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

Abstract

The present invention discloses a preparation method of a polymer dry powder directly prepared controllable variable-viscosity fracturing fluid. First, a thickening agent is prepared, which is made by polymerizing a hydrophilic monomer, a sulfonate-functional monomer, and a fluorine-functional monomer; the molecular weight of the polymer is 8 million to 25 million. Then, the thickening agent powder is mixed evenly with a nanomaterial and a surface modifier, and then dried, pulverized, and passed through a 100-200 mesh sieve to obtain a polymer dry powder; the polymer dry powder is mixed with water to form a polymer concentrate, and then the polymer concentrate is pumped into a sand mixing tank. At the same time, water is pumped into the sand mixing tank, and a fracturing additive is added to form a fracturing fluid, realizing the on-line direct preparation of the fracturing fluid. The addition mass concentration of the polymer dry powder in the fracturing fluid is 0.005% to 4%. By changing the addition amount of the polymer dry powder, the viscosity of the fracturing fluid can be adjusted in real time between 1 and 120 mPa·s, realizing seamless switching between the low viscosity of the slickwater and the high viscosity of the sand-carrying fluid, so as to meet the precise control of the viscosity of the fracturing fluid.

Description

Technical Field

[0001] The present invention relates to the technical field of fracturing stimulation in oil and gas reservoir development engineering, and in particular to a preparation method and application of a polymer dry powder direct mixing controllable variable viscosity fracturing fluid. Background Technique

[0002] Fracturing fluid is the "blood" of reservoir stimulation technology. Its function is to fracture the formation, form fractures, and carry proppants into the fractures, 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 stimulation effect. Fracturing fluid is required to have low friction resistance to meet large displacement, low viscosity to form complex fracture networks, rapid dissolution and on-line continuous preparation to meet the requirements of large fluid volume and large sand volume construction, and low cost to meet the economic development requirements of unconventional oil and gas. More than 95% of the fracturing fluids used are water-based fracturing fluids with excellent comprehensive performance, among which low-cost slickwater fracturing fluid accounts for more than 90%. The core component of slickwater fracturing fluid is drag reducer, including three types: synthetic emulsion, suspension and powder particles. Among them, the application proportion of suspension-type emulsion reaches more than 80%. In order to improve the efficiency of fracturing construction, reduce liquid loss, and meet the requirements of large fluid 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 on-line continuous preparation of high-quality fracturing fluid during construction. The efficient on-line continuous mixing process is an urgent requirement for the industrial operation of modern hydraulic fracturing.

[0003] At present, the emulsion and suspension drag reducers widely used in oil and gas fields meet the requirements of fracturing construction in terms of rapid dissolution, on-line continuous mixing, reducing friction resistance and increasing displacement, variable concentration and variable viscosity for creating fractures, and sand carrying. However, they still face challenges in application cost, storage stability, reservoir damage and potential environmental protection risks. Affected by its production process, the following problems exist: (1) The effective content is low and the salt resistance performance is poor. When using produced water for liquid preparation, the dosage will increase exponentially, greatly increasing the liquid cost and facing the challenge of high cost; (2) The emulsion and suspension contain 50-60% ineffective white oil and other additive components (organophilic clay, suspending agent), resulting in a great waste of cost and reservoir damage, and facing challenges in the clean-up of fracturing fluid; (3) The carriers of emulsion and suspension are oily solvents, and there are risks of leakage and combustion during transportation and storage. At the same time, the oil content in the produced water return exceeds 0.2%, and the hazardous waste produced water return increases the risk of treatment procedures, treatment costs and environmental pollution, facing greater QHSE risk challenges; (4) Facing the risks and costs of large-tonnage barrel storage and transportation costs, site occupation, hoisting equipment and costs, and poor storage stability and layering of emulsions or suspensions on site.

[0004] In the development process of unconventional oil and gas fields, these traditional emulsion and suspension fracturing fluid systems often have defects such as low effective concentration, a large amount of oil-phase pollution, poor salt tolerance, and high operating costs, which will affect the efficiency and safety of fracturing operations. Summary of the Invention

[0005] In order to realize the on-line direct preparation of a variable-viscosity fracturing fluid from dry powder, with low residue, clean and oil-phase-free, and meeting the requirements of large liquid volume, large displacement, and continuous construction, the present invention provides a method for preparing a polymer dry powder direct-mixing controllable variable-viscosity fracturing fluid.

[0006] The method for preparing a polymer dry powder direct-mixing controllable variable-viscosity fracturing fluid provided by the present invention comprises the following steps:

[0007] S1. Prepare a thickening agent powder;

[0008] The thickening agent is prepared by polymerizing 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-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-acrylamido-2-methylpropanesulfonic acid, 2-acrylamidododecanesulfonic acid, and sodium 5-sulfonatophthalate dimethyl ester.

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

[0012] The method for preparing the thickening agent powder is as follows:

[0013] S11. Mix the hydrophilic monomer, the sulfonate-functional monomer, the fluorine-functional monomer, a solubilizer, and water to obtain a reaction solution;

[0014] S12. After purging oxygen with nitrogen from the reaction solution, add an initiator, heat to 50-70 °C, and react for 8-12 hours to obtain a thickening agent gel;

[0015] S13. Cut, dry, and pulverize the thickening agent gel to obtain the thickening agent 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. 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 dosage is 10-20 times the mass of the fluorine-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: Directly prepare low-viscosity slickwater with polymer dry powder for large-displacement fracture creation to communicate microfractures, and use high-viscosity slickwater to carry sand for fracturing; the specific method is as follows:

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

[0029] (2) Inject the low-viscosity slickwater into the formation at a displacement of 18 - 20 m 3 / min as the preflush fluid to create fractures and communicate micro natural fractures;

[0030] (3) Increase the dosage of polymer concentrate and directly switch to prepare high-viscosity slickwater with a viscosity of 20 - 40 mPa·s, and inject it into the formation fractures as the sand-carrying fluid;

[0031] (4) Decrease the dosage of polymer concentrate and directly switch to prepare low-viscosity slickwater with a viscosity of 2 - 6 mPa·s, and displace the sand-carrying fluid in the wellbore into the formation to complete fracture creation and sand-carrying work;

[0032] (5) Shut in the well for 1 - 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the propped fractures are closed, and the proppants are effectively placed;

[0033] (6) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas are produced from the pore matrix, microfractures, propped fractures, and wellbore.

[0034] Application Process 2: Directly prepare high-viscosity slickwater with polymer dry powder for large-displacement main fracture creation, switch to low-viscosity slickwater to communicate microfractures, and then switch to high-viscosity slickwater to carry sand for fracturing; the specific method is as follows:

[0035] (1) Pump an appropriate amount of polymer concentrate and fracturing additives into the sand mixer to form high-viscosity slickwater with a viscosity of 20 - 40 mPa·s; inject the high-viscosity slickwater into the formation at a large displacement as the preflush fluid to create the main fracture;

[0036] (2) Decrease the dosage of polymer concentrate and directly switch to prepare low-viscosity slickwater with a viscosity of 2 - 6 mPa·s as the preflush fluid to continue fracture creation and communicate more natural microfractures;

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

[0038] (4) Increase the dosage of polymer concentrate and directly switch to prepare high-viscosity slickwater with a viscosity of 20 - 40 mPa·s, and inject it into the formation fractures as the sand-carrying fluid;

[0039] (5) Decrease the dosage of polymer concentrate and directly switch to prepare low-viscosity slickwater with a viscosity of 2 - 6 mPa·s, and displace the sand-carrying fluid in the wellbore into the formation to complete fracture creation and sand-carrying work;

[0040] (6) Shut in the well for 1 - 4 hours. Under the action of formation temperature and breaker, the fracturing fluid is completely broken, the propped fractures are closed, and the proppants are effectively placed.

[0041] (7) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas are produced from the pore matrix, micro - fractures, propped fractures, and wellbore.

[0042] The bactericides used in the present invention are mainly directed against sulfate - reducing bacteria, iron bacteria, saprophytic bacteria, etc., which can kill bacteria and inhibit serious corrosion and plugging problems that may occur in the fracturing environment. The bactericides are at least one of quaternary ammonium salts (such as dodecyltrimethylammonium chloride, dodecyldimethylbenzylammonium chloride, dodecyldimethylbenzylammonium bromide, etc.), polymeric quaternary ammonium salts, and other non - oxidizing bactericides. The flowback aid is a complex of cationic, anionic, non - ionic fluorocarbon surfactants, sulfonate surfactants, and co - solvents. The breaker is a liquid breaker, which is a complex of peroxide and organic additives. The temporary plugging agent is a water - soluble granular temporary plugging agent, and the particle size is 20 - 60 mesh. Clay stabilizers, demulsifiers, pH regulators, foaming agents, and defoaming agents and other common fracturing additives can also be used. These additives are all common types well - known to those skilled in the art and can be arbitrarily selected according to construction requirements.

[0043] Compared with the prior art, the advantages of the present invention are as follows:

[0044] (1) The polymer dry - powder thickener provided by the present invention can directly prepare a controllable variable - viscosity fracturing fluid online. The polymer dry - powder directly contacts, hydrates, rapidly dissolves, and thickens with the water in the shear flow field. There is no oil phase, no emulsifier, and no organoclay, eliminating the introduction of the oil phase from the source, reducing the damage of fracturing fluid residues, oil stains, and impurities to the reservoir and potential pollution to the environment, and also reducing the treatment cost required for the existence of oil stains in the flowback fluid, having the effect of environmental protection. It realizes the immediate supply, preparation, and injection of the fracturing fluid, with seamless switching, and can meet the requirements of large liquid volume, large displacement (2 - 20 m 3 / min pumping displacement), and continuous construction, achieving the purpose of improving construction efficiency and reducing comprehensive costs. For unconventional oil and gas, the real - time viscosity can be controllably changed from 1 to 120 mPa·s at a pumping displacement of 24 m 3 / min, realizing seamless switching between the low viscosity of slickwater and the high viscosity of sand - carrying fluid, so as to meet the precise control of the viscosity of the fracturing fluid.

[0045] (2) The polymer dry powder of the present invention has the characteristics of instant solubility, viscosity increase, and controllable rheological viscosity change, with good salt resistance adaptability, providing guarantee for on-line direct mixing of dry powder, efficiency improvement, and cost reduction. The polymer dry powder has a high solid content (greater than 88%), much higher than the 30 - 50% content of inverse emulsion and suspension emulsion polymers. To achieve the same liquid viscosity, direct mixing of polymer dry powder requires a lower polymer addition concentration, with the dosage reduced by 60 - 70%, reducing the fracturing cost; in comparison at the same usage concentration, direct mixing of polymer dry powder has a higher viscosity-increasing ability, and can better meet the requirements of large displacement and large sand volume in large-scale volume fracturing of unconventional oil and gas for the viscosity of fracturing fluid, realizing rapid switching between low viscosity, medium viscosity, and high viscosity.

[0046] (3) The molecular chain of the polymer dry powder of the present invention has special fluorine-containing hydrophobic, salt-resistant, and temperature-resistant functional groups; the polymer powder is treated with nanomaterials, surface modifiers, and particle fineness treatment, with better flow and dispersion properties, and can quickly disperse and hydrate in the sand mixing tank, forming a clean, fish-eye-free, and viscosity-controllable fracturing fluid, which shows good drag reduction performance (drag reduction rate greater than 70%), viscosity-increasing ability (adjustable from 1 to 120 mPa·s), sand-carrying ability (30% sand ratio fully suspended), and salt resistance ability in engineering applications; at a salinity of 100,000 mg / L, the viscosity retention rate is above 70%, the salt-resistant salinity reaches 300,000 mg / L, and the controllable viscosity change is 90 mPa·s, greatly improving the comprehensive performance of the polymer fracturing fluid, which is beneficial to the placement of proppants during the construction process and improves the fracturing construction effect.

[0047] (4) This fracturing fluid system is equipped with a multifunctional agent for sterilization, drainage assistance, and demulsification, a clay stabilizer, a high-efficiency gel breaker, and a temporary plugging agent. At a low addition concentration, the fracturing fluid has the ability to inhibit sulfate-reducing bacteria, demulsify and prevent water lock, reduce surface and interfacial tension, and stabilize clay. The fracturing fluid breaks gel completely, with low residue, even no residue, which can avoid the retention and blockage of the fracturing fluid in the oil and gas seepage channels, and is beneficial to improving oil and gas production capacity.

[0048] (5) For shale gas, tight gas, and coalbed methane wells, the returned fluid after fracturing with the direct mixing and controllable viscosity-changing fracturing fluid of polymer dry powder breaks gel completely and is clean without oil phase. Due to the good salt resistance of the polymer dry powder of the present invention, the returned fluid does not need to be treated on the ground to remove high-valent metal ions and oil stains, and can be directly reused for direct mixing fracturing construction of dry powder, reducing the potential environmental pollution of waste liquid and high-salt returned 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 drag reduction type thickening agent for the direct mixing and controllable viscosity-changing fracturing fluid of the polymer dry powder provided by the present invention is simple to prepare, has a high solid content, a low usage concentration, no oil phase, is clean without residue, is a green and environmentally friendly, low-cost water-based fracturing fluid, is an important product for increasing the single-well production capacity of unconventional oil and gas, extending the stable production period, and improving the recovery rate, and is a revolution in fracturing fluid preparation.

[0050] Other advantages, objectives and features of the present invention will be partially reflected by the following description and partially understood by those skilled in the art through the research and practice of the present invention. Description of the Drawings

[0051] Figure 1 Variation of the viscosity of the polymer dry powder of Example 1 with different concentrations over the swelling time.

[0052] Figure 2 Temperature and shear resistance rheological curve of the directly prepared fracturing fluid from the polymer dry powder of the present invention.

[0053] Figure 3 Photograph of the gel-breaking fluid of the polymer dry powder fracturing fluid of the present invention in an 80°C water bath.

[0054] Figure 4 Picture of the high-concentration gel solution tested by directly preparing the polymer dry powder of the present invention.

[0055] Figure 5 Comparison diagram of the flowback fluid after fracturing of the variable-viscosity fracturing fluid directly prepared from the polymer dry powder of the present invention and the flowback fluid of the inverse emulsion polymer fracturing of the adjacent well on the platform. The left figure is the flowback fluid corresponding to the polymer dry powder of the present invention, and the right figure is the flowback fluid corresponding to the inverse emulsion polymer. Detailed Embodiments

[0056] The following describes the preferred embodiments of the present invention with reference to 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 preparation method of a thickener for directly preparing a controllable variable-viscosity water-based fracturing fluid from dry powder:

[0059] (1) Take the hydrophilic monomer acrylamide, the salt-tolerant monomer 2-acrylamido-2-methylpropanesulfonic acid (AMPS), the fluorine-containing functional monomer 2,2,3,4,4,4-hexafluorobutyl methacrylate, and the solubilizer alkylphenol polyoxyethylene ether (OP-10) and add them to the reactor. Add the solvent water and quickly stir until the fluorine-containing functional monomer is completely and uniformly dispersed in the water to obtain a reaction solution. Among them, the dosage of the salt-tolerant monomer AMPS accounts for 13.3% of the total mass of the three monomers; the dosage of the 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 dosage of the 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 introducing nitrogen into the reaction solution for 20 min, an initiator (V50) was added, and the temperature was raised to 55 °C. The reaction was maintained at 55 °C for 10 h 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 with a thickness of 1 cm and dried at 70 - 75 °C for 8 h. The dried thickener was pulverized with a high-speed pulverizer to obtain polymer powder.

[0062] (4) Nano-silica (SiO2-NH2) and a surface modifier were added to the polymer powder and mixed evenly. After secondary drying and deep pulverization, it was passed through a 100 - 200 mesh sieve to obtain the finished polymer dry powder. The surface modifier was a compound of SDS and AEO-9 with a mass ratio of 6:4. The mixing mass ratio of polymer powder : nano material : surface modifier was 98.5 : 0.5 : 1.

[0063] Example 2

[0064] On the basis of Example 1, the dosage 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 the polymer dry powder.

[0065] Example 3

[0066] On the basis of Example 1, the dosage 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 the polymer dry powder.

[0067] Example 4

[0068] A preparation method of a thickener for a dry powder directly prepared controllable variable-viscosity water-based fracturing fluid:

[0069] (1) The hydrophilic monomer acrylamide, the salt-tolerant monomer sodium 5-sulfonatophthalate dimethyl ester, the fluorine-containing functional monomer 2,2,2-trifluoroethyl acrylate, and the solubilizer alkylphenol polyoxyethylene ether (OP-10) were weighed and added to a reactor. Solvent water was added, and it was rapidly stirred until the fluorine-containing functional monomer was uniformly dispersed in water to obtain a reaction solution. Among them, the dosage of the salt-tolerant monomer accounted for 12.5% of the total mass of the three monomers; the dosage of the fluorine-containing functional monomer accounted for 0.8% of the total mass of the three monomers, and the total of the three monomers was 100%; the dosage of the solubilizer was 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) accounted for 30% of the mass of the reaction solution.

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

[0071] (3) Cut the thickening agent gel into thin blocks with a thickness of 1 cm, dry them at 70 - 75 °C for 8 h, and crush the dried thickening agent with a high-speed crusher to obtain polymer powder.

[0072] (4) Add nano-silica (SiO₂-NH₂) and a surface modifier to the polymer powder and mix evenly. Then, perform secondary drying and deep crushing, and pass through a 100 - 200 mesh sieve to obtain the finished polymer dry powder. 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 : nano-material : surface modifier is 98.5 : 0.5 : 1.

[0073] Example 5

[0074] A preparation method of a thickening agent for a dry powder directly prepared controllable variable-viscosity water-based fracturing fluid:

[0075] (1) Weigh hydrophilic monomer sodium acrylate, salt-tolerant monomer 2-acrylamidododecanesulfonic acid, fluorine-containing functional monomer 2,2,3,4,4,4-hexafluorobutyl acrylate, and solubilizer alkylphenol polyoxyethylene ether (OP-10) and add them to a reactor. Add solvent water and quickly stir until the fluorine-containing functional monomer is uniformly dispersed in water to obtain a reaction solution. Among them, the dosage of the salt-tolerant monomer accounts for 12.5% of the total mass of the three monomers; the dosage 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 dosage 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, salt-tolerant monomer, and fluorine-containing functional monomer) accounts for 28% of the mass of the reaction solution.

[0076] (2) After introducing nitrogen into the reaction solution for 20 min, add an initiator (V50), and raise the temperature to 65 °C. Maintain the reaction at 65 °C for 10 h to obtain a polymer thickening agent gel. The mass of the initiator is 0.2% of the total mass of the three monomers.

[0077] (3) Cut the thickening agent gel into thin blocks with a thickness of 1 cm, dry them at 70 - 75 °C for 8 h, and crush the dried thickening agent with a high-speed crusher to obtain polymer powder.

[0078] (4) Add nano-silica (SiO₂-NH₂) and a surface modifier to the polymer powder and mix evenly. Then, perform secondary drying and deep crushing, and pass through a 100 - 200 mesh sieve to obtain the finished polymer dry powder. 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 : nano-material : surface modifier is 98.5 : 0.5 : 1.

[0079] Comparative Example 1

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

[0081] Comparative Example 2

[0082] On the basis of Example 1, replace the fluorine-containing functional monomer 2,2,3,4,4,4-hexafluorobutyl methacrylate with an equal amount of 1H,1H,2H,2H-heptadecafluorodecyl acrylate, and keep other steps 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, prepare an aqueous potassium chloride solution with a mass concentration of 1% in the laboratory. Measure 400 ml of the aqueous solution into a Wu Yin mixer, increase the rotation speed to 1000 rpm, and make the liquid flow vortex in the mixer bottom visible. After stirring the polymer dry powder of Example 1 at different dosage concentrations (mass percentage concentrations of 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.7%) for 40 seconds, use a six-speed rotational viscometer at a rotational speed of 100 rpm (shear rate 170S -1 ) to test the viscosity increasing ability at different times. The results are shown in Figure 1 . It can be seen from the experiment that the polymer dry powder disclosed in the present invention has good instant solubility and viscosity increasing effect in a 10000 mg / L salt water solution, and meets the viscosity increasing conditions for direct preparation of polymer dry powder.

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

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

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

[0088]

[0089] (4) The frictional resistance performance of the standard brine slickwater fracturing fluid of the polymer dry powder with a mass content of 0.05% was tested using a pipeline frictional resistance tester. In a pipeline with a diameter of 1 inch, simulating the linear velocity of the construction displacement of 20 m³ / min on-site during fracturing, the measured frictional resistance is shown in Table 2. The frictional resistance test results show that in the standard brine of 84000 mg / L, the polymers of Example 1 and Example 2 have good salt resistance, and the kinematic viscosity remains above 3.5 mm 2 / s, and they have excellent drag reduction rates, reaching more than 75%. However, for the polymer of the low-concentration fluorine-containing hydrophobic monomer in Example 3, the salt resistance is significantly deteriorated, closer to the viscosity of clear water, and the drag reduction rate only remains at 61%.

[0090] Table 2. Test results of frictional resistance performance

[0091]

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

[0093] Table 3. Test results of sand suspension performance

[0094]

[0095] (5) In an 80 °C water bath, 0.02% liquid breaker was added to the fracturing fluid, and the gel 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 respectively. The results are as Figure 3As shown, in Examples 1, 2, and 3, complete gel breaking can be achieved within 2 hours. The viscosities of the gel-breaking fluids are 1.27 mm 2 / s, 1.22 mm 2 / s, and 1.16 mm 2 / s respectively. The gel-breaking fluids are clear, without an oil phase, and there is no residue visible to the naked eye. In Comparative Examples 1 and 2, complete gel breaking could not be achieved within 2 hours, and the solution viscosities are 12.36 mm 2 / s and 18.90 mm 2 / s respectively. The gel-breaking fluids are turbid, and there is residue visible to the naked eye.

[0096] The surface tensions of the gel-breaking fluids were measured using an automatic surface tensiometer respectively, and the results are shown in Table 4. The experimental results show that the fluorinated monomers in the polymer dry powder of the present invention can effectively reduce the surface tension of the gel-breaking fluid, and with the increase of the concentration of the fluorinated monomers, the effect of reducing the surface tension becomes more obvious, which also verifies the important characteristic of the fluorinated hydrophobic monomers in improving the interfacial chemistry in the polymer.

[0097] Table 4. Viscosities and surface tensions of the gel-breaking fluids of fracturing fluids in different examples

[0098]

[0099]

[0100] (6) Test the ability of the polymer dry powder thickener prepared in Example 1 to directly prepare viscoelastic fracturing fluid gel from the dry powder. Using clear water with a total salinity of 268 mg / L, pump it into the sand mixing tank at a pipe flow rate of 1.0 m 3 / min, and at the same time pump in the dry powder concentrate of Example 1 at a dosage of 25 kg / min of the polymer dry powder to obtain a gel state with a mass concentration of 2.5% of the polymer dry powder. As Figure 4 shown, Figure (a) shows the gel state, (b) shows that the gel has excellent viscoelasticity, and (c) shows that the gel is uniform and without fish eyes. From the polymer dry powder to the polymer gel, it takes 25 seconds to obtain a high-concentration and high-viscosity gel, which is uniform, without fish eyes, and has good viscoelastic characteristics, which proves the feasibility of directly preparing viscoelastic fracturing fluid from the polymer dry powder.

[0101] (7) Field application tests were carried out on a shale gas fracturing platform well in the Sichuan Basin. The ability of the dry powder polymer thickener prepared in Example 1 of the test application to directly prepare a controllable viscosity-changing fracturing fluid from dry powder was tested. A mixed solution of fresh water and produced fluid (total salinity 12,500 mg / L) was used to prepare the fracturing fluid. After adding dry powder and mixing in a shear flow field for 17 seconds, it converged into the mixing tank of the sand mixer truck. For the low-viscosity slickwater with a dry powder concentration of 0.05%, the viscosity of the fracturing fluid was 4.8 mPa·s; for the medium-viscosity slickwater fracturing fluid with a dry powder concentration of 0.1%, the viscosity of the fracturing fluid was 13.8 mPa·s; for the high-viscosity slickwater fracturing fluid with a dry powder concentration of 0.2%, the viscosity of the fracturing fluid was 31.5 mPa·s. The maximum construction pressure was 75 MPa, the construction pressure was 60 - 75 MPa, the displacement was 12 - 14 m 3 / min, and the fluid volume was 57,024 m 3 . The maximum sand ratio reached 30%, and the average sand ratio was 220 kg / m 3 . The drag reduction rate of the slickwater fracturing fluid in the fracturing test was 75.8%. By adjusting the addition rate of the polymer dry powder, the variable viscosity of the slickwater fracturing fluid was quickly obtained. The formed slickwater fracturing fluid had stable performance, uniform gel, no fisheyes, and it was feasible to directly prepare a variable viscosity fracturing fluid from polymer dry powder.

[0102] Compared with the inverse emulsion polymer in the adjacent well of the platform, the fracturing fluid directly prepared from the polymer dry powder in this well was quickly soluble, easy to prepare, and had strong viscosity increasing and sand carrying capacities. The comparison of the produced fluid samples taken after the well operation in the oilfield is shown in Figure 5 . It can be seen that the variable viscosity fracturing fluid directly prepared from the polymer dry powder of the present invention had a good gel-breaking effect, was clear without impurities and oil phase, like fresh water, with a capillary viscosity of only 1.89 mPa·s, was green and environmentally friendly, and could be directly recycled and reused; while the produced fluid sample taken after the well operation of the emulsion drag reducer in the adjacent well was turbid, contained oil (more than 0.2%), and had a lot of flocculants and other residues. The produced fluid could not be directly recycled and reused and had to be treated as hazardous waste, increasing the treatment cost and environmental pollution risk.

[0103] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art, within the scope of the technical solution of the present invention, can make some changes or modifications to equivalent embodiments by using the above-disclosed technical content. However, as long as it does not deviate from the content of the technical solution of the present invention, any simple modification, equivalent change, and modification made to the above embodiments according to the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A preparation method of a polymer dry powder directly formulated controllable variable-viscosity 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; The sulfonate functional monomer is selected from any one of 2-acrylamide-2-methylpropanesulfonic acid, 2-acrylamide dodecanesulfonic acid, and dimethyl isophthalate 5-sodium sulfonate; The hydrophilic monomer is selected from any one of acrylamide, sodium acrylate and sodium methacrylate; The amount of the sulfonate functional monomer accounts for 11-15% of the total weight of the three monomers; the amount of the fluorine functional monomer accounts for 0.2-1.2% of the total weight of the three monomers; 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 preparation method of the polymer dry powder directly formulated controllable variable-viscosity fracturing fluid according to claim 1, 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.

3. The preparation method of the polymer dry powder directly formulated controllable variable viscosity fracturing fluid according to claim 2, characterized in that, The total mass of the three monomers accounts for 25-30% of the mass of the reaction solution.

4. The preparation method of the polymer dry powder directly formulated controllable variable-viscosity fracturing fluid according to claim 2, wherein, The solubilizer is alkylphenol polyoxyethylene ether, and its usage is 10 to 20 times the mass of the fluorine-containing functional monomer.

5. Use of a polymer dry powder directly prepared controllable variable viscosity fracturing fluid prepared by the preparation method according to any one of claims 1-4, characterized in that, The polymer dry powder is used in combination with fracturing additives to prepare functional water-based fracturing fluid; the fracturing fluid additives include bactericides, drainage aids, gel breakers and temporary plugging agents.

6. The application of the polymer dry powder directly formulated controllable variable viscosity fracturing fluid according to claim 5, 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) Inject low-viscosity slippery water into the formation at a displacement of 18 - 20 m 3 / min as the preflush fluid to form fractures and connect micro 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) Shut in the well for 1 to 4 hours. Under the formation temperature and the action of the gel breaker, the fracturing fluid is completely broken, the propped fractures close, and the proppants are effectively placed. (6) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas are produced from the pore matrix, microfractures, propped fractures, and wellbore.

7. Use of the polymer dry powder directly prepared controllable variable-viscosity fracturing fluid according to claim 5, characterized in that, Polymer dry powder is directly formulated into high-viscosity slickwater to create the main fracture at a large displacement, then switched to low-viscosity slickwater to communicate with microfractures, and then switched back to high-viscosity slickwater to carry sand for fracturing; the specific method is as follows: (1) Pump the polymer concentrate and fracturing additives into the sand mixing tank to form high-viscosity slickwater with a viscosity of 20 to 40 mPa·s; inject the high-viscosity slickwater into the formation at a large displacement as the preflush fluid to create the main fracture. (2) Reduce the dosage of the polymer concentrate and directly switch to formulate low-viscosity slickwater with a viscosity of 2 to 6 mPa·s as the preflush fluid to continue creating fractures and communicate more natural microfractures. (3) Repeat steps (1) and (2) alternately. (4) Increase the dosage of the polymer concentrate and directly switch to formulate high-viscosity slickwater with a viscosity of 20 to 40 mPa·s as the sand-carrying fluid and inject it into the formation fractures. (5) Reduce the dosage of the polymer concentrate and directly switch to formulate low-viscosity slickwater with a viscosity of 2 to 6 mPa·s to displace the sand-carrying fluid in the wellbore into the formation, completing the fracturing and sand-carrying work. (6) Shut in the well for 1 to 4 hours. Under the formation temperature and the action of the gel breaker, the fracturing fluid is completely broken, the propped fractures close, and the proppants are effectively placed. (7) Open the wellhead valve to allow the fracturing fluid to flow back, and oil and gas are produced from the pore matrix, microfractures, propped fractures, and wellbore.

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