Polymer for cleaning elastic sand-carrying fracturing fluid and preparation method thereof

Abstract

The invention provides a polymer for cleaning an elastic sand-carrying fracturing fluid and a preparation method of the polymer. The polymer is obtained by polymerizing acrylamide, acrylic acid, dimethyl diallyl ammonium chloride, sodium p-styrenesulfonate and dodecyl allyl ammonium bromide. By adopting an inverse emulsion polymerization technology, the functional monomer for enhancing the elasticity of the polymer is polymerized to macromolecules in a micro-block manner, so that the three-dimensional space network structure of the polymer is enhanced, and the elasticity of the fracturing fluid is enhanced. The dissolving time of the polymer in water is short, on-site continuous mixing integrated construction can be realized, and the process is simple and convenient. By carrying out shearing simulation, viscoelastic simulation and shearing simulation under construction conditions on a fracturing fluid system, the system is always kept in an elastomer structure, the structural viscosity is not easy to damage, and good shearing resistance is achieved; meanwhile, the high viscosity retention rate can still be achieved in a saline solution, on-site flow-back fluid can be recycled, a large amount of fracturing water is saved, and wide application prospects are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of oilfield chemistry, and particularly relates to a polymer for cleaning elastic sand-carrying fracturing fluid and a preparation method thereof. Background Art

[0002] At present, for the exploitation of tight oil and gas, a development method with large displacement and large liquid volume is trending. The purpose is to create more complex fractures, transport proppants to deep formations, and enhance the oil and gas diversion capacity. The sand suspension ability of fracturing fluid directly affects the sand filling method, and thus affects the diversion capacity of the supported fracture. The stronger the sand suspension ability of fracturing fluid, the more conducive it is to the uniform distribution of proppants in the fracture, and it is easier to form a supported fracture with high diversion capacity. If the sand suspension ability of fracturing fluid is insufficient, large particles of sand will settle first and small particles of sand will settle later, resulting in discontinuous settlement of proppants, and thus affecting the effect of fracturing construction.

[0003] The invention patent with the patent number 202310226063.8 and the patent name "A Clean Concentrated Fracturing Fluid Based on Water-Soluble Polymer" publicly disclosed by the State Intellectual Property Office on May 30, 2023. The polymer in this fracturing fluid is anionic modified polyacrylamide; the anionic modified polyacrylamide includes one or any combination of modified polyacrylamide obtained by homopolymerization, modified polyacrylamide obtained by post-hydrolysis, and modified polyacrylamide containing salt-tolerant monomers. The salt tolerance of both this polymer and guar gum is relatively poor, and the viscosity prepared in the flowback fluid is low, which cannot meet the use as a fracturing sand-carrying fluid. In addition, whether the fracturing fluid can break gel determines whether the fracturing fluid can be smoothly flowback and whether the supported fracture is effective. At the same time, the residue generated after the fracturing fluid breaks gel will reduce the diversion capacity of the supported fracture and may also cause blockage of matrix pores, thereby reducing the permeability of the formation. The residue generated by the fracturing fluid mainly comes from water-insoluble impurities and unflowed-back gel-breaking fluid contained in various additives. Therefore, selecting excellent polymers, formulating a scientific fracturing fluid formula, improving the gel-breaking degree, and reducing impurities in additives can effectively reduce the residue amount and reduce the damage to the formation. Summary of the Invention

[0004] The purpose of the present invention is to provide a polymer for cleaning elastic sand-carrying fracturing fluid to overcome the above-mentioned technical problems existing in the prior art.

[0005] Another purpose of the present invention is to provide a preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid. By using the inverse emulsion polymerization technology, the functional monomers enhancing the elasticity of the polymer are polymerized onto the macromolecule in a micro-block manner to enhance the three-dimensional network structure of the polymer, enhance the elasticity of the fracturing fluid, improve the salt tolerance of the system at the same time, and can meet the high sand-carrying performance requirements under different displacements.

[0006] Therefore, the technical solution provided by the present invention is as follows: A polymer for cleaning elastic sand-carrying fracturing fluid has the following general structural formula: Wherein, x:y:z:m:n = (0.45 - 0.65):(0.001 - 0.05):(0.0003 - 0.10):(0.0001 - 0.005):(0.00001 - 0.0006). A preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid includes the following steps: Step 1) Mix emulsified oil, emulsifier and oil-soluble initiator, and stir for 10 - 30 min to obtain an oil phase; Step 2) Prepare an aqueous solution of aqueous-phase reaction monomers, adjust the pH to 6.0 - 6.5, control the temperature ≤ 30 °C, and then add a water-soluble initiator and an oxidizing initiator to obtain an aqueous phase; Step 3) Mix the aqueous phase and the oil phase, stir, and emulsify for 40 - 60 min under the protection of an inert gas, and control the temperature of the reaction system to be 10 - 20 °C to obtain a first solution; Step 4) Add a reducing initiator to the first solution, control the temperature of the reaction system to be 40 - 50 °C, and the reaction time to be 4 - 6 h. After the reaction is completed, keep it warm to obtain a second solution; Step 5) Add a steering agent to the second solution and stir for 5 - 20 min to obtain the product; Based on the total amount of the prepared polymer, the oil phase is 35 - 45 wt%, the aqueous phase is 55 - 60 wt%, and the steering agent is 3 - 5 wt%; the addition amount of the oil-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, the addition amount of the reducing initiator is 0.5 - 1.5 wt% of the total amount of the polymer, the addition amount of the water-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, and the addition amount of the oxidizing initiator is 0.0001 - 0.05 wt% of the total amount of the polymer.

[0007] The emulsified oil is No. 3 white oil or No. 5 white oil.

[0008] The emulsifier is one or more of sorbitan monostearate, sorbitan monooleate, sorbitan trioleate, polyoxyethylene sorbitan monooleate, polyoxyethylene octylphenol ether, or polyethylene glycol 400 dioleate.

[0009] The steering agent is one or more of fatty alcohol polyoxyethylene ether AEO-9, isomeric tridecyl alcohol polyoxyethylene ether 1310, or polyoxyethylene sorbitan monooleate.

[0010] The oil-soluble initiator is azobisisobutyronitrile or azobisisoheptonitrile, and the water-soluble initiator is a cyclic azoamidine initiator VA-044 or V-50.

[0011] The reducing initiator is an aqueous solution of sodium bisulfite, and the oxidizing initiator is potassium persulfate.

[0012] The aqueous-phase reaction monomers are a mixture of acrylamide, acrylic acid, dimethyldiallylammonium chloride, sodium styrene sulfonate, and dodecyl allyl ammonium bromide, with a mass ratio of 32:18:5:2:1.

[0013] In step 1), the mass of the emulsifier is 20 - 30% of the mass of the emulsified oil.

[0014] The beneficial effects of the present invention are as follows: The polymer for cleaning elastic sand-carrying fracturing fluid provided by the present invention prepares a high-molecular emulsion polymer with a network structure by introducing dimethyldiallylammonium chloride with a relatively small molecular weight and the rigid group sodium styrene sulfonate. The purpose of introducing the structural monomer dimethyldiallylammonium chloride is to crosslink two molecular chains during the polymerization process, changing the traditional linear polyacrylamide into network polyacrylamide and increasing the molecular weight of the whole molecule; during the polymerization process, the salt-tolerant functional monomer sodium styrene sulfonate is introduced. This monomer has a rigid group. After being emulsified by the emulsifier, the hydrophobic group of the functional monomer enters the oil phase, and the hydrophilic group is preferably arranged in the water phase, which is conducive to grafting onto the polymer macromolecular chain. At the same time, it increases the hydrodynamic volume of the polymer in the solution, enhances the viscoelasticity, and endows it with excellent salt tolerance, realizing efficient fracturing.

[0015] The present invention adopts the inverse emulsion polymerization technology to polymerize the functional monomers enhancing the elasticity of the polymer onto the macromolecule in a micro-block manner, enhancing the three-dimensional network structure of the polymer, enhancing the elasticity of the fracturing fluid, and at the same time improving the salt tolerance of the system, and it can meet the high sand-carrying performance requirements under different displacement rates.

[0016] The polymer of the present invention has a short dissolution time in water, can realize the on-site continuous mixing and integration construction, and the process is simple. Through the shear, viscoelasticity and shear simulation under construction conditions of the fracturing fluid system, the system always maintains an elastomeric structure, and the structural viscosity is not easily destroyed, having good shear resistance; at the same time, it still has a high viscosity retention rate in the brine solution, and the on-site flowback fluid can be reused, saving a large amount of fracturing water, and having broad application prospects. Description of the Drawings

[0017] Figure 1 It is a scanning electron microscope comparison test chart of the polymer prepared in Example 3 of the present invention and a conventional polymer at a magnification of 1200 times and a size of 50 μm; Figure 2It is the rheological property test chart of the polymer prepared in Example 3 in clear water; Figure 3 It is the rheological property test chart of the polymer prepared in Example 3 at a salinity of 1000 ppm; Figure 4 It is the rheological property test chart of the polymer prepared in Example 3 at a salinity of 5000 ppm; Figure 5 It is the rheological property test chart of the polymer prepared in Example 3 at a salinity of 6000 ppm; Figure 6 It is the rheological property test chart of the polymer prepared in Example 3 at a salinity of 10000 ppm; Figure 7 It is the rheological property test chart of the polymer prepared in Example 3 at a salinity of 15000 ppm; Figure 8 It is the viscoelastic property test chart of the polymer prepared in Example 3 of the present invention; Figure 9 It is the solubility test chart of the polymer prepared in Example 3 of the present invention. Detailed implementation manners

[0018] The following specific embodiments illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0019] Now, refer to the accompanying drawings to introduce the exemplary implementation manners of the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely, and to fully convey the scope of the present invention to those skilled in the art. The terms in the exemplary implementation manners shown in the accompanying drawings are not limitations on the present invention.

[0020] Unless otherwise specified, the terms (including scientific and technical terms) used herein have the ordinary meaning understood by those skilled in the art. In addition, it can be understood that the terms defined in the commonly used dictionary should be understood to have a meaning consistent with the context of their related fields, and should not be understood as idealized or overly formal meanings.

[0021] Example 1 This example provides a polymer for a clean elastic sand-carrying fracturing fluid, and its structural general formula is as follows: Among them, x:y:z:m:n = (0.45 - 0.65):(0.001 - 0.05):(0.0003 - 0.10):(0.0001 - 0.005):(0.00001 - 0.0006). Example 2 This example provides a preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid, including the following steps: Step 1) Mix the emulsified oil, emulsifier and oil-soluble initiator, and stir for 10 - 30 min to obtain an oil phase; Step 2) Prepare an aqueous solution of the aqueous-phase reaction monomer, adjust the pH to 6.0 - 6.5, control the temperature ≤ 30°C, and then add a water-soluble initiator and an oxidizing initiator to obtain an aqueous phase; Step 3) Mix the aqueous phase and the oil phase, stir, and emulsify for 40 - 60 min under the protection of an inert gas, and control the temperature of the reaction system to be 10 - 20°C to obtain a first solution; Step 4) Add a reducing initiator to the first solution, control the temperature of the reaction system to be 40 - 50°C, and the reaction time to be 4 - 6 h. After the reaction is completed, keep it warm to obtain a second solution; Step 5) Add a diverting agent to the second solution and stir for 5 - 20 min to obtain it; Based on the total amount of the prepared polymer, the oil phase is 35 - 45 wt%, the aqueous phase is 55 - 60 wt%, and the diverting agent is 3 - 5 wt%; the addition amount of the oil-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, the addition amount of the reducing initiator is 0.5 - 1.5 wt% of the total amount of the polymer, the addition amount of the water-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, and the addition amount of the oxidizing initiator is 0.0001 - 0.05 wt% of the total amount of the polymer.

[0022] The polymer for cleaning elastic sand-carrying fracturing fluid provided by the present invention adopts an inverse emulsion polymerization process. By introducing dimethyldiallylammonium chloride with a relatively small molecular weight and sodium styrenesulfonate with a rigid group, a high-molecular emulsion polymer with a network structure is prepared. The purpose of introducing the structural monomer dimethyldiallylammonium chloride is to crosslink two molecular chains during the polymerization process, changing the traditional linear polyacrylamide into network polyacrylamide and increasing the molecular weight of the whole molecule; during the polymerization process, the salt-tolerant functional monomer sodium styrenesulfonate is introduced. This monomer has a rigid group. After being emulsified by the emulsifier, the hydrophobic group of the functional monomer enters the oil phase, and the hydrophilic group is preferably arranged in the aqueous phase, which is conducive to grafting onto the polymer macromolecular chain. At the same time, it increases the hydrodynamic volume of the polymer in the solution, enhances the viscoelasticity, and enables it to have excellent salt tolerance performance, realizing efficient fracturing.

[0023] Example 3 On the basis of Example 2, taking the preparation of 1 Kg of polymer as an example, this example provides a method for preparing a polymer for cleaning elastic sand-carrying fracturing fluid, which includes the following steps: Step 1) In jacketed reactor I, add 320 g of emulsified oil, 68 g of emulsifier, and 0.05 g of oil-soluble initiator, start stirring to obtain an oil phase; Step 2) In jacketed reactor II, add 280 g of water, 290 g of aqueous-phase reaction monomers and 48 g of sodium hydroxide, adjust the pH of the solution to 6.0, control the temperature ≤ 30 °C, then add 0.005 g of oxidizing initiator potassium persulfate and 0.05 g of water-soluble initiator VA-044 to obtain an aqueous phase; Step 3) Keep the rotation speed of reactor I at 320 r / min, slowly pour the aqueous phase in jacketed reactor II into oil-phase jacketed reactor I, turn on nitrogen for emulsification and deoxidation for 30 min, and control the system temperature at about 15 °C; Step 4) Use a metering pump to dropwise add 0.5 wt% sodium bisulfite solution for polymerization, control the reaction system temperature at 40 - 50 °C, and dropwise add 0.5 wt% sodium bisulfite solution until the reaction system temperature no longer rises, indicating the completion of the reaction; Step 5) After cooling the solution to room temperature, add the turning agent fatty alcohol polyoxyethylene ether AEO-9 and stir for 10 min to obtain a polymer for cleaning elastic sand-carrying fracturing fluid. The reaction process is as follows: In this example, the emulsified oil is No. 3 white oil; the emulsifier is a mixture of 13 g of sorbitan monostearate Span60, 30 g of sorbitan monooleate Span80 and 25 g of polyoxyethylene sorbitan monooleate Tween80, and the oil-soluble initiator is azobisisobutyronitrile; the aqueous-phase reaction monomers are composed of 160 g of acrylamide, 90 g of acrylic acid, 25 g of dimethyldiallylammonium chloride DMDAAC, 10 g of sodium styrenesulfonate and 5 g of dodecyl allyl ammonium bromide.

[0024] Example 4 On the basis of Example 2, taking the preparation of 1 Kg of polymer as an example, this example provides a method for preparing a polymer for cleaning elastic sand-carrying fracturing fluid, which includes the following steps: Step 1) In jacketed reactor I, add 330 g of emulsified oil, 68 g of emulsifier, and 0.05 g of oil-soluble initiator, start stirring to obtain an oil phase; Step 2) Add 280 g of water, 290 g of aqueous reaction monomers and 48 g of sodium hydroxide into the jacketed reactor II, adjust the pH of the solution to 6.5, control the temperature ≤ 30 °C, and then add 0.004 g of oxidizing initiator potassium persulfate and 0.05 g of water-soluble initiator VA-044 to obtain the aqueous phase; Step 3) Keep the rotation speed of the reactor I at 320 r / min, slowly pour the aqueous phase in the jacketed reactor II into the oil-phase jacketed reactor I, turn on nitrogen for emulsification and deoxidation for 30 min, and control the system temperature at about 15 °C; Step 4) Polymerize by dropwise adding 0.5 wt% sodium bisulfite solution, control the reaction system temperature at 40 - 50 °C, and dropwise add 0.5 wt% sodium bisulfite solution until the temperature of the reaction system no longer rises, indicating the completion of the reaction; Step 5) After the solution is cooled to room temperature, add the steering agent isomeric tridecyl alcohol polyoxyethylene ether 1310 and stir for 10 min to obtain the polymer for cleaning elastic sand-carrying fracturing fluid.

[0025] In this example, the emulsified oil is No. 5 white oil; the emulsifier is a mixture of 13 g of sorbitan monostearate Span60, 30 g of polyoxyethylene octylphenol ether OP-10 and 25 g of polyoxyethylene sorbitan monooleate Tween80, and the oil-soluble initiator is azobisisobutyronitrile; the aqueous reaction monomers are composed of 160 g of acrylamide, 90 g of acrylic acid, 25 g of dimethyldiallylammonium chloride DMDAAC, 10 g of sodium styrenesulfonate and 5 g of dodecyl allyl ammonium bromide.

[0026] Example 5 Based on Example 2, taking the preparation of 1 Kg of polymer as an example, this example provides a preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid, including the following steps: Step 1) In the jacketed reactor I, add 340 g of emulsified oil, 68 g of emulsifier, 0.05 g of oil-soluble initiator, turn on the stirring to obtain the oil phase; Step 2) Add 280 g of water, 290 g of aqueous reaction monomers and 48 g of sodium hydroxide into the jacketed reactor II, adjust the pH of the solution to 6.0, control the temperature ≤ 30 °C, and then add 0.004 g of oxidizing initiator potassium persulfate and 0.05 g of water-soluble initiator VA-044 to obtain the aqueous phase; Step 3) Keep the rotation speed of the reactor I at 320 r / min, slowly pour the aqueous phase in the jacketed reactor II into the oil-phase jacketed reactor I, turn on nitrogen for emulsification and deoxidation for 30 min, and control the system temperature at about 15 °C; Step 4) Polymerization is carried out by dropwise adding a 0.5 wt% sodium bisulfite solution using a metering pump, controlling the temperature of the reaction system at 40 - 50 °C, and dropping the 0.5 wt% sodium bisulfite solution until the temperature of the reaction system no longer rises, indicating the completion of the reaction; Step 5) After the solution is cooled to room temperature, add the turning agent polyoxyethylene sorbitan monooleate Tween 80 and stir for 10 min to obtain the polymer for cleaning elastic sand-carrying fracturing fluid.

[0027] In this example, the emulsified oil is 3# white oil; the emulsifier is a mixture of 13 g sorbitan monostearate Span60, 30 g sorbitan monooleate Span80, and 25 g polyoxyethylene sorbitan monooleate Tween 80, the oil-soluble initiator is azobisisobutyronitrile; the aqueous-phase reaction monomers are composed of 160 g acrylamide, 90 g acrylic acid, 25 g dimethyldiallylammonium chloride DMDAAC, 10 g sodium styrenesulfonate, and 5 g dodecyl allyl ammonium bromide.

[0028] Example 6 Based on Example 2, taking the preparation of 1 Kg of polymer as an example, this example provides a preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid, including the following steps: Step 1) In jacketed reactor I, add 350 g of emulsified oil, 68 g of emulsifier, 0.05 g of oil-soluble initiator, turn on the stirring to obtain the oil phase; Step 2) In jacketed reactor II, add 280 g of water, 290 g of aqueous-phase reaction monomers, and 48 g of sodium hydroxide, adjust the pH of the solution to 6.0, control the temperature ≤ 30 °C, then add 0.005 g of oxidizing initiator potassium persulfate and 0.05 g of water-soluble initiator VA-044 to obtain the aqueous phase; Step 3) Keep the rotation speed of reactor I at 320 r / min, slowly pour the aqueous phase in jacketed reactor II into oil-phase jacketed reactor I, turn on nitrogen for emulsification and deoxidation for 30 min, and control the system temperature at about 15 °C; Step 4) Polymerization is carried out by dropwise adding a 0.5 wt% sodium bisulfite solution using a metering pump, controlling the temperature of the reaction system at 40 - 50 °C, and dropping the 0.5 wt% sodium bisulfite solution until the temperature of the reaction system no longer rises, indicating the completion of the reaction; Step 5) After the solution is cooled to room temperature, add the turning agent fatty alcohol polyoxyethylene ether and stir for 10 min to obtain the polymer for cleaning elastic sand-carrying fracturing fluid.

[0029] In this embodiment, the emulsified oil is 3# white oil; the emulsifier is a mixture of 13 g of sorbitan monostearate Span60, 30 g of sorbitan monooleate Span80, and 25 g of polyoxyethylene sorbitan monooleate Tween80; the oil-soluble initiator is azobisisobutyronitrile; the aqueous-phase reaction monomers are composed of 160 g of acrylamide, 90 g of acrylic acid, 25 g of dimethyldiallylammonium chloride DMDAAC, 10 g of sodium styrenesulfonate, and 5 g of dodecyl allyl ammonium bromide.

[0030] Comparative example: Compared with Example 3, the difference in the preparation process of the polymer is that the aqueous-phase monomer is an acrylic acid copolymer. The obtained polymer is formulated into an aqueous solution with a mass fraction of 0.5 wt%, denoted as Sample 1.

[0031] Performance test: Perform performance tests on the polymers prepared in Example 3 and the comparative example.

[0032] The polymer prepared in Example 3 is formulated into polymer solutions with mass fractions of 0.3 wt%, 0.4 wt%, and 0.5 wt% respectively, to obtain Sample 2, Sample 3, and Sample 4.

[0033] 1. Scanning electron microscope After the 4 samples are frozen with liquid nitrogen, they are then vacuum-dried at room temperature. A small amount of each sample is taken and placed on the sample stage. After sputter-coating the surface with gold for 1 minute, it is placed in a scanning electron microscope to observe the microscopic morphology and structural characteristics of the samples. As Figure 1 shown, compared with the three-dimensional network structure formed by dissolving a conventional linear polymer (comparative example) in deionized water, the higher the concentration of the polymer solution, the tighter and denser the three-dimensional network structure, enhancing the viscoelasticity of the solution and endowing it with excellent salt tolerance, thus achieving efficient fracturing.

[0034] 2. Rheology Use a Hake Mars rotational rheometer to test the rheological properties of the polymer solution. According to the industry standard "SY / T5107-2016 Evaluation Method for the Performance of Water-Based Fracturing Fluids", conduct rheological shear performance tests on the synthesized polymer for clean elastic sand-carrying fracturing fluid at a temperature of 70 °C and a shear rate of 170 s -1 . As Figure 2-7 shown, through this test, it is verified that under the water quality conditions with different salinities, by adjusting the addition amount of the polymer, the performance requirements of the fracturing fluid can be met, solving the problem that the polymer cannot meet the fracturing requirements due to the high salinity of the on-site water quality. Figure 3 In Figure 3 , 1000 ppm refers to the salinity, Figure 4-7 and in Figure 4-7 , the same refers to the salinity.

[0035] 3. Viscoelasticity The viscoelasticity of the polymer was tested using a Hake Mars rotational rheometer. As Figure 8 shown, the polymer exhibited better viscoelasticity in the brine medium. Among them, the viscoelasticity of the polymer in the 20000 mg / L NaCl solution was better than that in the 10000 mg / L NaCl solution. In pure water, since the polymer molecular chains mainly exhibited intermolecular interactions, the solution changed from an elastomer to a viscous body during the stress sweep. Therefore, in the brine medium, G′ > G″ (G′ is the storage modulus and G″ is the loss modulus), and the solution exhibited good elasticity. The intermolecular hydrogen bond force of the polymer was greater than the intramolecular hydrogen bond force, forming a denser and more complex spatial network structure. The structural viscosity of the polymer solution increased, the elasticity became larger, and the value of G′ also increased, indicating that the polymer could still maintain good viscoelastic properties under high salinity conditions, solving the problem that the polymer molecular chains were prone to chain curling when encountering metal salt ions.

[0036] 4. Apparent viscosity Samples 2 - 4 were tested. The dissolution time was recorded with a stopwatch, and the apparent viscosity was measured using an electric six-speed viscometer. As Figure 9 shown, the dissolution time of the elastic high-carrying sand fracturing fluid at different concentrations was less than 30 s, and the time required for the apparent viscosity to reach the final state was 50 s. Through this test, the dissolution performance of the polymer was verified, solving the problems of long dissolution time and low liquid preparation efficiency during on-site mixing of the polymer.

[0037] 5. Viscosity and gel-breaking time of the gel-breaking fluid Fresh water fracturing fluid was prepared. Among them, the polymer was the polymer prepared in Examples 3 - 6 and Comparative Examples, the solvent was laboratory-grade secondary pure water, and their addition amounts were 0.5 wt% and 0.7 wt% respectively. Then, ammonium persulfate was used for gel breaking, and the addition amount of ammonium persulfate was 0.4 wt%. At temperatures of 50 °C, 60 °C, and 70 °C, the viscosity and gel-breaking time of the gel-breaking fluid were recorded, and the test data are shown in Table 1.

[0038] Table 1 Viscosity and gel-breaking time of the fresh water fracturing fluid prepared in Examples 3 - 5 Table 2 Viscosity and gel-breaking time of the fresh water fracturing fluid prepared in Example 6 and Comparative Examples As can be seen from Table 1, within a certain time, the fresh water fracturing fluid prepared with the polymer system of the present invention could be gel-broken, with a viscosity lower than 5 mPa·s, a surface tension less than 28 mN / m, an interfacial tension less than 2 mN / m, and a residue content less than 50 mg / L, meeting the gel-breaking requirements of water-based fracturing fluids.

[0039] The above examples are only illustrative of the present invention and do not constitute a limitation on the scope of protection of the present invention. Any design identical or similar to the present invention falls within the scope of protection of the present invention.

Claims

1. A polymer for cleaning elastic proppant-carrying fracturing fluid, characterized in that: The structural general formula is as follows: wherein, x:y:z:m:n = (0.45 - 0.65):(0.001 - 0.05):(0.0003 - 0.10):(0.0001 - 0.005):(0.00001 - 0.0006).

2. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 1, characterized in that: It includes the following steps: Step 1) Mix the emulsified oil, emulsifier and oil-soluble initiator, and stir for 10 - 30 min to obtain an oil phase; Step 2) Prepare an aqueous solution of the water-phase reaction monomer, adjust the pH to 6.0 - 6.5, control the temperature ≤ 30°C, and then add a water-soluble initiator and an oxidizing initiator to obtain a water phase; Step 3) Mix the water phase and the oil phase, stir, and emulsify for 40 - 60 min under the protection of an inert gas, and control the temperature of the reaction system to be 10 - 20°C to obtain a first solution; Step 4) Add a reducing initiator to the first solution, control the temperature of the reaction system to be 40 - 50°C, and the reaction time to be 4 - 6 h. After the reaction is completed, keep it warm to obtain a second solution; Step 5) Add a steering agent to the second solution and stir for 5 - 20 min to obtain the product; Based on the total amount of the prepared polymer, the oil phase is 35 - 45 wt%, the water phase is 55 - 60 wt%, and the steering agent is 3 - 5 wt%; the addition amount of the oil-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, the addition amount of the reducing initiator is 0.5 - 1.5 wt% of the total amount of the polymer, the addition amount of the water-soluble initiator is 0.01 - 0.05 wt% of the total amount of the polymer, and the addition amount of the oxidizing initiator is 0.0001 - 0.05 wt% of the total amount of the polymer.

3. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 2, characterized in that: The emulsified oil is No. 3 white oil or No. 5 white oil.

4. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 2, characterized in that: The emulsifier is one or more of sorbitan monostearate, sorbitan monooleate, sorbitan trioleate, polyoxyethylene sorbitan monooleate, polyoxyethylene octylphenol ether or polyethylene glycol 400 dioleate.

5. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 2, characterized in that: The steering agent is one or more of fatty alcohol polyoxyethylene ether AEO-9, isomeric tridecyl alcohol polyoxyethylene ether 1310 or polyoxyethylene sorbitan monooleate.

6. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 2, characterized in that: The oil-soluble initiator is azobisisobutyronitrile or azobisisoheptonitrile, and the water-soluble initiator is a cyclic azoamidine initiator VA-044 or V-50.

7. A preparation method of a polymer for cleaning elastic proppant-carrying fracturing fluid according to claim 2, It is characterized in that: The reducing initiator is an aqueous solution of sodium bisulfite, and the oxidizing initiator is potassium persulfate.

8. The preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid according to claim 2, It is characterized in that: The aqueous-phase reaction monomers are a mixture of acrylamide, acrylic acid, dimethyldiallylammonium chloride, sodium styrenesulfonate, and dodecyl allyl ammonium bromide, and the mass ratio is 32:18:5:2:

1.

9. The preparation method of a polymer for cleaning elastic sand-carrying fracturing fluid according to claim 2, It is characterized in that: In step 1), the mass of the emulsifier is 20-30% of the mass of the emulsified oil.

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