Salt-tolerant concentrated solution, salt-tolerant fracturing fluid for deep coal bed gas flowback fluid and preparation methods of salt-tolerant concentrated solution and salt-tolerant fracturing fluid

Abstract

The invention provides a salt-tolerant concentrated solution, a salt-tolerant fracturing fluid for deep coal bed gas flow-back fluid and preparation methods of the salt-tolerant concentrated solution and the salt-tolerant fracturing fluid, and the salt-tolerant concentrated solution and the salt-tolerant fracturing fluid are prepared from the following raw materials in percentage by mass: 55-75% of an oil phase, 1.5-2.5% of a stabilizer, 0.45-1% of an activating agent, 25-35% of a thickening agent and 3-5% of a phase inversion agent, wherein the thickening agent is formed by polymerizing acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid and a hydrophobic monomer, and the hydrophobic monomer is octadecyl allyl dimethyl ammonium chloride or alkyl polyoxyethylene ether acrylate. The salt-tolerant concentrated solution and the fracturing fluid have the advantages of high salt tolerance, good resistance reduction performance, good interworking performance, low residue rate (low damage) and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and particularly relates to a salt-resistant concentrate, a salt-resistant fracturing fluid for deep coalbed methane produced water and preparation methods thereof. Background Art

[0002] During the development of deep coalbed methane, hydraulic fracturing technology is required to transform the gas-producing formation, generating a large amount of fracturing flowback waste liquid. The fracturing flowback liquid is characterized by high liquid volume, high salinity, and high calcium and magnesium ion content, making it difficult to reuse. To achieve the reuse of the flowback liquid, the fracturing flowback liquid can undergo a series of water treatment technologies, such as oxidative gel-breaking technology, enhanced coagulation technology, highly efficient coupling of nanofiltration technology, oxidation-flocculation technology, desalination technology, etc., so as to improve the water quality of the fracturing fluid flowback liquid. However, for large-scale fracturing processes of deep coalbed methane, the treatment of the flowback liquid will cause a sharp increase in fracturing costs, and the water quality improvement effect is also extremely limited. Therefore, the use of salt-resistant thickeners is one of the key methods to solve this problem.

[0003] Currently, the salt-resistant polymers in the salt-resistant fracturing fluid system have poor thickening effects under high salinity, especially in fracturing flowback fluids with high calcium and magnesium ion contents, and cannot meet the requirements of large-scale fracturing processes, resulting in the difficulty of applying existing fracturing flowback liquid reuse technologies in deep coalbed methane. Therefore, the reuse of fracturing flowback liquid has become a difficult problem. Summary of the Invention

[0004] Currently, the salt-resistant fracturing fluid system has insufficient calcium and magnesium resistance, and the performance of the fracturing fluid in fracturing flowback fluids with high calcium and magnesium ion contents is poor, unable to meet the requirements of large-scale fracturing processes for deep coalbed methane. To solve the above technical problems, the purpose of the present invention is to provide a salt-resistant concentrate, a salt-resistant fracturing fluid for deep coalbed methane produced water and preparation methods thereof, so as to overcome the problem that it is difficult to reuse the high-salinity produced water of deep coalbed methane.

[0005] To achieve the above purpose, the present invention provides a salt-resistant concentrate, which, by mass percentage, comprises: 55-75% of an oil phase, 1.5-2.5% of a stabilizer, 0.45-1% of an activator, 25-35% of a thickener, and 3-5% of a phase inversion agent;

[0006] Wherein, the thickener is polymerized from acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid and a hydrophobic monomer, and the hydrophobic monomer is octadecyl allyl dimethyl ammonium chloride and / or alkyl polyoxyethylene ether acrylate.

[0007] The salt-tolerant concentrate of the present invention and the fracturing fluid prepared therefrom have high salt tolerance, good drag reduction performance, good compatibility, and low residue rate, and are suitable for the reuse of high salinity backflow fluid in deep coalbed methane. In the thickening agent of the present invention, the alkyl polyoxyethylene ether acrylate monomer can greatly improve the salt tolerance of the thickening agent, and further improve the salt tolerance and stability of the concentrate and the fracturing fluid.

[0008] In the above-mentioned salt-tolerant concentrate, preferably, the preparation method of the alkyl polyoxyethylene ether acrylate is as follows: in anhydrous dichloromethane, a combination of alkyl polyoxyethylene ether or alkylphenol polyoxyethylene ether and acryloyl chloride with a mass ratio of 6-10:1 is added. Under ice-water bath conditions, a dichloromethane solution containing triethylamine with a concentration of 0.01-0.03 g / mL is added. After reacting for 4-6 h, the precipitate in the reaction solution is filtered off, the filtrate is taken, washed, and evaporated to dryness to obtain an oily liquid, which is the alkyl polyoxyethylene ether acrylate.

[0009] In the above-mentioned salt-tolerant concentrate, preferably, the preparation method of the alkyl polyoxyethylene ether acrylate is as follows: in anhydrous dichloromethane, a combination of alkyl polyoxyethylene ether or alkylphenol polyoxyethylene ether and acryloyl chloride with a mass ratio of 6-10:1 is added. Under ice-water bath conditions, a dichloromethane solution containing triethylamine with a concentration of 0.01-0.03 g / mL is added. After reacting for 4-6 h, the precipitate in the reaction solution is filtered off, the filtrate is washed successively with saturated sodium bicarbonate solution and dilute hydrochloric acid, separated by liquid separation, the lower organic layer is taken, and evaporated to dryness to obtain an oily liquid, which is the alkyl polyoxyethylene ether acrylate.

[0010] In the above-mentioned salt-tolerant concentrate, preferably, the preparation method of the thickening agent includes:

[0011] Under the protection of nitrogen and the action of an initiator, acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid, and the hydrophobic monomer with a molar ratio of 80-90:8-10:0.6-1:0.2-0.5 are subjected to a copolymerization reaction in water to obtain the thickening agent.

[0012] In the above-mentioned salt-tolerant concentrate, preferably, in the preparation method of the thickening agent, the temperature of the copolymerization reaction is 50-60 °C and the time is 5-7 h.

[0013] In the above-mentioned salt-tolerant concentrate, preferably, in the preparation method of the thickening agent, the dosage of the initiator is 0.1-0.15% of the total mass of the monomers; more preferably, the initiator is initiator V50.

[0014] According to a specific embodiment of the present invention, preferably, the preparation method of the thickening agent is as follows: First, acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid are added, acrylamide and a hydrophobic monomer are added, a certain amount of water is added, and the mixture is stirred evenly; after deoxygenation by nitrogen bubbling, an initiator is added to initiate a polymerization reaction to obtain a quaternary copolymer, which is the thickening agent.

[0015] In the above-mentioned salt-tolerant concentrate, preferably, the stabilizer is poly(methacrylic acid-co-octadecyl methacrylate), and its structural formula is (C4H6O2) a -(C 21 H 40 O2) b , where a is the degree of polymerization of the acrylic acid monomer, b is the degree of polymerization of the octadecyl methacrylate monomer, a = 100 - 150, b = 200 - 250, and a + b = 350 - 400; more preferably, the structural formula of the stabilizer is (C4H6O2) 100 -(C 21 H 40 O2) 250 . The stabilizer of the present invention is a polymer containing octadecyl methacrylate and acrylic acid structural units.

[0016] Currently, the suspension fracturing fluid system stabilized by bentonite will inevitably result in a high content of solid residues, thereby increasing reservoir damage. The stabilizer of the present invention is an oil-soluble polymer. Compared with using bentonite as a stabilizer in the prior art, the fracturing fluid prepared from the concentrate stabilized by the oil-soluble polymer of the present invention, after complete gel breaking, only a small amount of oil-like substances appear on the upper layer of the gel-breaking fluid, and no solid precipitates will appear. The residue content is low, and the damage to the reservoir caused by preparing the fracturing fluid with high salinity flowback fluid can be significantly reduced. The stabilizer of the present invention can effectively reduce the content of solid residues in the flowback fluid, thereby providing a clean fracturing fluid system.

[0017] In the above-mentioned salt-tolerant concentrate, preferably, the activator is a lower alcohol with less than 5 carbon atoms.

[0018] In the above-mentioned salt-tolerant concentrate, preferably, the activator is one or a combination of two or more of methanol, ethanol, propanol, and isopropanol.

[0019] In the above-mentioned salt-tolerant concentrate, preferably, the mass of the activator is 30 - 40% of the stabilizer.

[0020] In the above-mentioned salt-tolerant concentrate, preferably, the phase inversion agent is an oil-in-water (O / W) surfactant. More preferably, the oil-in-water surfactant includes one or a combination of two or more of OPE-10, NP-6, NP-8, NP-10, and NP-OPE surfactants.

[0021] In the above-mentioned salt-tolerant concentrated liquid, preferably, the oil phase includes white oil. More preferably, it includes one or a combination of more than two of 3# white oil, 5# white oil, 7# white oil, 10# white oil, and 15# white oil.

[0022] The salt-tolerant concentrated liquid of the present invention has good compatibility and good fluidity, ensuring the on-site pumping construction requirements.

[0023] The present invention also provides a preparation method of the above-mentioned salt-tolerant concentrated liquid, which includes the following steps: dissolving the stabilizer in the oil phase, and then adding the activator, thickening agent, and phase inversion agent, and stirring evenly to obtain the salt-tolerant concentrated liquid.

[0024] In the preparation method of the above-mentioned salt-tolerant concentrated liquid, preferably, the stirring speed is 3000-6000 r / min, and the stirring time is 30-50 min.

[0025] In the preparation method of the above-mentioned salt-tolerant concentrated liquid, preferably, the raw material addition order of the salt-tolerant concentrated liquid is white oil, stabilizer, activator, thickening agent, and phase inversion agent.

[0026] The present invention also provides a salt-tolerant fracturing fluid for deep coalbed methane flowback fluid, which is prepared from the above-mentioned salt-tolerant concentrated liquid, flowback fluid, and additives.

[0027] In the above-mentioned salt-tolerant fracturing fluid for deep coalbed methane flowback fluid, preferably, the additives include one or a combination of more than two of swelling inhibitor, drainage aid, and breaker.

[0028] In the above-mentioned salt-tolerant fracturing fluid for deep coalbed methane flowback fluid, preferably, the salinity of the flowback fluid is 200000-250000 ppm.

[0029] In the above-mentioned salt-tolerant fracturing fluid for deep coalbed methane flowback fluid, preferably, the flowback fluid contains calcium and magnesium ions, the calcium ion content is 30000-40000 ppm, and the magnesium ion content is 3000-5000 ppm.

[0030] In some specific embodiments, when the salinity of the flowback fluid reaches 250,000 ppm, the calcium ion content is 30,000 ppm, and the magnesium ion content is 5,000 ppm, the viscosity of the salt-tolerant fracturing fluid of the present invention remains above 38 mPa·s. In some other specific embodiments, when the salinity of the flowback fluid reaches 200,000 ppm, the drag reduction rate of the salt-tolerant fracturing fluid of the present invention is distributed between 64% and 70%, and the overall value is greater than 60%, meeting the on-site fracturing use standard for deep coalbed methane. According to the specific embodiments of the present invention, preferably, based on the total mass of the fracturing fluid, the salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid includes the following raw materials in mass percentages: 0.1-2% of salt-tolerant concentrate, 0.1-0.25% of swelling inhibitor, 0.1-0.25% of flowback aid, and 0.03-0.5% of breaker.

[0031] In the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the flowback aid is a combination of a non-ionic surfactant and an anionic surfactant with a mass ratio of 1:1-3.

[0032] In the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the non-ionic surfactant includes one or a combination of two or more of Tween 80, nonylphenol polyoxyethylene ether, and lauryl alcohol polyoxyethylene ether.

[0033] In the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the anionic surfactant includes potassium dodecylbenzenesulfonate and / or sodium dodecylphenol polyoxyethylene ether sulfonate.

[0034] In the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the swelling inhibitor includes one or a combination of two or more of tris(hydroxymethyl)aminomethane hydrochloride, hydroxyethyl trimethyl ammonium chloride, swelling inhibitor JN-2, ammonium chloride, and potassium chloride.

[0035] In the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the breaker includes ammonium persulfate and / or potassium persulfate.

[0036] Breaking gel is the process in which after adding a breaker during the preparation of the fracturing fluid and placing it at the target temperature, the gel becomes an aqueous solution (viscosity lower than 5 mP.s). The salt-tolerant fracturing fluid of the present invention has good gel-breaking performance, low residue content, and no obvious solid particles. In some specific embodiments, at a temperature of 60 °C, the gel breaks completely in 6 hours. The residue content of the fracturing fluid of the present invention is 76 mg / L, < 600 mg / L, meeting the standard of SY / T 7627-2021 "Technical Requirements for Water-Based Fracturing Fluids".

[0037] The present invention also provides a preparation method for the above-mentioned salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, which includes the following steps:

[0038] Add the salt-tolerant concentrate to the flowback fluid and stir evenly; then add an anti-swelling agent, a flowback aid, and a gel breaker, and stir for 0.5 - 1 h at a stirring speed of 3000 - 6000 r / min to obtain a salt-tolerant fracturing fluid;

[0039] In the method for preparing the salt-tolerant fracturing fluid for the deep coalbed methane flowback fluid, preferably, the stirring speed is 3000 - 6000 r / min.

[0040] The method for preparing the salt-tolerant fracturing fluid of the present invention is simple in operation, easy to scale up for processing and production, and meets the requirements of on-site large-displacement continuous mixing and fracturing construction.

[0041] The technical solution provided by the present invention has the following beneficial effects:

[0042] The salt-tolerant concentrate and fracturing fluid of the present invention have the advantages of high salt tolerance, good drag reduction performance, good compatibility, and low residue rate (low damage). By synthesizing a new type of thickening agent, the fracturing fluid can maintain a high viscosity under high salinity conditions, which can meet the repeated utilization of high-salinity flowback fluid in deep coalbed methane, reduce the disposal cost of flowback fluid, and reduce the secondary pollution of flowback fluid to the environment. In addition, by using an oil-soluble polymer as a stabilizer, the residue content after the gel breaking of the fracturing fluid is significantly lower than that of the existing fracturing fluid system with bentonite stable suspension, thereby reducing reservoir damage and providing a new method for solving the problem of reusing and formulating the flowback fluid of deep coalbed methane. Description of the Drawings

[0043] Figure 1 Salt tolerance test results of the thickening agent prepared in Example 1;

[0044] Figure 2 Appearance picture of the salt-tolerant fracturing fluid after gel breaking in Example 3, where the stabilizer is a polymer;

[0045] Figure 3 Appearance picture of the fracturing fluid in Comparative Example 1, where the stabilizer is bentonite;

[0046] Figure 4a Viscoelasticity test results of the fracturing fluid (salinity 20W) in Example 3, where G' is the elastic modulus and G" is the viscous modulus;

[0047] Figure 4b Viscoelasticity test results of the fracturing fluid (salinity 0, fresh water) in Example 5, where G' is the elastic modulus and G" is the viscous modulus;

[0048] Figure 4c Viscoelasticity test results of the fracturing fluid (salinity 20W) in Example 6, where G' is the elastic modulus and G" is the viscous modulus;

[0049] Figure 5 Shear thinning behavior test results of the fracturing fluids (with different salinities) in Examples 3, 5 - 7;

[0050] Figure 6 Test results of the viscosity of the fracturing fluid varying with the thickener content in Experimental Example 5. Detailed implementation manners

[0051] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solution of the present invention will be described in detail below, but it should not be construed as a limitation on the implementable scope of the present invention.

[0052] Example 1

[0053] This example provides a salt - resistant concentrate, and its preparation method includes the following steps:

[0054] (1) Prepare a thickener (polymerized from four monomers: acrylamide, acrylic acid, 2 - acrylamido - 2 - methylpropanesulfonic acid, and alkyl polyoxyethylene ether acrylate):

[0055] Synthesize alkyl polyoxyethylene ether acrylate: Add alkyl polyoxyethylene ether and acryloyl chloride in a mass ratio of 6:1 in anhydrous dichloromethane. Under an ice - water bath condition, add a dichloromethane solution containing triethylamine at 0.02 g / mL, stir for 6 h, filter off the precipitate, take the filtrate, wash it three times successively with saturated sodium bicarbonate solution and dilute hydrochloric acid, separate the layers, take the lower organic layer, and evaporate to dryness to obtain an oily liquid, which is alkyl polyoxyethylene ether acrylate;

[0056] Add acrylamide, acrylic acid, 2 - acrylamido - 2 - methylpropanesulfonic acid, and alkyl polyoxyethylene ether acrylate in a molar ratio of 90:10:1:0.2, add a certain amount of water, and stir evenly; after bubbling with nitrogen to remove oxygen, add initiator V50 (0.1% of the total mass of the four monomers), and initiate aqueous solution polymerization at 60 °C to obtain a quaternary copolymer, which is the thickener.

[0057] (2) Prepare the salt - resistant concentrate

[0058] By mass percentage, the raw materials of the salt - resistant concentrate include: 65% oil phase, 1.5% stabilizer, 0.45% isopropanol activator, 30% thickener obtained in step (1), and 3.25% OPE - 10 phase - inversion agent; among them, the stabilizer is poly(methacrylic acid - co - octadecyl methacrylate), and its structural formula is (C4H6O2) 100 -(C 21 H 40 O2) 250 .

[0059] Dissolve the stabilizer in the oil phase, then sequentially add the activator, thickening agent, and phase inversion agent, and stir at 3500 r / min for 50 min to prepare a salt-tolerant concentrate.

[0060] In this example, the salt tolerance of the thickening agent obtained in step (1) was also tested under brines with different salinities (thickening agent concentration 0.5 wt%), and the results are as Figure 1 shown.

[0061] The test results show that in brine with a salinity of 200,000 ppm (calcium ion content 30,000 ppm, magnesium ion content 5,000 ppm), the viscosity of the 0.5 wt% thickening agent solution is still higher than 40 mPa·s.

[0062] Example 2

[0063] This example provides a salt-tolerant concentrate, and its preparation method is the same as that of Example 1, except that the structural formula of the stabilizer in this example is (C4H6O2) 150 -(C 21 H 40 O2) 200 .

[0064] Comparative Example 1

[0065] This comparative example provides a salt-tolerant concentrate, and its preparation method is the same as that of Example 1, except that the structural formula of the stabilizer in this example is (C4H6O2) 100 -(C 21 H 40 O2) 200 .

[0066] Comparative Example 2

[0067] This example provides a salt-tolerant concentrate, and its preparation method is the same as that of Example 1, except that the structural formula of the stabilizer in this example is (C4H6O2) 150 -(C 21 H 40 O2) 250 .

[0068] The properties of the salt-tolerant concentrates of the above Examples 1-2 and Comparative Examples 1-2 are shown in Table 1. The results show that the stabilizers (C4H6O2) 100 -(C 21 H 40 O2) 250 、(C4H6O2) 150 -(C 21 H 40 O2) 200 can be used to prepare a concentrate system with good fluidity and stabilizer.

[0069] Table 1 Properties of the Concentrate

[0070]

[0071] Example 3

[0072] This example provides a salt - resistant fracturing fluid for deep - layer coalbed methane flow - back fluid, which is prepared by using the salt - resistant concentrate of Example 1. The preparation method is as follows:

[0073] By mass percentage, the raw materials of the salt - resistant fracturing fluid include: 1.42% of the salt - resistant concentrate of Example 1 (the thickener concentration is converted to 0.5 wt%), 0.2% of potassium chloride swelling inhibitor, 0.25% of a combination of nonylphenol polyoxyethylene ether and sodium dodecylphenol polyoxyethylene ether sulfonate (mass ratio is 1:1, as a flow - back aid), 0.03% of ammonium persulfate breaker, and the balance is 200000 ppm of mineralized water (simulating flow - back fluid). The 200000 ppm of mineralized water contains 30000 ppm of calcium ions and 5000 ppm of magnesium ions;

[0074] Add the salt - resistant concentrate to 200000 ppm of mineralized water, add potassium chloride, nonylphenol polyoxyethylene ether, sodium dodecylphenol polyoxyethylene ether sulfonate and ammonium persulfate at a stirring rate of 3500 r / min, and continue to mix and stir until the solution is uniform to obtain the salt - resistant fracturing fluid for deep - layer coalbed methane flow - back fluid. Measure its apparent viscosity with a six - speed viscometer, and the value is 42 mP.s.

[0075] It is observed that there is no precipitation and no flocculation in the fracturing fluid, indicating that this fracturing fluid has good compatibility with the additives.

[0076] After placing the fracturing fluid of this example in an oven at 60 °C for 6 h, observe the gel - breaking situation of the fracturing fluid, as Figure 2 shown. After gel - breaking, there is no obvious residue in the solution, and a small amount of oily substance appears on the upper layer.

[0077] Example 4

[0078] This example provides a salt - resistant fracturing fluid for deep - layer coalbed methane flow - back fluid. Its preparation method is the same as that of Example 3, except that the salt - resistant concentrate of Example 2 is used.

[0079] Comparative Example 3

[0080] This comparative example provides a thickener. Its preparation method is the same as Step 1 of Example 1, except that alkyl polyoxyethylene ether acrylate monomer is not introduced into the thickener.

[0081] This comparative example also tested the salt - resistant performance of the obtained thickener in brines with different salinities (thickener concentration 0.5 wt%). The results are shown in Table 2.

[0082] Table 2 Apparent Viscosity of Thickener Solutions under Different Mineralized Waters

[0083] Salinity (ppm) Fresh water 1W 2W 3W 4W 5W 10W 20W 25W Viscosity (mPa·s) 65 20 13 8 5 4 3 3 2

[0084] The test results show that in 200,000 ppm mineralized brine (with calcium ion content of 30,000 ppm and magnesium ion content of 5,000 ppm), the viscosity of the 0.5 wt% thickener solution is 3 mPa·s, far lower than the viscosity of the thickener prepared in Example 1 (40 mPa·s).

[0085] Comparative Example 4

[0086] This comparative example provides a concentrate, the preparation method of which is the same as that of Example 1, except that the stabilizer is bentonite. The stability and viscosity of the concentrate in this example were tested. It was found that the viscosity of the concentrate in this comparative example was 64 mPa·s, and solid-liquid two-phase stratification occurred after standing for 15 days.

[0087] Comparative Example 5

[0088] This comparative example provides a fracturing fluid, the preparation method of which is the same as that of Example 3, except that the stabilizer is bentonite. The apparent viscosity of the fracturing fluid in this example was measured with a six-speed viscometer to be 28 mP·s.

[0089] After placing the fracturing fluid in this comparative example in an oven at 60 °C for 6 h, the gel-breaking situation of the fracturing fluid was observed, as Figure 3 shown, the solution was generally turbid, and obvious solid residues appeared on the upper layer.

[0090] Experimental Example 1

[0091] This experimental example evaluated the apparent viscosity of the fracturing fluid in Example 3, and the results are shown in Table 3.

[0092] Table 3 Apparent Viscosity of Formulated Fracturing Fluids under Different Mineralized Waters

[0093]

[0094]

[0095] The results show that under 25W mineralization, the viscosity of the formulated fracturing fluid can still reach 38 mPa·s, showing excellent salt tolerance performance.

[0096] Example 5

[0097] This example provides a salt-tolerant fracturing fluid for deep coalbed methane flowback fluid, the preparation method of which is the same as that of Example 3, except that 200,000 ppm mineralized water is replaced with fresh water.

[0098] Example 6

[0099] This embodiment provides a salt-resistant fracturing fluid for deep coalbed methane flowback fluid. Its preparation method is the same as that of Example 3, except that 200,000 ppm of mineralized water is replaced with 10,000 ppm of mineralized water.

[0100] Example 7

[0101] This embodiment provides a salt-resistant fracturing fluid for deep coalbed methane flowback fluid. Its preparation method is the same as that of Example 3, except that 200,000 ppm of mineralized water is replaced with 40,000 ppm of mineralized water.

[0102] Experimental Example 2

[0103] This experimental example is used to evaluate the salt tolerance and shear resistance of the salt-resistant fracturing fluid for deep coalbed methane flowback fluid in Examples 3, 5 - 7.

[0104] Figure 4a , Figure 4b , Figure 4c They are respectively the test results of the viscoelasticity of the fracturing fluids in Examples 3, 5, and 6. The results show that the elastic modulus (G') of the three groups of fracturing fluids is greater than the viscous modulus (G"), indicating good viscoelasticity.

[0105] Figure 5 They are the test results of the shear resistance of the fracturing fluids in Examples 3, 5 - 7. It can be seen that with the increase of salinity, the viscoelastic properties of the fracturing fluid decrease. When the salinity is greater than 40,000, the viscoelastic properties of the drag reducer solution change little, and it still has good viscoelastic properties at a salinity of 200,000. With the change of shear time, the viscosity remains almost unchanged, basically remaining above 36 mPa·s, indicating that this suspension fracturing fluid system still has good salt tolerance and shear resistance at high salinity.

[0106] Experimental Example 3

[0107] This experimental example is used to evaluate the drag reduction performance of the salt-resistant fracturing fluid for deep coalbed methane flowback fluid in Example 3.

[0108] According to SY / T 5107 - 2016 "Evaluation Method for Water-Based Fracturing Fluid Performance", the drag reduction property of the fracturing fluid in Example 3 is tested, and the results are shown in Table 4.

[0109] Table 4 Drag Reduction Rate of Integrated Suspension Fracturing Fluid

[0110]

[0111]

[0112] The results show that the drag reduction rate of the fracturing fluid is distributed between 64 - 70%, and the overall is greater than 60%, meeting the requirements of the slickwater system for drag reduction rate in project fracturing.

[0113] Experimental Example 4

[0114] This experimental example is used to evaluate the gel-breaking performance of the salt-tolerant fracturing fluid for deep coalbed methane flowback fluid in Example 3.

[0115] After placing the fracturing fluid of this Example 3 in an oven at 60 °C for 6 h, the fracturing fluid was completely gel-broken. There was no obvious residue in the gel-broken solution, and a small amount of oily substance appeared on the upper layer. After testing, the viscosity of the gel-broken fluid was 3.1 mPa·s.

[0116] Example 3 uses a polymer-stabilized concentrated solution. The residue content after gel-breaking of the fracturing fluid system is 54 mg / L, which is far lower than the residue content of 389 mg / L after gel-breaking of the fracturing fluid system prepared with the traditional bentonite-stabilized suspension, meeting the standard residue content of less than 600 mg / L in SY / T 7627--2021 "Technical Requirements for Water-Based Fracturing Fluids".

[0117] Therefore, the fracturing fluid of the present invention can meet the repeated utilization of high salinity flowback fluid, reduce the disposal cost of flowback fluid, reduce the secondary pollution of flowback fluid to the environment, and the residue content after gel-breaking is lower than that of the existing fracturing fluid system prepared with bentonite-stabilized suspension, which can reduce reservoir damage and provide a new method for solving the problem of reusing the flowback fluid for deep coalbed methane.

[0118] Experimental Example 5

[0119] This experimental example is used to explore the dosage of thickening agent in the fracturing fluid.

[0120] Based on the fracturing fluid of Example 3, the concentration of the thickening agent in the fracturing fluid was changed, and the viscosities of each fracturing fluid were tested. The results are as Figure 6 shown.

[0121] From Figure 6 it can be seen that when the dosage of the thickening agent is above 0.2%, the viscosity-starting performance of the solution can reach above 12 mPa·s; when the dosage of the thickening agent is above 0.4%, the starting viscosity of the solution can reach above 36 mPa·s, and it has good salt tolerance. After being placed in an oven at 60 °C for 6 h, it was completely gel-broken. A small amount of oily substance appeared on the upper layer of the gel-broken fluid, and no solid residue appeared. The residue content was 76 mg / L.

[0122] Experimental Example 6

[0123] This experimental example is used to explore the optimal ratio of stabilizer, activator, and phase inversion agent.

[0124] Sixteen groups of concentrated solutions were provided. Their raw materials and preparation methods were the same as those in Example 1, except that the contents of the stabilizer, activator, and phase inversion agent were as shown in Table 5 respectively.

[0125] After the above-mentioned concentrated liquid was centrifuged at 3000 r / min for 30 min and then allowed to stand for 10 days, the stability of the concentrated liquid was judged by observing the liquid-phase precipitation rate of the concentrated liquid. The results are shown in Table 5.

[0126] Table 5 Orthogonal experimental design for optimizing the contents of stabilizer, activator and phase-inverting agent

[0127]

[0128] The results show that the liquid-phase precipitation rate of the concentrated liquid prepared in Group 11 is the lowest, so it has the best stability. The composition of this concentrated liquid is: 59.25% white oil, 2% stabilizer, 3% phase-inverting agent, 0.75% activator, and 35% drag reducer.

[0129] Example 8

[0130] This example provides a salt-resistant concentrated liquid, and its preparation method is the same as that of Example 1, except that when preparing the thickening agent, the hydrophobic monomer alkyl polyoxyethylene ether acrylate is replaced by octadecyl allyl dimethyl ammonium chloride.

[0131] The salt-resistant concentrated liquid of this example was prepared into a salt-resistant fracturing fluid according to the method of Example 3. After testing and verification, the salt-resistant concentrated liquid and salt-resistant fracturing fluid of this example also have good salt resistance.

Claims

1. A salt-tolerant concentrate, by mass percentage, its raw materials include: The oil phase is 55 - 75%, the stabilizer is 1.5 - 2.5%, the activator is 0.45 - 1%, the thickening agent is 25 - 35%, and the phase inversion agent is 3 - 5%; Among them, the thickening agent is polymerized from acrylamide, acrylic acid, 2 - acrylamido - 2 - methylpropanesulfonic acid and a hydrophobic monomer, and the hydrophobic monomer is octadecyl allyl dimethyl ammonium chloride or alkyl polyoxyethylene ether acrylate.

2. The salt-tolerant concentrate according to claim 1, wherein, The preparation method of the alkyl polyoxyethylene ether acrylate is as follows: Add a combination of alkyl polyoxyethylene ether or alkylphenol polyoxyethylene ether and acryloyl chloride with a mass ratio of 6 - 10:1 in anhydrous dichloromethane. Under the condition of an ice - water bath, add a dichloromethane solution containing triethylamine at 0.01 - 0.03 g / mL. After reacting for 4 - 6 h, filter the reaction solution to remove the precipitate, take the filtrate, wash and evaporate to dryness to obtain an oily liquid, which is the alkyl polyoxyethylene ether acrylate.

3. The salt-tolerant concentrate according to claim 1, wherein, The preparation method of the thickening agent includes: Under the protection of nitrogen and the action of an initiator, acrylamide, acrylic acid, 2 - acrylamido - 2 - methylpropanesulfonic acid, and the hydrophobic monomer with a molar ratio of 80 - 90:8 - 10:0.6 - 1:0.2 - 0.5 are copolymerized in water to obtain the thickening agent.

4. The salt-tolerant concentrate according to claim 3, wherein, The temperature of the copolymerization reaction is 50 - 60 °C, and the time is 5 - 7 h; Preferably, the dosage of the initiator is 0.1 - 0.15% of the total mass of the monomers.

5. The salt-tolerant concentrate according to claim 1, wherein, The stabilizer is poly(methacrylic acid-co-stearyl methacrylate), and its structural formula is (C4H6O2) a -(C 21 H 40 O2) b , where a is the degree of polymerization of the acrylic acid monomer, b is the degree of polymerization of the stearyl methacrylate monomer, a = 100 - 150, b = 200 - 250, and a + b = 350 - 400; More preferably, the structural formula of the stabilizer is (C4H6O2) 100 -(C 21 H 40 O2) 250 .

6. The salt-tolerant concentrate according to claim 1, wherein, The activator is a lower alcohol with the number of carbon atoms < 5; Preferably, the activator includes one or a combination of two or more of methanol, ethanol, propanol, and isopropanol; Preferably, the mass of the activator is 30 - 40% of the mass of the stabilizer.

7. The salt-tolerant concentrate according to claim 1, wherein, The phase inversion agent is an oil - in - water surfactant; Preferably, the oil - in - water surfactant includes one or a combination of two or more of OPE - 10, NP - 6, NP - 8, NP - 10, and NP - OPE surfactants.

8. The salt-tolerant concentrate according to claim 1, wherein, The oil phase includes white oil.

9. A preparation method of the salt - resistant concentrate according to any one of claims 1 - 8, which includes the following steps: Dissolve the stabilizer in the oil phase, then add the activator, thickening agent, and phase inversion agent, and stir evenly to obtain the salt - resistant concentrate; Preferably, the stirring speed is 3000 - 6000 r / min, and the stirring time is 30 - 50 min.

10. A salt - resistant fracturing fluid for deep coalbed methane flowback fluid, which is prepared from the salt - resistant concentrate according to any one of claims 1 - 8, the flowback fluid, and additives; Preferably, the additives include one or a combination of two or more of an anti - swelling agent, a drainage aid, and a gel breaker.

11. The salt-tolerant fracturing fluid for deep coalbed methane flowback fluid according to claim 10, wherein, The salinity of the flowback fluid is 200000 - 250000 ppm; Preferably, the flowback fluid contains calcium and magnesium ions, the calcium ion content is 30000 - 40000 ppm, and the magnesium ion content is 3000 - 5000 ppm.

12. The salt-resistant fracturing fluid for deep coalbed methane flowback fluid according to claim 10, wherein, Based on the total mass of the fracturing fluid, it includes the following raw materials in mass percentages: the salt - resistant concentrate is 0.1 - 2%, the anti - swelling agent is 0.1 - 0.25%, the drainage aid is 0.1 - 0.25%, and the gel breaker is 0.03 - 0.5%.

13. The salt-tolerant fracturing fluid for deep coalbed methane flowback fluid according to claim 12, wherein, The flowback aid is a combination of a non-ionic surfactant and an anionic surfactant with a mass ratio of 1:1-3; Preferably, the non-ionic surfactant includes one or a combination of more than two of Tween 80, nonylphenol polyoxyethylene ether, and lauryl alcohol polyoxyethylene ether; Preferably, the anionic surfactant includes potassium dodecylbenzenesulfonate and / or sodium dodecylphenol polyoxyethylene ether sulfonate.

14. The salt-tolerant fracturing fluid for deep coalbed methane flowback fluid according to claim 12, wherein, The swelling inhibitor includes one or a combination of more than two of trimethylamine hydrochloride, hydroxyethyl trimethyl ammonium chloride, swelling inhibitor JN-2, ammonium chloride, and potassium chloride; Preferably, the breaker includes ammonium persulfate and / or potassium persulfate.

15. A preparation method of a salt-tolerant fracturing fluid for deep coalbed methane flowback fluid according to any one of claims 10-14, which comprises the following steps: Adding the salt-tolerant concentrate to the flowback fluid and stirring evenly; then adding the swelling inhibitor and the flowback aid, mixing evenly and then adding the breaker, and stirring evenly to obtain the salt-tolerant fracturing fluid; Preferably, the stirring speed is 3000-6000 r / min.

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