Preparation method and application of polymer dry powder directly-prepared controllable variable-viscosity fracturing fluid

Abstract

The invention discloses a preparation method of a polymer dry powder directly-prepared controllable variable-viscosity fracturing fluid, which comprises the following steps: firstly, preparing a thickening agent which is prepared by carrying out polymerization reaction on a hydrophilic monomer, a sulfonate-containing functional monomer and a fluorine-containing functional monomer; and the molecular weight of the polymer is 8000000-25000000. Uniformly mixing the thickening agent powder with a nano material and a surface modifier, drying and crushing, and sieving with a 100-200-mesh sieve to prepare dry polymer powder; the method comprises the following steps: mixing dry polymer powder with water to form a polymer concentrated solution, pumping the polymer concentrated solution into a sand mixing tank, pumping water into the sand mixing tank, and adding an additive for fracturing to form a fracturing fluid, thereby realizing on-line direct preparation of the fracturing fluid. The adding amount mass concentration of the polymer dry powder in the fracturing fluid is 0.005%-4%, the viscosity of the fracturing fluid can be adjusted in real time within 1-120 mPa.s by changing the adding amount of the polymer dry powder, seamless switching between low viscosity of slick water and high viscosity of sand-carrying fluid is achieved, and therefore accurate control over the viscosity of the fracturing fluid is met.

Description

Technical Field

[0001] The invention relates to the technical field of fracturing and production increase in oil and gas reservoir development engineering, in particular to a preparation method of a polymer dry powder directly mixed with a controllable variable viscosity fracturing fluid and application thereof. Background Art

[0002] Fracturing fluid is the "blood" of reservoir reconstruction technology. Its function is to press open the formation, form cracks, carry proppant into the cracks, lay and break the gel to form a fracture system with high conductivity. The quality of its performance directly affects the success or failure of fracturing construction and the difference in production increase effect. Fracturing fluid requires low friction to meet large displacement, low viscosity to meet the construction of complex fracture networks, fast dissolving online continuous preparation to meet the construction of large liquid and sand volumes, and low cost to meet the requirements of unconventional oil and gas economic development. More than 95% of the fracturing fluids used are water-based fracturing fluids with excellent comprehensive performance, of which low-cost slick water fracturing fluids account for more than 90%. The core component of slick water fracturing fluid is drag reducer, including three types: synthetic emulsions, suspensions and powdered particles, of which suspension emulsions account for more than 80%. In order to improve the efficiency of fracturing construction, reduce liquid loss, and meet the requirements of large liquid volume and large displacement in oil and gas production for environmental protection and cost reduction, the on-site liquid preparation process of modern hydraulic fracturing has developed from batch preparation in a large number of storage tanks before construction to continuous preparation of high-quality fracturing fluid online during construction. Efficient online continuous mixing technology is an urgent requirement for modern hydraulic fracturing factory operations.

[0003] At present, the emulsion and suspension drag reducers widely used in oil and gas fields meet the needs of fracturing construction in terms of quick dissolution, online continuous mixing, friction reduction and displacement increase, variable concentration and viscosity fracture creation and sand carrying. However, they still face challenges in application cost, placement stability, damage to reservoirs and potential environmental risks. Affected by its production process, there are the following problems: (1) The effective content is low and the salt resistance is poor. When using return water to prepare the fluid, the dosage will increase exponentially, which greatly increases the liquid cost and faces high cost challenges; (2) Emulsions and suspensions contain 50-60% invalid white oil and other additives (organic soil, suspending agent), which causes great waste of cost and reservoir damage, and the cleanliness of fracturing fluid faces challenges; (3) The emulsion and suspension carriers are oily solvents, and there is a risk of leakage and combustion during transportation and storage. At the same time, the oil content in the return fluid exceeds 0.2%, and the hazardous waste return fluid increases the processing procedures, processing costs and environmental pollution risks, facing greater QHSE risk challenges; (4) There are risks and cost challenges of on-site storage and transportation costs of large quantities of emulsions or suspensions, occupied sites, lifting equipment and costs, and poor placement stability and stratification.

[0004] In the development process of unconventional oil and gas fields, these traditional emulsion and suspended emulsion fracturing fluid systems often have defects such as low effective concentration, large amount of oil phase contamination, poor salt resistance and high operating costs. These problems will affect the efficiency and safety of fracturing operations. Summary of the invention

[0005] In order to realize the online direct preparation of variable viscosity fracturing fluid with dry powder, low residue, clean and oil-free phase, and meet the requirements of large liquid volume, large displacement, and continuous and uninterrupted construction, the present invention provides a preparation method of polymer dry powder directly mixed with controllable variable viscosity fracturing fluid.

[0006] The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid provided by the present invention comprises the following steps:

[0007] S1. preparing thickener powder;

[0008] The thickener is prepared by polymerization of a hydrophilic monomer, a sulfonate functional monomer and a fluorine functional monomer; the molecular weight of the polymer is 8 million to 25 million.

[0009] The fluorine-containing functional monomer is selected from any one of 2,2,3,4,4,4-hexafluorobutyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl acrylate, and 2,2,3,4,4,4-hexafluorobutyl acrylate.

[0010] The sulfonate functional monomer is selected from at least one of 2-acrylamide-2-methylpropanesulfonic acid, 2-acrylamide dodecanesulfonic acid, and sodium dimethyl isophthalate 5-sulfonate.

[0011] The hydrophilic monomer is selected from at least one of acrylamide, sodium acrylate and sodium methacrylate.

[0012] The preparation method of the thickener powder is as follows:

[0013] S11, mixing a hydrophilic monomer, a sulfonate-containing functional monomer, a fluorine-containing functional monomer, a solubilizing agent and water to prepare a reaction solution;

[0014] S12, adding an initiator after nitrogen is passed through the reaction solution to remove oxygen, heating the solution to 50-70° C. and reacting the solution for 8-12 hours to obtain a thickener gel;

[0015] S13, cutting, drying and crushing the thickener gel into pieces to obtain a thickener powder.

[0016] Preferably, in step S1, the sulfonate functional monomer accounts for 11-15% of the total mass of the three monomers, the fluorine functional monomer accounts for 0.2-1.2% of the total mass of the three monomers, and the total mass of the three monomers accounts for 25-30% of the mass of the reaction solution.

[0017] The solubilizer is alkylphenol polyoxyethylene ether, and its usage is 10 to 20 times the mass of the fluorine-containing functional monomer.

[0018] The initiator is azobisisobutylamidine hydrochloride (V-50), and the amount of the initiator used is 0.1-0.2% of the total mass of the three types of monomers.

[0019] S2. Evenly mix the thickener powder with the nanomaterial and the surface modifier, then dry, crush, and pass through a 100-200 mesh screen to obtain a polymer dry powder. The selective hydrophilic modification of the surface of the nanomaterial and the surface modifier, and the secondary crushing improve the surface energy of the polymer powder, thereby improving the dispersibility of the powder, preventing agglomeration, and rapidly dissolving and thickening. The polymer dry powder has significant thickening ability and drag reduction performance.

[0020] The nano material is nano silicon dioxide, preferably SiO2-NH2.

[0021] The surface modifier is a compound of anionic surfactant and nonionic surfactant; specifically, the surface modifier is one of a compound of sodium dodecyl sulfate (SDS) and fatty alcohol polyoxyethylene ether (AEO-9), a compound of sodium dodecyl sulfate and Tween 80, and a compound of sodium dodecyl sulfate and Triton X-100. Preferably, the surface modifier is a compound of SDS and AEO-9 in a mass ratio of 6:4.

[0022] The mixed mass percentages of polymer powder, nanomaterials and surface modifiers are as follows:

[0023] Polymer powder 97.0% to 99.7%, nanomaterial = 0.1% to 1%, surface modifier 0.2% to 2%.

[0024] Preferably, the mixing mass ratio of polymer powder: nanomaterial: surface modifier is 98.5:0.5:1.

[0025] S3. Add water to the polymer dry powder to form a flowable polymer concentrate, then pump the polymer concentrate into a sand mixing tank, pump water into the sand mixing tank at the same time, and add fracturing additives to form a fracturing fluid, so as to achieve online direct preparation of the fracturing fluid; the added mass concentration of the polymer dry powder in the fracturing fluid is 0.005% to 4%. By changing the added amount of the polymer dry powder, the viscosity of the fracturing fluid can be adjusted in real time between 1 and 120 mPa·s, and seamless switching can be achieved between the low viscosity of slippery water and the high viscosity of sand-carrying fluid.

[0026] The present invention also provides an application of polymer dry powder directly mixed with controllable viscosity-changing fracturing fluid: the polymer dry powder is used in combination with fracturing additives to prepare functional water-based fracturing fluid; fracturing fluid additives include but are not limited to bactericides, drainage aids, debonding agents, and temporary plugging agents. It mainly includes the following two application processes:

[0027] Application process 1: polymer dry powder is directly mixed with low-viscosity slippery water to create large-volume fractures, connect fine cracks, and use high-viscosity slippery water to carry sand for fracturing; the specific methods are as follows:

[0028] (1) Pump a small amount of polymer concentrate and fracturing additives into the sand mixing tank to form low-viscosity slippery water with a viscosity of 2 to 6 mPa·s;

[0029] (2) Low viscosity slippery water is discharged at 18 to 20 m 3 / min displacement is injected into the formation as a pre-fluid to form fractures and connect fine natural fractures;

[0030] (3) Increase the amount of polymer concentrate and directly switch to form a highly viscous slippery water with a viscosity of 20 to 40 mPa·s, which is injected into the formation fractures as a sand-carrying fluid;

[0031] (4) Reduce the amount of polymer concentrate added and directly switch to prepare low-viscosity slippery water with a viscosity of 2 to 6 mPa·s to displace the sand-carrying fluid in the wellbore into the formation to complete the fracturing and sand-carrying work;

[0032] (5) The well is shut in for 1 to 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the proppant fracture is closed, and the proppant is effectively laid.

[0033] (6) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas will be generated from the pore matrix, fine cracks, propped cracks, and wellbore.

[0034] Application process 2: polymer dry powder is directly mixed with high-viscosity slippery water to create a large displacement of the main fracture, switch to low-viscosity slippery water to connect the fine cracks, and then switch to high-viscosity slippery water to carry sand for fracturing; the specific method is as follows:

[0035] (1) Pumping an appropriate amount of polymer concentrate and fracturing additives into the sand mixing tank to form high-viscosity slippery water with a viscosity of 20 to 40 mPa·s; injecting a large amount of high-viscosity slippery water into the formation as a pre-fluid to create the main fracture;

[0036] (2) Reduce the amount of polymer concentrate and directly switch to the preparation of low-viscosity slippery water with a viscosity of 2 to 6 mPa·s, which is used as the pre-fluid to continue to create cracks and connect more natural micro-cracks;

[0037] (3) Repeat steps (1) and (2) alternately;

[0038] (4) Increase the amount of polymer concentrate and directly switch to form a highly viscous slippery water with a viscosity of 20 to 40 mPa·s, which is injected into the formation fractures as a sand-carrying fluid;

[0039] (5) Reduce the amount of polymer concentrate added and directly switch to prepare low-viscosity slippery water with a viscosity of 2 to 6 mPa·s to displace the sand-carrying fluid in the wellbore into the formation to complete the fracturing and sand-carrying work;

[0040] (6) The well is shut in for 1 to 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the proppant fracture is closed, and the proppant is effectively laid.

[0041] (7) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas will be generated from the pore matrix, microcracks, propped cracks, and wellbore.

[0042] The bactericide used in the present invention is mainly for sulfate-reducing bacteria, iron bacteria and saprophytes, etc., and the bactericidal inhibition may cause serious corrosion and clogging problems in the fracturing environment. The bactericide is at least one of a quaternary ammonium salt (such as dodecyl trimethyl ammonium chloride, dodecyl dimethyl benzyl ammonium chloride, dodecyl dimethyl benzyl ammonium bromide, etc.), a polymeric quaternary ammonium salt and other non-oxidizing bactericides. The drainage aid is a complex of a cationic, anionic, non-ionic fluorocarbon surfactant with a sulfonate surfactant and a cosolvent. A gel breaker is a liquid gel breaker, which is a complex of a peroxide and an organic auxiliary agent. A temporary plugging agent is a water-soluble granular temporary plugging agent with a particle size of 20-60 meshes. Common fracturing additives such as clay stabilizers, demulsifiers, pH regulators, foaming agents and defoamers can also be used. These additives are all commonly used species known to those skilled in the art and can be selected arbitrarily according to construction requirements.

[0043] Compared with the prior art, the present invention is beneficial in that:

[0044] (1) The polymer dry powder thickener provided by the present invention can realize the online direct preparation of controllable variable viscosity fracturing fluid. The polymer dry powder is in direct contact with the water in the shear flow field, hydrated, dissolved quickly, and thickened. It has no oil phase, no emulsifier, and no organic soil. The introduction of the oil phase is eliminated from the source, reducing the damage to the reservoir and the potential pollution to the environment caused by fracturing fluid residues, oil pollution, and impurities. It also reduces the treatment cost of oil pollution in the return fluid, and has the effect of green environmental protection. It realizes the instant supply, preparation, and injection of fracturing fluid, seamless switching, and can meet the needs of large liquid volume and large displacement (2-20m 3 / min pumping displacement), continuous and uninterrupted construction, to achieve the purpose of improving construction efficiency and reducing comprehensive costs. For unconventional oil and gas, 3 The real-time viscosity can be controlled to change from 1 to 120 mPa·s at a flow rate of 1 / min, achieving seamless switching between the low viscosity of slick water and the high viscosity of sand-carrying fluid, thereby meeting the precise control of the viscosity of the fracturing fluid.

[0045] (2) The polymer dry powder of the present invention has the characteristics of rapid dissolution and thickening and controllable rheological viscosity change, and has good salt resistance and adaptability, which provides a guarantee for online direct mixing of dry powder, improving efficiency and reducing costs. The polymer dry powder has a high solid content (greater than 88%), which is much higher than the 30-50% content of inverse emulsion and suspended emulsion polymers. To achieve the same liquid viscosity, direct mixing of polymer dry powder requires a lower polymer addition concentration, and the dosage is reduced by 60-70%, reducing the cost of fracturing; compared with the same usage concentration, direct mixing of polymer dry powder has a higher thickening ability, which can better meet the requirements of large-scale volume fracturing of unconventional oil and gas, large displacement, and large sand volume for fracturing fluid viscosity, and realize rapid switching between low viscosity, medium viscosity, and high viscosity.

[0046] (3) The polymer dry powder of the present invention has special fluorine-containing hydrophobic and salt-resistant and temperature-resistant functional groups in its molecular chain; the polymer powder is treated with nanomaterials and surface modifiers as well as the particle size is treated, so that the flow dispersion is better and the hydration can be quickly dispersed in the sand mixing tank to form a clean fracturing fluid with controllable viscosity without fisheyes. In engineering applications, the fracturing fluid has good drag reduction performance (drag reduction rate greater than 70%), viscosity increasing ability (adjustable from 1 to 120 mPa·s), sand carrying capacity (30% sand ratio full suspension) and salt resistance; at a mineralization of 100,000 mg / L, the viscosity retention rate is above 70%, the salt resistance mineralization reaches 300,000 mg / L, and the viscosity can be controlled to 90 mPa·s, which greatly improves the comprehensive performance of the polymer fracturing fluid. This is beneficial to the laying of the proppant during the construction process and improves the fracturing construction effect.

[0047] (4) The fracturing fluid system is equipped with a multifunctional agent for sterilization, drainage and demulsification, a clay stabilizer, a high-efficiency demulsifier and a temporary plugging agent. When added at a low concentration, the fracturing fluid has the ability to inhibit sulfate-reducing bacteria, demulsify and prevent water locks, reduce surface interfacial tension and stabilize clay. The fracturing fluid degels thoroughly, has low residue, or even no residue, which can avoid fracturing fluid retention and blocking the oil and gas seepage channel, which is beneficial to increasing oil and gas production capacity.

[0048] (5) For shale gas, tight gas and coal-rock gas wells, the return fluid after fracturing with the dry powder directly mixed with the controllable viscosity-changing fracturing fluid is completely debonded and clean without oil phase. Since the polymer dry powder of the present invention has good salt tolerance, the return fluid does not need to be treated on the ground to remove high-valent metal ions and oil stains, and can be directly reused in the dry powder direct mixing fracturing construction, reducing the potential environmental pollution of waste liquid and high-salt return fluid, being green and environmentally friendly, and eliminating the environmental protection cost of wastewater treatment, which can improve the economic benefits of fracturing.

[0049] In summary, the polymer dry powder directly mixed with the controllable variable viscosity fracturing fluid drag reducing thickener provided by the present invention is simple to prepare, has a high solid content, a low usage concentration, no oil phase, is clean and residue-free, is a green, environmentally friendly, low-cost water-based fracturing fluid, is an important product for increasing the production capacity of unconventional oil and gas single wells, extending the stable production period, and improving the recovery rate, and is a revolution in the preparation of fracturing fluids.

[0050] Other advantages, objectives and features of the present invention will be embodied in part through the following description, and in part will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 The viscosity of the polymer powder of Example 1 at different concentrations varies with swelling time.

[0052] Figure 2 It is the temperature and shear resistance rheological curve of the polymer dry powder directly mixed with the fracturing fluid of the present invention.

[0053] Figure 3 This is a photo of the polymer dry powder fracturing fluid of the present invention being debonded in a water bath at 80°C.

[0054] Figure 4 This is a picture of the high-concentration glue solution tested by direct mixing of the polymer dry powder of the present invention.

[0055] Figure 5 This is a comparison of the flowback fluid after fracturing with the polymer dry powder of the present invention directly mixed with the variable viscosity fracturing fluid and the flowback fluid of the inverse emulsion polymer fracturing in the adjacent well of the platform. The left picture is the flowback fluid corresponding to the polymer dry powder of the present invention, and the right picture is the flowback fluid corresponding to the inverse emulsion polymer. DETAILED DESCRIPTION

[0056] The preferred embodiments of the present invention are described below in conjunction with the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0057] Example 1

[0058] A method for preparing a thickener for a dry powder direct-mixed controllable viscosity-changing water-based fracturing fluid:

[0059] (1) Add hydrophilic monomer acrylamide, salt-tolerant monomer 2-acrylamide-2-methylpropanesulfonic acid (AMPS), fluorine-containing functional monomer methacrylate-2,2,3,4,4,4-hexafluorobutyl ester and solubilizer alkylphenol polyoxyethylene ether (OP-10) into a reactor, add solvent water, and stir quickly until the fluorine-containing functional monomers are all evenly dispersed in the water to obtain a reaction solution. Among them, the amount of salt-tolerant monomer AMPS accounts for 13.3% of the total mass of the three monomers; the amount of fluorine-containing functional monomer accounts for 1.2% of the total mass of the three monomers, and the total of the three monomers is 100%; the amount of solubilizer is 10 times the mass of the fluorine-containing functional monomer. The total mass of the three monomers (i.e., the total mass of the hydrophilic monomer, the sulfonate-containing salt-tolerant functional monomer, and the fluorine-containing functional monomer) accounts for 30% of the mass of the reaction solution.

[0060] (2) After nitrogen was introduced into the reaction solution for 20 minutes, an initiator (V50) was added, and the temperature was raised to 55° C. and maintained at 55° C. for 10 hours to obtain a polymer thickener gel. The mass of the initiator was 0.15% of the total mass of the three monomers.

[0061] (3) The thickener gel was cut into thin blocks of 1 cm thick, dried at 70-75° C. for 8 h, and the dried thickener was crushed with a high-speed crusher to obtain polymer powder.

[0062] (4) Add nano-silica (SiO2-NH2) and a surface modifier to the polymer powder, mix them evenly, dry them twice and crush them deeply, and pass them through a 100-200 mesh sieve to obtain a polymer dry powder product. The surface modifier is a compound of SDS and AEO-9 in a mass ratio of 6:4. The mixing mass ratio of polymer powder: nanomaterial: surface modifier is 98.5:0.5:1.

[0063] Example 2

[0064] On the basis of Example 1, the amount of the fluorine-containing functional monomer was adjusted to 0.8% of the total mass of the three monomers, and the others remained unchanged to obtain a polymer dry powder.

[0065] Example 3

[0066] On the basis of Example 1, the amount of the fluorine-containing functional monomer was adjusted to 0.2% of the total mass of the three monomers, and the others remained unchanged to obtain a polymer dry powder.

[0067] Example 4

[0068] A method for preparing a thickener for a dry powder direct-mixed controllable viscosity-changing water-based fracturing fluid:

[0069] (1) Weigh the hydrophilic monomer acrylamide, the salt-tolerant monomer sodium dimethyl isophthalate 5-sulfonate, the fluorine-containing functional monomer acrylic acid-1,1,1,3,3,3-hexafluoroisopropyl ester and the solubilizing agent alkylphenol polyoxyethylene ether (OP-10) and add them into the reactor, add the solvent water, and stir quickly until the fluorine-containing functional monomer is evenly dispersed in the water to obtain a reaction solution. Among them, the amount of the salt-tolerant monomer accounts for 12.5% ​​of the total mass of the three monomers; the amount of the fluorine-containing functional monomer accounts for 0.8% of the total mass of the three monomers, and the total of the three monomers is 100%; the amount of the solubilizing agent is 15 times the mass of the fluorine-containing functional monomer. The total mass of the three monomers (i.e., the total mass of the hydrophilic monomer, the salt-tolerant monomer, and the fluorine-containing functional monomer) accounts for 30% of the mass of the reaction solution.

[0070] (2) After nitrogen was introduced into the reaction solution for 20 minutes, an initiator (V50) was added, and the temperature was raised to 65° C. and maintained at 65° C. for 10 hours to obtain a polymer thickener gel. The mass of the initiator was 0.2% of the total mass of the three monomers.

[0071] (3) The thickener gel was cut into thin blocks of 1 cm thick, dried at 70-75° C. for 8 h, and the dried thickener was crushed with a high-speed crusher to obtain polymer powder.

[0072] (4) Add nano-silica (SiO2-NH2) and a surface modifier to the polymer powder, mix them evenly, dry them twice and crush them deeply, and pass them through a 100-200 mesh sieve to obtain a polymer dry powder product. The surface modifier is a compound of SDS and AEO-9 in a mass ratio of 6:4. The mixing mass ratio of polymer powder: nanomaterial: surface modifier is 98.5:0.5:1.

[0073] Example 5

[0074] A method for preparing a thickener for a dry powder direct-mixed controllable viscosity-changing water-based fracturing fluid:

[0075] (1) Weigh the hydrophilic monomer sodium acrylate, the salt-tolerant monomer 2-acrylamidododecanesulfonic acid, the fluorine-containing functional monomer acrylate-2,2,3,4,4,4-hexafluorobutyl ester and the solubilizer alkylphenol polyoxyethylene ether (OP-10) and add them into a reactor, add solvent water, and stir quickly until the fluorine-containing functional monomers are all evenly dispersed in the water to obtain a reaction solution. Among them, the amount of the salt-tolerant monomer accounts for 12.5% ​​of the total mass of the three monomers; the amount of the fluorine-containing functional monomer accounts for 0.2% of the total mass of the three monomers, and the total of the three monomers is 100%; the amount of the solubilizer is 20 times the mass of the fluorine-containing functional monomer. The total mass of the three monomers (i.e., the total mass of the hydrophilic monomer, the salt-tolerant monomer, and the fluorine-containing functional monomer) accounts for 28% of the mass of the reaction solution.

[0076] (2) After nitrogen was introduced into the reaction solution for 20 minutes, an initiator (V50) was added, and the temperature was raised to 65° C. and maintained at 65° C. for 10 hours to obtain a polymer thickener gel. The mass of the initiator was 0.2% of the total mass of the three monomers.

[0077] (3) The thickener gel was cut into thin blocks of 1 cm thick, dried at 70-75° C. for 8 h, and the dried thickener was crushed with a high-speed crusher to obtain polymer powder.

[0078] (4) Add nano-silica (SiO2-NH2) and a surface modifier to the polymer powder, mix them evenly, dry them twice and crush them deeply, and pass them through a 100-200 mesh sieve to obtain a polymer dry powder product. The surface modifier is a compound of SDS and AEO-9 in a mass ratio of 6:4. The mixing mass ratio of polymer powder: nanomaterial: surface modifier is 98.5:0.5:1.

[0079] Comparative Example 1

[0080] On the basis of Example 1, the fluorine-containing functional monomer 2,2,3,4,4,4-hexafluorobutyl methacrylate was replaced with an equal amount of 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl methacrylate, and the other steps remained unchanged to prepare a polymer dry powder as Comparative Sample 1.

[0081] Comparative Example 2

[0082] On the basis of Example 1, the fluorine-containing functional monomer methacrylate-2,2,3,4,4,4-hexafluorobutyl ester was replaced with an equal amount of propylene-1H,1H,2H,2H-heptadecafluorodecyl ester, and the other steps remained unchanged to prepare a polymer dry powder as comparative sample 2.

[0083] The performance tests of the polymer dry powders prepared in the above examples and comparative examples are as follows:

[0084] (1) First, a 1% potassium chloride aqueous solution was prepared in the laboratory. 400 ml of the aqueous solution was measured and placed in a Wu Yin mixer. The speed was increased to 1000 rpm so that the mixer vortex could see the bottom. The polymer powder of Example 1 was tested by stirring for 40 seconds at different addition concentrations (mass percentage concentration 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%). The viscosity of the polymer powder was measured by a six-speed rotational viscometer at a speed of 100 rpm (shear rate 170S -1 ) and tested the viscosity increasing ability at different times. The results are shown in Figure 1 The experiment shows that the polymer dry powder disclosed in the present invention has good instant solubility and viscosity-increasing effect in a 10000 mg / L saline solution, and has the viscosity-increasing conditions for direct preparation of the polymer dry powder.

[0085] (2) Thermal shear stability test of polymer dry powder mixed with variable viscosity fracturing fluid: Using a controlled stress rheometer, the temperature was 90°C and 100S -1 At a shear rate of 0.6%, a continuous shear test was conducted for 120 minutes on the polymer dry powder aqueous solution of Example 2 with a mass concentration of 0.6%. The experimental results are shown in Figure 2 The initial viscosity reaches 150-175 mPa·s, and the viscosity is maintained at 125 mPa·s after 60 minutes of thermal shearing at 90°C, 110 mPa·s after 90 minutes of shearing, and 106 mPa·s after 120 minutes of shearing. This shows that the polymer dry powder directly mixed with the variable viscosity fracturing fluid of the present invention has excellent thermal shear stability.

[0086] (3) In a standard salt water (mineralization: 85000 mg / L) medium, the swelling viscosity of the polymer dry powder of Examples 1 to 5 and Comparative Examples 1 and 2 at a mass concentration of 0.3% was tested respectively, and the experimental results are shown in Table 1. The results show that: ① In a high-salinity solution of 85000 mg / L, with the increase of the proportion of hexafluoro functional monomer, the solution viscosity increases significantly, and also shows good drag reduction performance, sand carrying capacity and special effects of reducing surface tension. ② In a high-salinity solution of 85000 mg / L, under the same functional monomer dosage conditions, among different types of fluorine-containing monomers, the hexafluoro functional monomer has the best performance, while the high-fluorine functional monomer has problems such as incomplete swelling, easy appearance of "fish eyes" and difficult to break the gel. This shows that the hexafluoro functional monomer used in the present invention can effectively improve the viscosity and salt resistance of the polymer, and compared with other fluorine-containing functional monomers, the hexafluoro functional monomer shows the best performance.

[0087] Table 1. Salt resistance of different types of polymer dry powders

[0088]

[0089] (4) The friction resistance of the standard brine slick water fracturing fluid with a polymer dry powder content of 0.05% by mass was tested using a pipeline friction tester. In a 1-inch diameter pipeline, the linear velocity of the construction displacement of 20 cubic meters / minute was simulated for on-site fracturing, and the friction resistance was measured and shown in Table 2. The friction test results show that in 84000 mg / L standard brine, the polymers of Examples 1 and 2 have good salt resistance, and the kinematic viscosity is maintained at 3.5 mm 2 / s or more, and has an excellent drag reduction rate of more than 75%. However, the low-concentration fluorine-containing hydrophobic monomer polymer of Example 3 has significantly poor salt resistance, closer to the viscosity of clean water, and the drag reduction rate is only maintained at 61%.

[0090] Table 2. Friction performance test results

[0091]

[0092] 20% quartz sand was added to standard brine, and the sand suspension performance of Example 1, Example 2 and Example 3 at a polymer dry powder concentration of 0.3% by mass was tested. The results are shown in Table 3.

[0093] Table 3. Suspended sand performance test results

[0094]

[0095] (5) In a water bath at 80°C, 0.02% liquid breaker was added to the fracturing fluid, and the breaking performance of Example 1, Example 2, Example 3, Comparative Example 1 and Comparative Example 2 at a polymer concentration of 0.3% was tested. The results are shown in Figure 3As shown, Example 1, Example 2 and Example 3 can completely break the gel within 2 hours, and the viscosity of the broken gel solution is 1.27 mm 2 / s、1.22mm 2 / s and 1.16mm 2 / s, the gel-breaking liquid was clear, without oil phase, and no residue was visible to the naked eye. Comparative Examples 1 and 2 failed to completely break the gel within 2 hours, and the solution viscosities were: 12.36 mm 2 / s, 18.90mm 2 / s, the gel-breaking liquid is turbid and residues can be seen by naked eyes.

[0096] The surface tension of the gel-breaking liquid was tested using a fully automatic surface tension meter, and the results are shown in Table 4. The experimental results show that the fluorinated monomer in the polymer dry powder of the present invention can effectively reduce the surface tension of the gel-breaking liquid, and as the concentration of the fluorinated monomer increases, the effect of reducing the surface tension becomes more obvious, which also verifies the important property of the fluorinated hydrophobic monomer in the polymer to improve the interfacial chemistry.

[0097] Table 4. Viscosity and surface tension of the fracturing fluids of different embodiments

[0098]

[0099]

[0100] (6) The ability of the polymer dry powder thickener prepared in Example 1 to be directly mixed with viscoelastic fracturing fluid gel from dry powder was tested. Clean water with a total mineralization of 268 mg / L was used with a pipe flow rate of 1.0 m 3 / min pumped into the sand mixing tank, and at the same time pumped into the dry powder concentrate of Example 1 at an addition rate of 25kg / min of polymer dry powder to obtain a gel state with a mass concentration of 2.5% of the polymer dry powder, such as Figure 4 As shown, Figure (a) shows the glue liquid state, (b) shows that the glue liquid has excellent viscoelasticity, and (c) shows that the glue liquid is uniform and has no fisheyes. From polymer dry powder to polymer glue liquid, the time is 25 seconds, and a high-concentration and high-viscosity glue liquid is obtained. The glue liquid is uniform, has no fisheyes, and has good viscoelastic characteristics, which proves the feasibility of directly mixing polymer dry powder with viscoelastic fracturing fluid.

[0101] (7) A field application test was conducted in a shale gas fracturing platform well in the Sichuan Basin. The dry powder polymer thickener prepared in Example 1 was tested for its ability to directly mix controllable variable viscosity fracturing fluid from dry powder. A mixture of clean water and return fluid (total mineralization 12500 mg / L) was used to prepare the fracturing fluid. After the dry powder was added and mixed in the shear flow field for 17 seconds, the mixture was merged in the mixing tank of the mixing truck. The fracturing fluid viscosity of the low-viscosity slippery water with a dry powder concentration of 0.05% was 4.8 mPa·s; the fracturing fluid viscosity of the medium-viscosity slippery water fracturing fluid with a dry powder concentration of 0.1% was 13.8 mPa·s; the fracturing fluid viscosity of the high-viscosity slippery water fracturing fluid with a dry powder concentration of 0.2% was 31.5 mPa·s. The maximum construction pressure is 75 MPa, the construction pressure is 60-75 MPa, and the displacement is 12-14 m 3 / min, liquid volume 57024m 3 , the highest sand ratio is 30%, the average sand ratio is 220kg / m 3 The fracturing test showed that the drag reduction rate of the slick water fracturing fluid was 75.8%. By adjusting the addition rate of the polymer dry powder, the variable viscosity of the slick water fracturing fluid was quickly obtained. The slick water fracturing fluid formed had stable performance, uniform gel, and no fish eyes. It is feasible to directly mix the polymer dry powder with the variable viscosity fracturing fluid.

[0102] Compared with the inverse emulsion polymer in the adjacent wells of the platform, the polymer dry powder in this well is directly mixed with the fracturing fluid and is quick-dissolving, easy to prepare, and has strong viscosity-increasing and sand-carrying capabilities. Figure 5 It can be seen that the polymer dry powder directly mixed with the variable viscosity fracturing fluid of the present invention has a good gel breaking effect, is clear and free of impurities and oil phase, such as water, and has a capillary viscosity of only 1.89 mPa·s. It is green and environmentally friendly and can be directly recycled and reused; while the return fluid sampled after the adjacent well emulsion drag reducer is turbid, contains oil (greater than 0.2%), and has a lot of flocculent residues. The return fluid cannot be directly recycled and reused, and must be treated as hazardous waste, which increases the treatment cost and environmental pollution risk.

[0103] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification 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 falls within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid, characterized in that: The following steps are involved: S1. preparing thickener powder; The thickener is prepared by polymerization of a hydrophilic monomer, a sulfonate-containing functional monomer and a fluorine-containing functional monomer; the molecular weight of the polymer is 8 million to 25 million; the fluorine-containing functional monomer is selected from any one of 2,2,3,4,4,4-hexafluorobutyl methacrylate, 1,1,1,3,3,3-hexafluoroisopropyl acrylate and 2,2,3,4,4,4-hexafluorobutyl acrylate; S2, mixing the thickener powder with the nanomaterial and the surface modifier uniformly, then drying, crushing, and passing through a 100-200 mesh sieve to obtain a polymer dry powder; the surface modifier is a compound of anionic surfactant and nonionic surfactant; S3. Premix the polymer dry powder with water to form a flowable polymer concentrate, then pump the polymer concentrate into a sand mixing tank, and at the same time pump water into the sand mixing tank, and add fracturing additives to form a fracturing fluid, so as to realize online direct preparation of the fracturing fluid; the added mass concentration of the polymer dry powder in the fracturing fluid is 0.005% to 4%. By changing the added amount of the polymer dry powder, the viscosity of the fracturing fluid can be adjusted in real time between 1 and 120 mPa·s, and seamless switching can be achieved between the low viscosity of slippery water and the high viscosity of sand-carrying fluid.

2. The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid according to claim 1, characterized in that: The sulfonate functional monomer is selected from any one of 2-acrylamide-2-methylpropanesulfonic acid, 2-acrylamide dodecanesulfonic acid, and sodium dimethyl isophthalate 5-sulfonate.

3. The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid according to claim 2, characterized in that: The hydrophilic monomer is selected from any one of acrylamide, sodium acrylate and sodium methacrylate.

4. The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid according to claim 3, characterized in that: The preparation method of the thickener powder is as follows: S11, mixing a hydrophilic monomer, a sulfonate-containing functional monomer, a fluorine-containing functional monomer, a solubilizing agent and water to prepare a reaction solution; S12, adding an initiator after nitrogen is passed through the reaction solution to remove oxygen, heating the solution to 50-70° C. and reacting the solution for 8-12 hours to obtain a thickener gel; S13, cutting, drying and crushing the thickener gel into pieces to obtain a thickener powder.

5. The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid according to claim 4, characterized in that: The amount of the sulfonate functional monomer accounts for 11-15% of the total mass of the three monomers; the amount of the fluorine functional monomer accounts for 0.2-1.2% of the total mass of the three monomers; and the total mass of the three monomers accounts for 25-30% of the mass of the reaction solution.

6. The method for preparing a polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid according to claim 4, characterized in that: The solubilizer is alkylphenol polyoxyethylene ether, and its usage is 10 to 20 times the mass of the fluorine-containing functional monomer.

7. An application of the polymer dry powder directly mixed with a controllable viscosity-changing fracturing fluid as claimed in any one of claims 1 to 6, characterized in that: The polymer dry powder is used in combination with fracturing additives to prepare a functional water-based fracturing fluid; the fracturing fluid additives include but are not limited to bactericides, drainage aids, gel breakers, and temporary plugging agents.

8. The use of the polymer dry powder directly mixed with the controllable viscosity-changing fracturing fluid as claimed in claim 7, characterized in that: The polymer dry powder is directly mixed with low-viscosity slippery water to create large-volume fractures, connect fine cracks, and high-viscosity slippery water carries sand for fracturing; the specific methods are as follows: (1) Pumping polymer concentrate and fracturing additives into the sand mixing tank to form low-viscosity slippery water with a viscosity of 2 to 6 mPa·s; (2) Low viscosity slippery water at 18-20m 3 / min displacement is injected into the formation as a pre-fluid to form fractures and connect fine natural fractures; (3) Increase the amount of polymer concentrate and directly switch to form a highly viscous slippery water with a viscosity of 20 to 40 mPa·s, which is injected into the formation fractures as a sand-carrying fluid; (4) Reduce the amount of polymer concentrate added and directly switch to prepare low-viscosity slippery water with a viscosity of 2 to 6 mPa·s to displace the sand-carrying fluid in the wellbore into the formation to complete the fracturing and sand-carrying work; (5) The well is shut in for 1 to 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the proppant fracture is closed, and the proppant is effectively laid. (6) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas will be generated from the pore matrix, fine fractures, propping fractures, and wellbore.

9. The use of the polymer dry powder directly mixed with the controllable viscosity-changing fracturing fluid as claimed in claim 7, characterized in that: The polymer dry powder is directly mixed with high-viscosity slippery water to create the main fracture in large volume, and then the low-viscosity slippery water is switched to connect the fine cracks, and then the high-viscosity slippery water is switched to carry sand for fracturing; the specific method is as follows: (1) Pumping polymer concentrate and fracturing additives into the sand mixing tank to form high-viscosity slippery water with a viscosity of 20 to 40 mPa·s; injecting the high-viscosity slippery water into the formation in large quantities as a pre-fluid to create the main fracture; (2) Reduce the amount of polymer concentrate and directly switch to the preparation of low-viscosity slippery water with a viscosity of 2 to 6 mPa·s, which is used as the pre-fluid to continue to create cracks and connect more natural micro-cracks; (3) Repeat steps (1) and (2) alternately; (4) Increase the amount of polymer concentrate and directly switch to form a highly viscous slippery water with a viscosity of 20 to 40 mPa·s, which is injected into the formation fractures as a sand-carrying fluid; (5) Reduce the amount of polymer concentrate added and directly switch to prepare low-viscosity slippery water with a viscosity of 2 to 6 mPa·s to displace the sand-carrying fluid in the wellbore into the formation to complete the fracturing and sand-carrying work; (6) The well is shut in for 1 to 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the proppant fracture is closed, and the proppant is effectively laid. (7) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas will be generated from the pore matrix, fine fractures, propped fractures, and wellbore.

Images