Sand-carrying oil displacement fracturing fluid based on physical crosslinking and preparation method and application thereof

Abstract

The invention provides a sand-carrying oil displacement fracturing fluid based on physical crosslinking as well as a preparation method and application of the sand-carrying oil displacement fracturing fluid. The preparation method comprises the following steps: carrying out polymerization reaction on acrylamide, acrylic acid, 2-acrylamide-2-methylpropanesulfonic acid and polyoxypropylene to obtain a fracturing fluid main agent; the solution of the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer is mixed with a fracturing fluid main agent, and the sand-carrying oil displacement fracturing fluid based on physical crosslinking is obtained. The sand-carrying oil displacement fracturing fluid provided by the invention is prepared by the preparation method. The invention also provides application of the sand-carrying oil displacement fracturing fluid in shale oil exploitation. The sand-carrying oil displacement fracturing fluid has the advantages of high sand-carrying property, low damage, high oil displacement property and the like, can realize integration of pressure and displacement, is suitable for shale oil exploitation, and can effectively reduce the liquid cost in the fracturing construction process and enhance the fracturing oil displacement effect.

Description

Technical Field

[0001] The present invention relates to a sand-carrying displacement fracturing fluid based on physical crosslinking, a preparation method thereof and an application thereof, and belongs to the technical field of fracturing fluids for oil and gas field development. Background Art

[0002] Shale oil exploitation mainly adopts the cluster perforation technology for operation. The fracturing fluid system mainly uses polyacrylamide polymers. However, with the in-depth development of oil and gas resources, in order to meet the exploitation requirements of unconventional oil and gas fields, new fracturing systems have been developed one after another, becoming potential means for the exploitation of high-temperature and high-salt reservoirs. It is worth noting that most of these drag-reducing polymer agents have low viscosity and poor crosslinking performance. Some slickwater fracturing fluid systems use metal crosslinking to improve the sand-carrying capacity of the system, but the subsequent residues of the system will cause secondary damage to the reservoir, and the backflow of the breaker will also affect the formation permeability. Therefore, the integrated fracturing and displacement process is proposed. The integrated fracturing and displacement means that the fracturing fluid does not need to be backflowed and directly undergoes imbibition displacement of oil through the production well, which solves the contradiction between sand-carrying and backflow to a certain extent. At present, the essence of the integrated fracturing and displacement technology is to introduce a surfactant-containing substance on the basis of traditional fracturing fluids. After the fracturing construction is completed, the surfactant adsorbs on the rock surface, changes the rock wettability, and then realizes the integrated fracturing and displacement of oil by replacing the capillary force of the reservoir from resistance to driving force and replacing the oil and gas.

[0003] The fracturing displacement working fluid is an important component for realizing the integrated fracturing and displacement process. The fracturing fluid flowback fluid displacement technology uses viscoelastic surfactants as the main agent. Since surfactant molecules have both hydrophilic and hydrophobic groups, when their concentration exceeds the critical micelle concentration, various self-assembled structures can be formed according to their chemical structure and solvent environment, such as spherical, disc-shaped, worm-like, and vesicles (Hoffmann H. Fascinating phenomena in Surfactant Chemistry[J]. Adv Mater, 1994, 6(2): 116-29.). When the surfactant molecules in the fracturing fluid system self-assemble into micelles, the spatial network structure makes the solution exhibit excellent rheological properties. Such fluids can be used as fracturing fluids to increase formation fracturing production by carrying sand and gravel. In the reservoir environment, the micelles swell and decompose, and displacement agents are in-situ formed by releasing small molecule surfactants, and the purpose of increasing oil displacement production is achieved through adsorption and imbibition. CN107828399A discloses a displacement agent prepared from fracturing fluid flowback fluid and its preparation method, which uses fracturing fluid flowback fluid as the base fluid and adds various types of surface active additives to construct the displacement agent. For reservoirs with a permeability of 4000-16000 mD, this system can increase the recovery rate by 28%-32%. Wang Huan et al. constructed a clean fracturing fluid system RSH-2 with octadecyl trimethyl ammonium chloride. After the system is broken, when it is compounded with α-olefin sulfonate, the recovery rate can be further increased by 11.8% on the basis of water flooding (Wang Huan, You Qing, Han Kun, etc. Construction and performance evaluation of a compound displacement system for clean fracturing flowback fluid[J]. Oilfield Chemistry, 2018, 35(2): 302-307.). Zhou Wensheng et al. constructed a surfactant displacement system based on clean fracturing fluid flowback fluid with a quaternary ammonium salt surfactant containing a stearic acid hydrophobic link. This system can cause wetting reversal on the surface of oil-wet quartz chips, and increase the crude oil recovery rate by 12.5% on the basis of water flooding (Zhou Wensheng, Wang Kai, Liu Chen, etc. Research on the reuse displacement system of clean fracturing fluid flowback fluid[J]. Lithologic Reservoirs, 2017, 29(2): 160-166.). Wu Xinmin et al. constructed a clean fracturing fluid system with Gemini cationic surfactant as the thickening agent. The formation breakage fluid of the Gemini cationic surfactant clean fracturing fluid system has obvious pressure reduction and injection increase effects and oil displacement ability, and can increase the recovery rate by 10.69%-12.50% (Wu Xinmin, Chen Yanan. Feasibility experimental study on oil displacement by a high-performance clean fracturing fluid flowback fluid[J]. Science Technology and Engineering, 2017, 17(32): 245-250.).Although the displacement fracturing fluids constructed based on clean fracturing fluids have good oil displacement effects, they have poor temperature and salt tolerance, a large amount of surfactant is used, the cost of large-displacement volume fracturing is high, and the drag reduction effect decreases significantly during shearing, which is not suitable for large-scale exploitation and popularization of shale oil (Zhang Zhisheng. Multifunctional surfactant displacement fracturing fluid system for tight sandstone reservoirs [J]. Petroleum Geology & Oilfield Development in Daqing, 2020, 39(1): 169-174.; CAO Huihui, NI Yunfeng. Laboratory experimental study on a new type of compound fracturing fluid for polymer flooding wells [C]. China Key Laboratory of Oil and Gas Reservoir Reconstruction. Proceedings of the 4th National Symposium on Fracturing and Acidizing Technology for Low Permeability Oil and Gas Reservoirs. 2010: 509-513.).

[0004] Based on the stronger temperature and salt tolerance of polymers, CN110791277A discloses a fracturing fluid for oil fields that can be converted into an oil displacement agent without flowback and its preparation method. The displacement fracturing fluid is prepared from polyacrylamide, organic zirconium salt and / or organic aluminum salt or phenolic resin, and persulfate. The displacement fracturing fluid can use a catalyst to catalyze the oxidation of persulfate to break the molecular chain of the polymer in the reservoir, and no insoluble substances are generated during the gel-breaking process. This system can increase the oil recovery rate by more than 12% on the basis of water flooding. CN106905947A discloses a displacement fracturing fluid, its preparation method and application, and an oxidation amine surfactant, hydrophobic polyacrylamide and ethylene glycol monobutyl ether are used to construct a displacement fracturing fluid system. The gel-breaking fluid of this displacement fracturing fluid system has a more significant effect of reducing the oil-water interfacial tension compared with the gel-breaking fluid of conventional fracturing fluid, the cetyl sulfonate oil displacement agent solution and the sodium dodecyl benzene sulfonate oil displacement agent solution commonly used in oil fields. The oil displacement efficiency can be increased by 48.86% in the core flooding experiment. However, the polymer dosage of the above fracturing fluids is high, resulting in a large amount of residue and high damage. Moreover, in essence, they are still a mixture of multiple agents, and there is no mutually beneficial effect on sand carrying and oil displacement effects, and the integration has not been truly achieved (He Xiaodong, Zhu Jiawei, Shi Shanzhi, etc. Nano-displacement slickwater system for Mahu tight glutenite reservoir [J]. Drilling Fluid & Completion Fluid, 2019, 36(5): 629-633.). Summary of the Invention

[0005] To solve the above technical problems, the object of the present invention is to provide a sand-carrying displacement fracturing fluid based on physical crosslinking, its preparation method and application. The sand-carrying displacement fracturing fluid of the present invention has the advantages of high sand-carrying capacity, low damage and high oil displacement property, and can realize the integration of fracturing and displacement.

[0006] To achieve the above object, the first aspect of the present invention provides a preparation method of a sand-carrying displacement fracturing fluid based on physical crosslinking, which includes the following steps:

[0007] (1) Polymerize acrylamide (AM), acrylic acid (AA), 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and polyoxypropylene to obtain the main agent of the fracturing fluid;

[0008] (2) Mix the solution of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer with the main agent of the fracturing fluid to obtain the sand-carrying displacement fracturing fluid based on physical crosslinking.

[0009] In the above preparation method, preferably, in step (1), the polymerization reaction is carried out by the method of inverse emulsion polymerization. More preferably, step (1) specifically includes: (a) Add acrylamide, acrylic acid and 2-acrylamido-2-methylpropanesulfonic acid to water and dissolve them by stirring to obtain a solution of hydrophilic monomers; (b) Add polyoxypropylene to the solution of hydrophilic monomers, and add a dispersant to fully disperse polyoxypropylene after ultrasonic treatment, and then adjust the pH of the solution to 5.8-7.0 to form the aqueous phase of the inverse emulsion; (c) Add an emulsifier to the oil phase solvent, and fully stir and dissolve it to form the oil phase of the inverse emulsion; (d) Mix the oil phase with the aqueous phase, and emulsify for a period of time to form a stable inverse emulsion; (e) In a protective gas environment, carry out a free radical polymerization reaction on the inverse emulsion through a redox initiator system to obtain the main agent of the fracturing fluid.

[0010] In the above preparation method, preferably, in step (1), the molecular weight of the polyoxypropylene is 200-4000.

[0011] In the above preparation method, preferably, in step (1), the mass ratio of acrylamide, acrylic acid, 2-acrylamido-2-methylpropanesulfonic acid and polyoxypropylene is 1:0.4-0.6:0.15-0.3:0.01-0.05.

[0012] In the above preparation method, preferably, in step (1), the ratio of the total mass of monomers in the aqueous phase to the total mass of the aqueous phase is 40-50%.

[0013] In the above preparation method, preferably, in step (1), the dispersant includes one or a combination of several of Tween 80, Tween 60, AEO-7, AEO-9, OP-9, OP-10, etc. More preferably, the addition amount of the dispersant is in a mass ratio of 0.2-0.4:1 to the polyoxypropylene.

[0014] In the above preparation method, preferably, in step (1), adjusting the pH of the solution to 5.8-7.0 is carried out using an aqueous solution of one or several of sodium bicarbonate, acetic acid, hydrochloric acid, sodium hydroxide, etc.

[0015] In the above preparation method, preferably, in step (1), the oil-phase solvent includes one or a combination of several of solvent oil No. 5, liquid paraffin, solvent oil No. 7, n-heptane, etc.

[0016] In the above preparation method, preferably, in step (1), the emulsifier includes one or a combination of several of Span80, Span60, Tween 80, alkylphenol polyoxyethylene ethers, etc.

[0017] In the above preparation method, preferably, in step (1), the mass ratio of the emulsifier to the oil-phase solvent is 0.1-0.2:1.

[0018] In the above preparation method, preferably, in step (1), the mixing volume ratio of the oil phase to the water phase is 0.2-0.5:1.

[0019] In the above preparation method, preferably, in step (1), after mixing the oil phase and the water phase and emulsifying for 1-5 min, a stable inverse emulsion is formed.

[0020] In the above preparation method, preferably, in step (1), the redox initiator system includes a combination of tert-butyl hydroperoxide and sodium metabisulfite, a combination of hydrogen peroxide and sodium sulfite, or a combination of sodium persulfate and sodium sulfite, etc. More preferably, the addition amount of the redox initiator system is in a mass ratio of 0.01-0.03:1 to the total amount of monomers (i.e., the total amount of AM, AA, AMPS, and polyoxypropylene). The ratio of the oxidant to the reductant in the redox initiator system can be adjusted conventionally by those skilled in the art, and the present invention does not impose special restrictions on it.

[0021] In the above preparation method, preferably, in step (1), the temperature of the free radical polymerization reaction is 35-45 °C, and the time is 1-3 h.

[0022] In the above preparation method, preferably, in step (2), the mass concentration of the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer solution is 0.1-0.2%.

[0023] In the above preparation method, preferably, in step (2), the mass ratio of the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer to the main fracturing fluid agent is 0.2-1:1.

[0024] In the above preparation method, preferably, in step (2), the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer includes one or a combination of several of F127, P123, P84, P65, etc.

[0025] In the above preparation method, preferably, in step (2), mixing the solution of the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer with the main fracturing fluid agent specifically includes: stirring the solution of the polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer at a high speed of 500-1000 r / min to form a vortex, and adding the main fracturing fluid agent into the vortex at room temperature, and stirring until completely dissolved to obtain the sand-carrying oil-displacing fracturing fluid based on physical crosslinking.

[0026] The second aspect of the present invention provides a sand-carrying oil-displacing fracturing fluid based on physical crosslinking, which is prepared by the above preparation method of the sand-carrying oil-displacing fracturing fluid based on physical crosslinking.

[0027] The third aspect of the present invention provides the application of the above sand-carrying oil-displacing fracturing fluid based on physical crosslinking in shale oil exploitation.

[0028] According to the specific embodiments of the present invention, preferably, the application includes: fracturing the shale oil reservoir with the sand-carrying oil-displacing fracturing fluid based on physical crosslinking, and then without flowback, directly using the gel-breaking fluid after gel-breaking of the sand-carrying oil-displacing fracturing fluid based on physical crosslinking as an oil-displacing agent to displace the shale oil reservoir to achieve integrated fracturing and displacement. Among them, the gel-breaking agent used for gel-breaking can be a gel-breaking agent in the prior art, and the present invention does not specifically limit it.

[0029] In the present invention, through the method of inverse emulsion polymerization, hydrophilic monomers AA, AM, AMPS and the surface-active hydrophobic monomer polyoxypropylene (block monomer) are used for quaternary free radical polymerization to prepare an emulsion-type polymer as the main fracturing fluid agent. An aqueous solution formed by dissolving Pluronic triblock copolymer in water is used as a nano-micelle supramolecular crosslinking agent solution. The present invention modifies conventional polyacrylamide polymers with surface-active hydrophobic monomers to enhance the shear resistance of the polymers, and at the same time introduces a small amount of Pluronic triblock copolymer surfactant to form a nano-micelle supramolecular crosslinking agent to jointly form a supramolecular-polymer network structure based on physical crosslinking. The branched chains of polymer molecules are embedded in the nano-micelles to construct a sand-carrying oil-displacing fracturing fluid.

[0030] According to the specific embodiments of the present invention, the schematic diagram of the formation of the sand-carrying displacement fracturing fluid and the gel-breaking displacement agent provided by the present invention is as Figure 1 shown. The sand-carrying displacement fracturing fluid of the present invention uses a Pluronic triblock copolymer surfactant to form a nano-micelle supramolecular crosslinking agent. The branched chains of the polymer molecules prepared by polymerizing AA, AM, AMPS and polyoxypropylene are embedded in the nano-micelles, obtaining a sand-carrying displacement fracturing fluid based on physical crosslinking. After the sand-carrying displacement fracturing fluid breaks the gel, the surface-active hydrophobic monomers after the polymer chains break and the surfactant molecules after the disassembly of the nano-micelle supramolecular crosslinking agent serve as the gel-breaking displacement agent, and imbibition displacement of oil can be directly carried out without flowback.

[0031] The technical solution of the present invention has at least the following beneficial effects:

[0032] Benefiting from the supramolecular-polymer network structure, the fracturing fluid of the present invention has excellent sand-carrying effect. Benefiting from physical crosslinking, the fracturing fluid of the present invention breaks the gel completely and has low damage. At the same time, the surface-active hydrophobic monomers after the polymer chains break and the disassembled nano-micelle supramolecular crosslinking agent serve as displacement agents to assist in the exploitation of crude oil, realizing the integration of pressure displacement. The sand-carrying displacement fracturing fluid of the present invention has excellent sand-carrying effect, and can suspend sand for 300 - 700 min at room temperature; based on physical crosslinking, it has less gel-breaking residue and the core damage is <20%; the oil washing efficiency of the gel-breaking fluid is higher than 90% (under the condition of 70 - 90 °C), the imbibition displacement oil rate can be increased by 1 - 2 times compared with water, and the displacement efficiency is increased by more than 10%. Therefore, the sand-carrying displacement fracturing fluid based on physical crosslinking of the present invention is a new type of multifunctional sand-carrying displacement fracturing fluid with high sand-carrying property, low damage and high oil displacement property. It will become a sharp weapon for shale oil exploitation, and can effectively reduce the liquid cost during the fracturing construction process and enhance the fracturing displacement effect. Description of the Drawings

[0033] Figure 1 It is a schematic diagram of the formation of the sand-carrying displacement fracturing fluid and the gel-breaking displacement agent provided by the specific embodiments of the present invention.

[0034] Figure 2 It is the SEM images of the fracturing fluids prepared in Comparative Examples 1 - 2 and the fracturing fluids PN-PPS-n prepared in Examples 1 - 3.

[0035] Figure 3 It is the curve of the change of the fracturing fluid viscosity with time of the fracturing fluid PN-PPS-1 prepared in Example 1 at 25 °C and a constant shear rate of 170 S -1 condition.

[0036] Figure 4 It is the dynamic rheological curves of the fracturing fluid prepared in Comparative Example 1 and the fracturing fluid PN-PPS-1 prepared in Example 1.

[0037] Figure 5 Sand-carrying time and ceramsite settlement velocity diagrams of the fracturing fluid prepared in Comparative Example 1 and the fracturing fluid PN-PPS-1 prepared in Example 1.

[0038] Figure 6 Viscosity and residue content diagrams of the gel-breaking oil displacement agent formed after gel-breaking of the fracturing fluid PN-PPS-1 prepared in Example 1 at different APS concentrations.

[0039] Figure 7 Curves of pressure and recovery rate varying with the injection volume of the gel-breaking oil displacement agent of Example 1 in the core flooding experiment. Detailed implementation mode

[0040] 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.

[0041] Example 1

[0042] This example provides a sand-carrying and oil-displacing fracturing fluid based on physical crosslinking, and its preparation method includes the following steps:

[0043] (1) Preparation of the main agent of the polymer fracturing fluid

[0044] (a) Add AM, AA, and AMPS to water and dissolve them by stirring to obtain a solution of hydrophilic monomers; (b) Add polyoxypropylene with a molecular weight of 400 to the solution of hydrophilic monomers, and add the dispersant Tween 80. The addition amount of the dispersant is 0.2:1 by mass ratio to polyoxypropylene, and make polyoxypropylene fully dispersed after ultrasonic treatment. Subsequently, adjust the pH of the solution to 6.5 with 30 wt% sodium hydroxide aqueous solution to form the aqueous phase of the inverse emulsion. Among them, the mass ratio of AM, AA, AMPS, and polyoxypropylene is 1:0.5:0.2:0.01, and the ratio of the total mass of monomers in the aqueous phase to the total mass of the aqueous phase is 42.85%; (c) Add the emulsifier Span80 to the 5# solvent oil, and the mass ratio of the emulsifier to the 5# solvent oil is 0.1:1. After fully stirring and dissolving, form the oil phase of the inverse emulsion; (d) Mix the oil phase and the aqueous phase at a volume ratio of 3:7, and emulsify for 2 min to form a stable inverse emulsion; (e) In an N2 environment, use a combination of tert-butyl hydroperoxide and sodium metabisulfite as the redox initiator system. The mass ratio of the redox initiator system to the total amount of monomers is 0.01:1, and make the inverse emulsion undergo a free radical polymerization reaction. The reaction temperature is 40 °C and the time is 2 h to obtain the main agent PPS-1 of the polymer fracturing fluid;

[0045] (2) Preparation of the sand-carrying and oil-displacing fracturing fluid

[0046] The polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer P123 was dissolved in water by magnetic stirring to obtain a nano-micelle supramolecular crosslinking agent solution with a mass concentration of 0.1%. The nano-micelle supramolecular crosslinking agent solution was subjected to high-speed stirring at a speed of 800 r / min to form a vortex. Under room temperature conditions, the emulsion after inverse emulsion polymerization in step (1) (i.e., the main agent of the polymer fracturing fluid PPS-1) was added to the vortex so that the mass concentration of the main agent of the polymer fracturing fluid PPS-1 in the solution was 0.5%. After stirring until completely dissolved, a sand-carrying displacement fracturing fluid based on physical crosslinking was obtained, denoted as PN-PPS-1.

[0047] Example 2

[0048] This example provides a sand-carrying displacement fracturing fluid based on physical crosslinking, and its preparation method includes the following steps:

[0049] (1) Preparation of the main agent of the polymer fracturing fluid

[0050] (a) AM, AA, and AMPS were added to water and dissolved by stirring to obtain a solution of hydrophilic monomers; (b) polyoxypropylene with a molecular weight of 1000 was added to the solution of hydrophilic monomers, and dispersant OP-10 was added. The addition amount of the dispersant was 0.3:1 by mass ratio to polyoxypropylene, so that polyoxypropylene was fully dispersed after ultrasonic treatment. Subsequently, the pH of the solution was adjusted to 6.2 with an aqueous sodium bicarbonate solution to form the aqueous phase of the inverse emulsion. Among them, the mass ratio of AM, AA, AMPS, and polyoxypropylene was 1:0.5:0.3:0.02, and the ratio of the total mass of monomers in the aqueous phase to the total mass of the aqueous phase was 45.5%; (c) emulsifier Span60 was added to 5# solvent oil, and the mass ratio of the emulsifier to 5# solvent oil was 0.15:1. After fully stirring and dissolving, the oil phase of the inverse emulsion was formed; (d) the oil phase and the aqueous phase were mixed at a volume ratio of 3:7, and after emulsification for 5 min, a stable inverse emulsion was formed; (e) in an N2 environment, a redox initiator system composed of sodium persulfate and sodium sulfite was used, and the mass ratio of the redox initiator system to the total amount of monomers was 0.02:1. The inverse emulsion was subjected to a free radical polymerization reaction at a reaction temperature of 35 °C for 3 h to obtain the main agent of the polymer fracturing fluid PPS-2;

[0051] (2) Preparation of the sand-carrying displacement fracturing fluid

[0052] The polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer P123 was dissolved in water by magnetic stirring to obtain a nano-micelle supramolecular crosslinking agent solution with a mass concentration of 0.2%. The nano-micelle supramolecular crosslinking agent solution was stirred at a high speed of 800 r / min to form a vortex. Under room temperature conditions, the emulsion after inverse emulsion polymerization in step (1) (i.e., the main agent of the polymer fracturing fluid PPS-2) was added to the vortex so that the mass concentration of the main agent of the polymer fracturing fluid PPS-2 in the solution was 0.5%. Stir until completely dissolved to obtain a sand-carrying oil-displacing fracturing fluid based on physical crosslinking, denoted as PN-PPS-2.

[0053] Example 3

[0054] This example provides a sand-carrying oil-displacing fracturing fluid based on physical crosslinking, and its preparation method includes the following steps:

[0055] (1) Preparation of the main agent of the polymer fracturing fluid

[0056] (a) AM, AA, and AMPS were added to water and dissolved by stirring to obtain a solution of hydrophilic monomers; (b) polyoxypropylene with a molecular weight of 4000 was added to the solution of hydrophilic monomers, and dispersant OP-10 was added. The addition amount of the dispersant was 0.2:1 in mass ratio to polyoxypropylene, so that polyoxypropylene was fully dispersed after ultrasonic treatment. Subsequently, the pH of the solution was adjusted to 6.2 with 30 wt% alkali solution to form the aqueous phase of the inverse emulsion. Among them, the mass ratio of AM, AA, AMPS, and polyoxypropylene was 1:0.6:0.3:0.02, and the ratio of the total mass of monomers in the aqueous phase to the total mass of the aqueous phase was 48%; (c) emulsifier Span60 was added to 5# solvent oil, and the mass ratio of the emulsifier to 5# solvent oil was 0.15:1. After fully stirring and dissolving, the oil phase of the inverse emulsion was formed; (d) the oil phase and the aqueous phase were mixed at a volume ratio of 3:7 and emulsified for 2 min to form a stable inverse emulsion; (e) in an N2 environment, a redox initiator system composed of tert-butyl hydroperoxide and sodium metabisulfite was used, and the mass ratio of the redox initiator system to the total amount of monomers was 0.01:1. The inverse emulsion was subjected to a free radical polymerization reaction. The reaction temperature was 43 °C and the time was 2 h to obtain the main agent of the polymer fracturing fluid PPS-3;

[0057] (2) Preparation of the sand-carrying oil-displacing fracturing fluid

[0058] The polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer F127 was dissolved in water by magnetic stirring to obtain a nano-micelle supramolecular crosslinking agent solution with a mass concentration of 0.2%. The nano-micelle supramolecular crosslinking agent solution was stirred at a high speed of 800 r / min to form a vortex. Under room temperature conditions, the emulsion after inverse emulsion polymerization in step (1) (i.e., the polymer fracturing fluid main agent PPS-3) was added to the vortex so that the mass concentration of the polymer fracturing fluid main agent PPS-3 in the solution was 0.5%. Stir until completely dissolved to obtain a sand-carrying oil-displacing fracturing fluid based on physical crosslinking, denoted as PN-PPS-3.

[0059] Comparative Example 1

[0060] This comparative example provides a polymer fracturing fluid, which is used as a comparison with Example 1. The polymer fracturing fluid main agent PPS-1 prepared in Example 1 was used, but the triblock copolymer P123 was not used.

[0061] The preparation method of the polymer fracturing fluid in this comparative example includes the following steps: Pure water was stirred at a high speed of 800 r / min to form a vortex. Under room temperature conditions, the polymer fracturing fluid main agent PPS-1 was added to the vortex so that the mass concentration of the polymer fracturing fluid main agent PPS-1 in the solution was 0.5%. Stir until completely dissolved to obtain the polymer fracturing fluid.

[0062] Comparative Example 2

[0063] This comparative example provides a sand-carrying oil-displacing fracturing fluid based on physical crosslinking, which is used as a comparison with Example 1.

[0064] The preparation method of the fracturing fluid in this comparative example is basically the same as that in Example 1, except that: In the preparation process of the polymer fracturing fluid main agent, polyoxypropylene monomer was not used, and the remaining steps were the same as those in Example 1.

[0065] Test Example

[0066] The structures and properties of the fracturing fluids prepared in the above examples and comparative examples were evaluated as follows.

[0067] 1. Microstructure

[0068] The fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluids PN-PPS-1, PN-PPS-2, and PN-PPS-3 prepared in Examples (1, 2, 3) were subjected to SEM detection, and the results are as Figure 2 shown. Figure 2 In (a) is the SEM image of the fracturing fluid PN-PPS-1 in Example 1, Figure 2 In (b) is the SEM image of the fracturing fluid PN-PPS-2 in Example 2, Figure 2Among them, (c) is the SEM image of the fracturing fluid PN-PPS-3 in Example 3, Figure 2 and (d) among them is the SEM image of the fracturing fluid in Comparative Example 1, Figure 2 and (e) among them is the SEM image of the fracturing fluid in Comparative Example 2. It can be seen from Figure 2 that a spatial network structure exists in all five fracturing fluid systems. The network formed by the PN-PPS system with the introduction of the nano-micelle supramolecular crosslinking agent is denser, with smaller and denser pore networks, which confirms the enhancing effect of the nano-micelle supramolecular crosslinking agent on the network structure and explains the reason why the viscosity of the crosslinked system is always higher than that of the non-crosslinked system.

[0069] 2. Shear resistance

[0070] The shear resistance of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluids PN-PPS-n prepared in Examples (1, 2, 3) was tested under the conditions of 25 °C and a constant shear rate of 170S -1 . The results are shown in Table 1 and Figure 3 . It can be seen from Table 1 and Figure 3 that the viscosity of the fracturing fluid in Example 1 remained at 76 - 77 mPa·s within 30 min under the conditions of 25 °C and a constant shear rate of 170S -1 , which confirmed that the fracturing fluid has excellent shear resistance. Moreover, compared with Comparative Examples 1 and 2, each example of the present invention has better shear resistance.

[0071] Table 1 Shear resistance of different fracturing fluids (170s -1 )

[0072] Number Initial Viscosity (mPa.s) Viscosity after Shearing for 30 min (mPa.s) Comparative Example 1 46 41 Comparative Example 2 44 38 Example 1 77 76 Example 2 64 61 Example 3 53 53

[0073] 3. Viscoelasticity

[0074] Viscoelasticity is an important index for evaluating fracturing fluids. Good viscoelastic properties can enable it to achieve a good stable suspension effect even under low viscosity (≤30 mPa·s) conditions. In order to further explore the viscoelasticity of the fracturing fluid, the dynamic rheological curves of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluids PN-PPS-n prepared in Examples (1, 2, 3) were tested. The test steps are as follows: Use a Platte P60 Ti L parallel plate rotor with a spacing of 1.000 mm; Take 2 mL of the prepared sample on the test platform of the rheometer, lower the rotor to the spacing, wait for 5 min, and start the test. Viscosity test: Temperature 25 °C, frequency range 0.01 - 100 Hz. Viscoelasticity test: Temperature 25 °C, fixed stress 1 Pa, frequency range 0.01 - 100 Hz. Strain test: Temperature 25 °C, fixed frequency 1 Hz, stress range 0.1 - 1000 Pa. The results are shown in Table 2 and Figure 4As shown. Throughout the entire frequency range, compared with the fracturing fluids of the comparative examples and other embodiments, the viscoelasticity of the fracturing fluid PN-PPS-1 of Example 1 is significantly increased, and the G' of PN-PPS-1 is always greater than G", showing the characteristics of a typical viscoelastic fluid; in the high-frequency range, the G' and G" of PN-PPS-1 are stably about 10.6 Pa and 2.5 Pa respectively, meeting the industry standard SY / 6376-2008 (G'≥2.0 Pa; G"≥0.3 Pa). It shows that the fracturing fluid PN-PPS-1 of Example 1 has good viscoelastic properties. Moreover, compared with Comparative Examples 1 and 2, each embodiment of the present invention has better viscoelastic properties.

[0075] Table 2 Viscoelasticity of Different Fracturing Fluids (10 Hz)

[0076]

[0077]

[0078] 4. Proppant Carrying Capacity

[0079] The proppant carrying capacity is one of the most important properties of fracturing fluids. The proppant carrying time and the settling velocity of ceramsite of the fracturing fluids prepared in the comparative examples (1, 2) and the fracturing fluids PN-PPS-n prepared in the examples (1, 2, 3) were tested. The specific steps are as follows: For the settling velocity of ceramsite, add 100 mL of fracturing fluid into a graduated cylinder at room temperature, take a ceramsite with a particle size range of 0.425 - 0.500 mm and place it below the liquid surface (about 2 cm), let it settle naturally, and then immediately start recording the time t taken for the ceramsite to settle a certain distance L. The settling velocity of the ceramsite is the distance settled per unit time; for the proppant carrying time of the fracturing fluid, measure 100 mL of fracturing fluid at room temperature, pour it into a stirrer and add ceramsite (particle size 0.425 - 0.500 mm) according to a sand-fluid ratio of 20%. After stirring thoroughly for 5 min, quickly pour the fracturing fluid into a 100 mL graduated cylinder, and start recording the time required for all the ceramsite to settle to the bottom of the container. The results are shown in Table 3 and Figure 5 as shown. From Table 3 and Figure 5 it can be seen that compared with the fracturing fluids of the comparative examples and other embodiments, the complete settling time of the ceramsite of the fracturing fluid PN-PPS-1 of Example 1 increased from 327 min to 510 min, and the settling velocity of the ceramsite decreased from 0.097 cm / min to 0.007 cm / min, indicating that the fracturing fluid PN-PPS-1 of Example 1 has good proppant carrying capacity. Moreover, compared with Comparative Examples 1 and 2, each embodiment of the present invention has better proppant carrying capacity.

[0080] Table 3 Proppant Carrying Capacity of Different Fracturing Fluids

[0081] Number Carrying Sand Time (min) Ceramsite Settling Velocity (cm / min) Comparative Example 1 327 0.097 Comparative Example 2 173 0.13 Example 1 510 0.007 Example 2 427 0.014 Example 3 411 0.009

[0082] 5. Gel-breaking property

[0083] Using ammonium persulfate (APS) as the gel breaker, the gel-breaking properties of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluid PN-PPS-n prepared in Examples (1, 2, 3) were investigated with the viscosity of the gel-breaking displacement agent and the residue content of the gel-breaking displacement agent as the indexes. The results are shown in Table 4 and Figure 6 as follows. Figure 6 In (a) of the figure, it is the viscosity of the gel-breaking displacement agent formed after gel-breaking of the fracturing fluid PN-PPS-1 prepared in Example 1 at different APS concentrations, Figure 6 and in (b) of the figure, it is the residue content of the gel-breaking displacement agent formed after gel-breaking of the fracturing fluid PN-PPS-1 prepared in Example 1 at different APS concentrations. The sand-carrying displacement fracturing fluid of the present invention in-situ forms an active gel-breaking displacement agent from the surface-active hydrophobic monomers that break after gel-breaking and the dis-assembled nano-micelle supramolecular crosslinking agent. The viscosity and interfacial tension of the active gel-breaking displacement agent decrease with the increase of the APS concentration. The fracturing fluid PN-PPS-1 of Example 1 is completely gel-broken after the APS concentration (i.e., the addition amount) reaches 0.1 wt%, and the viscosity is lower than 5 mPa·s. The residue content of the fracturing fluid PN-PPS-1 of Example 1 after gel-breaking is lower than 35 mg / L, which is much less than 600 mg / g required by the standard "General Technical Conditions for Fracturing Fluids" SY / T 6376-2008, confirming that the fracturing fluid PN-PPS-1 of Example 1 has good gel-breaking properties. Moreover, all examples of the present invention have good gel-breaking properties.

[0084] Table 4 Gel-breaking properties of different fracturing fluids (0.1 wt% APS, 90 °C, 2 h)

[0085] Number Gel-Breaking Fluid Viscosity (mPa.s) Residue Content (mg / L) Comparative Example 1 4.92 46 Comparative Example 2 3.75 32 Example 1 3.94 35 Example 2 4.72 48 Example 3 4.77 37

[0086] 6. Core damage

[0087] The gel-breaking fluid of the fracturing fluid may cause plugging damage to the pore throats and the outside of the internal matrix of the formation, and the damage to low-permeability reservoirs will be more obvious. The core damage rates of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluid PN-PPS-n prepared in Examples (1, 2, 3) were tested through displacement experiments. The specific steps are as follows: The core permeability damage simulation experiment was carried out according to the SY / T 6376-2008 standard; the core damage rate was calculated through the core permeability before and after injecting the gel-breaking fluid of the water-based fracturing fluid; Experimental steps: (1) With a pressure difference of 3.5 MPa and a flow rate of 1 mL / min, first inject water into the core and measure the permeability as K1; (2) Then displace with the gel-breaking fluid and measure the permeability as K2; (3) Then, use Equation (1) to calculate the core damage rate (μ):

[0088]

[0089] The fracturing fluids of all comparative examples and all examples were broken by using 0.1 wt% APS. The broken gel fluid was named as broken gel oil displacement agent, and the damage performance to core permeability was tested. The results are shown in Table 5.

[0090] Table 5 Core damage rate of broken gel fluids of different fracturing fluids

[0091] Number Core Damage Rate (%) Comparative Example 1 16.7 Comparative Example 2 19.5 Example 1 16.67 Example 2 19.5 Example 3 19.6

[0092] As can be seen from Table 5, after displacement by clear water and broken gel fluid, it is obtained that the core damage rates of the broken gel fluids of all comparative example fracturing fluids and the fracturing fluid PN-PPS-n of all examples are less than 20%, far lower than the core damage rate of the industry standard SY / T6376 - 2008. The above results show that the broken gel fluids of the fracturing fluid PN-PPS-n of each example of the present invention have low core damage, and fully meet the actual application standards of fracturing fluids. In addition, the composition of the broken gel fluid is simple, with low damage and no pollution. Therefore, the fracturing fluids of each example can be further used as oil displacement agents after gel breaking without need for flowback.

[0093] 7. Oil washing efficiency

[0094] The oil washing efficiency of the broken gel oil displacement agents obtained after gel breaking (using 0.1 wt% APS) of the fracturing fluids prepared in comparative examples (1, 2) and the fracturing fluid PN-PPS-n prepared in examples (1, 2, 3) was tested at different temperatures (50°C, 70°C, and 90°C). The experimental results are shown in Table 6.

[0095] Table 6 Oil washing efficiency of broken gel oil displacement agents

[0096]

[0097] As can be seen from Table 6, with the increase of temperature, the oil washing efficiency of the broken gel oil displacement agent of Example 1 gradually increases. The temperature promotes the desorption behavior of crude oil from the surface of sand and gravel, improving the cleaning efficiency. Under the condition of 90°C, the oil washing efficiency of the broken gel oil displacement agent of Example 1 can reach up to 99.57%. And, compared with Comparative Examples 1 and 2, the oil washing efficiency of each example of the present invention is significantly improved.

[0098] 8. Imbibition oil displacement efficiency

[0099] Core spontaneous imbibition experiments were conducted on cores soaked in the gel-breaking oil displacement agents obtained after gel breaking of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluids PN-PPS-n prepared in Examples (1, 2, 3) (using 0.1 wt% APS) and water at room temperature, respectively. The specific steps are as follows: Six oil-saturated cores were placed in the gel-breaking oil displacement agents of the comparative examples and examples and in water, and left standing at room temperature for a period of time. The oil seeped to the liquid surface, and the volume of the seeped oil was read. The imbibition oil displacement rate was obtained through formula (2), as shown in Table 7.

[0100]

[0101] ω: Imbibition oil displacement rate, %;

[0102] V: Volume of oil dialyzed out, mL;

[0103] m1: Mass of the core after being saturated with oil, g;

[0104] m2: Mass of the dried core, g.

[0105] Table 7 Imbibition oil displacement efficiency of the gel-breaking oil displacement agent

[0106]

[0107] As can be seen from Table 7, the imbibition oil displacement rate of the core in water is 10.32%, and the imbibition oil displacement rate of the core in the gel-breaking oil displacement agent of Example 1 is 20.46%, and the imbibition oil displacement rate has increased by nearly 1 time. Thus, it can be seen that due to the presence of a large amount of surface active substances in the gel-breaking oil displacement agent of the present invention, under the action of surfactant molecules, spontaneous imbibition is likely to occur, displacing the oil in the pore throats, which helps to improve the oil displacement efficiency.

[0108] 9. Core oil displacement efficiency

[0109] The improvement effect of the gel-breaking oil displacement agents obtained after gel breaking of the fracturing fluids prepared in Comparative Examples (1, 2) and the fracturing fluids PN-PPS-n prepared in Examples (1, 2, 3) (using 0.1 wt% APS) on the crude oil recovery rate was tested through core displacement experiments. The specific experimental steps are as follows: The oil-saturated core was placed in a core holder, first water flooding was carried out at a flow rate of 2 cm / min for displacement until the water content at the outlet of the core holder reached 98%, then water flooding was stopped, and the amount of oil produced was recorded; then the gel-breaking oil displacement agent was used for displacement until the water content of the last drop of liquid at the outlet reached 98%, then displacement was stopped, and the amount of oil produced by the gel-breaking oil displacement agent was recorded. The results are shown in Table 8 and Figure 7As shown. Compared with water flooding, the gel-breaking oil displacement agent of each embodiment of the present invention can greatly improve the crude oil recovery rate. Based on water flooding, the crude oil recovery rate can be increased by 10.31 - 12.54%, which proves the feasibility of in-situ oil displacement by the gel-breaking oil displacement agent formed after the gel-breaking of the fracturing fluid PN-PPS-n in the verified embodiment.

[0110] Table 8 Oil displacement efficiency of the gel-breaking oil displacement agent

[0111]

[0112]

[0113] Obviously, the above-mentioned embodiments of the present invention are only examples for clearly explaining the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation manners here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A method for preparing a sand-carrying oil-displacing fracturing fluid based on physical cross-linking, comprising the following steps: (1) polymerizing acrylamide, acrylic acid, 2-acrylamide-2-methylpropane sulfonic acid and polyoxypropylene to obtain a fracturing fluid main agent; (2) Mixing a solution of polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymer with the main agent of the fracturing fluid to obtain the physically cross-linked sand-carrying oil displacement fracturing fluid.

2. The preparation method of the proppant-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 1, wherein, In step (1), the polymerization reaction is carried out by inverse emulsion polymerization; Preferably, step (1) specifically comprises: (a) adding acrylamide, acrylic acid and 2-acrylamide-2-methylpropane sulfonic acid to water and dissolving them by stirring to obtain a solution of hydrophilic monomers; (b) adding polyoxypropylene to the solution of the hydrophilic monomers, and adding a dispersant to fully disperse the polyoxypropylene after ultrasonic treatment, and then adjusting the pH value of the solution to 5.8-7.0 to form an aqueous phase of an inverse emulsion; (c) adding an emulsifier to the oil phase solvent, stirring and dissolving them sufficiently to form an oil phase of an inverse emulsion; (d) mixing the oil phase with the aqueous phase, and emulsifying them for a period of time to form a stable inverse emulsion; (e) in a protective gas environment, subjecting the inverse emulsion to a free radical polymerization reaction by a redox initiator system to obtain the main agent of the fracturing fluid.

3. The preparation method of the proppant-carrying displacement fracturing fluid based on physical crosslinking according to claim 1 or 2, wherein, In step (1), the molecular weight of the polyoxypropylene is 200-4000.

4. The preparation method of the sand-carrying displacement hydraulic fracturing fluid based on physical crosslinking according to claim 1 or 2, wherein, In step (1), the mass ratio of the acrylamide, the acrylic acid, the 2-acrylamide-2-methylpropanesulfonic acid and the polyoxypropylene is 1:0.4-0.6:0.15-0.3:0.01-0.

05.

5. The preparation method of the sand-carrying displacement hydraulic fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the ratio of the total mass of the monomers in the aqueous phase to the total mass of the aqueous phase is 40-50%; Preferably, in step (1), the dispersant comprises one or a combination of Tween 80, Tween 60, AEO-7, AEO-9, OP-9 and OP-10; more preferably, the mass ratio of the added amount of the dispersant to the polyoxypropylene is 0.2-0.4:

1.

6. The preparation method of the sand-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the pH of the solution is adjusted to 5.8-7.0 by using an aqueous solution of one or more of sodium bicarbonate, acetic acid, hydrochloric acid and sodium hydroxide.

7. The preparation method of the sand-carrying displacement hydraulic fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the oil phase solvent includes one or a combination of 5# solvent oil, liquid paraffin, 7# solvent oil and n-heptane; Preferably, in step (1), the emulsifier comprises one or a combination of Span80, Span60, Tween 80 and alkylphenol polyoxyethylene ethers; Preferably, in step (1), the mass ratio of the emulsifier to the oil phase solvent is 0.1-0.2:

1.

8. The preparation method of the sand-carrying oil-displacing fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the volume ratio of the oil phase to the water phase is 0.2-0.5:1; Preferably, in step (1), the oil phase is mixed with the water phase and emulsified for 1-5 minutes to form a stable inverse emulsion.

9. The preparation method of the sand-carrying oil-displacing fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the redox initiator system includes a combination of tert-butyl hydroperoxide and sodium metabisulfite, a combination of hydrogen peroxide and sodium sulfite, or a combination of sodium persulfate and sodium sulfite; preferably, the addition amount of the redox initiator system is a mass ratio of 0.01-0.03:1 to the total amount of monomers.

10. The preparation method of the proppant-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 2, wherein, In step (1), the temperature of the free radical polymerization reaction is 35-45 °C and the time is 1-3 h.

11. The preparation method of the sand-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 1, wherein, In step (2), the mass concentration of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer solution is 0.1-0.2%. Preferably, in step (2), the mass ratio of the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer to the main fracturing fluid agent is 0.2-1:

1.

12. The preparation method of the sand-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 1, wherein, In step (2), the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer includes one or a combination of several of F127, P123, P84, and P65.

13. The preparation method of the sand-carrying oil displacement fracturing fluid based on physical crosslinking according to claim 1, wherein, In step (2), mixing the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer solution with the main fracturing fluid agent specifically includes: stirring the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer solution at a rotational speed of 500-1000 r / min to form a vortex, and adding the main fracturing fluid agent into the vortex under room temperature conditions, and stirring until completely dissolved to obtain the sand-carrying displacement fracturing fluid based on physical crosslinking.

14. A sand-carrying displacement fracturing fluid based on physical crosslinking, which is prepared by the preparation method of the sand-carrying displacement fracturing fluid based on physical crosslinking according to any one of claims 1-13.

15. Application of the sand-carrying displacement fracturing fluid based on physical crosslinking according to claim 14 in shale oil exploitation; Preferably, the application includes: Using the sand-carrying displacement fracturing fluid based on physical crosslinking to fracture the shale oil reservoir, and then without flowback, directly using the gel-breaking fluid after gel-breaking of the sand-carrying displacement fracturing fluid based on physical crosslinking as a displacement agent to displace the shale oil reservoir to achieve the integration of fracturing and displacement.

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