Method for preparing degradable polyacrylamide fracturing fluid by RAFT (reversible addition-fragmentation chain transfer) polymerization method

Abstract

The invention relates to a method for preparing a degradable polyacrylamide fracturing fluid through an RAFT (Reversible Addition-Fragmentation Chain Transfer) polymerization method. According to the method, a chain transfer reagent-RAFT reagent is used during synthesis of a macromolecular chain, and after the chain transfer reagent is added, the reaction rate can be effectively reduced, and gelation is avoided; the control on the molecular weight can be ensured; after the primary amine is added, a C-S bond in the RAFT reagent is converted into sulfydryl which is further oxidized into a dynamic disulfide bond in air, the molecular weight of the fracturing fluid is doubled, and the drag reduction performance of the fracturing fluid is effectively improved; after the reducing agent is added, the disulfide bond is broken, the molecular weight is rapidly reduced, and after the oxidizing agent is added, the molecular weight can be rapidly increased. The obtained linear drag reducer can reach the degradation rate of 87% or above, and has important significance on exploration and development of oil and gas resources.

Description

Technical field:

[0001] The invention relates to the technical field of drag reducer slick water, and in particular to a method for preparing a degradable polyacrylamide fracturing fluid by a RAFT polymerization method. Background technology:

[0002] With the continuous exploration and development of oil and gas resources, fracturing has been rapidly developed and widely used as the main measure to increase the production and injection of oil and gas reservoirs. Fracturing transformation technology is an offensive measure for the development of low-permeability, unconventional and other oil and gas reservoirs, and has become the main technology for increasing the production of single wells. With the continuous deepening of oil and gas exploration and development, the proportion of ultra-low permeability reservoirs and tight oil and gas reservoirs has increased, and the difficulty of transformation has become increasingly high. Many downhole fracturing has basically no production, and the scale has gradually increased. The fracturing fluid is an important part of the fracturing technology and is the key to determining the 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 that is widely used in construction operations is water-based fracturing. Slippery water is a major category of common water-based fracturing fluids. It is formed by adding drag reducers, filtration reducers, proppants, surfactants, bactericides, clay stabilizers, etc. to clean water. The water content is 96%-99%, so it is also called clean water fracturing fluid or drag-reducing water fracturing fluid. Slippery water is a non-Newtonian fluid that exhibits viscoelasticity and has a significant drag reduction effect. It can be operated in a large-volume, large-scale, and high-pressure pumping mode to connect natural fractures in the formation to achieve reservoir transformation. It has the characteristics of easy flowback, reusability, difficulty in forming filter cakes, and little damage to the formation. Among them, drag reducers, as the most important component in slippery water, are currently a hot topic in fracturing fluid research. During the oil extraction process, the flow of oil and gas will show turbulence, which is a very complex internal motion state of the fluid. When the fluid in the pipeline flows in a turbulent state, due to the impact of fluids in different layers on the inner wall of the pipeline, under the existence of action and reaction forces and near-wall defects, friction resistance will be formed between the inner wall of the pipeline and the fluid and between different layers of the fluid. These internal friction resistances seriously affect the construction efficiency and increase energy consumption. The use of slick water fracturing fluid in the shale gas volume fracturing construction process greatly reduces the friction resistance during the pumping process and reduces the pressure on the ground pumping equipment.

[0003] At present, the most widely used sand-carrying fluid for shale oil volume fracturing is variable viscosity slippery water or high viscosity slippery water. However, the common polyacrylamide fracturing fluid cannot be completely removed or dispersed after the operation is completed, and may form a high-concentration liquid area in the formation, affecting the permeability of the formation and the fluidity of gas / oil. Some chemicals may dissolve, corrode or other chemical reactions on rocks or minerals in the formation, affecting the stability and permeability of the formation. Variable viscosity slippery water fracturing fluid has low viscosity, poor sand carrying capacity, great adsorption damage to the reservoir, and incomplete gel breaking. At present, the most widely used sand-carrying fluid for shale oil volume fracturing is variable viscosity slippery water or high viscosity slippery water. When the slippery water concentration is 0.1%, the shale permeability is reduced by 40%. Polyacrylamide slippery water has great adsorption damage to the reservoir. During the construction process, a large number of shale oil wells appeared. When the wells were blown out after pressure, the mixture formed by polyacrylamide slippery water viscosity, crude oil, and proppant blocked the wellbore and oil nozzle, increasing the difficulty of wellbore treatment. At present, the scale of shale water fracturing operations is constantly expanding, and the demand for fresh water for construction is constantly increasing. However, the produced water has complex composition and high mineralization. The drag reducer molecules are easily shielded by metal cations, which will cause curling and precipitation, and the drag reduction performance will be greatly reduced or even completely disappear. Polyacrylamide slick water fracturing fluid has a great impact on the environment. It is a chemical substance that may have a negative impact 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 production has achieved good results, it also has some significant disadvantages. First, it is easily degraded under high temperature and high shear force environment, resulting in a decrease in its viscosity and affecting the fracturing effect. Secondly, polyacrylamide has poor adaptability to hydrolysis, brine and high pH environment, which may cause molecular chain breakage or performance degradation. In addition, polyacrylamide also has problems in compatibility with certain additives, which may affect the overall effect. Although it is considered relatively environmentally friendly, its degradation process in water bodies may have a certain impact on the environment.

[0004] Fracturing fluids synthesized by traditional polymerization methods are often difficult to degrade. In the patent (CN 117986488A), polyethylene glycol, acrylamide, 2-acrylamide-2-methylpropanesulfonic acid and methylallyl hexadecyl ammonium chloride are used to synthesize a fracturing fluid with high drag reduction rate, good natrium and salt resistance, but it has a large molecular weight, is difficult to degrade, and is highly harmful to the environment.

[0005] In view of this, a method for preparing degradable and recyclable polyacrylamide fracturing fluid by RAFT polymerization method is urgently needed to be developed. Summary of the invention:

[0006] The content of the present invention is to provide a method for preparing a degradable and recyclable polyacrylamide fracturing fluid by RAFT polymerization method in view of the problems of fast reaction speed, easy gelation and difficult degradation after use in the synthesis process of polyacrylamide fracturing fluid in the prior art. The method uses a chain transfer agent-RAFT agent when synthesizing the 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 also be guaranteed. After adding the primary amine, the CS bond in the RAFT agent is converted into a thiol group, and finally further oxidized into a dynamic disulfide bond in the air, and the molecular weight of the fracturing fluid is doubled, which effectively improves the drag reduction performance of the fracturing fluid. After adding the reducing agent, the disulfide bond is broken and the molecular weight is rapidly reduced, so that a decomposable polyacrylamide fracturing fluid that can be reused is realized; and a small amount of additives such as a fluid loss reducer, a proppant, a clay stabilizer, a bactericide and a surfactant are added to prepare a slippery water.

[0007] The technical solution of the present invention:

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

[0009] S1: The emulsion is prepared by one of the following two methods:

[0010] Method 1, 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, cross-linking monomer, and RAFT agent to the dispersant solution, stir for 5 to 10 minutes, then add the initiator, and react at 30 to 50° C. for 3 to 6 hours to obtain a polyacrylamide w / w emulsion;

[0011] Wherein, 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, cross-linking 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] Alternatively, method 2, preparation of reverse emulsion: dissolve the emulsifier in solvent oil, emulsify for 30-50 minutes under nitrogen atmosphere and stirring at 30-50°C to obtain an oil phase; separately dissolve acrylamide, salt-resistant monomer, cross-linking monomer, and RAFT agent in water and add dropwise to the oil phase, stir for 5-10 minutes, then add the initiator, and then react at 30-50°C for 3-6 hours to obtain a polyacrylamide reverse emulsion;

[0015] Among them, 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;

[0016] The mass ratio of acrylamide, salt-resistant monomer, cross-linking monomer and RAFT agent 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 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;

[0021] The cross-linking monomer in method 1 or method 2 is one or both of acrylic acid and hydroxyethyl methacrylate;

[0022] The RAFT agent in method 1 or method 2 is preferably a RAFT agent A synthesized from bis(carboxymethyl) trithiocarbonate, 4-cyano-4-((phenylthio)thio)pentanoic acid, 1-(dimethylamino)-2-((phenylthio)thiocarbonyl)-acetone, 2-((ethylthio)carbonylthio)propionic acid or 2-((ethylthio)thiocarbonyl)propionic acid and ethylene glycol;

[0023] The initiators in method 1 or method 2 are ammonium persulfate and sodium bisulfite; the stirring speed is 200-1000r / min.

[0024] S2: adding the primary amine to the emulsion obtained in S1, reacting for 30-60 minutes under stirring to obtain a drag reducer polymer;

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

[0026] The primary amine is tetraethylenepentamine or ethylenediamine.

[0027] S3: The polymer obtained in S2 is allowed to stand 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 reducer, a clay stabilizer, a bactericide and a surfactant;

[0029] The mass proportions of the fluid loss reducer, clay stabilizer, fungicide and surfactant in the degradable polyacrylamide fracturing fluid are 0.1-0.5%, 0.1-0.3%, 0.05-0.1% and 0.1-0.3% respectively;

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

[0031] The clay stabilizer is specifically acetic acid, aluminum sulfate or hexadecyltrimethylammonium bromide;

[0032] The fungicide is specifically hexadecyltrimethylammonium chloride, benzoic acid, p-chlorobenzoic acid, chlorothioxanthene or benzimidazole;

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

[0034] The essential features of the present invention are:

[0035] The present invention aims at the technical problems of the polyacrylamide fracturing fluid system, reacts a RAFT agent, a salt-resistant monomer with acrylamide, and synthesizes a linear polymer under the initiation of initiators ammonium persulfate and sodium bisulfite. Primary amines are added, and primary amines can be used as nucleophiles to attack these thiocyanates, resulting in the formation of sulfhydryl groups (-SH). Sulfhydryl groups (-SH) can be oxidized to disulfide bonds (-SS-) in the air. Compared with the polyacrylamide fracturing fluid synthesized by the traditional process, the molecular weight of this polyacrylamide fracturing fluid increases rapidly after the disulfide bonds are formed, which can effectively improve the drag reduction effect of the fracturing fluid. After adding a reducing agent, the molecular weight decreases rapidly, and small molecules can be degraded faster. If conditions permit, the disulfide bonds can be restored by adding an oxidant, so that the polyacrylamide fracturing fluid can be recycled.

[0036] The beneficial effects of the present invention are:

[0037] The present invention synthesizes a linear polyacrylamide polymer from acrylamide, a salt-resistant component and a RAFT agent through a RAFT emulsion polymerization method. The presence of the salt-resistant component can make the fracturing fluid have excellent drag reduction performance in a high-salt environment. The RAFT agent can not only control the reaction rate and prevent explosion polymerization, but also generate thiol groups under the action of primary amines, which are oxidized to disulfide bonds in the air, and the molecular weight of the fracturing fluid increases rapidly, thereby improving the drag reduction performance. Under certain conditions, it can also be recycled. Some additives such as filtration reducers, proppants, clay stabilizers, bactericides and surfactants are added to prepare slippery water. Therefore, the linear drag reducer that is easy to decompose and has good drag reduction performance through RAFT water-in-water emulsion polymerization can achieve a degradation rate of more than 87%, which is of great significance for the research and development of slippery water. Description of the drawings:

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

[0039] Figure 2 is the hydrogen nuclear magnetic resonance spectrum of S5 in Example 2; Specific implementation method:

[0041] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific implementation described herein is only used to explain the present invention and is not intended to limit the present invention.

[0042] The main function of polyacrylamide fracturing fluid drag reducer is to act as a drag reducer and carrier to reduce friction in shale oil exploitation and carry proppant to fill the gaps in the shale layer, thereby reducing the difficulty of drilling. They have high drag reduction performance, good salt resistance, and high viscosity, which can reduce the difficulty of drilling and are widely used in shale oil exploitation.

[0043] The present invention can stably synthesize a polyacrylamide fracturing fluid with good drag reduction performance and reusability through the RAFT emulsion polymerization method. Through this method, the drag reduction performance can be improved and the fracturing fluid can be conveniently used.

[0044]

[0045] The reaction process of the present invention is shown in the above formula: RAFT agent, acrylic acid, acrylamide, 2-acrylamide-2-methylpropanesulfonic acid are used to synthesize polyacrylamide linear copolymer under the initiation of initiator, and then primary amine is added. The primary amine can be used as a nucleophilic agent to attack these thiocyanates, resulting in the formation of thiol groups (-SH). The thiol groups (-SH) can be oxidized to disulfide bonds (-SS-) in the air, and the molecular weight of the polyacrylamide molecular chain is increased to obtain polyacrylamide fracturing fluid.

[0046] The water-soluble RAFT agents of the present invention are as follows, but not limited thereto:

[0047]

[0048] The following example is a method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization:

[0049] Example 1

[0050] S1: Preparation of acrylamide w / w emulsion: 15g ammonium sulfate and 13g PEG2000 were added to 100ml water in sequence, and stirred at 300r / min for 30min with mechanical stirring under N2 environment, and then 30g acrylamide, 2.4g 2-acrylamide-2-methylpropanesulfonic acid, 6g acrylic acid, and 0.2g 4-cyano-4-((phenylthio)thio)pentanoic acid were added in sequence. 0.05g ammonium persulfate and 0.05g sodium bisulfite initiator were added under N2 environment, and the reaction was carried out at 35°C for 4h to obtain polyacrylamide w / w emulsion.

[0051] S2: Preparation of drag reducer: 3 g of tetraethylenepentamine was added to the polyacrylamide w / w emulsion obtained in S1, and the mixture was stirred for reaction for 1 h.

[0052] S3: The drag reducer obtained above is allowed to stand and air is introduced for 10 minutes to obtain the required polyacrylamide fracturing fluid.

[0053] S4: Add 0.3 g of calcium chloride, 0.2 g of aluminum sulfate, 0.1 g of benzoic acid and 0.2 g of polyoxyethylene behenyl ether to prepare slippery water.

[0054] like Figure 1 As shown, 3440cm -1 is the stretching vibration peak of -NH2; 3200cm -1 is the stretching vibration of -NH; 2950cm -1 It is the saturation characteristic absorption peak of -CH2; 2400cm -1 is the stretching vibration peak of -SH; 1675cm -1 It is the characteristic absorption peak of O=C=NH; 1450cm -1 It is the characteristic absorption peak of -CH; 1320cm -1 is the characteristic absorption peak of -CO-; 1200cm -1 It is the characteristic absorption peak of S=O in AMPS; 1040cm -1 It is the characteristic absorption peak of -SO in AMPS; 540cm -1 It is the characteristic absorption peak of CS in AMPS, which proves that S3 synthesized the polymer chain.

[0055] Example 2

[0056] S1: Preparation of oil phase: 2.2 g OP-3 emulsifier and 1.5 g span-80 emulsifier were added to 40 g white oil and stirred at 400 r / min for 40 min at 30 °C;

[0057] S2: Preparation of aqueous phase: 25 g acrylamide, 2.4 g 2-acrylamide-2-methylpropanesulfonic acid, 4 g acrylic acid, and 0.1 g 4-cyano-4-((phenylthio)thio)pentanoic acid were added to 40 ml water in sequence;

[0058] S3: Preparation of polyacrylamide inverse 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°C. After nitrogen was passed through for 10 min, 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°C for 4 h to obtain the polyacrylamide inverse emulsion.

[0059] S4: Preparation of drag reducer: Add 1 g of tetraethylenepentamine to the acrylamide polymer in S3 and react for 1 hour.

[0060] S5: Pass the drag reducer into air for 10 minutes to obtain the required polyacrylamide fracturing fluid.

[0061] S6: Add 0.3 g of calcium chloride, 0.15 g of aluminum sulfate, 0.08 g of benzoic acid and 0.15 g of polyoxyethylene behenyl ether to prepare slippery water.

[0062] like Figure 2 As shown, in the polymer synthesized by 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 agent 4-cyano-4-((phenylthio)thio)pentanoic acid; the peak at δ = 1.65 ppm comes from -CH in AM, AA and AMPS. In addition, the resonance peak at δ = 3.5 ppm comes from -C(CH3)2 in AMPS. It proves that S5 synthesized the desired polymer chain.

[0063] Example 3

[0064] The other steps are the same as those in Example 1, except that 2.4 g of 2-acrylamide-2-methylpropanesulfonic acid is replaced by 4.2 g of 2-acrylamide-2-methylpropanesulfonic acid.

[0065] Example 4

[0066] The other steps are the same as those in Example 1, except that 0.1 g of 4-cyano-4-((phenylthio)thio)pentanoic acid is replaced with 0.1 g of RAFT agent A;

[0067] Example 5

[0068] The other steps are the same as those in Example 1, except that 15 g of ammonium sulfate is replaced with 10 g of ammonium sulfate;

[0069] Example 6

[0070] The other steps are the same as those in Example 1, except that 15 g of ammonium sulfate is replaced with 20 g of ammonium sulfate;

[0071] Example 7

[0072] The other steps are the same as those in Example 2, except that 2.2 g OP-3 emulsifier and 1.5 g span-80 emulsifier are replaced with 1.8 g OP-3 emulsifier and 1.9 g span-80 emulsifier;

[0073] Comparative Example 1

[0074] A method for preparing a linear drag reducer with good drag reduction performance by water-in-water emulsion polymerization, comprising the following steps:

[0075] S1: Preparation of acrylamide w / w emulsion: 15g ammonium sulfate and 13g PEG2000 were added to 100ml water in sequence, and stirred at 300r / min for 30min with mechanical stirring under N2 environment, and then 30g acrylamide, 2.4g 2-acrylamide-2-methylpropanesulfonic acid and 6g acrylic acid were added in sequence. 0.05g ammonium persulfate and 0.05g sodium bisulfite initiator were added under N2 environment, and reacted at 35°C for 4h to obtain polyacrylamide w / w emulsion;

[0076] S2: 0.3 g calcium chloride, 0.2 g aluminum sulfate, 0.1 g benzoic acid and 0.2 g polyoxyethylene behenyl ether were added to the polyacrylamide w / w emulsion to prepare slippery water.

[0077] Comparative Example 2

[0078] A method for inverse emulsion polymerization of a linear drag reducer that is easily degradable and has good drag reduction performance comprises the following steps:

[0079] S1: Preparation of oil phase: 2.2 g OP-3 and 1.5 g span-80 were added to 40 g white oil and emulsified at 30 °C and 400 r / min for 40 min;

[0080] S2: Preparation of aqueous phase: 13 g acrylamide, 1.2 g 2-acrylamide-2-methylpropanesulfonic acid and 4 g acrylic acid were added to 40 ml aqueous solution in sequence;

[0081] S3: Preparation of polyacrylamide inverse 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°C. After nitrogen was passed through for 10 min, 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°C for 4 h to obtain the polyacrylamide inverse emulsion.

[0082] S4: Add 0.3 g of calcium chloride, 0.15 g of aluminum sulfate, 0.08 g of benzoic acid and 0.15 g of polyoxyethylene behenyl ether to the polyacrylamide reverse emulsion to prepare slippery water.

[0083] The RAFT emulsion polymerization of the present invention has a degradable, low-damage, high salt resistance and high shear resistance polyacrylamide drag reducer that is used in petrochemical industry and can be formulated into slippery water. Examples 1, 2, 3 and 4 are the preparations of the RAFT emulsion polymerization of the present invention has a degradable, low-damage, high salt resistance and high shear resistance branched polyacrylamide drag reducer, comparative example 1 is a traditional w / w emulsion polymerization synthesis of linear polyacrylamide drag reducer, and comparative example 2 is a traditional reverse emulsion polymerization synthesis of linear polyacrylamide drag reducer.

[0084] In order to verify the properties of the materials obtained in Examples 1, 2, 3 and Comparative Examples 1 and 2, the following phase performance tests were performed:

[0085] Molecular weight test: the sample was dried and dissolved in water, and tested using a gel permeation chromatography (GPC) tester, model Waters GPC 1515;

[0086] Viscosity test: Dissolve the polyacrylamide fracturing fluid in water at a mass ratio of 1:100, and use a MK-03 six-speed rotary viscometer to perform the viscosity test;

[0087] Drag reduction rate: measured using laboratory-made equipment, calculated as k = (P2-P1) / P1, where P1 is the pressure difference when clean water flows through the drag reduction device, and P2 is the pressure difference when the fracturing fluid flows through the drag reduction device.

[0088] Shear resistance test: HAAKE MARS60 rheometer is used, and the experimental parameters of the HAAKE rheometer are rotor shear rate of 0-180s -1 .

[0089] Degradation rate test: Place the polyacrylamide fracturing fluid in a high temperature environment, adjust the pH to acidic, and test its molecular weight after one week. The degradation rate of polyacrylamide can be determined from the change in molecular weight.

[0090] Performance Testing

[0091]

[0092] Through the experimental data, it can be analyzed that the performance of various embodiments is better than that of the drag reducer of the comparative example in any aspect. From the molecular weight, it can be intuitively seen that the application successfully achieved a multiple increase in molecular weight, and the molecular weight dropped sharply after adding the reducing agent. The designed fracturing fluid has a relatively obvious improvement in degradation rate, drag reduction rate and salt resistance.

[0093] In order to verify the recyclability of the polyacrylamide fracturing fluid, a reusable ring performance test was conducted on Example 4, where the reducing agent used was sodium persulfate with a mass concentration of 3% in the fracturing fluid, and the oxidizing agent used was sodium persulfate with a mass concentration of 6% in the fracturing fluid.

[0094] Example 4 Repeatability Test

[0095]

[0096] The cyclic performance test shows that the molecular weight of the synthetic polyacrylamide fracturing fluid can be reduced after adding a reducing agent, which can reduce the viscosity and facilitate the discharge of the fracturing fluid from the well. After adding an oxidant, the molecular weight of the fracturing fluid increases to maintain the performance of the fracturing fluid.

[0097] The above are only several preferred embodiments of the present invention, but the present invention is not limited to the above several specific implementations. The above specific implementations are illustrative rather than restrictive. Under the inspiration of the present invention and under the condition of following the spirit and principles of the present invention, researchers in this field can also make improvements and perfections, which are all within the protection scope of the present invention.

[0098] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization, characterized in that: The method comprises the following steps: S1: The emulsion is prepared by one of the following two methods: Method 1, 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, cross-linking monomer, and RAFT agent to the dispersant solution, stir for 5 to 10 minutes, then add the initiator, and react at 30 to 50° C. for 3 to 6 hours to obtain a polyacrylamide w / w emulsion; Wherein, 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, cross-linking 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; The dispersant is polyethylene glycol, polyvinyl alcohol or poly-2-acrylamide-2-methylpropane sulfonic acid; the molecular weight is 200-20000; Alternatively, method 2, preparation of reverse emulsion: dissolve the emulsifier in solvent oil, emulsify for 30-50 minutes at 30-50°C under nitrogen atmosphere with stirring to obtain an oil phase; separately dissolve acrylamide, salt-resistant monomer, cross-linking monomer, and RAFT agent in water and add dropwise to the oil phase, stir for 5-10 minutes, then add the initiator, and react at 30-50°C for 3-6 hours to obtain a polyacrylamide reverse emulsion; Among them, 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, cross-linking 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 cross-linking monomer in method 1 or method 2 is one or both of acrylic acid and hydroxyethyl methacrylate; The RAFT agent in method 1 or method 2 is a water-soluble RAFT agent. S2: adding the primary amine to the emulsion obtained in S1, reacting for 30-60 minutes under stirring 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: The polymer obtained in S2 is passed through air for 5-10 minutes to obtain a degradable polyacrylamide fracturing fluid.

2. The method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization as claimed in claim 1, characterized in that: RAFT agent A synthesized from the RAFT agent bis(carboxymethyl) trithiocarbonate, 4-cyano-4-((phenylthio)thio)pentanoic acid, 1-(dimethylamino)-2-((phenylthio)thiocarbonyl)-acetone, 2-((ethylthio)carbonylthio)propionic acid or 2-((ethylthio)thiocarbonyl)propionic acid in method one or method two and ethylene glycol.

3. The method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization as claimed in 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 initiators are ammonium persulfate and sodium bisulfite in a mass ratio of 1:1; the stirring speed in step S1 is 200-1000 r / min.

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

5. The method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization as claimed in claim 1, characterized in that: The degradable polyacrylamide fracturing fluid further comprises one or more of a fluid loss reducer, a clay stabilizer, a bactericide and a surfactant; The mass proportions of the fluid loss reducer, clay stabilizer, bactericide and surfactant in the degradable polyacrylamide fracturing fluid are 0.1-0.5%, 0.1-0.3%, 0.05-0.1% and 0.1-0.3% respectively.

6. The method for preparing a degradable polyacrylamide fracturing fluid by RAFT polymerization as claimed in claim 1, characterized in that: The fluid loss reducer is specifically polyvinyl alcohol, calcium chloride, sodium chloride or guar gum; The clay stabilizer is specifically acetic acid, aluminum sulfate or hexadecyltrimethylammonium bromide; The fungicide is specifically hexadecyltrimethylammonium chloride, benzoic acid, p-chlorobenzoic acid, chlorothioxanthene or benzimidazole; The surfactant is specifically polyoxyethylene behenyl ether, alkyl dimethyl benzotriazene, polyoxyethylene lauryl alcohol ether or sodium alkyl benzene sulfonate.

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