Reusable clean fracturing fluid system and preparation method thereof

Abstract

The invention discloses a reusable clean fracturing fluid system and a preparation method thereof. When the emulsion is mixed with water, a strong association effect is generated, a high-viscosity weak-elasticity sand-carrying fluid with a net structure is formed, and the proppant is brought into a crack. Under the action of formation temperature, pressure and a gel breaker, the fracturing fluid breaks gel, original macromolecular aggregates cannot be mutually entangled, and a viscoelastic macromolecular structure can be degraded into a micromolecular structure and then flows back to the ground. Therefore, dual protection significance of reservoir protection, environmental protection and the like is realized, and the method has a relatively good application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development in oil exploration and development, and particularly relates to a reusable clean fracturing fluid system and a preparation method thereof. Background Art

[0002] Traditional fracturing fluid systems, such as guar gum, emulsion polymer and other fracturing fluid systems, will cause a certain degree of damage to the fractured reservoir, and the drainage rate is relatively low during the oil and gas blowout test. In addition, the unbroken gelled high-viscosity gel and the large molecular weight polymer with unbroken high molecular chains will also block the oil and gas channels, resulting in a reduction in oil and gas recovery rate, affecting the efficiency of daily oil exploration operations and the process of exploration and development. Summary of the Invention

[0003] The purpose of the present invention is to provide a reusable clean fracturing fluid system and a preparation method thereof. The reusable clean fracturing fluid system is a high drag reduction, high salt resistance, high sand-carrying fracturing fluid system that can be continuously mixed online and recycled. And the clean fracturing fluid system is an environment-friendly water-based fracturing fluid system. When the emulsion is mixed with water, a strong association occurs, forming a high-viscosity and weak-elastic sand-carrying fluid with a network structure, which brings the proppant into the fracture. Under the action of formation temperature, pressure and breaker, the fracturing fluid breaks gel. The original polymer aggregates cannot entangle with each other, and the viscoelastic macromolecular structure will degrade into small molecular structures, and then flow back to the ground, thus realizing double protection of reservoir protection and environmental protection.

[0004] To solve the above technical problems, the technical solution provided by the present invention is as follows:

[0005] In the first aspect, the present invention provides a reusable clean fracturing fluid system, which comprises the following components in parts by weight: 20-50 parts of thickening agent by weight ratio, 3-15 parts of high-efficiency clay stabilizer by weight ratio, 5-20 parts of viscoelastic surfactant by weight ratio, and the rest is white oil and water;

[0006] Wherein, the thickening agent is a thickening agent polymer prepared by inverse emulsion polymerization of a high drag reduction, high salt resistance, high sand-carrying type thickening agent molecule for fracturing fluid, and the thickening agent polymer is an anionic polymer.

[0007] The high drag reduction, high salt resistance, high sand-carrying type thickening agent for fracturing fluid is based on acrylamide and / or AMPS, and the molecular structure is designed by SYBYL / BASE software. One or more heterocyclic structures such as aromatic ring, five-membered heterocyclic ring and six-membered heterocyclic ring are introduced into the structure, and a molecular structure of a high drag reduction, high salt resistance, high sand-carrying type thickening agent for fracturing fluid is designed.

[0008] Further,

[0009] The molecular structure of the high drag reduction, high salt resistance, and high sand-carrying thickening agent for fracturing fluid is a linear molecular structure, with fewer branched chains or no branched-chain structure.

[0010] Furthermore,

[0011] The five-membered heterocycle is one or more of pyrrolidone and thiophene.

[0012] Furthermore,

[0013] The six-membered heterocycle is one or more of pyridine and its derivatives.

[0014] Furthermore,

[0015] The high drag reduction, high salt resistance, and high sand-carrying thickening agent polymer after inverse emulsion polymerization is an anionic polymer, with a molecular weight between 12 million and 18 million.

[0016] Furthermore,

[0017] The polymerization reaction temperature of the high drag reduction, high salt resistance, and high sand-carrying thickening agent polymer after inverse emulsion polymerization is between 50°C and 80°C.

[0018] Furthermore,

[0019] The polymerization reaction time of the high drag reduction, high salt resistance, and high sand-carrying thickening agent polymer after inverse emulsion polymerization is between 3h and 6h.

[0020] Furthermore,

[0021] The high-efficiency clay stabilizer is a small molecule organic quaternary ammonium salt substance, which is formed by the reaction of ethylenediamine and glacial acetic acid, and its molecular weight is not greater than 100.

[0022] Furthermore,

[0023] The viscoelastic surfactant is a Gemini cationic surfactant.

[0024] In a second aspect, the present invention provides a method for preparing the reusable clean fracturing fluid system, including the following steps:

[0025] Using acrylamide and / or AMPS as the matrix, first design the molecular structure through SYBYL / BASE software, and then introduce one or more heterocyclic structures of aromatic rings, five-membered heterocycles, and six-membered heterocycles into the structure to design a molecular structure of a high drag reduction, high salt resistance, and high sand-carrying thickening agent for fracturing fluid;

[0026] The designed thickening agent molecules with high drag reduction, high salt resistance, and high sand-carrying capacity are prepared into an anionic polymer through inverse emulsion polymerization at a certain temperature and time, and are compounded with an efficient clay stabilizer and a viscoelastic surfactant. The proportions are 20 - 50 parts by weight of the thickening agent, 3 - 15 parts by weight of the efficient clay stabilizer, and 5 - 20 parts by weight of the VES, with the rest being white oil and water.

[0027] Among them, when the viscoelastic surfactant (VES) is compounded with the synthesized thickening agent, a highly viscous and weakly elastic sand-carrying fluid with a network structure can be formed, enabling the clean fracturing fluid system to have the properties of high drag reduction and shear resistance.

[0028] The clean fracturing fluid system with high drag reduction, high salt resistance, and high sand-carrying capacity that can be reused only needs to filter out mechanical impurities after fracturing flowback, without the need to add chemical agents for treatment and biological treatment processes. After gel breaking, the fracturing flowback fluid can be reused for formulating fluid again, and its performance is basically fully maintained.

[0029] Based on the above technical solutions, the technical effects that the present invention can achieve are as follows:

[0030] 1. Different from traditional fracturing fluid systems, the clean fracturing fluid uses a viscoelastic bio-based thickening agent as the main base fluid, which can overcome the disadvantages of conventional fracturing fluids causing greater damage to the reservoir and being difficult to drain completely. At the same time, the advantages of this fracturing fluid system are good drag reduction effect, high drainage efficiency, shear resistance, high salt resistance, and good sand-carrying effect, which is suitable for the overall construction trend of future oil and gas well fracturing operations.

[0031] 2. The present invention designs the molecular structure through SYBYL / BASE software, then introduces aromatic rings, aromatic heterocycles and other structures into the structure, designs a molecular structure of a high-temperature and high-salt-resistant thickening agent for fracturing fluid, and prepares a polymer with a special molecular structure through inverse emulsion polymerization at a certain temperature and time. At the same time, it is compounded with an efficient clay stabilizer and a surfactant, and finally a recyclable clean fracturing fluid system with high drag reduction, high salt resistance, and high temperature resistance is obtained. The protection of the reservoir and the environment is achieved during the injection process, and its characteristics are low residue, easy flowback, and efficient reuse. In addition, online continuous mixing, high-efficiency sand-carrying, swelling prevention ability, high salt and temperature resistance during the injection process broaden the application range of this system, and the reuse realizes good economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the specific embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 1 (where the test temperature is 90 °C, 170 reciprocal seconds, shearing test for 1 hour, 1.2% thickening agent + 0.3% crosslinking agent);

[0034] Figure 2 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 1 (where the test temperature is 120 °C, 170 reciprocal seconds, shearing test for 1 hour, 1.5% thickening agent + 0.5% crosslinking agent);

[0035] Figure 3 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 2 (where the temperature is 140 °C, 170 reciprocal seconds, 1.2% thickening agent + 0.5% crosslinking agent);

[0036] Figure 4 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 3 (where the test temperature is 120 °C, 170 reciprocal seconds, shearing test for 1 hour, 1.0% thickening agent + 0.5% crosslinking agent);

[0037] Figure 5 Drag reduction rate change diagram at different displacement rates;

[0038] Figure 6 Experimental condition diagram during the high salt resistance performance test;

[0039] Figure 7 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 3;

[0040] Figure 8 Physical diagram of the effect of static sand carrying performance;

[0041] Figure 9 Physical diagram of the sand effect within 2 hours during the evaluation of static sand carrying performance;

[0042] Figure 10 Physical diagram of the effect of dynamic sand carrying performance;

[0043] Figure 11 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 1 (first mixing of the fracturing fluid flowback fluid);

[0044] Figure 12 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 1 (third mixing of the fracturing fluid flowback fluid);

[0045] Figure 13Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 1 (the fourth formulation of the fracturing fluid flowback fluid);

[0046] Figure 14 Viscosity-temperature change curve of the viscosity reduction experiment for the formulation of the fracturing fluid flowback fluid;

[0047] Figure 15 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 2 (where the test temperature is 150 °C, 170 reciprocal seconds, shearing test for 1 hour, 1.5% thickening agent + 0.3% crosslinking agent);

[0048] Figure 16 Viscosity-temperature change curve of the fracturing fluid system during the shearing process of the product in Example 2 (where the test temperature is 180 °C, 170 reciprocal seconds, shearing test for 1 hour, 1.8% thickening agent + 0.5% crosslinking agent). Detailed implementation manners

[0049] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments.

[0050] Therefore, the following detailed description of the provided embodiments of the present invention is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0051] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified; the reagents, materials, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0052] Example 1:

[0053] The preparation method of the reusable clean fracturing fluid system with high drag reduction, high salt resistance and high temperature resistance of the present invention is:

[0054] The molecular structure was designed by SYBYL / BASE software. A sulfonic acid group was introduced into acrylamide, and then a five-membered heterocyclic structure (pyrrole or pyrrolidone) was introduced into the structure. The reaction time was 4 hours and the reaction temperature was 70 °C to produce an anionic polymer with a molecular weight of 16 million. At the same time, a highly efficient clay stabilizer and a surfactant were compounded. This clean fracturing fluid system is an environmentally friendly water-based fracturing fluid system. The ratio is 40 parts by weight of thickening agent, 5 parts by weight of highly efficient clay stabilizer, 12 parts by weight of VES (viscoelastic surfactant), and the rest is white oil and water. It has good effects when used as a fracturing fluid system for shale oil wells.

[0055] Evaluation test:

[0056] The rheological properties of the thickening agent were tested using simulated brine with a total salinity of 30080 mg / L, Ca 2+ 3500 mg / L, Fe 2+ / 3+ , Mg 2+ 2500 mg / L.

[0057] Refer to Figure 1 , Figure 1 The detection results were obtained using a Haake M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 90 °C, 170 s -1 , Figure 1 is the viscosity-temperature change curve after 1 hour of shear test. The test object was the clean fracturing fluid system prepared in Example 1. The main agent of the system tested was 1.2% thickening agent + 0.3% crosslinking agent. The thickening agent was a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight of 16 million and a concentration of 1.2%. The crosslinking agent was the crosslinking agent used in conjunction with this system. The crosslinking ratio was 100:0.3. From Figure 1 the curve, it can be seen that after 1 hour of shear, the viscosity was 69 mPa·s, and its viscosity was greater than 50 mPa·s, meeting the standards for fracturing fluid performance detection tests. Figure 1 The temperature curve in Figure 1 shows that the test temperature was 90 °C, and

[0058] Refer to Figure 2 , Figure 2 The detection results were obtained using a Haake M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 120 °C, 170 s -1 , Figure 2Viscosity-temperature change curve after 1 hour of shear test. The test object is the clean fracturing fluid system prepared in Example 1. The main agent of the tested system is 1.5% thickening agent + 0.5% crosslinking agent. The thickening agent is a high drag reduction, high salt resistance, high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight of 16 million and a concentration of 1.5%. The crosslinking agent is the crosslinking agent used in conjunction with this system. The crosslinking ratio is 100:0.5. From Figure 2 the curve, it can be seen that after 1 hour of shear, the viscosity is 61 mP.s, and its viscosity is greater than 50 mP.s, meeting the standards for fracturing fluid performance detection tests. Figure 2 In the temperature curve, it shows that the test temperature is 120 °C, Figure 2 In the viscosity curve, it shows the viscosity change of the fracturing fluid system during 1 hour of shear.

[0059] When the test temperature rises, simulating the situation of rising formation temperature, if the concentration of the thickening agent wants to reach the detection standard, it will also increase correspondingly, and the concentration of the crosslinking agent will increase correspondingly with the increase in the concentration of the thickening agent. From 1 and Figure 2 the data, it can be seen that when sheared for 1 h at 90 °C and 120 °C, the viscosity is greater than 50 mP.s, showing good shear stability.

[0060] Example 2:

[0061] The molecular structure was designed by SYBYL / BASE software. Sulfonic acid groups were introduced into acrylamide, and then a five-membered heterocyclic structure (thiophene) was introduced into the structure. The reaction time was 3 hours and the reaction temperature was 80 °C to generate an anionic polymer with a molecular weight of 17 million. At the same time, a highly efficient clay stabilizer and a surfactant were compounded. This clean fracturing fluid system is an environmentally friendly water-based fracturing fluid system. The ratio is 32 parts by weight of the thickening agent, 7 parts by weight of the highly efficient clay stabilizer, 15 parts by weight of VES, and the rest is white oil and water. It has good effects when used as a fracturing fluid system for shale oil wells.

[0062] Refer to Figure 3 , Figure 3 The detection results were obtained using a Haake M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 140 °C, 170 s -1 , Figure 3 is the viscosity-temperature change curve after 1 hour of shear test. The test object is the clean fracturing fluid system prepared in Example 2. The main agent of the tested system is 1.2% thickening agent + 0.5% crosslinking agent. The thickening agent is a high drag reduction, high salt resistance, high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight of 17 million and a concentration of 1.2%. The crosslinking agent is the crosslinking agent used in conjunction with this system. The crosslinking ratio is 100:0.5. From Figure 3As can be seen from the curve, after shearing for 1 hour, the viscosity is 101 mP.s, and its viscosity is greater than 50 mP.s, meeting the standards for the performance detection and testing of fracturing fluids. Figure 3 The temperature curve in Figure 3 shows that the test temperature is 140 °C.

[0063] Example 3:

[0064] The molecular structure was designed through SYBYL / BASE software. A sulfonic acid group was introduced into acrylamide, and then a six-membered heterocyclic structure (pyridine) was introduced into the structure. The reaction time was 6 hours, and the reaction temperature was 60 °C to generate an anionic polymer with a molecular weight of 18 million. At the same time, a highly efficient clay stabilizer and a surfactant were compounded. This clean fracturing fluid system is an environmentally friendly water-based fracturing fluid system. The ratio is 30 parts by weight of thickening agent, 8 parts by weight of highly efficient clay stabilizer, 13 parts by weight of VES, and the rest is white oil and water. Its application as a shale oil fracturing fluid system in Well Guan 13-6-1 of a certain oilfield has shown good results.

[0065] Refer to Figure 4 , Figure 4 The detection results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 120 °C, 170 s -1 , Figure 4 The viscosity-temperature change curve after shearing for 1 hour, with the test object being the clean fracturing fluid system prepared in Example 3. The main agent of the tested system is 1.0% thickening agent + 0.5% crosslinking agent. The thickening agent is a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight of 18 million and a concentration of 1.0%. The crosslinking agent is the crosslinking agent used in conjunction with this system. The crosslinking ratio is 100:0.5. From Figure 4 the curve, it can be seen that after shearing for 1 hour, the viscosity is 115 mP.s, and its viscosity is greater than 50 mP.s, meeting the standards for the performance detection and testing of fracturing fluids. Figure 4 The temperature curve in Figure 4 shows the viscosity change of the fracturing fluid system during the 1-hour shearing process.

[0066] It can be seen from the comparative examples that the clean fracturing fluid systems at different concentrations and different temperatures all meet the requirements of fracturing construction. The higher the test temperature and the higher the concentrations of the thickening agent and crosslinking agent, the requirements of the construction are met. When the concentration of the thickening agent is reduced and the concentration of the crosslinking agent remains unchanged, the viscosity increases after shearing.

[0067] Next, a series of performance tests were conducted on the clean fracturing fluid system provided by the present invention.

[0068] 1. Basic performance:

[0069] 2. Evaluation test of drag reduction performance:

[0070] In the laboratory, a pipe with an inner diameter of 10 mm and a length of 2 m is used. By making the flow velocity in the pipe similar, the drag reduction rate of the oil pipe with an inner diameter of 62 mm commonly used in fracturing construction under different displacement rates is simulated. Tap water at a temperature of 20 °C is used in the experiment. Referring to Figure 5 , the concentration of the thickener in the clean fracturing fluid system prepared by the present invention is 0.10%, and the displacement rate is 3.0 m 3 / min, and the drag reduction rate reaches over 75%. At a relatively low concentration (0.1 - 0.3%), it is a very excellent slickwater system.

[0071] 3. High salt resistance performance:

[0072] Referring to Figure 6 and Figure 7 , a thickener with a concentration of 0.6% is prepared with high salinity water (32085 ppm) on site. After shearing at 100 °C for 120 min, the viscosity is stabilized above 21 mPa·s. The above experiments were evaluated by the Engineering Institute of Dagang Oilfield.

[0073] Specifically, Figure 7 The detection results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 100 °C, 170 s -1 , Figure 7 is the viscosity-temperature change curve after 1 hour of shear test. The test object is the clean fracturing fluid system prepared in Example 3. The main agent of the tested system is 0.6% thickener. The thickener is a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million, and a concentration of 0.6%. From Figure 7 the curve, it can be seen that after shearing for 2 hours, the viscosity is 21 mP.s, and its viscosity is greater than 20 mP.s, meeting the standards for fracturing fluid performance detection tests. Figure 7 The temperature curve in Figure 7 shows that the test temperature is 100 °C, and the viscosity curve in

[0074] 4. Sand-carrying performance (static)

[0075] Referring to Figure 8 and Figure 9, the test object was the clean fracturing fluid system prepared in Example 3, where the thickener concentration in the system was 0.6%. 40% ceramsite with a particle size of 40 - 70 mesh was added to the clean fracturing fluid system, and the suspension dispersion was very good. No precipitation occurred within 2 hours, indicating good proppant-carrying performance.

[0076] Proppant-carrying performance (dynamic):

[0077] Refer to Figure 10 It can be seen that the fracturing fluid with a low-concentration clean fracturing fluid thickener has good proppant-carrying performance, uniform sand placement, and no sand plugging phenomenon.

[0078] 5. Reusability performance evaluation test:

[0079] Refer to Figure 11 , the first formulation of the fracturing fluid flowback fluid: 2.0% thickener + 200000 ppm NaCl + 2000 ppm CaCl 2 . Figure 11 The test results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 100 °C, 170 s -1 , the viscosity-temperature change curve after 1.5 hours of shear testing. The test object was the clean fracturing fluid system prepared in Example 1, where the main agent of the tested system was 2.0% thickener. The thickener was a high drag-reducing, high salt-resistant, high proppant-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million, and a concentration of 2.0%. From Figure 11 The curve shows that after 1.5 hours of shear, the viscosity was 64 mPa·s, and its viscosity was greater than 50 mPa·s, meeting the standards for fracturing fluid performance testing. Figure 11 The temperature curve, showing the test temperature was 100 °C, Figure 11 The viscosity curve, showing the viscosity change of the fracturing fluid system during 1.5 hours of shear.

[0080] Refer to Figure 12 , the third formulation of the fracturing fluid flowback fluid: 2.0% thickener + 200000 ppm NaCl + 2000 ppm CaCl 2 . Figure 12 The test results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific. The test temperature was set at 100 °C, 170 s -1 , the viscosity-temperature change curve after 1.5 hours of shear testing. The test object was the clean fracturing fluid system prepared in Example 1, where the main agent of the tested system was 2.0% thickener. The thickener was a high drag-reducing, high salt-resistant, high proppant-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million, and a concentration of 2.0%. FromFigure 12 As can be seen from the curve, after shearing for 1.5 hours, the viscosity is 63.3 mP.s, and its viscosity is greater than 50 mP.s, meeting the standard of fracturing fluid performance detection test. Figure 12 The temperature curve shows that the test temperature is 100 °C. Figure 12 The viscosity curve shows the viscosity change of the fracturing fluid system during the 1.5-hour shearing process.

[0081] Refer to Figure 13 , the fourth formulation of the fracturing fluid flowback fluid: 2.0% thickening agent + 200000 ppm NaCl + 2000 ppm CaCl 2 . Figure 13 The detection results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific, with the test temperature set at 100 °C and 170 s -1 , the viscosity-temperature change curve after 1.5 hours of shearing test. The test object is the clean fracturing fluid system prepared in Example 1. The main agent of the tested system is 2.0% thickening agent. The thickening agent is a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million and a concentration of 2.0%. From Figure 13 As can be seen from the curve, after shearing for 1.5 hours, the viscosity is 63.8 mP.s, and its viscosity is greater than 50 mP.s, meeting the standard of fracturing fluid performance detection test. Figure 13 The temperature curve shows that the test temperature is 100 °C. Figure 13 The viscosity curve shows the viscosity change of the fracturing fluid system during the 1.5-hour shearing process.

[0082] Refer to Figure 14 , the formulation of the fracturing fluid flowback fluid (viscosity reduction experiment): The clean fracturing fluid system of Example 1 was used for testing, where the concentration of the thickening agent in the clean fracturing fluid system is 1.5%, 1.5% thickening agent + flowback fluid.

[0083] 6. High temperature and high salt resistance performance:

[0084] Refer to Figure 15 , Figure 15 The detection results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific, with the test temperature set at 150 °C and 170 s -1 , Figure 15The viscosity-temperature change curve after 1 hour of shear test, with the test object being the clean fracturing fluid system prepared in Example 2. The main agent of the tested system is 1.5% thickening agent + 0.3% crosslinking agent. The thickening agent is a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million and a concentration of 1.5%. The crosslinking agent is the crosslinking agent used in conjunction with this system. The crosslinking ratio is 100:0.3. From Figure 15 the curve, it can be seen that after 1 hour of shear, the viscosity is 60 mP.s, and its viscosity is greater than 50 mP.s, meeting the standards for fracturing fluid performance detection tests. Figure 15 The temperature curve shows that the test temperature is 150 °C, Figure 15 and the viscosity curve shows the viscosity change of the fracturing fluid system during 1 hour of shear.

[0085] Refer to Figure 16 , Figure 16 The detection results were obtained using a HAAKE M60 high-temperature and high-pressure rheometer from Thermo Fisher Scientific, with the test temperature set at 180 °C and 170 s -1 , Figure 16 The viscosity-temperature change curve after 1 hour of shear test, with the test object being the clean fracturing fluid system prepared in Example 2. The main agent of the tested system is 1.8% thickening agent + 0.5% crosslinking agent. The thickening agent is a high drag reduction, high salt resistance, and high sand-carrying polymer after inverse emulsion polymerization, with a molecular weight between 12 million and 18 million and a concentration of 1.8%. The crosslinking agent is the crosslinking agent used in conjunction with this system. The crosslinking ratio is 100:0.5. From Figure 16 the curve, it can be seen that after 1 hour of shear, the viscosity is 71 mP.s, and its viscosity is greater than 50 mP.s, meeting the standards for fracturing fluid performance detection tests. Figure 16 The temperature curve shows that the test temperature is 180 °C, Figure 16 and the viscosity curve shows the viscosity change of the fracturing fluid system during 1 hour of shear.

[0086] For the high salinity and high temperature conditions of seawater, it is necessary to increase the dosage of the thickening agent to meet the requirements of fracturing fluid performance.

[0087] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A reusable clean fracturing fluid system, characterized in that, it comprises the following components in parts by weight: 20 - 50 parts of thickening agent by weight ratio, 3 - 15 parts of high-efficiency clay stabilizer by weight ratio, 5 - 20 parts of viscoelastic surfactant by weight ratio, and the rest is white oil and water; wherein, the thickening agent is a thickening agent polymer prepared by inverse emulsion polymerization of a high drag reduction, high salt resistance, and high sand-carrying type thickening agent molecule for fracturing fluid, and the thickening agent polymer is an anionic polymer; The high drag reduction, high salt resistance, and high sand-carrying type thickening agent for fracturing fluid is based on acrylamide and / or AMPS, and the molecular structure is designed by SYBYL / BASE software, and one or more heterocyclic structures among aromatic rings, five-membered heterocycles, and six-membered heterocycles are introduced into the structure to design a molecular structure of a high drag reduction, high salt resistance, and high sand-carrying type thickening agent for fracturing fluid.

2. The reusable clean fracturing fluid system according to claim 1, characterized in that, the molecular structure of the high drag reduction, high salt resistance, and high sand-carrying type thickening agent for fracturing fluid is a linear molecular structure, with fewer or no branched chain structures.

3. The reusable clean fracturing fluid system according to claim 1, characterized in that, the five-membered heterocycle is one or more of pyrrolidone and thiophene.

4. The reusable clean fracturing fluid system according to claim 1, characterized in that, the six-membered heterocycle is one or more of pyridine and its derivatives.

5. The reusable clean fracturing fluid system according to claim 1, characterized in that, the high drag reduction, high salt resistance, and high sand-carrying type thickening agent polymer after inverse emulsion polymerization is an anionic polymer, and its molecular weight is between 12 million and 18 million.

6. The reusable clean fracturing fluid system according to claim 1, characterized in that, the polymerization reaction temperature of the high drag reduction, high salt resistance, and high sand-carrying type thickening agent polymer after inverse emulsion polymerization is between 50 - 80 °C.

7. The reusable clean fracturing fluid system according to claim 1, characterized in that, the polymerization reaction time of the high drag reduction, high salt resistance, and high sand-carrying type thickening agent polymer after inverse emulsion polymerization is between 3 - 6 h.

8. The reusable clean fracturing fluid system according to claim 1, characterized in that, the high-efficiency clay stabilizer is a small molecule organic quaternary ammonium salt substance, which is formed by the reaction of ethylenediamine and glacial acetic acid, and its molecular weight is not more than 100.

9. The reusable clean fracturing fluid system according to claim 1, characterized in that, the viscoelastic surfactant is a Gemini cationic surfactant.

10. A preparation method of the reusable clean fracturing fluid system according to any one of claims 1 - 9, characterized in that, it comprises the following steps: Based on acrylamide and / or AMPS as the matrix, first design the molecular structure through SYBYL / BASE software, and then introduce one or more heterocyclic structures such as aromatic rings, five-membered heterocycles, and six-membered heterocycles into the structure to design a molecular structure of a thickening agent for fracturing fluid with high drag reduction, high salt resistance, and high proppant-carrying capacity; The designed thickening agent molecules with high drag reduction, high salt resistance, and high proppant-carrying capacity are prepared into an anionic polymer through inverse emulsion polymerization at a certain temperature and time, and are compounded with an efficient clay stabilizer and a viscoelastic surfactant. The ratio is 20-50 parts by weight of the thickening agent, 3-15 parts by weight of the efficient clay stabilizer, 5-20 parts by weight of VES, and the rest is white oil and water.

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