A multi-crosslinking point drag reducer, variable viscosity fracturing fluid and preparation method thereof

Abstract

The invention discloses a multi-crosslinking point drag reducer, a variable viscosity fracturing fluid and a preparation method thereof, and belongs to the technical field of oil extraction. The multi-crosslinking point drag reducer comprises, by weight percentage, 8-25% acrylamide, 3-15% N-isopropyl acrylamide, 2-8% acrylic acid, 4-10% methacryloyloxybutyrolactone, 20-35% mineral oil, 0.5-5% primary emulsifier, 1-5% cosolvent, 0.5-2% initiator, 1-3.5% post-treatment emulsifier, and the balance is water; the variable viscosity fracturing fluid comprises, by weight percentage, 0.1-1.0 parts of drag reducer, 0.1-0.6 parts of crosslinking agent, 0.1-0.8 parts of bactericide, 0.1-0.8 parts of drainage aid, 0.1-0.8 parts of clay stabilizer, 0.1-0.5 parts of gel breaker and 100 parts of water, and the drag reducer is the multi-crosslinking point drag reducer. The invention can solve the problems of unstable viscosity and uneven cross-linking structure that occur when a conventional drag reducer and a cross-linking agent are added simultaneously.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum extraction, and in particular to a multi-crosslinking point drag reducer, a variable viscosity fracturing fluid and a preparation method thereof. Background Art

[0002] In hydraulic fracturing technology, the performance of fracturing fluid directly affects the formation of cracks and the efficiency of oil and gas extraction. The preparation of variable viscosity fracturing fluid gel usually requires the addition of drag reducers and cross-linkers to optimize the fluid's fluidity and enhance its stability in the underground environment, and improve the ability to create main fractures. Drag reducers are mainly used to reduce the resistance of liquid flow and improve the rheological properties of the fluid, while the main function of cross-linkers is to enhance the structural stability and temperature resistance of drag reducers.

[0003] In traditional drag reducer and cross-linker formulations, the role of the cross-linker is usually to form a cross-linked network between the drag reducer molecular chains to improve its stability in extreme environments. However, in actual use, when the drag reducer and the cross-linker are added at the same time to simulate the fracturing construction mode, the chemical compatibility between the drag reducer and the cross-linker will also affect the formation of the cross-linked structure, which is manifested as a decrease in the viscosity of the sliding water, poor cross-linking effect, and the inability to form a stable structure of the jelly under the same amount of drag reducer. Certain functional groups in the cross-linker may react adversely with the drag reducer, resulting in a decrease in the activity of the cross-linker, thereby affecting the cross-linking effect. The incompatibility of chemical reactions may lead to the formation of by-products of the cross-linking reaction, affecting the stability of the cross-linked structure. Summary of the invention

[0004] In view of the above problems, the present invention aims to provide a multi-crosslinking point drag reducer, a variable viscosity fracturing fluid and a preparation method thereof.

[0005] The technical solution of the present invention is as follows:

[0006] On the one hand, a multi-crosslinking point drag reducer is provided, which includes, by weight percentage, 8-25% of acrylamide, 3-15% of N-isopropylacrylamide, 2-8% of acrylic acid, 4-10% of methacryloyloxybutyrolactone, 20-35% of mineral oil, 0.5-5% of a primary emulsifier, 1-5% of a cosolvent, 0.5-2% of an initiator, 1-3.5% of a post-treatment emulsifier, and the balance is water.

[0007] Preferably, the composition comprises, by weight percentage, 8-15% acrylamide, 4-10% N-isopropylacrylamide, 2-6% acrylic acid, 4-8% methacryloyloxybutyrolactone, 20-30% mineral oil, 1-3% primary emulsifier, 2-4% cosolvent, 0.5-1.2% initiator, 1-3% post-treatment emulsifier, and the balance is water.

[0008] Preferably, the mineral oil is selected from any one or more of light mineral oil, heavy mineral oil and liquid paraffin.

[0009] Preferably, the primary emulsifier and the post-treatment emulsifier are respectively selected from any one or more of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, Tween-80, Tween-20, Span-80, polyvinyl alcohol, polyoxyethylene ether, polyvinyl alcohol copolymer, polyacrylate, sodium long-chain olefin sulfonate, polyoxyethylene-polyoxypropylene copolymer, epoxidized dodecenylphenol, OP-9, OP-12, octylphenol ethoxylate, nonylphenol polyoxyethylene ether and AEO series.

[0010] Preferably, the co-solvent is selected from any one or more of dimethyl sulfoxide, dimethylsulfamide, isopropanol, ethylene glycol, acetone, ethyl acetate, propylene glycol and glycerol.

[0011] Preferably, the initiator is an initiator of a redox initiation system.

[0012] Preferably, the oxidant in the redox initiation system is selected from any one of ammonium persulfate, sodium persulfate, hydrogen peroxide, acetone peroxide and benzoyl peroxide, and the reducing agent in the redox initiation system is selected from any one of sodium bisulfite, ascorbic acid, sodium thiosulfate, sodium hypophosphite and ethanolamine.

[0013] On the other hand, a method for preparing the multi-crosslinking point drag reducer described in any one of the above is also provided, comprising the following steps:

[0014] S1: Weigh acrylamide, N-isopropylacrylamide, acrylic acid and methacryloxybutyrolactone according to weight percentage, and dissolve them in water accounting for 96% of the total water consumption to obtain a mixed solution;

[0015] S2: Weighing a co-solvent according to weight percentage, and adding the co-solvent to the mixed solution, stirring continuously, to obtain an aqueous phase;

[0016] S3: Weighing mineral oil and a primary emulsifier according to weight percentage, and mixing the mineral oil and the primary emulsifier to obtain an oil phase;

[0017] S4: adding the water phase to the oil phase under stirring conditions, and continuing stirring to obtain an inverse emulsion;

[0018] S5: transferring the inverse emulsion to a reactor and deoxygenating it under stirring using a nitrogen atmosphere;

[0019] S6: weighing an initiator by weight percentage, and dissolving the initiator in water accounting for 4% of the total water usage to obtain an initiator solution;

[0020] S7: adding the initiator solution into the reactor, maintaining a nitrogen atmosphere for protection, and performing a polymerization reaction;

[0021] S8: After the polymerization reaction is completed, the temperature is increased and stirring is continued, and then a post-treatment emulsifier is added to the reactor according to a weight percentage, and stirring is continued;

[0022] S9: Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0023] Preferably, in step S7, when the polymerization reaction is carried out, the temperature is 35-40°C and the time is 3-4h; in step S8, when the temperature is increased and stirring is continued, the temperature is increased to 55-65°C and stirring is continued for 15-30min; after adding the post-treatment emulsifier, stirring is continued for 30-40min.

[0024] On the other hand, a variable viscosity fracturing fluid is also provided, which comprises, by weight, 0.1-1.0 parts of a drag reducer, 0.1-0.6 parts of a cross-linking agent, 0.1-0.8 parts of a bactericide, 0.1-0.8 parts of a drainage aid, 0.1-0.8 parts of a clay stabilizer, 0.1-0.5 parts of a breaker and 100 parts of water, wherein the drag reducer is a multi-cross-linking point drag reducer as described above.

[0025] On the other hand, a method for preparing the variable viscosity fracturing fluid described in any one of the above is also provided, comprising the following steps:

[0026] S1': mixing a fungicide, a drainage aid, a clay stabilizer and water in parts by weight to obtain a liquid preparation water;

[0027] S2': adding the drag reducer and the cross-linking agent to the prepared water in parts by weight under stirring conditions, and continuously stirring to obtain an initial variable viscosity fracturing fluid;

[0028] S3': After the initial variable viscosity fracturing fluid forms a cross-linked gel, a gel breaker is added according to parts by weight to obtain the variable viscosity fracturing fluid.

[0029] The beneficial effects of the present invention are:

[0030] The multi-crosslinking point drag reducer of the present invention introduces multiple crosslinking points and enhances the temperature control effect through reaction activation treatment. The introduction of a post-treatment emulsifier avoids particle aggregation or precipitation caused by the increase in system temperature during the temperature increase activation process, improves the stability and dispersibility of the emulsion drag reducer, and increases the solubility and chemical interference resistance of the drag reducer. It avoids the situation in which the drag reducer reacts with the crosslinker in advance during the water-soluble thickening process due to the simultaneous addition of the drag reducer and the crosslinking agent during the preparation of the fracturing fluid, and due to the chemical interference between them, the drag reducer reacts with the crosslinking agent in advance during the water-soluble thickening process, and low-viscosity crosslinking occurs locally, the final viscosity of the fracturing fluid becomes low, and the crosslinking structure is unstable.

[0031] The multi-crosslinking point emulsion drag reducer of the present invention strengthens the crosslinking network between polymer chains, which makes the viscosity more stable under high temperature conditions. Compared with the traditional single crosslinking point drag reducer, it can better maintain the rheological properties of the liquid and reduce the viscosity fluctuation caused by temperature changes during fracturing operations.

[0032] The variable viscosity fracturing fluid of the present invention can maintain the viscosity of the fracturing fluid and the stability of the cross-linking network under high temperature environment through the multi-cross-linking point emulsion drag reducer. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0034] Figure 1 This is a schematic diagram of the rheological test results of the fracturing fluid gel formed by the multi-crosslinking point drag reducer in Example 1 at 90°C;

[0035] Figure 2 Schematic diagram of rheological test results of the fracturing fluid gel formed by the multi-crosslinking point drag reducer in Example 1 at 120°C. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0037] On the one hand, the present invention provides a multi-crosslinking point drag reducer, which includes, by weight percentage, 8-25% of acrylamide, 3-15% of N-isopropyl acrylamide, 2-8% of acrylic acid, 4-10% of methacryloyloxybutyrolactone, 20-35% of mineral oil, 0.5-5% of a primary emulsifier, 1-5% of a cosolvent, 0.5-2% of an initiator, 1-3.5% of a post-treatment emulsifier, and the balance is water.

[0038] In the present invention, a polyacrylamide emulsion drag reducer with multiple crosslinking points is formed by introducing a variety of monomers and functional additives. These multiple crosslinking points can increase the crosslinking density between polymer chains, thereby enhancing the viscosity stability of the drag reducer in a high temperature environment. Compared with the traditional single crosslinking point drag reducer, the multiple crosslinking point design of the drag reducer of the present invention can better maintain the rheological properties of the fracturing fluid, reduce the viscosity change caused by temperature fluctuations, and thus improve the stability and efficiency of the fracturing operation.

[0039] In the present invention, by using temperature-responsive monomers (such as N-isopropylacrylamide), the formation of cross-linking points is increased by later temperature activation, thereby improving the viscosity controllability and stability of the drag reducer at high temperatures, and being able to better control the timing and degree of the cross-linking reaction under actual fracturing conditions, and avoiding the problem of inefficient cross-linking of the drag reducer with the cross-linking agent in advance during the preparation of the fracturing fluid. Existing drag reducers are mostly completed by one-time cross-linking, which cannot achieve the adjustment and optimization of cross-linking points, while the present invention can enhance the temperature resistance and viscosity retention ability during use by thermally stimulating the interaction between monomers.

[0040] In a specific embodiment, the multi-crosslinking point drag reducer includes, by weight percentage, 8-15% acrylamide, 4-10% N-isopropyl acrylamide, 2-6% acrylic acid, 4-8% methacryloyloxybutyrolactone, 20-30% mineral oil, 1-3% primary emulsifier, 2-4% cosolvent, 0.5-1.2% initiator, 1-3% post-treatment emulsifier, and the balance is water.

[0041] In a specific embodiment, the mineral oil is selected from any one or more of light mineral oil, heavy mineral oil and liquid paraffin; the primary emulsifier and the post-treatment emulsifier are respectively selected from any one or more of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, Tween-80, Tween-20, Span-80, polyvinyl alcohol, polyoxyethylene ether, polyvinyl alcohol copolymer, polyacrylate, sodium long-chain alkenyl sulfonate, polyoxyethylene-polyoxypropylene copolymer, epoxidized dodecenylphenol, OP-9, OP-12, octylphenol ethoxylate, nonylphenol polyoxyethylene ether and AEO series; the cosolvent is selected from any one or more of dimethyl sulfoxide, dimethyl sulfamide, isopropanol, ethylene glycol, acetone, ethyl acetate, propylene glycol and glycerol; the initiator is an initiator of a redox initiation system.

[0042] Optionally, the oxidant in the redox initiation system is selected from any one of ammonium persulfate, sodium persulfate, hydrogen peroxide, acetone peroxide and benzoyl peroxide, and the reducing agent in the redox initiation system is selected from any one of sodium bisulfite, ascorbic acid, sodium thiosulfate, sodium hypophosphite and ethanolamine.

[0043] On the other hand, the present invention also provides a method for preparing the multi-crosslinking point drag reducer described in any one of the above, comprising the following steps:

[0044] S1: Weigh acrylamide, N-isopropylacrylamide, acrylic acid and methacryloxybutyrolactone according to weight percentage, and dissolve them in water accounting for 96% of the total water consumption to obtain a mixed solution;

[0045] S2: Weighing a co-solvent according to weight percentage, and adding the co-solvent to the mixed solution, stirring continuously, to obtain an aqueous phase;

[0046] S3: Weighing mineral oil and a primary emulsifier according to weight percentage, and mixing the mineral oil and the primary emulsifier to obtain an oil phase;

[0047] S4: adding the water phase to the oil phase under stirring conditions, and continuing stirring to obtain an inverse emulsion;

[0048] S5: transferring the inverse emulsion to a reactor and deoxygenating it under stirring using a nitrogen atmosphere;

[0049] S6: weighing an initiator by weight percentage, and dissolving the initiator in water accounting for 4% of the total water usage to obtain an initiator solution;

[0050] S7: adding the initiator solution into the reactor, maintaining a nitrogen atmosphere for protection, and performing a polymerization reaction;

[0051] S8: After the polymerization reaction is completed, the temperature is increased and stirring is continued, and then a post-treatment emulsifier is added to the reactor according to a weight percentage, and stirring is continued;

[0052] S9: Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0053] In the present invention, the emulsifier is added twice, wherein the main emulsifier of step S3 is used for emulsification, and the post-treatment emulsifier of step S8 is used to stabilize the reaction system, improve the dispersibility of the product, increase the water solubility, and optimize the rheological properties. The present invention can improve the stability of the emulsion by using the added post-treatment emulsifier, optimize the solubility and chemical interference resistance of the emulsion, and further reduce the chemical interference between the drag reducer and the cross-linking agent, thereby avoiding the low viscosity and unstable structure problems of the fracturing fluid caused by premature cross-linking during the water-soluble viscosity increase process.

[0054] In a specific embodiment, when the polymerization reaction is carried out, the temperature is 35-40°C and the time is 3-4h; in step S8, when the temperature is increased and stirring is continued, the temperature is increased to 55-65°C and stirring is continued for 15-30min; after adding the post-treatment emulsifier, stirring is continued for 30-40min.

[0055] On the other hand, the present invention also provides a variable viscosity fracturing fluid, which comprises, by weight, 0.1-1.0 parts of a drag reducer, 0.1-0.6 parts of a cross-linking agent, 0.1-0.8 parts of a bactericide, 0.1-0.8 parts of a drainage aid, 0.1-0.8 parts of a clay stabilizer, 0.1-0.5 parts of a breaker and 100 parts of water, wherein the drag reducer is a multi-cross-linking point drag reducer as described above.

[0056] In a specific embodiment, the variable viscosity fracturing fluid includes, by weight, 0.1-0.6 parts of drag reducer, 0.1-0.3 parts of cross-linking agent, 0.1-0.3 parts of fungicide, 0.2-0.5 parts of drainage aid, 0.2-0.5 parts of clay stabilizer, 0.1-0.4 parts of breaker and 100 parts of water.

[0057] In a specific embodiment, the crosslinking agent is an organic zirconium crosslinking agent.

[0058] In a specific embodiment, the organic zirconium crosslinking agent includes, by weight, 5-12 parts of an organic zirconium salt, 10-25 parts of a weak acid, 10-50 parts of a diluent, 0.5-1.5 parts of an anionic surfactant, and 20-70 parts of water.

[0059] In a specific embodiment, the organic zirconium crosslinking agent includes, by weight, 5-10 parts of an organic zirconium salt, 10-20 parts of a weak acid, 15-45 parts of a diluent, 0.5-1.0 parts of an anionic surfactant, and 25-55 parts of water.

[0060] In a specific embodiment, the organic zirconium salt is selected from any one or more of zirconium propoxide, zirconium isopropoxide and zirconium ethoxy chloride; the weak acid is selected from any one or more of acetic acid, lactic acid, citric acid, formic acid and tartaric acid; the diluent is selected from any one or more of ethanol, isopropanol, methanol, butanol and ethyl acetate; the anionic surfactant is selected from any one or more of nonylphenol polyoxyethylene ether, alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether and sodium dodecyl sulfate; the bactericide is an aldehyde bactericide; the drainage agent is a fluorocarbon surfactant; the clay stabilizer is an aqueous solution of any one or more of potassium chloride, calcium chloride and ammonium chloride, and the effective content of solute in the aqueous solution is 10-20wt%; the breaker is an aqueous solution of any one or more of ammonium persulfate, potassium persulfate and sodium persulfate, and the effective content of solute in the aqueous solution is 10-20wt%.

[0061] Optionally, the aldehyde bactericide is formaldehyde and / or glutaraldehyde; the fluorocarbon surfactant is any one or more of potassium perfluorooctane sulfonate, ammonium perfluorooctanoate, sodium perfluorohexane sulfonate, perfluorooctane sulfonyl fluoride and perfluorooctane sulfonyl carbamate.

[0062] On the other hand, the present invention also provides a method for preparing the variable viscosity fracturing fluid described in any one of the above, comprising the following steps:

[0063] S1': mixing a fungicide, a drainage aid, a clay stabilizer and water in parts by weight to obtain a liquid preparation water;

[0064] S2': adding the drag reducer and the cross-linking agent to the prepared water in parts by weight under stirring conditions, and continuously stirring to obtain an initial variable viscosity fracturing fluid;

[0065] S3': After the initial variable viscosity fracturing fluid forms a cross-linked gel, a gel breaker is added according to parts by weight to obtain the variable viscosity fracturing fluid.

[0066] In the present invention, when preparing the variable viscosity fracturing fluid, different drag reducer dosages can be added according to the different viscosity requirements of the target variable viscosity fracturing fluid. By adjusting the dosage, the viscosity of the fracturing fluid can be linearly changed, and the fracturing fluid can be divided into low viscosity, medium viscosity, and high viscosity fracturing fluids according to the viscosity. The present invention can form a fracturing fluid gel by delaying crosslinking through temperature control, and maintain the gel form until the gel breaker breaks the gel. The delayed crosslinking time control can be adjusted by the temperature and the amount of crosslinking agent, and the crosslinking time is 3 to 10 minutes.

[0067] Example 1

[0068] A multi-crosslinking point drag reducer is prepared by the following steps:

[0069] (1) Weigh 150 g of acrylamide, 50 g of N-isopropylacrylamide, 30 g of acrylic acid, and 70 g of methacryloyloxybutyrolactone, and dissolve them in 500 g of deionized water to obtain a mixed solution;

[0070] (2) adding 30 g of dimethyl sulfoxide to the mixed solution and continuing stirring to obtain an aqueous phase;

[0071] (3) Weigh 300 g of liquid paraffin and add 20 g of sodium dodecyl sulfate, mix and dissolve evenly to obtain an oil phase;

[0072] (4) adding the aqueous phase dropwise to the oil phase under continuous mechanical stirring at 500 rpm, and continuing stirring until a stable inverse emulsion is formed;

[0073] (5) Transfer the inverse emulsion to a reactor, adjust the stirring speed to 300-400 rpm, and bubble with nitrogen for 30 min;

[0074] (6) Weigh 5 g of ammonium persulfate and 5 g of sodium bisulfite, and dissolve them in 10 g of deionized water to obtain an ammonium persulfate solution and a sodium bisulfite solution, respectively;

[0075] (7) The ammonium persulfate solution and the sodium bisulfite solution were slowly dripped into the reactor through a drip pump, the reaction temperature was controlled to 35° C., nitrogen protection was maintained, and the reaction was carried out for 4 hours;

[0076] (8) After the polymerization reaction was completed, the temperature was raised to 55 °C and stirred at 200 rpm for 30 min. Then, 10 g of AEO-9 was added and the mixture was stirred slowly for 30 min.

[0077] (9) Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0078] Example 2

[0079] A multi-crosslinking point drag reducer is prepared by the following steps:

[0080] (1) Weigh 120 g of acrylamide, 70 g of N-isopropylacrylamide, 45 g of acrylic acid, and 50 g of methacryloyloxybutyrolactone, and dissolve them in 500 g of deionized water to obtain a mixed solution;

[0081] (2) adding 40 g of propylene glycol to the mixed solution and continuing stirring to obtain an aqueous phase;

[0082] (3) Weigh 250 g of white oil and add 25 g of sodium dodecyl sulfate, mix and dissolve evenly to obtain an oil phase;

[0083] (4) adding the aqueous phase dropwise into the oil phase at a rotation speed of 500-700 rpm, and continuously stirring until a stable inverse emulsion is formed;

[0084] (5) transferring the inverse emulsion to a reactor and deoxygenating it for 30 min under nitrogen protection;

[0085] (6) Weigh 6 g of ammonium persulfate and 3 g of ascorbic acid, and dissolve them in 10 g of deionized water to obtain an ammonium persulfate solution and an ascorbic acid solution, respectively;

[0086] (7) Slowly dripping the ammonium persulfate solution and the ascorbic acid solution into the reactor through a drip pump, respectively, controlling the reaction temperature to 40° C., maintaining nitrogen protection, and reacting for 4 hours;

[0087] (8) After the polymerization reaction was completed, the temperature was raised to 60 °C and stirred at 200 rpm for 15 min. Then, 15 g of AEO-7 was added and the mixture was stirred slowly for 40 min.

[0088] (9) Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0089] Example 3

[0090] A multi-crosslinking point drag reducer is prepared by the following steps:

[0091] (1) Weigh 110 g of acrylamide, 65 g of N-isopropylacrylamide, 55 g of acrylic acid, and 60 g of methacryloyloxybutyrolactone, and dissolve them in 450 g of deionized water to obtain a mixed solution;

[0092] (2) adding 25 g of propylene glycol to the mixed solution and continuing stirring to obtain an aqueous phase;

[0093] (3) Weigh 150 g of liquid paraffin and 100 g of white oil, mix them in a ratio of 3:2 to obtain mineral oil, and add 18 g of sodium dodecylbenzene sulfonate to the mineral oil, mix and dissolve them evenly, to obtain an oil phase;

[0094] (4) adding the aqueous phase dropwise to the oil phase under mechanical stirring at 700 rpm, and continuing stirring until a stable inverse emulsion is formed;

[0095] (5) The inverse emulsion was transferred to a reactor and deoxygenated for 25 minutes under nitrogen protection;

[0096] (6) Weigh 5 g of ammonium persulfate and 4 g of sodium hypophosphite, and dissolve them in 10 g of deionized water to obtain an ammonium persulfate solution and a sodium hypophosphite solution, respectively;

[0097] (7) The ammonium persulfate solution and the sodium hypophosphite solution were slowly added into the reactor through a drip pump, respectively, the reaction temperature was controlled to 40° C., the stirring speed was 400 rpm, nitrogen protection was maintained, and the reaction was carried out for 3.5 hours;

[0098] (8) After the polymerization reaction was completed, the temperature was raised to 55 °C and stirred for 20 min. Then, 20 g of nonylphenol polyoxyethylene ether (NP-10) was added and stirred for 30 min.

[0099] (9) Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0100] Example 4

[0101] A multi-crosslinking point drag reducer is prepared by the following steps:

[0102] (1) Weigh 130 g of acrylamide, 85 g of N-isopropylacrylamide, 60 g of acrylic acid, and 70 g of methacryloyloxybutyrolactone, and dissolve them in 600 g of deionized water to obtain a mixed solution;

[0103] (2) adding 35 g of isopropanol to the mixed solution and continuing stirring to obtain an aqueous phase;

[0104] (3) Weigh 200 g of liquid paraffin and 150 g of white oil, mix them in a ratio of 4:3 to obtain mineral oil, add 25 g of Span 80 to the mineral oil, mix and dissolve them evenly, and obtain an oil phase;

[0105] (4) adding the aqueous phase dropwise into the oil phase at 500-700 rpm, and continuously stirring until a stable inverse emulsion is formed;

[0106] (5) transferring the inverse emulsion to a reactor and continuously deoxygenating the reactor under a nitrogen atmosphere for 30 min;

[0107] (6) Weigh 4 g of benzoyl peroxide (BPO) and 5 g of ethanolamine (MEA), and dissolve them in 10 g of deionized water to obtain benzoyl peroxide solution and ethanolamine solution, respectively;

[0108] (7) The benzoyl peroxide solution and the ethanolamine solution were slowly dripped into the reaction kettle through a drip pump, the reaction temperature was controlled to 38° C., the rotation speed was adjusted to 400 rpm, nitrogen protection was maintained, and the reaction was carried out for 3 hours;

[0109] (8) After the polymerization reaction was completed, the temperature was raised to 65 °C and maintained for 25 min. Then, 25 g of octylphenol ethoxylate (TX-100) was added and stirred for 40 min.

[0110] (9) Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

[0111] Comparative Example 1

[0112] A conventional drag reducing agent is prepared by the following steps:

[0113] (1) Dissolve 60 g of acrylamide, 12 g of acrylic acid, and 8 g of 2-acrylamido-2-methylpropanesulfonic acid in 80 g of deionized water to prepare an aqueous phase solution;

[0114] (2) Dissolve 3 g of Span 80 in 100 g of white oil to prepare an oil phase solution;

[0115] (3) Slowly add the aqueous phase solution dropwise into the oil phase using a high-speed stirrer and continue stirring to form a stable inverse emulsion;

[0116] (4) Under nitrogen protection, heat the inverse emulsion to 50°C;

[0117] (5) Dissolve ammonium persulfate and sodium bisulfite in 10 g of deionized water respectively as initiators;

[0118] (6) Slowly add the initiator dropwise into the inverse emulsion, maintain the temperature at 50°C, the rotation speed at 500 rpm, and continue the reaction for 4 hours;

[0119] (7) After the reaction is completed, the heating is stopped and the product is cooled to room temperature to obtain the conventional drag reducer.

[0120] Comparative Example 2

[0121] A drag reducing agent is prepared by the following steps:

[0122] First, 75 g of acrylamide, 39 g of N-isopropylacrylamide, 30 g of acrylic acid, 180 g of dispersion medium (20 wt% ethanol + 20 wt% potassium chloride + 60 wt% deionized water) and 4.5 g of polydimethyldiallylammonium chloride were added to the reactor in sequence to form a mixture, stirred evenly, and nitrogen was passed through. The stirring speed was adjusted to 500 rpm and the reaction temperature was 45 °C. Then, 10 g of initiator solution prepared by 0.75 g of sodium persulfate and 0.75 g of sodium sulfite were added dropwise to the mixture, and the polymerization reaction was carried out for 10 hours. After cooling to room temperature, a water-in-water type drag reducer was obtained.

[0123] Comparative Example 3

[0124] A drag reducing agent is prepared by the following steps:

[0125] (1) Weigh 170 g of acrylamide, 40 g of acrylic acid, and 90 g of methacryloyloxybutyrolactone, and dissolve them in 500 g of deionized water to obtain a mixed solution;

[0126] (2) adding 30 g of dimethyl sulfoxide to the mixed solution and continuing stirring to obtain an aqueous phase;

[0127] (3) Weigh 300 g of liquid paraffin and add 20 g of sodium dodecyl sulfate, mix and dissolve evenly to obtain an oil phase;

[0128] (4) adding the aqueous phase dropwise into the oil phase at 500 rpm, and continuously stirring until a stable inverse emulsion is formed;

[0129] (5) Transfer the inverse emulsion to a reactor, adjust the stirring speed to 300-400 rpm, and bubble with nitrogen for 30 min;

[0130] (6) Weigh 5 g of ammonium persulfate and 5 g of sodium bisulfite, and dissolve them in 10 g of deionized water to obtain an ammonium persulfate solution and a sodium bisulfite solution, respectively;

[0131] (7) The ammonium persulfate solution and the sodium bisulfite solution were slowly dripped into the reactor through a drip pump, the reaction temperature was controlled to 35° C., nitrogen protection was maintained, and the reaction was carried out for 4 hours;

[0132] (8) After the polymerization reaction was completed, the temperature was raised to 55 °C and stirred at 200 rpm for 30 min. Then, 10 g of AEO-9 was added and stirring was continued for 30 min.

[0133] (9) Cooling the product to room temperature to obtain the drag reducing agent.

[0134] Comparative Example 4

[0135] A drag reducing agent is prepared by the following steps:

[0136] (1) Weigh 150 g of acrylamide, 50 g of N-isopropylacrylamide, 30 g of acrylic acid, and 70 g of methacryloyloxybutyrolactone, and dissolve them in 500 g of deionized water to obtain a mixed solution;

[0137] (2) adding 30 g of dimethyl sulfoxide to the mixed solution and continuing stirring to obtain an aqueous phase;

[0138] (3) Weigh 300 g of liquid paraffin and add 20 g of sodium dodecyl sulfate, mix and dissolve evenly to obtain an oil phase;

[0139] (4) adding the aqueous phase dropwise to the oil phase under continuous mechanical stirring at 500 rpm, and continuing stirring until a stable inverse emulsion is formed;

[0140] (5) Transfer the inverse emulsion to a reactor, adjust the stirring speed to 300-400 rpm, and bubble with nitrogen for 30 min;

[0141] (6) Weigh 5 g of ammonium persulfate and 5 g of sodium bisulfite, and dissolve them in 10 g of deionized water to obtain an ammonium persulfate solution and a sodium bisulfite solution, respectively;

[0142] (7) The ammonium persulfate solution and the sodium bisulfite solution were slowly dripped into the reactor through a drip pump, the reaction temperature was controlled to 35° C., nitrogen protection was maintained, and the reaction was carried out for 4 hours;

[0143] (8) Cooling the product to room temperature to obtain the drag reducing agent.

[0144] Test Example 1

[0145] At a stirring speed of 500 rpm, a certain amount of drag reducers of Examples 1 to 4 and Comparative Examples 1 to 4 were added to clean water, respectively, and stirring was continued for 3 minutes. After the stirring was completed, the viscosity of the fracturing fluid was immediately tested using a capillary viscometer or a digital six-speed viscometer. The results are shown in Table 1:

[0146] Table 1 Fracturing fluid viscosity test results

[0147]

[0148] At a stirring speed of 500 rpm, a certain amount of drag reducers of Example 1 and Comparative Examples 1 to 4 were added to clean water, and 0.5% of a cross-linking agent was added at the same time. The mixture was stirred for 3 minutes. After the stirring was completed, the viscosity of the fracturing fluid was tested using a capillary viscometer or a digital six-speed viscometer. The results are shown in Table 2:

[0149] Table 2 Fracturing fluid viscosity test results

[0150]

[0151] It can be seen from Table 1 and Table 2 that the multi-crosslinking point drag reducer of the present invention has better water solubility and molecular chain stretchability in clear water, and has a better viscosity-increasing effect on the fracturing fluid than the conventional drag reducer; and under the experimental condition of adding the crosslinking agent at the same time, the crosslinking agent will not excessively participate in the molecular swelling process of the multi-crosslinking point drag reducer of the present invention, and will not affect the viscosity development of the fracturing fluid.

[0152] In addition, the water in the experimental conditions of the above test example 1 and comparative example 1 was replaced with produced water (total mineralization 20000~30000ppm, Ca2+ Content greater than 150ppm, Mg 2+ Content is greater than 150ppm), the test results are shown in Table 3:

[0153] Table 3 Viscosity test results of fracturing fluid prepared with produced water

[0154]

[0155] It can be seen from Table 3 that the multi-crosslinking point drag reducer of the present invention has a better viscosity increasing effect in high-mineralization or high-salinity produced water rich in calcium and magnesium ions, and has better salt resistance and better water quality adaptability than conventional drag reducers.

[0156] Test Example 2

[0157] The five fracturing fluids with 0.5% drag reducer and 0.5% cross-linking agent added and clean water in Test Example 1 were placed in a 90°C water bath to test the gel formation time and gel state. The results are shown in Table 4:

[0158] Table 4 Crosslinking test results at 90°C

[0159]

[0160] As can be seen from Table 4, compared with Comparative Examples 1-4, the fracturing fluid prepared with the multi-crosslinking point drag reducer of the present invention can achieve delayed crosslinking at 90°C. After crosslinking to form a gel, the gel viscosity is higher, and it can remain stable at 90°C, and the gel will not dissolve or rehydrate. This stability enables the drag reducer to exhibit excellent temperature resistance in high-temperature fracturing operations, thereby ensuring that the proppant in the cracks is suspended for a longer time and reducing the risk of liquid loss. In addition, compared with traditional drag reducers, the gel structure is destroyed due to temperature increase, so that the fracturing fluid can still maintain a high viscosity in a high temperature environment, ensuring the effectiveness and sustainability of the operation.

[0161] Test Example 3

[0162] The rheological test of the jelly formed by cross-linking the multi-crosslinking point drag reduction prepared slick water in Test Example 2 was carried out at 90°C and 120°C. The results are as follows: Figure 1 and Figure 2 As shown. Figure 1 and Figure 2It can be seen that after 200 minutes of rheological experiments at 90°C and 120°C, the gel viscosity is still greater than 200 mPa·s, indicating that the design of increasing the polymer cross-linking points of the multi-cross-linking point drag reducer of the present invention forms a more stable three-dimensional network structure. This uniform and tight cross-linking structure will not easily break or dissolve at high temperatures, thereby avoiding the sharp drop in viscosity of traditional drag reducers under high temperature environments; it also shows that it has more stable rheological properties under underground working conditions, can effectively support the opening of cracks and the suspension of proppants, and meet the needs of actual construction.

[0163] In summary, the present invention can form a polyacrylamide emulsion drag reducer with multiple crosslinking points by introducing multiple monomers and functional additives. These multiple crosslinking points can increase the crosslinking density between polymer chains and enhance the viscosity stability of the drag reducer in a high temperature environment.

[0164] The multi-crosslinking point drag reducer of the present invention has a temperature-responsive monomer (such as NIPAAm) added to the formula, and the formation of more crosslinking points is triggered by the later temperature rise activation during the preparation process. The introduction of a post-treatment emulsifier helps to further disperse the polymer chains of the drag reducer during the temperature rise activation process, reduce local aggregation, and make the crosslinking points more evenly distributed in the entire polymer network, thereby forming a more stable three-dimensional structure, improving the viscosity controllability and stability of the drag reducer at high temperatures, and being able to better control the timing and degree of the crosslinking reaction under actual fracturing conditions, avoiding the problem of inefficient crosslinking of the drag reducer with the crosslinking agent in advance during the preparation process. In the past, drag reducers were mostly completed by one-time crosslinking, which could not achieve the adjustment and optimization of crosslinking points. In the preparation method of the present invention, the interaction between monomers is thermally stimulated, and the temperature resistance and viscosity retention ability can be enhanced during use.

[0165] The present invention can reduce the chemical interference between the drag reducer and the cross-linking agent by designing a multi-cross-linking point drag reducer formula, activating the temperature reaction during the preparation process, and optimizing the post-treatment emulsifier, thereby avoiding the low viscosity and unstable structure problems of the fracturing fluid caused by premature cross-linking during the water-soluble viscosity increasing process.

[0166] Compared with the traditional single cross-linking point drag reducer, the multi-cross-linking point design of the present invention can better maintain the rheological properties of the fracturing fluid, reduce the viscosity change caused by temperature and pressure fluctuations, and thus improve the stability and efficiency of the fracturing operation.

[0167] The above description is only a representative embodiment of the present invention and does not limit the present invention in any form. Any technician familiar with the profession, without departing from the scope of the technical solution of the present invention, uses the above-disclosed technical contents to make some changes or modifications to the embodiments are equivalent embodiments of the present invention. However, any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A multi-crosslinking point drag reducer, characterized in that: Calculated by weight percentage, it includes 8-25% acrylamide, 3-15% N-isopropyl acrylamide, 2-8% acrylic acid, 4-10% methacryloyloxybutyrolactone, 20-35% mineral oil, 0.5-5% primary emulsifier, 1-5% auxiliary solvent, 0.5-2% initiator, 1-3.5% post-treatment emulsifier, and the balance is water.

2. The multi-crosslinking point drag reducer according to claim 1, characterized in that: Calculated by weight percentage, it includes 8-15% acrylamide, 4-10% N-isopropyl acrylamide, 2-6% acrylic acid, 4-8% methacryloyloxybutyrolactone, 20-30% mineral oil, 1-3% primary emulsifier, 2-4% auxiliary solvent, 0.5-1.2% initiator, 1-3% post-treatment emulsifier, and the balance is water.

3. The multi-crosslinking point drag reducer according to claim 1 or 2, characterized in that: The mineral oil is selected from any one or more of light mineral oil, heavy mineral oil and liquid paraffin.

4. The multi-crosslinking point drag reducer according to claim 1 or 2, characterized in that: The primary emulsifier and the post-treatment emulsifier are respectively selected from any one or more of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, Tween-80, Tween-20, Span-80, polyvinyl alcohol, polyoxyethylene ether, polyvinyl alcohol copolymer, polyacrylate, sodium long-chain olefin sulfonate, polyoxyethylene-polyoxypropylene copolymer, epoxidized dodecenylphenol, octylphenol ethoxylate, nonylphenol polyoxyethylene ether and AEO series.

5. The multi-crosslinking point drag reducer according to claim 1 or 2, characterized in that: The co-solvent is selected from any one or more of dimethyl sulfoxide, dimethylsulfamide, isopropanol, ethylene glycol, acetone, ethyl acetate, propylene glycol and glycerol.

6. The multi-crosslinking point drag reducer according to claim 1 or 2, characterized in that: The initiator is an initiator of a redox initiation system, the oxidant in the redox initiation system is selected from any one of ammonium persulfate, sodium persulfate, hydrogen peroxide, acetone peroxide and benzoyl peroxide, and the reducing agent in the redox initiation system is selected from any one of sodium bisulfite, ascorbic acid, sodium thiosulfate, sodium hypophosphite and ethanolamine.

7. The method for preparing a multi-crosslinking point drag reducer according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1: Weigh acrylamide, N-isopropylacrylamide, acrylic acid and methacryloxybutyrolactone according to weight percentage, and dissolve them in water accounting for 96% of the total water consumption to obtain a mixed solution; S2: Weighing a co-solvent according to weight percentage, and adding the co-solvent to the mixed solution, stirring continuously, to obtain an aqueous phase; S3: Weighing mineral oil and a primary emulsifier according to weight percentage, and mixing the mineral oil and the primary emulsifier to obtain an oil phase; S4: adding the water phase to the oil phase under stirring conditions, and continuing stirring to obtain an inverse emulsion; S5: transferring the inverse emulsion to a reactor and deoxygenating it under stirring using a nitrogen atmosphere; S6: weighing an initiator by weight percentage, and dissolving the initiator in water accounting for 4% of the total water usage to obtain an initiator solution; S7: adding the initiator solution into the reactor, maintaining a nitrogen atmosphere for protection, and performing a polymerization reaction; S8: After the polymerization reaction is completed, the temperature is increased and stirring is continued, and then a post-treatment emulsifier is added to the reactor according to a weight percentage, and stirring is continued; S9: Cooling the product to room temperature to obtain the multi-crosslinking point drag reducer.

8. The method for preparing a multi-crosslinking point drag reducer according to claim 7, characterized in that: In step S7, when the polymerization reaction is carried out, the temperature is 35-40°C and the time is 3-4h; in step S8, when the temperature is increased and stirring is continued, the temperature is increased to 55-65°C and stirring is continued for 15-30min; after adding the post-treatment emulsifier, stirring is continued for 30-40min.

9. A variable viscosity fracturing fluid, characterized in that: The invention comprises, by weight, 0.1-1.0 parts of drag reducer, 0.1-0.6 parts of cross-linking agent, 0.1-0.8 parts of fungicide, 0.1-0.8 parts of drainage aid, 0.1-0.8 parts of clay stabilizer, 0.1-0.5 parts of degumming agent and 100 parts of water. The drag reducer is the multi-cross-linking point drag reducer according to any one of claims 1 to 6.

10. The method for preparing the variable viscosity fracturing fluid according to claim 9, characterized in that: The following steps are involved: S1': mixing a fungicide, a drainage aid, a clay stabilizer and water in parts by weight to obtain a liquid preparation water; S2': adding the drag reducer and the cross-linking agent to the prepared water in parts by weight under stirring conditions, and continuously stirring to obtain an initial variable viscosity fracturing fluid; S3': After the initial variable viscosity fracturing fluid forms a cross-linked gel, a gel breaker is added according to parts by weight to obtain the variable viscosity fracturing fluid.

Images