multiple components, the thickening system provides viscosity support, the crosslinking system enhances elasticity, the breaking system controls flowback, the surfactant and the clay stabilizer protect the reservoir, and the bactericide ensures the stability of the system.

[0029] 2. The composite fracturing fluid for shale oil reservoirs according to the present application, the composite thickening agent is obtained by compounding a modified polyacrylamide thickening agent and hydroxypropyl guar gum. On the one hand, hydroxypropyl guar gum is compounded with the modified polyacrylamide, and the network structure is enhanced through hydrogen bonds and physical entanglement to improve the rheological properties of the fracturing fluid. On the other hand, hydroxypropyl guar gum provides an initial and relatively fast thickening effect, while the modified polyacrylamide ensures the viscosity of the fracturing fluid over a long time and under complex working conditions due to its stable structure. The two can cooperate with each other in terms of thickening performance to maintain an appropriate viscosity level of the fracturing fluid at different stages and in different environments.

[0030] 3. The composite fracturing fluid for shale oil reservoirs according to the present application, through specific crosslinking agent, breaker, bactericide, surfactant and clay stabilizer, cooperate with the composite thickening agent to adapt to the application environment of shale oil reservoirs.

[0031] 4. The preparation method of the composite fracturing fluid for shale oil reservoirs according to the present application has a simple preparation method, is easy to operate, and is convenient for popularization and utilization. Specific Embodiments

[0032] The present application will be described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0033] Unless otherwise specified, the raw materials in the embodiments and comparative examples of the present application are all purchased through commercial channels.

[0034] Unless otherwise specified, the methods used in the embodiments and comparative examples of the present application are conventional methods in the prior art. The preparation method of the modified nano-titanium dioxide in the following embodiments and comparative examples includes the following steps: Dissolve cetyltrimethylammonium bromide in water to obtain a cetyltrimethylammonium bromide solution with a concentration of 0.1 mol / L; ultrasonically disperse nano-titanium dioxide in deionized water to obtain a nano-titanium dioxide suspension; add the cetyltrimethylammonium bromide solution to the nano-titanium dioxide suspension, and react at 50 °C for 3 h to obtain modified nano-titanium dioxide; wherein, the mass of cetyltrimethylammonium bromide is 4% of the mass of nano-titanium dioxide; the particle size of nano-titanium dioxide is 40 nm.

[0035] Example 1 A preparation method of a composite fracturing fluid for shale oil reservoirs: (1) Weigh each raw material by weight percentage. Among them, the composite thickening agent is 0.3%, and the composite thickening agent includes a modified polyacrylamide thickening agent and hydroxypropyl guar gum, and the mass ratio of the modified polyacrylamide thickening agent to hydroxypropyl guar gum is 1:1; the crosslinking agent zirconium oxychloride is 0.15%, the breaker is 0.08%, and the breaker includes ammonium persulfate and modified nano-titanium dioxide, and the mass ratio of modified nano-titanium dioxide to ammonium persulfate is 1:7; the bactericide dodecyldimethylbenzylammonium chloride is 0.03%, the surfactant is 0.15%, and the surfactant includes bis(dodecyldiphenyl ether) disulfonate and perfluorooctanoate, and the mass ratio of the two is 2:1; the clay stabilizer is 0.25%, and the balance is water; add the composite thickening agent to water, stir evenly until completely dissolved; (2) Then add the crosslinking agent, breaker, bactericide, surfactant, and clay stabilizer in sequence, and continuously stir until all raw materials are evenly dispersed to obtain a composite fracturing fluid.

[0036] The preparation method of the clay stabilizer includes the following steps: S001 Add polydimethyldiallylammonium chloride to deionized water, stir until completely dissolved, and then add polyethylene glycol monomethyl ether; the mass of polyethylene glycol monomethyl ether is 15% of the mass of polydimethyldiallylammonium chloride; the total concentration of the polymer and the monomer is 10%; S002 Under the conditions of continuous stirring and nitrogen protection, add an initiator, and the mass of the initiator is 0.5% of the sum of the masses of polyethylene glycol monomethyl ether and polydimethyldiallylammonium chloride; the initiator is a potassium persulfate-sodium sulfite redox system, and the mass ratio of sodium persulfate to sodium sulfite is 1:1, and carry out graft polymerization reaction at 50 °C for 3 h; S003 After the reaction is completed, carry out alcohol precipitation washing, filtration, drying and pulverization to obtain the clay stabilizer.

[0037] The preparation method of the modified polyacrylamide thickening agent includes the following steps: S01 Add acrylamide, styrene and 2-vinylpyridine to deionized water in sequence. The molar ratio of acrylamide to styrene is 8:2; the molar amount of 2-vinylpyridine is 5% of the total molar amount of acrylamide and styrene; stir evenly to obtain a monomer solution; add a buffer to the monomer solution, and the buffer is a sodium dihydrogen phosphate-disodium hydrogen phosphate buffer system, and continue to stir to adjust the pH value to 6; S02 Under the conditions of continuous stirring and nitrogen protection, add an initiator. The initiator is an ammonium persulfate-sodium bisulfite redox system, and the mass ratio of ammonium persulfate to sodium bisulfite is 1:1.5, and carry out polymerization reaction at 60 °C for 5 h; S03 After the reaction is completed, cool, then carry out alcohol precipitation, washing, filtration, drying and pulverization to obtain the modified polyacrylamide thickening agent.

[0038] Example 2 Preparation method of a composite fracturing fluid for shale oil reservoirs: (1) Weigh each raw material according to weight percentage. Among them, the composite thickening agent is 0.6%, and the composite thickening agent includes a modified polyacrylamide thickening agent and hydroxypropyl guar gum, and the mass ratio of the modified polyacrylamide thickening agent to hydroxypropyl guar gum is 3:1; the crosslinking agent zirconium oxychloride is 0.25%, the breaker is 0.12%, and the breaker includes ammonium persulfate and modified nano-titanium dioxide, and the mass ratio of modified nano-titanium dioxide to ammonium persulfate is 1:10; the bactericide dodecyl dimethyl benzyl ammonium chloride is 0.06%, the surfactant is 0.25%, and the surfactant includes dioctyl sodium sulfosuccinate and perfluorooctane sulfonate, and the mass ratio of the two is 4:1; the clay stabilizer is 0.35%, and the balance is water; add the composite thickening agent to water, stir evenly and dissolve completely; (2) Then add the crosslinking agent, breaker, bactericide, surfactant, and clay stabilizer in sequence, and continuously stir until all raw materials are evenly dispersed to obtain the composite fracturing fluid.

[0039] The preparation method of the clay stabilizer includes the following steps: S001 Add polydimethyldiallylammonium chloride to deionized water, stir until completely dissolved, and then add polyethylene glycol monomethyl ether; the mass of polyethylene glycol monomethyl ether is 20% of the mass of polydimethyldiallylammonium chloride; the total concentration of the polymer and the monomer is 30%; S002 Under the conditions of continuous stirring and nitrogen protection, add an initiator, and the mass of the initiator is 2% of the sum of the masses of polyethylene glycol monomethyl ether and polydimethyldiallylammonium chloride; the initiator is a potassium persulfate-sodium sulfite redox system, and the mass ratio of sodium persulfate to sodium sulfite is 1:1, and carry out graft polymerization reaction at 70 °C for 6 h; S003 After the reaction is completed, carry out alcohol precipitation washing, filtration, drying and pulverization to obtain the clay stabilizer.

[0040] The preparation method of the modified polyacrylamide thickening agent includes the following steps: S01 Add acrylamide, styrene and 2-vinylpyridine to deionized water in sequence. The molar ratio of acrylamide to styrene is 8:2; the molar amount of 2-vinylpyridine is 10% of the total molar amount of acrylamide and styrene; stir evenly to obtain a monomer solution; add a buffer to the monomer solution, and the buffer is a sodium dihydrogen phosphate-disodium hydrogen phosphate buffer system, and continue to stir to adjust the pH value to 8; S02 Under the conditions of continuous stirring and nitrogen protection, add an initiator. The initiator is an ammonium persulfate-sodium bisulfite redox system, and the mass ratio of ammonium persulfate to sodium bisulfite is 1:1.5, and carry out polymerization reaction at 70 °C for 7 h; After the S03 reaction is completed, after cooling, alcohol precipitation, washing, filtration, drying, and pulverization are carried out to obtain the modified polyacrylamide thickening agent.

[0041] Example 3 A preparation method of a composite fracturing fluid for shale oil reservoirs: (1) Weigh each raw material according to the weight percentage. Among them, the composite thickening agent is 0.5%, and the composite thickening agent includes a modified polyacrylamide thickening agent and hydroxypropyl guar gum, and the mass ratio of the modified polyacrylamide thickening agent to hydroxypropyl guar gum is 2:1; the crosslinking agent zirconium oxychloride is 0.2%, the breaker is 0.1%, and the breaker includes ammonium persulfate and modified nano-titanium dioxide, and the mass ratio of modified nano-titanium dioxide to ammonium persulfate is 1:8; the bactericide dodecyl dimethyl benzyl ammonium chloride is 0.05%, the surfactant is 0.2%, and the surfactant includes sodium dodecyl diphenyl ether dicarboxylate and sodium perfluorooctanoate, and the mass ratio of the two is 3:1; the clay stabilizer is 0.3%, and the balance is water; add the composite thickening agent to water and stir evenly until it is completely dissolved; (2) Then, add the crosslinking agent, breaker, bactericide, surfactant, and clay stabilizer in sequence and continuously stir until all raw materials are evenly dispersed to obtain the composite fracturing fluid.

[0042] The preparation method of the clay stabilizer includes the following steps: S001 Add polydimethyldiallylammonium chloride to deionized water, and after stirring until it is completely dissolved, add polyethylene glycol monomethyl ether; the mass of polyethylene glycol monomethyl ether is 18% of the mass of polydimethyldiallylammonium chloride; the total concentration of the polymer and the monomer is 20%; S002 Under the conditions of continuous stirring and nitrogen protection, add an initiator, and the mass of the initiator is 1% of the sum of the masses of polyethylene glycol monomethyl ether and polydimethyldiallylammonium chloride; the initiator is a potassium persulfate-sodium sulfite redox system, and the mass ratio of sodium persulfate to sodium sulfite is 1:1, and carry out graft polymerization reaction at 60 °C for 5 h; S003 After the reaction is completed, carry out alcohol precipitation washing, filtration, drying and pulverization to obtain the clay stabilizer.

[0043] The preparation method of the modified polyacrylamide thickening agent includes the following steps: S01 Add acrylamide, styrene and 2-vinylpyridine to deionized water in sequence, and the molar ratio of acrylamide to styrene is 8:2; the molar amount of 2-vinylpyridine is 8% of the total molar amount of acrylamide and styrene; stir evenly to obtain a monomer solution; add a buffer to the monomer solution, and the buffer is a sodium dihydrogen phosphate-disodium hydrogen phosphate buffer system, and continue to stir to adjust the pH value to 7; Under continuous stirring and nitrogen protection, an initiator was added. The initiator was an ammonium persulfate-sodium bisulfite redox system, and the mass ratio of ammonium persulfate to sodium bisulfite was 1:1.5. The polymerization reaction was carried out at 65 °C for 6 h; After the reaction ended, it was cooled, then subjected to alcohol precipitation, washing, filtration, drying, and pulverization to obtain the modified polyacrylamide thickener.

[0044] Example 4 The difference between Example 4 and Example 3 is that the surfactant is 0.2%, and the surfactant is sodium dodecyl diphenyl ether dicarboxylate, and the rest are the same.

[0045] Example 5 The difference between Example 5 and Example 3 is that the surfactant is 0.2%, and the surfactant is sodium perfluorooctanoate, and the rest are the same.

[0046] Example 6 The difference between Example 6 and Example 3 is that the mass ratio of sodium dodecyl diphenyl ether dicarboxylate to sodium perfluorooctanoate is 1:1, and the rest are the same.

[0047] Example 7 The difference between Example 7 and Example 3 is that the clay stabilizer is poly(dimethyldiallylammonium chloride) without graft treatment, and the rest are the same.

[0048] Example 8 The difference between Example 8 and Example 3 is that the breaker is 0.1%, and the breaker is ammonium persulfate, and the rest are the same.

[0049] Example 9 The difference between Example 9 and Example 3 is that the mass ratio of modified nano-titanium dioxide to ammonium persulfate is 1:1, and the rest are the same.

[0050] Example 10 The difference between Example 10 and Example 3 is that the nano-titanium dioxide is not modified.

[0051] Example 11 The difference between Example 11 and Example 3 is that the mass ratio of the modified polyacrylamide thickener to hydroxypropyl guar gum is 5:1, and the rest are the same.

[0052] Comparative Example 1 The difference between Comparative Example 1 and Example 3 is that a polyacrylamide thickener was used to replace the modified polyacrylamide thickener, and the rest are the same.

[0053] Comparative Example 2 The difference between Comparative Example 2 and Example 3 is that the composite thickener is 0.5%, and the composite thickener is the modified polyacrylamide thickener.

[0054] Comparative Example 3 The difference between Comparative Example 3 and Example 3 lies in that the composite thickener is 0.5%, and the composite thickener is modified hydroxypropyl guar gum.

[0055] Test Example 1 The composite fracturing agents obtained in Examples 1 to 11 and Comparative Examples 1 to 3 were subjected to temperature resistance performance tests, sand-carrying performance tests, water breaker time tests, and salt tolerance performance tests. The test results are shown in Table 1.

[0056] Temperature resistance performance test: According to the test method specified in SY / T5107-2016 "Evaluation Method for Water-based Fracturing Fluid Performance", the viscosities of the test samples were tested at 70°C, 110°C, and 160°C respectively.

[0057] Sand-carrying performance test: Take the fracturing fluids prepared in Examples 1 to 11 and Comparative Examples 1 to 3, fill a 10 cm stoppered graduated cylinder, put ceramsite or glass beads with a diameter of 0.4 - 0.8 cm into the stoppered graduated cylinder, and measure the time it takes for the ceramsite or glass beads to reach the bottom of the graduated cylinder, which is the sedimentation time.

[0058] Salt tolerance performance test: Take the fracturing fluids prepared in Examples 1 to 11 and Comparative Examples 1 to 3, and test their apparent viscosities at different salinities at 25°C. The test results are shown in Table 2.

[0059] Table 1 Performance test results

[0060] Table 2 Salt tolerance performance test results

[0061] The composite fracturing agent provided by this application has good high-temperature resistance performance, salt tolerance performance, and good sand-carrying performance, is suitable for the shale oil reservoir environment, has good fracturing effect and is beneficial to the long-term stability of the reservoir.

[0062] As mentioned above, only the embodiments of this application are concerned. The protection scope of this application is not limited by these specific embodiments, but is determined by the claims of this application. For those skilled in the art, various changes and modifications can be made to this application. Any modification, equivalent replacement, improvement, etc. made within the technical idea and principle of this application should be included within the protection scope of this application.

Claims

1. A composite fracturing agent for shale oil reservoirs, characterized in that, By weight percentage, it includes 0.3 - 0.6% of a composite thickener, 0.15 - 0.25% of a crosslinking agent, 0.08 - 0.12% of a breaker, 0.03 - 0.06% of a bactericide, 0.15 - 0.25% of a surfactant, and 0.25 - 0.35% of a clay stabilizer; the balance is water; the composite thickener includes a modified polyacrylamide thickener and hydroxypropyl guar gum; The preparation of the modified polyacrylamide thickener includes the following steps: S01 Add acrylamide, styrene, and 2-vinylpyridine to deionized water in sequence, stir evenly to obtain a monomer solution; add a buffer to the monomer solution, continue stirring, and adjust the pH value to 6 - 8; S02 Under continuous stirring and nitrogen protection, add an initiator and carry out a polymerization reaction at 60 - 70 °C for 5 - 7 h; S03 After the reaction ends, cool, carry out alcohol precipitation, washing, filtration, drying, and pulverization to obtain the modified polyacrylamide thickener.

2. The composite fracturing fluid for a shale oil reservoir according to claim 1, wherein The surfactant includes an anionic gemini surfactant and a fluorocarbon surfactant, and the mass ratio of the anionic gemini surfactant to the fluorocarbon surfactant is (2 - 4):

1.

3. The composite fracturing fluid for a shale oil reservoir according to claim 1, wherein The clay stabilizer is obtained by grafting a polyethylene glycol monomethyl ether segment onto poly(dimethyldiallylammonium chloride), and the specific preparation method includes the following steps: S001 Add poly(dimethyldiallylammonium chloride) to deionized water, stir until completely dissolved, and then add polyethylene glycol monomethyl ether; S002 Under continuous stirring and nitrogen protection, add an initiator and carry out a graft polymerization reaction at 50 - 70 °C for 3 - 6 h; S003 After the reaction ends, carry out alcohol precipitation washing, filtration, drying, and pulverization to obtain the clay stabilizer.

4. The composite fracturing fluid for shale oil reservoir according to claim 3, characterized in that The mass of the polyethylene glycol monomethyl ether is 15 - 20% of the mass of poly(dimethyldiallylammonium chloride); the mass of the initiator is 0.5% - 2% of the sum of the masses of polyethylene glycol monomethyl ether and poly(dimethyldiallylammonium chloride); the total concentration of the polymer and monomer in S01 is 10 - 30%.

5. The composite fracturing fluid for a shale oil reservoir according to claim 1, wherein, The crosslinking agent is zirconium oxychloride; the breaker is ammonium persulfate and modified nano-titanium dioxide; the bactericide is dodecyldimethylbenzylammonium chloride.

6. The composite fracturing fluid for shale oil reservoir according to claim 5, wherein The mass ratio of the modified nano-titanium dioxide to ammonium persulfate is 1:(7 - 10); the modified nano-titanium dioxide is prepared by surface modification of nano-titanium dioxide with cetyltrimethylammonium bromide.

7. The composite fracturing fluid for shale oil reservoirs according to claim 1, characterized in that, The mass ratio of the modified polyacrylamide thickener to hydroxypropyl guar gum is (1 - 3):

1.

8. The composite fracturing fluid for shale oil reservoir according to claim 2, characterized in that, The anionic gemini surfactant includes bis(dodecyl diphenyl ether disulfonate), bis(octyl sulfosuccinate), or dodecyl diphenyl ether dicarboxylate; the fluorocarbon surfactant includes sodium perfluorooctanoate or sodium perfluorooctanesulfonate.

9. A preparation method of a composite fracturing fluid for a shale oil reservoir according to any one of claims 1 to 8, characterized in that, It includes the following steps: (1) Weigh each raw material according to the weight percentage, add the composite thickener to water, and stir evenly until completely dissolved; (2) Then add the crosslinking agent, breaker, bactericide, surfactant, and clay stabilizer in sequence, and continuously stir until all raw materials are evenly dispersed to obtain a composite fracturing fluid.

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