A method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method

Abstract

The application is a method for preparing degradable polyacrylamide fracturing fluid by a RAFT polymerization method. The method uses a chain transfer agent, a RAFT agent, when synthesizing a polymer chain. After adding the chain transfer agent, the reaction rate can be effectively reduced to avoid gelation; the control of the molecular weight can be ensured; after adding a primary amine, the C-S bond in the RAFT agent is converted into a mercapto group, which is further oxidized into a dynamic disulfide bond in the air, the molecular weight of the fracturing fluid is doubled, and the drag reduction performance of the fracturing fluid is effectively improved; after adding a reducing agent, the disulfide bond is broken, the molecular weight is rapidly reduced, and after adding an oxidizing agent, the molecular weight can also be rapidly increased, so that the repeated use can be realized. The linear drag reducer obtained by the application can achieve a degradation rate of more than 87%, which has important significance for oil and gas resource exploration and development.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of friction-reducing agent slick water, and particularly relates to a method for preparing degradable polyacrylamide fracturing fluid by a RAFT polymerization method. BACKGROUND

[0002] With continuous exploration and development of oil and gas resources, fracturing, as a main measure for increasing production and injection of oil and gas reservoirs, has been rapidly developed and widely applied. Fracturing reconstruction technology is an offensive measure for development of low-permeability and unconventional oil and gas reservoirs, and has become a main technology for improving single-well production. With continuous deepening of oil and gas exploration and development, the proportion of super-low-permeability reservoirs and tight oil and gas reservoirs is increasing, and the reconstruction difficulty is higher and higher, and many downhole fracturing operations have basically no production, and the scale is gradually increasing. Fracturing fluid is an important component of fracturing technology, and is a key to success or failure of fracturing. There are many types of fracturing technologies, including water-based fracturing, oil-based fracturing, alcohol-based fracturing, foam fracturing, low-temperature fracturing, pulse fracturing and supercritical carbon dioxide fracturing. At present, the technology widely applied in construction operation is water-based fracturing. Slick water is a large category of common water-based fracturing fluid, which is formed by adding friction-reducing agents, fluid loss agents, proppants, surfactants, bactericides, clay stabilizers and the like in clear water, and has a water content of 96% to 99%, and is therefore also called clear water fracturing fluid or friction-reducing water fracturing fluid. Slick water belongs to non-Newtonian fluid, shows viscoelasticity, has a significant friction-reducing effect, can adopt an operation mode of large displacement, large scale and high pump pressure, and can communicate natural fractures of a formation to realize reservoir reconstruction, and has the characteristics of easy flowback, reusability, difficulty in forming a filter cake and small damage to the formation. Among them, the friction-reducing agent, as the most important component in the slick water, is a hot topic in the research of fracturing fluid. In the process of oil exploitation, oil and gas flow shows a turbulent flow phenomenon, which is a very complex internal motion state of fluid. When the fluid in a pipeline flows in a turbulent flow state, due to impacts of different layers of fluid on the inner wall of the pipeline, under the existence of action and reaction forces and near-wall defects, the inner wall of the pipeline and the fluid and different layers of the fluid will form friction, and the internal frictional resistance seriously affects the construction efficiency and increases energy consumption. The use of slick water fracturing fluid in the process of volume fracturing of shale gas greatly reduces the frictional resistance in the process of pump injection and reduces the pressure of the ground pumping equipment.

[0003] Currently, the most widely used sand-carrying fluid for shale oil volume fracturing is variable viscosity slick water or high viscosity slick water. However, after the operation of common polyacrylamide fracturing fluid is completed, it cannot be completely removed or dispersed, which may form a high-concentration liquid area in the formation, affecting the permeability of the formation and the flowability of gas / oil. Some chemicals may cause dissolution, corrosion or other chemical reactions to the rocks or minerals in the formation, affecting the stability and permeability of the formation. Variable viscosity slick water fracturing fluid has low viscosity, poor sand-carrying property, large adsorption damage to the reservoir, and incomplete gel breaking. Currently, the most widely used sand-carrying fluid for shale oil volume fracturing is variable viscosity slick water or high viscosity slick water. When the concentration of slick water is 0.1%, the shale permeability is reduced by 40%. The adsorption damage of polyacrylamide slick water to the reservoir is large. During the construction process, a large number of shale oil wells are blocked by the mixture of polyacrylamide slick water, crude oil and proppant after fracturing and flowback, which increases the difficulty of wellbore treatment. The current shale hydraulic fracturing operation scale is continuously expanding, and the demand for fresh water for construction is increasing. However, the composition of produced water is complex and the salinity is high. The shielding effect of metal cations on the molecules of the drag reduction agent may cause the molecules to curl and precipitate, resulting in a significant reduction or even complete disappearance of the drag reduction performance. The polyacrylamide slick water fracturing fluid has a greater impact on the environment. It is a chemical substance that may have negative effects on groundwater and soil. When using polyacrylamide slick water fracturing fluid, a series of environmental protection measures need to be taken to reduce the harm to the environment. Although the application of existing polyacrylamide (PAM) fracturing fluid in oil and gas exploitation has achieved good results, it also has some significant shortcomings. First, it is prone to degradation under high temperature and high shear force environment, leading to a decrease in viscosity and affecting the fracturing effect. Second, polyacrylamide has poor adaptability to hydrolysis, salt water and high pH environment, which may cause molecular chain breakage or performance degradation. In addition, polyacrylamide has problems in compatibility with some additives, which may affect the overall effect. Although it is considered relatively environmentally friendly, its degradation process in water bodies may have some impact on the environment.

[0004] Traditional polymerization methods often result in fracturing fluids that are difficult to degrade. In the patent (CN 117986488A), a fracturing fluid with high drag reduction rate, good viscosity and salt tolerance is synthesized using polyethylene glycol, acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and methyl allyl hexadecyl ammonium chloride. However, its molecular weight is large, making it difficult to degrade and posing a greater threat to the environment.

[0005] Therefore, a method for preparing degradable and recyclable polyacrylamide fracturing fluid through RAFT polymerization is urgently needed. SUMMARY

[0006] The present application is directed to the problems of fast reaction speed, easy gelation and difficult degradation after use in the synthesis process of the polyacrylamide fracturing fluid in the prior art, and provides a method for preparing a degradable and recyclable polyacrylamide fracturing fluid by a RAFT polymerization method. The method uses a chain transfer agent, a RAFT agent, when synthesizing a polymer chain, and after adding the chain transfer agent, the reaction rate can be effectively reduced to avoid gelation; and the control of the molecular weight can also be ensured. After adding a primary amine, the C-S bond in the RAFT agent is converted into a mercapto group, and finally further oxidized into a dynamic disulfide bond in the air, so that the molecular weight of the fracturing fluid is doubled, and the drag reduction performance of the fracturing fluid is effectively improved. After adding a reducing agent, the disulfide bond is broken, and the molecular weight is rapidly reduced, so that the degradable polyacrylamide fracturing fluid which can be reused is realized; and after adding a small amount of additives such as a fluid loss additive, a proppant, a clay stabilizer, a bactericide and a surfactant, slickwater can be prepared.

[0007] The technical scheme of the present application is as follows:

[0008] A method for preparing a degradable polyacrylamide fracturing fluid by a RAFT polymerization method, the method comprising the following steps:

[0009] S1: preparation of an emulsion, which is one of the following two methods:

[0010] Method one, configuration of a w / w emulsion: dispersant and ammonium sulfate are dissolved in deionized water, and after mixing, a dispersion solution is obtained, then acrylamide, initiator, salt-resistant monomer, crosslinking monomer and RAFT agent are added to the dispersant solution, stirring for 5-10 min, then adding the initiator, and then reacting at 30-50 DEG C for 3-6 h to obtain a polyacrylamide w / w emulsion;

[0011] In the w / w emulsion, the mass percentage concentration of the dispersant is 5-25%, and the mass percentage concentration of ammonium sulfate is 5-30%; in the polyacrylamide w / w emulsion, the mass percentage concentration of acrylamide is 10-30%.

[0012] The mass ratio of acrylamide, salt-resistant monomer, crosslinking monomer and RAFT agent is (60-80):(5-20):(15-30):(0.1-10); the mass of the initiator is 0.01-10% of the mass of the acrylamide monomer.

[0013] The dispersant is polyethylene glycol, polyvinyl alcohol or polyacrylamide-methylpropane sulfonic acid; the molecular weight is 200-20000.

[0014] Or, method two, the preparation of reverse emulsion: dissolve the emulsifier in solvent oil, emulsify under nitrogen atmosphere, stirring, 30-50℃ for 30-50min, to get oil phase; Another acrylamide, anti-salt monomer, crosslinking monomer, RAFT reagent is dissolved in water and then added dropwise to the oil phase, stirring for 5-10min, then add initiator, and then add to the reaction at 30-50℃ for 3-6h, to get polyacrylamide reverse emulsion;

[0015] Among them, the emulsifier accounts for 1-8% of the total mass of the reverse emulsion, the solvent oil accounts for 30-60% of the total mass of the reverse emulsion, and the mass of acrylamide accounts for 10-30% of the total mass of the reverse emulsion;

[0016] The mass ratio of acrylamide, anti-salt monomer, crosslinking monomer and RAFT reagent is (60-80):(5-20):(15-30):(0.1-10);

[0017] The mass of the initiator is 0.01-10% of the mass of the acrylamide monomer.

[0018] The emulsifier is one or more of OP-3, OP-10, ODEA, Tween-80, and Span-80;

[0019] The solvent oil is white oil or solvent oil D110.

[0020] The anti-salt monomer in method one or method two is one or more of 2-acrylamide-2-methylpropane sulfonic acid, N-hydroxymethyl acrylamide, and N,N-dimethyl acrylamide;

[0021] The crosslinking monomer in method one or method two is one or both of acrylic acid or hydroxyethyl methacrylate;

[0022] The RAFT reagent in method one or method two is preferably trithiocarbonic acid bis(carboxymethyl) ester, 4-cyano-4-((phenylthio)thio) pentanoic acid, 1-(dimethylamino)-2-((phenylthio)thiocarbonyl)-propanone, 2-((ethylthio)carbonylthio) propionic acid, or 2-((ethylthio)thiocarbonyl) propionic acid and ethylene glycol synthesized RAFT reagent A;

[0023] The initiator in method one or method two is ammonium persulfate and sodium bisulfite; the stirring speed is 200-1000r / min.

[0024] S2: add primary amine to the emulsion obtained in S1, stir for 30-60min to obtain drag reducer polymer;

[0025] The mass ratio of the primary amine to the RAFT reagent in the emulsion is (5-100):1;

[0026] The primary amine is tetraethylenepentamine or ethylenediamine.

[0027] S3: The polymer obtained in S2 is placed, and air is introduced for 5-10 minutes to obtain a degradable polyacrylamide fracturing fluid;

[0028] The degradable polyacrylamide fracturing fluid further comprises one or more of a fluid loss additive, a clay stabilizer, a bactericide and a surfactant.

[0029] The mass ratio of the fluid loss additive, the clay stabilizer, the bactericide and the surfactant in the degradable polyacrylamide fracturing fluid is 0.1-0.5%, 0.1-0.3%, 0.05-0.1% and 0.1-0.3%, respectively.

[0030] The fluid loss additive is specifically polyvinyl alcohol, calcium chloride, sodium chloride or guar gum.

[0031] The clay stabilizer is specifically acetic acid, aluminum sulfate or cetyltrimethylammonium bromide.

[0032] The bactericide is specifically cetyltrimethylammonium chloride, benzoic acid, p-chlorobenzoic acid, chlorpromazine or benzimidazole.

[0033] The surfactant is specifically polyoxyethylene docosyl ether, alkyl dimethyl benzotriazene, polyoxyethylene lauryl ether or sodium alkyl benzene sulfonate.

[0034] The substantial features of the present application are:

[0035] The present application aims at the technical problem of the polyacrylamide fracturing fluid system, and a linear polymer is synthesized by reacting a RAFT agent, a salt-resistant monomer and acrylamide under the initiation of an initiator ammonium persulfate and sodium bisulfite. The primary amine can act as a nucleophile to attack the thiocyanate to form a mercapto group (-SH). The mercapto group (-SH) can be oxidized to a disulfide bond (-S-S-) in the air. Compared with the polyacrylamide fracturing fluid synthesized by the conventional process, the molecular weight of the polyacrylamide fracturing fluid increases rapidly after the formation of the disulfide bond, which can effectively improve the drag reduction effect of the fracturing fluid. After the addition of a reducing agent, the molecular weight decreases rapidly, and the small molecules can be degraded faster. If the conditions permit, the disulfide bond can be restored by adding an oxidizing agent, so as to realize the recycling of the polyacrylamide fracturing fluid.

[0036] The present application has the following beneficial effects:

[0037] The present application synthesizes polyacrylamide linear polymer by the method of RAFT emulsion polymerization of acrylamide, salt-resistant component and RAFT agent. The presence of the salt-resistant component can make the fracturing fluid have excellent drag reduction performance in high salt environment. The RAFT agent can not only control the reaction rate and prevent explosive polymerization, but also generate mercapto group under the action of primary amine and be oxidized into disulfide bond in air, so that the molecular weight of the fracturing fluid increases rapidly and the drag reduction performance is improved. The fracturing fluid can also be recycled under certain conditions. By adding some additives such as filtrate reducer, proppant, clay stabilizer, bactericide and surfactant, slick water can be prepared. Therefore, the linear drag reducer with easy decomposition and good drag reduction performance synthesized by the method of RAFT water-in-water emulsion polymerization can achieve a degradation rate of more than 87%, which has important significance for the research and development of slick water. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is the infrared spectrum obtained by S3 in Example 1;

[0039] Figure 2 is the nuclear magnetic resonance hydrogen spectrum of S5 in Example 2; DETAILED DESCRIPTION

[0040] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific implementation described herein is only used to explain the present application and does not limit the present application.

[0041] The main function of the polyacrylamide fracturing fluid drag reducer is to act as a drag reducer and carrier to reduce friction in the exploitation of shale oil and carry proppant to fill the cracks in the shale layer, thereby reducing the difficulty of drilling. They have high drag reduction performance, good salt resistance and large viscosity, can reduce the difficulty of drilling and are widely used in the exploitation of shale oil.

[0042] The present application can stably synthesize a polyacrylamide fracturing fluid with good drag reduction performance and reusability by the method of RAFT emulsion polymerization. By this method, the drag reduction performance can be improved and the fracturing fluid can be conveniently used.

[0043]

[0044] The reaction process of the present application is shown in the above formula: the RAFT agent, acrylic acid, acrylamide and 2-acrylamide-2-methylpropanesulfonic acid are synthesized into polyacrylamide linear copolymer under the initiation of the initiator. Then, the primary amine is added. The primary amine can act as a nucleophile to attack these thiocyanate esters to form mercapto group (-SH). The mercapto group (-SH) can be oxidized into disulfide bond (-S-S-) in air. At this time, the molecular weight of the polyacrylamide molecular chain increases, and the polyacrylamide fracturing fluid is obtained.

[0045] The water-soluble RAFT reagent involved in the present application is as follows, but is not limited thereto:

[0046]

[0047] The following example is a method for preparing a degradable polyacrylamide fracturing fluid by a RAFT polymerization method:

[0048] Example 1

[0049] S1: Preparation of an acrylamide w / w emulsion: 100 ml of water was sequentially added with 15 g of ammonium sulfate, 13 g of PEG2000, and stirred at a speed of 300 r / min for 30 min under N2 environment, then sequentially added with 30 g of acrylamide, 2.4 g of 2-acrylamido-2-methylpropanesulfonic acid, 6 g of acrylic acid, and 0.2 g of 4-cyano-4-((phenylthio)thio)pentanoic acid. 0.05 g of ammonium persulfate and 0.05 g of sodium bisulfite initiators were added under N2 environment, and reacted at 35°C for 4 h to obtain a polyacrylamide w / w emulsion.

[0050] S2: Preparation of a friction reducer: 3 g of tetraethylenepentamine was added to the polyacrylamide w / w emulsion obtained in S1, and stirred and reacted for 1 h.

[0051] S3: The friction reducer obtained above was left to stand, and air was introduced for 10 min to obtain a required polyacrylamide fracturing fluid.

[0052] S4: 0.3 g of calcium chloride, 0.2 g of aluminum sulfate, 0.1 g of benzoic acid, and 0.2 g of polyoxyethylene lauryl ether were further added to prepare slick water.

[0053] As shown in Figure 1 , 3440 cm -1 is a stretching vibration peak of -NH2; 3200 cm -1 is a stretching vibration of -NH; 2950 cm -1 is a saturated characteristic absorption peak of -CH2; 2400 cm -1 is a stretching vibration peak of -SH; 1675 cm -1 is a characteristic absorption peak of O=C=NH; 1450 cm -1 is a characteristic absorption peak of -CH; 1320 cm -1 is a characteristic absorption peak of -C-O-; 1200 cm -1 is a characteristic absorption peak of S=O in AMPS; 1040 cm -1 is a characteristic absorption peak of -S-O in AMPS; 540 cm -1 is a characteristic absorption peak of C-S in AMPS. It is proved that S3 synthesizes a polymer chain.

[0054] Example 2

[0055] S1: Preparation of the oil phase: Add 2.2g of OP-3 emulsifier and 1.5g of span-80 emulsifier to 40g of white oil, and emulsify by stirring at 400r / min at 30℃ for 40min.

[0056] S2: Preparation of the aqueous phase: Add 25g acrylamide, 2.4g 2-acrylamide-2-methylpropanesulfonic acid, 4g acrylic acid, and 0.1g 4-cyano-4-((phenylthio)thio)valerate to 40ml of water in sequence;

[0057] S3: Preparation of polyacrylamide reverse emulsion: The aqueous phase obtained in S2 was added dropwise to the oil phase obtained in S1. During this process, the oil phase was stirred at 500 r / min at 30℃. After purging with nitrogen for 10 min, 0.03 g of ammonium persulfate and 0.03 g of sodium bisulfite initiator were added. The reaction was then carried out at 40℃ for 4 h to obtain polyacrylamide reverse emulsion.

[0058] S4: Preparation of drag-reducing agent: Add 1g of tetraethylenepentamine to the acrylamide polymer in S3 and react for 1h.

[0059] S5: Pass the drag-reducing agent into the air for 10 minutes to obtain the required polyacrylamide fracturing fluid.

[0060] S6: Add 0.3g calcium chloride, 0.15g aluminum sulfate, 0.08g benzoic acid and 0.15g polyoxyethylene dodecyl ether to make a slippery liquid.

[0061] like Figure 2 As shown, in the polymer synthesized in S5, the resonance peak at δ = 4.75 ppm is attributed to the solvent D₂O; the resonance peak at δ = 7.35 ppm is attributed to the resonance peak on the benzene ring in the RAFT reagent 4-cyano-4-((phenylthio)thio)pentanoic acid; and the peak at δ = 1.65 ppm originates from the -CH group in AM, AA, and AMPS. Furthermore, the resonance peak at δ = 3.5 ppm originates from the -C(CH₃)₂ group in AMPS. This demonstrates that S5 synthesized the desired polymer chain.

[0062] Example 3

[0063] The other steps are the same as in Example 1, except that 2.4g of 2-acrylamide-2-methylpropanesulfonic acid is replaced with 4.2g of 2-acrylamide-2-methylpropanesulfonic acid.

[0064] Example 4

[0065] The other steps are the same as in Example 1, except that 0.1g of 4-cyano-4-((phenylthio)thio)valerate is replaced with 0.1g of RAFT reagent A;

[0066] Example 5

[0067] Other steps are the same as example 1, the difference is that 15g of ammonium sulfate is changed to 10g of ammonium sulfate;

[0068] Example 6

[0069] Other steps are the same as example 1, the difference is that 15g of ammonium sulfate is changed to 20g of ammonium sulfate;

[0070] Example 7

[0071] Other steps are the same as example 2, the difference is that 2.2g OP-3 emulsifier, 1.5g span-80 emulsifier is changed to 1.8g OP-3 emulsifier, 1.9g span-80 emulsifier;

[0072] Comparative example 1

[0073] A method for preparing a linear drag reducer with good degradability and drag reduction performance by water-in-water emulsion polymerization, comprising the following steps:

[0074] S1: Preparation of acrylamide w / w emulsion: 100ml water is sequentially added with 15g of ammonium sulfate, 13g of PEG2000, and stirred at a speed of 300r / min for 30min under N2 environment, then 30g of acrylamide, 2.4g of 2-acrylamide-2-methylpropane sulfonic acid, and 6g of acrylic acid are sequentially added. 0.05g of ammonium persulfate and 0.05g of sodium bisulfite initiator are added under N2 environment, and the reaction is carried out at 35℃ for 4h to obtain a polyacrylamide w / w emulsion;

[0075] S2: 0.3g of calcium chloride, 0.2g of aluminum sulfate, 0.1g of benzoic acid, and 0.2g of polyoxyethylene lauryl ether are added to the polyacrylamide w / w emulsion to prepare a slippery water.

[0076] Comparative example 2

[0077] A method for preparing a linear drag reducer with good degradability and drag reduction performance by inverse emulsion polymerization, comprising the following steps:

[0078] S1: Preparation of oil phase: 2.2g of OP-3 and 1.5g of span-80 are added to 40g of white oil, and emulsified at 400r / min for 40min at 30℃;

[0079] S2: Preparation of water phase: 40ml of water solution is sequentially added with 13g of acrylamide, 1.2g of 2-acrylamide-2-methylpropane sulfonic acid, and 4g of acrylic acid;

[0080] S3: Preparation of polyacrylamide reverse emulsion: The water phase obtained in S2 was added dropwise into the oil phase obtained in S1, and the oil phase was stirred at 500 r / min at 30℃ during the process. After 10 min of nitrogen blowing, 0.03 g of ammonium persulfate and 0.03 g of sodium bisulfite initiator were added, and then the reaction was carried out at 40℃ for 4 h to obtain the polyacrylamide reverse emulsion.

[0081] S4: 0.3 g of calcium chloride, 0.15 g of aluminum sulfate, 0.08 g of benzoic acid, and 0.15 g of polyoxyethylene lauryl ether were further added to the polyacrylamide reverse emulsion to prepare slick water.

[0082] The RAFT emulsion polymerization of the present application has degradable, low-damage, high salt resistance, and high shear resistance polyacrylamide friction reducer, which is applied in petroleum and chemical industry and can be prepared into slick water. Examples 1, 2, 3, and 4 are the preparation of the RAFT emulsion polymerization of the present application, which has degradable, low-damage, high salt resistance, and high shear resistance branched polyacrylamide friction reducer. Comparative example 1 is the traditional w / w emulsion polymerization to synthesize linear polyacrylamide friction reducer, and comparative example 2 is the traditional reverse emulsion polymerization to synthesize linear polyacrylamide friction reducer.

[0083] In order to verify the properties of the materials obtained in examples 1, 2, 3, and comparative examples 1 and 2, the following performance tests were carried out:

[0084] Molecular weight test: After drying, the sample was dissolved in water, and a gel permeation chromatography (GPC) tester was used for testing, the model being Waters GPC 1515;

[0085] Viscosity test: The polyacrylamide fracturing fluid was dissolved in water at a mass ratio of 1:100, and a MK-03 six-speed rotational viscometer was used for viscosity test;

[0086] Friction reduction rate: A self-made device was used for measurement, and the calculation method was k=(P2-P1) / P1, wherein P1 was the pressure difference when water flowed through the friction reduction device, and P2 was the pressure difference when the fracturing fluid flowed through the friction reduction device.

[0087] Shear resistance test: A HAAKE MARS60 rheometer was used, and the experimental parameters of the HAAKE rheometer were rotor shear rate of 0-180 s -1 .

[0088] Degradation rate test: The polyacrylamide fracturing fluid was placed in a high-temperature environment, the PH was adjusted to be acidic, and the molecular weight was tested after one week of placement, so as to determine the degradation rate of the polyacrylamide according to the change of the molecular weight.

[0089] Performance test

[0090]

[0091] Through the experimental data, it can be analyzed that the performance of various embodiments is better than that of the resistance-reducing agent in any aspect. The application can be directly observed from the molecular weight that the molecular weight is multiplied, the molecular weight is sharply reduced after the reducing agent is added, and the designed fracturing fluid has a relatively obvious improvement in degradation rate, resistance reduction rate and salt tolerance.

[0092] In order to verify the recyclable performance of the polyacrylamide fracturing fluid, a reusability cycle performance test was conducted on Example 4, the reducing agent used was sodium bisulfate with a mass concentration of 3% of the fracturing fluid, and the oxidizing agent was sodium persulfate with a mass concentration of 6% of the fracturing fluid.

[0093] Reusability performance test of Example 4

[0094]

[0095] Through the cycle performance test, it is shown that the synthesized polyacrylamide fracturing fluid can reduce the molecular weight after the reducing agent is added, can reduce the viscosity and is beneficial to the discharge of the fracturing fluid from the well, and the molecular weight of the fracturing fluid is increased after the oxidizing agent is added, so as to maintain the use performance of the fracturing fluid.

[0096] The above only describes several preferred embodiments of the present application, but the present application is not limited to the above several specific ways. The above specific embodiments are illustrative and not restrictive, and researchers in the field can make improvements and refinements under the inspiration of the present application, under the condition of following the spirit and principles of the present application, and all belong to the protection scope of the present application.

[0097] The remaining matters of the present application are known technologies.

Claims

1. A method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method, characterized in that, The method comprises the following steps: S1: preparation of the emulsion, which is one of the following two methods: Method one, preparation of w / w emulsion: dissolve the dispersant and ammonium sulfate in deionized water, mix to obtain a dispersion, then add acrylamide, initiator, salt-resistant monomer, crosslinking monomer and RAFT agent to the dispersant solution, stir for 5-10 min, then add initiator, and react at 30-50℃ for 3-6 h to obtain a polyacrylamide w / w emulsion; In the w / w emulsion, the mass percentage concentration of the dispersant is 5-25%, and the mass percentage concentration of ammonium sulfate is 5-30%; in the polyacrylamide w / w emulsion, the mass percentage concentration of acrylamide is 10-30%; The mass ratio of acrylamide, salt-resistant monomer, crosslinking monomer and RAFT agent is (60-80):(5-20):(15-30):(0.1-10); the mass of initiator is 0.01-10% of the mass of acrylamide monomer; The dispersant is polyethylene glycol, polyvinyl alcohol or poly 2-acrylamide-2-methylpropanesulfonic acid; the molecular weight is 200-20000; Alternatively, method two, reverse emulsion preparation: dissolve the emulsifier in solvent oil, under nitrogen atmosphere, stirring, 30-50 o C emulsification for 30-50 min to obtain the oil phase; dissolve acrylamide, salt-resistant monomer, crosslinking monomer, and RAFT reagent in water, then add them dropwise to the oil phase, stir for 5-10 min, add the initiator, and then react at 30-50°C for 3-6 h to obtain the polyacrylamide reverse emulsion; In the inverse emulsion, the emulsifier accounts for 1-8% of the total mass of the inverse emulsion, the solvent oil accounts for 30-60% of the total mass of the inverse emulsion, and the mass of acrylamide accounts for 10-30% of the total mass of the inverse emulsion; The mass ratio of acrylamide, salt-resistant monomer, crosslinking monomer and RAFT agent is (60-80):(5-20):(15-30):(0.1-10); The salt-resistant monomer in method one or method two is one or more of 2-acrylamide-2-methylpropanesulfonic acid, N-hydroxymethyl acrylamide and N,N-dimethyl acrylamide; The crosslinking monomer in method one or method two is one or both of acrylic acid and hydroxyethyl methacrylate; The RAFT agent in method one or method two is a water-soluble RAFT agent; S2: add primary amine to the emulsion obtained in S1, stir and react for 30-60 min to obtain a drag reducer polymer; The mass ratio of the primary amine to the RAFT agent in the emulsion is (5-100):1; S3: pass the polymer obtained in S2 into air for 5-10 min to obtain a degradable polyacrylamide fracturing fluid.

2. The method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method according to claim 1, characterized in that, The RAFT agent in method one or method two is trithiocarbonic acid bis(carboxymethyl) ester, 4-cyano-4-((phenylthio)thio) pentanoic acid, 1-(dimethylamino)-2-((phenylthio)thiocarbonyl)-propanone, 2-((ethylthio)carbonylthio)propanoic acid or 2-((ethylthio)thiocarbonyl)propanoic acid and ethylene glycol to synthesize a RAFT agent A.

3. The method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method according to claim 1, characterized in that, The emulsifier is one or more of OP-3, OP-10, ODEA, Tween-80 and Span-80; The solvent oil is white oil or solvent oil D110; The initiator is ammonium persulfate and sodium bisulfite, and the mass ratio is 1:1; the stirring speed in step S1 is 200-1000 r / min.

4. The method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method according to claim 1, characterized in that, The primary amine is tetraethylenepentamine or ethylenediamine.

5. The method for preparing degradable polyacrylamide fracturing fluid by RAFT polymerization method according to claim 1, characterized in that, The degradable polyacrylamide fracturing fluid further comprises one or more of a fluid loss additive, a clay stabilizer, a bactericide and a surfactant. The mass ratio of the fluid loss additive, clay stabilizer, bactericide and surfactant in the degradable polyacrylamide fracturing fluid is 0.1-0.5%, 0.1-0.3%, 0.05-0.1% and 0.1-0.3% respectively.

6. The method for preparing biodegradable polyacrylamide fracturing fluid by RAFT polymerization as described in claim 5, characterized in that, The fluid loss additive is specifically polyvinyl alcohol, calcium chloride, sodium chloride or guar gum; The clay stabilizer is specifically acetic acid, aluminum sulfate or cetyltrimethylammonium bromide; The bactericide is specifically cetyltrimethylammonium chloride, benzoic acid, p-chlorobenzoic acid, chlorothixol or benzimidazole; The surfactant is specifically polyoxyethylene docosyl ether, alkyl dimethyl benzotriazene, polyoxyethylene dodecanol ether or sodium alkyl benzene sulfonate.

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