A dynamic water plugging method for fractured reservoirs based on salt response and its application

Abstract

The present invention belongs to the technical field of oilfield development, and relates to a dynamic water plugging method for fractured reservoirs based on salt response and the application of this method. A salt-responsive gel microsphere product is prepared, and the particle size and strength of the salt-responsive gel microspheres are controlled by regulating the salinity of the injected water, so as to achieve dynamic plugging of fractures. The method provided by the present invention is based on the fact that the salt-responsive gel microspheres have different particle sizes and strengths in salt water with different salinities, so that the migration and plugging of the gel microspheres in the fractures can be controlled timely by regulating the salinity of the injected water, ensuring that while plugging the water channel of the fractures, the function of the fracture channel as an oil drainage channel is retained, not only minimizing the flow resistance of the fluid in the reservoir to the greatest extent, but also expanding the swept volume of the injected water in a larger range, and ultimately achieving a substantial improvement in the water injection development effect of fractured reservoirs.

Description

Technical Field

[0001] The present invention belongs to the technical field of oilfield development, and relates to a dynamic water plugging method for fractured reservoirs based on salt response and the application of this method. Background Art

[0002] Fractured reservoirs account for a significant proportion in China's oilfields. For example, the main areas of super-large oilfields with a production capacity of tens of millions of tons such as Changqing Oilfield and Yanchang Oilfield are mainly such reservoirs. In such reservoirs, due to the widespread development of fractures, "fracture water channeling" is likely to occur during water injection development. The reservoir enters the high water cut stage from the low water cut stage in a short period, and a large amount of remaining oil in the reservoir matrix is difficult to be effectively developed. To address this problem, using water plugging technology to reasonably plug fractures is the most reliable production-increasing measure. Since fractures are both important oil drainage channels and water channeling channels, the focus of plugging lies in how to ensure that the fractures are "plugged but not dead". Among many traditional water plugging systems, gel microspheres integrate the functions of migration and plugging, and to a certain extent, achieve the goal of "plugging but not dead" for fractures, so they are widely used in high water cut fractured low permeability reservoirs.

[0003] For traditional gel microspheres, their migration ability and plugging ability are dialectically unified. A strong migration ability requires low strength or small particle size of the gel microspheres, which results in poor plugging ability. On the contrary, a strong plugging ability requires high strength or large particle size of the gel microspheres, which results in poor migration ability. Currently, since it is difficult to accurately and quantitatively balance the migration ability and plugging ability of gel microspheres in fractures, situations where the migration ability and plugging ability are too high or too low are likely to occur, leading to two common problems in the gel microsphere water plugging construction of fractured reservoirs: ① After construction in some wells, there is an increase in production, but the construction pressure is too high, and subsequent injection is difficult; ② The construction of some wells is smooth, but only the decline rate is moderately slowed down, and no obvious production-increasing effect is achieved. Therefore, how to break through the performance constraints of traditional gel microspheres is a new problem faced by the efficient development of fractured reservoirs.

[0004] Compared with traditional materials, intelligent materials have functional elements such as sensing, driving, and control. Special structural features enable intelligent materials to make precise, efficient, and appropriate responses to changes in environmental conditions and internal states. Based on the response function of intelligent materials, their performance can be adjusted timely. For a given reservoir, among the changeable environmental conditions, the salinity of the injected water is the most convenient, controllable, and low-cost. This shows that developing intelligent gel microspheres with salt response is the most technically and economically feasible. Salt-responsive gel microspheres can adjust parameters such as their particle size and strength by controlling the salinity of the injected water.

[0005] The intelligentization in the field of oil and gas exploitation is a major trend in the industry's development. However, the research and application of intelligent materials in the field of oil exploitation are still in the initial stage of development, and corresponding technological inventions are urgently needed. In view of the new problems faced by water plugging and production increase in fractured reservoirs, a new dynamic water plugging method for fractured reservoirs based on salt response is invented. The present invention can not only provide strong technical support for the production increase of fractured reservoirs, but also provide new methods and means for the production increase of other heterogeneous reservoirs, and can also promote the intelligent development of oilfield chemistry. Summary of the Invention

[0006] The object of the present invention is to provide a new dynamic water plugging method for fractured reservoirs based on salt response. This method is based on the fact that salt-responsive gel microspheres have different particle sizes and strengths in salt water with different salinities, so that the migration and plugging of gel microspheres in fractures can be controlled timely by regulating the salinity of the injected water, ensuring that while plugging the water channel in the fracture, the fracture channel is retained as the function of the oil drainage channel, not only minimizing the flow resistance of the fluid in the reservoir, but also expanding the swept volume of the injected water in a larger range, and finally achieving a substantial improvement in the water injection development effect of fractured reservoirs.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is a dynamic water plugging method for fractured reservoirs based on salt response, which is characterized by including the following steps:

[0008] Step 1. Mix components with a mass fraction ratio of 1% - 4% of acrylamide, 1% - 4% of anionic vinyl monomer, 1% - 4% of cationic vinyl monomer, 0.1% - 0.6% of nanoparticles, 0.1% - 0.5% of N,N'-methylenebisacrylamide, 0.01 - 0.03% of persulfate, and the rest being water, stir evenly and add NaOH dropwise to adjust the pH value to neutral, then add white oil dissolved with Tween-80 and Span-80, stir and fill with pure N 2 Remove oxygen, raise the temperature to initiate the polymerization reaction, and after reacting for a period of time, demulsify to obtain a salt-responsive gel microsphere product;

[0009] Step 2. Prepare a salt-responsive gel microsphere solution with a concentration range of 5% - 15% from the salt-responsive gel microsphere product obtained in Step 1, and use an injection device to inject a salt-responsive gel microsphere solution with a volume that is times the fracture volume into the fractured reservoir, shut down the injection device, and as the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes;

[0010] Step 3. Inject simulated formation water into the fractured reservoir using an injection device. When the water cut at the outlet end increases to 98%, switch to injecting low salinity brine and shut down the injection device. As the low salinity brine gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously decreases and the strength continuously declines until they can migrate smoothly in the fractures. Among them, when 25000 mg / L ≤ the salinity of the simulated formation water ≤ 40000 mg / L, the anionic vinyl monomer used is acrylic acid (AA); when the salinity of the simulated formation water ≥ 40000 mg / L, the anionic vinyl monomer used is 2-acrylamido-2-methylpropanesulfonic acid (AMPS).

[0011] Step 4. Inject simulated formation water with a volume times that of the fracture volume into the fractured reservoir using an injection device, and then shut down the injection device. As the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes.

[0012] Step 5. Repeat Step 3 and Step 4 for a total of n times to achieve dynamic plugging of the entire fractured reservoir.

[0013] Moreover, in Step 1, the cationic vinyl monomer is dimethyldiallylammonium chloride (DMDAAC) or acryloyloxyethyltrimethylammonium chloride (DMAEA-Q), and the nanoparticles are metal-based nanomaterials, inorganic-based nanomaterials, or organic-based nanomaterials; the mass fraction ratio of Tween-80 in white oil is 0.5%, the mass fraction ratio of Span-80 is 0.5%, and the ratio of white oil to water is 1-2:1.

[0014] Moreover, the specific conditions for the polymerization reaction to occur during stirring, filling with pure N 2 deoxygenation, and temperature increase in Step 1 are high-speed shear stirring for 10-30 min, filling with pure nitrogen for 15-20 min, heating to 45-60 °C, and reacting for 5-6 h.

[0015] Moreover, the 5-15% salt-responsive gel microsphere solution in Step 2 is prepared using tap water, and the time to shut down the injection device is 5-6 h; and the fracture aperture of the fractured reservoir is 100-1000 μm.

[0016] Moreover, in Step 2, n ≥ 2; for reservoirs with a permeability ≤ 50×10 -3 μm 2 n = 4-6; for reservoirs with a permeability ≥ 50×10 -3 μm 2 n = 3-4.

[0017] Moreover, in Step 3, the salinity of the low-salinity brine is ≤ 5000 mg / L, and the salinity of the simulated formation water is ≥ 25000 mg / L; when the water cut increases to 98%, the volume of the low-salinity brine injected is 1 - 2 times that of the salt-responsive gel microsphere solution, and the injection equipment is shut down for 0.5 - 1 h.

[0018] Moreover, in Step 4, the injection equipment is shut down for 5 - 6 h.

[0019] Moreover, in Step 1, the synthesized salt-responsive gel microspheres have a particle size of 100 - 800 μm and a strength of 10 - 25 Pa in low-salinity brine; the synthesized salt-responsive gel microspheres have a particle size of 250 - 2000 μm and a strength of 45 - 70 Pa in simulated formation water.

[0020] Moreover, for reservoirs with a permeability ≤ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low-salinity brine is ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water is ≥ 2 times the fracture aperture of the reservoir and the strength is ≥ 55 Pa; for reservoirs with a permeability ≥ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low-salinity brine is ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water is ≥ 1.6 times the fracture aperture of the reservoir and the strength is ≥ 45 Pa.

[0021] And the application of the above method in the dynamic water plugging of fractured reservoirs. During the entire dynamic plugging process, only the section where the salt-responsive gel microsphere solution is located in the entire fracture is plugged, and the remaining section still provides a flow channel for the fluid.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: First, the practicability of the method is verified through experiments. In the formation water of the simulated reservoir, the gel microspheres have a large particle size and high strength, and can plug the water flow channel in the fracture, forcing the subsequent injected water to enter the non-fracture area of the reservoir, thereby expanding the water flooding swept volume. In low-salinity water, the gel microspheres have a small particle size and low strength, and can migrate smoothly in the fracture until they reach the plugging area of the next cycle. The salt-responsive gel microspheres can split the unity of the migration ability and plugging ability of traditional gel microspheres into two independent functions, which do not affect each other. After injecting a small slug of salt-responsive gel microspheres into a fractured reservoir, by appropriately adjusting the salinity of the injected water, it can be ensured that the salt-responsive gel microspheres shrink and migrate, expand and plug, shrink and migrate again, expand and plug again, and so on. Through the dynamic sequential sectional water plugging of the fracture, the purpose of significantly improving the recovery rate of the entire reservoir can be achieved.

[0023] II. The dynamic water plugging method in the present invention controls the particle size and strength of the gel microspheres by regulating the salinity of the injected water, so as to achieve the dynamic plugging of fractures. Using this method, not only the water channeling channels are plugged, but also the fracture channels are retained as the function of the oil drainage channels. It not only minimizes the flow resistance of the fluid in the reservoir and greatly reduces the injection pressure, but also can expand the swept volume of the injected water in a larger range, and finally achieve a significant improvement in the water injection development effect of the fractured reservoir.

[0024] III. The preparation component ratios of different salt-responsive gel microsphere products are adapted to different fractured reservoir conditions. When 25000 mg / L ≤ the salinity of the simulated formation water ≤ 40000 mg / L, acrylic acid (AA) is used as the anionic vinyl monomer; when the salinity of the simulated formation water ≥ 40000 mg / L, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) is used as the anionic vinyl monomer.

[0025] IV. The injection amounts of different salt-responsive gel microsphere solutions are adjusted according to different fractured reservoir conditions. A salt-responsive gel microsphere solution with a volume that is times (n ≥ 2) the fracture volume is injected into the fractured reservoir. For reservoirs with a permeability ≤ 50×10 -3 μm 2 , n = 4 - 6; for reservoirs with a permeability ≥ 50×10 -3 μm 2 , n = 3 - 4.

[0026] V. The synthesized salt-responsive gel microspheres have a particle size of 100 - 800 μm and a strength of 10 - 25 Pa in low-salinity brine; and a particle size of 250 - 2000 μm and a strength of 45 - 70 Pa in the simulated formation water. For reservoirs with a permeability ≤ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low-salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in the simulated formation water ≥ 2 times the fracture aperture of the reservoir and the strength ≥ 55 Pa; for reservoirs with a permeability ≥ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low-salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in the simulated formation water ≥ 1.6 times the fracture aperture of the reservoir and the strength ≥ 45 Pa. It is precisely because the salt-responsive gel microspheres have these characteristics that the dynamic water plugging of the fractured reservoir can be achieved. Description of the Drawings

[0027] Figure 1Showing the particle size and strength of salt-responsive gel microspheres in tap water and simulated formation water (25,000 mg / L ≤ salinity of reservoir formation water ≤ 40,000 mg / L) (a: particle size; b: strength)

[0028] Figure 2 Showing the dynamic plugging experiment process of salt-responsive gel microspheres in fractured reservoirs (25,000 mg / L ≤ salinity of reservoir formation water ≤ 40,000 mg / L)

[0029] Figure 3 Showing the particle size and strength of salt-responsive gel microspheres in tap water and simulated formation water (salinity of reservoir formation water ≥ 40,000 mg / L) (a: particle size; b: strength)

[0030] Figure 4 Showing the dynamic plugging experiment process of salt-responsive gel microspheres in fractured reservoirs (salinity of reservoir formation water ≥ 40,000 mg / L)

[0031] Figure 5 Schematic diagram of the dynamic water plugging of salt-responsive gel microspheres in fractured reservoirs (●: salt-responsive gel microspheres; Water flow; a: shrinkage, migration; b: swelling, plugging; c: re-shrinkage, re-migration; d: re-swelling, re-plugging; e: re-shrinkage, re-migration; f: re-swelling, re-plugging) Detailed implementation manners

[0032] The present invention will be described in detail below in conjunction with the drawings and embodiments, and the content of the present invention is not limited to the following embodiments.

[0033] The steps of the dynamic water plugging method for fractured reservoirs based on salt response involved in the present invention are as follows:

[0034] Step 1. Mix components with a mass fraction ratio of 1% - 4% acrylamide, 1% - 4% anionic vinyl monomer, 1% - 4% cationic vinyl monomer, 0.1% - 0.6% nanoparticles, 0.1% - 0.5% N,N'-methylenebisacrylamide, 0.01 - 0.03% persulfate, and the rest being water, stir evenly and add NaOH dropwise to adjust the pH value to neutral, then add white oil dissolved with Tween-80 and Span-80, control the oil-water ratio, and after high-speed shear stirring for a certain time, fill with pure N 2 Remove oxygen, then raise the temperature to initiate the polymerization reaction, and after the polymerization reaction for a period of time, demulsify to obtain a salt-responsive gel microsphere product.

[0035] To further improve the technical effect, preferably, the mass fraction of acrylamide is 2% - 3%, the mass fraction of anionic vinyl monomer is 2% - 3%, the mass fraction of cationic vinyl monomer is 2% - 3%, the mass fraction of nanoparticles is 0.3% - 0.5%, the mass fraction of N,N'-methylenebisacrylamide is 0.2% - 0.4%, the mass fraction of persulfate is 0.01 - 0.03%, and the rest is water.

[0036] Among them, both the anionic vinyl monomer and the cationic vinyl monomer are selected with a flexible chain structure to avoid a rigid structure, so as to enhance their stretching and swelling properties. The cationic vinyl monomer can be dimethyldiallylammonium chloride (DMDAAC) or acryloyloxyethyltrimethylammonium chloride (DMAEA-Q); preferably acryloyloxyethyltrimethylammonium chloride (DMAEA-Q). The nanoparticles can be metal-based nanomaterials, inorganic nanomaterials, organic nanomaterials, etc., preferably nano-silica.

[0037] The anionic vinyl monomer is acrylic acid (AA) or 2-acrylamido-2-methylpropanesulfonic acid (AMPS). Among them, the salt tolerance of acrylic acid (AA) itself is relatively weak, and it is not very salt-tolerant. When the salinity is too high, it will cause flocculation, sedimentation and other phenomena. Therefore, when 25000mg / L ≤ the salinity of the simulated formation water ≤ 40000mg / L, acrylic acid (AA) is preferred. The salt tolerance of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) itself is good, and it is relatively salt-tolerant. Even when the salinity is relatively high, it is relatively stable. Therefore, when the salinity of the simulated formation water ≥ 40000mg / L, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) is preferred.

[0038] The finally prepared salt-responsive gel microsphere product has a particle size of 100 - 800μm and a strength of 10 - 25Pa in low salinity brine; and a particle size of 250 - 2000μm and a strength of 45 - 70Pa in simulated formation water.

[0039] The principle of the salt-responsive gel microsphere product is that the gel microspheres made of this component carry both negative and positive charges and are electrically neutral externally. Due to the electrostatic attraction, the structure is shrunk. At high salinity, due to the strong action of ions, the original electrostatic adsorption will be destroyed, resulting in the expansion of the structure, and thus the particle size of the gel microspheres increases and the strength becomes higher.

[0040] Step 2. Prepare a salt-responsive gel microsphere solution with a concentration range of 5 - 15% from the salt-responsive gel microsphere product obtained in Step 1 using low salinity brine, and use an injection device to inject into the fractured reservoir a volume equal to the fracture volume A salt-responsive gel microsphere solution with a certain multiple is prepared. The injection equipment is shut down. As the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes. The shutdown time is 5 - 6 hours to ensure that the formation water fully invades the gel microsphere solution and the gel microspheres fully expand and increase in strength.

[0041] Step 3. Inject simulated formation water into the fractured reservoir using the injection equipment. When the water cut at the outlet end increases to 98%, switch to injecting low salinity brine and shut down the injection equipment. As the low salinity brine gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously decreases and the strength continuously decreases until they can migrate smoothly forward in the fracture. The volume of the low salinity brine for the transfer injection is 2 times that of the salt-responsive gel microsphere solution, and the shutdown time is 0.5 - 1 hour to ensure that the low salinity brine invades the gel microsphere solution and the gel microspheres fully shrink.

[0042] Step 4. Inject simulated formation water with a volume that is times the fracture volume into the fractured reservoir using the injection equipment. The injection equipment is shut down. As the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes. The shutdown time is 5 - 6 hours to ensure that the formation water fully invades the gel microsphere solution and the gel microspheres fully expand and increase in strength.

[0043] Step 5. Repeat Step 3 and Step 4 for a total of n times to achieve dynamic plugging of the entire fractured reservoir and achieve the effect of production increase. The schematic diagram of the dynamic plugging is as shown in Figure 5 shown.

[0044] During the entire dynamic plugging process, only the section where the salt-responsive gel microsphere solution is located in the entire fracture is plugged, and the remaining section still provides a flow channel for the fluid. Therefore, the flow resistance of the fluid in the reservoir is minimized and the injection pressure is greatly reduced, solving the problem of "too high construction pressure and difficult subsequent injection" existing in the current conventional water plugging construction.

[0045] And through a large number of experimental verifications, the optimal particle size of the salt-responsive gel microspheres for reservoirs with different permeabilities is summarized as follows: for reservoirs with a permeability ≤ 50×10 -3 μm 2 the particle size of the salt-responsive gel microspheres in low salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water ≥ 2 times the fracture aperture of the reservoir and the strength ≥ 55 Pa; for reservoirs with a permeability ≥ 50×10 -3 μm 2For the reservoir, the particle size of the salt-responsive gel microspheres in low salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water ≥ 1.6 times the fracture aperture of the reservoir, with a strength ≥ 45 Pa.

[0046] Example 1 (25000 mg / L ≤ salinity of reservoir formation water ≤ 40000 mg / L)

[0047] I. Preparation of salt-responsive gel microspheres

[0048] Step 1: Add 2 g of acrylamide, 3 g of acrylic acid, 2 g of acryloyloxyethyltrimethylammonium chloride, 0.4 g of nano-silica particles, 0.3 g of N,N'-methylenebisacrylamide, and 0.02 g of persulfate to 92.3 g of tap water, stir well until homogeneous, and then adjust the pH value to neutral with 1% NaOH solution.

[0049] Step 2: Add 0.5 g of Tween-80 and 0.5 g of Span-80 to 99 mL of white oil.

[0050] Step 3: Mix the aqueous solution in Step 1 with the white oil in Step 2, stir at high speed for 15 min, fill with pure N 2 Remove oxygen for 15 min, then raise the temperature to 45 °C and react for 5 h.

[0051] Step 4: Demulsify and separate the reaction product in Step 3, and prepare aqueous solutions of gel microspheres with a concentration of 15% using tap water and simulated formation water with a salinity of 30000 mg / L respectively.

[0052] Figure 1 The test results of the particle size and strength of the salt-responsive gel microspheres involved in Example 1 in waters with different salinities. In tap water, the average particle size of the gel microspheres is 230 μm and the strength is 20 Pa. In simulated formation water with a salinity of 30000 mg / L, the average particle size of the gel microspheres is 450 μm and the strength is 55 Pa.

[0053] II. Dynamic water plugging method for fractured reservoirs based on salt response

[0054] Step 1: Prepare a 15% salt-responsive gel microsphere solution with tap water.

[0055] Step 2: Select an artificial cubic rock core with a length of 100 cm, a width of 4.5 cm, and a height of 4.5 cm. The porosity of the core is 13.5% (the pore volume of the core is 273.4 cm 3 ), the water permeability of the core measured by water is 106.6×10 -3 μm 2 , and the oil saturation is 67.26%.

[0056] Step 3: Cut the artificial cubic core into two equal pieces along the length direction in the middle (each piece has a size of 100 cm in length, 2.25 cm in width, and 4.5 cm in height). Subsequently, place a rubber band with a thickness of 240 μm along the length edge on the surface of one core piece, fix the rubber band with epoxy resin, and then cover the other core piece on the rubber band. After aligning the two core pieces, a fractured core with a fracture aperture of 240 μm can be formed, which is used to simulate the actual fractured reservoir.

[0057] Step 4: Place the fabricated fractured core into a matching core holder, and then connect the displacement experiment device according to the standard GB / T28912-2012 "Determination Method for Relative Permeability of Two-Phase Fluids in Rock" (including three intermediate containers with a volume of 500 cm 3 , which are respectively used to store the aqueous solution of gel microspheres with a concentration of 15%, tap water, and simulated formation water with a salinity of 30000 mg / L).

[0058] Step 5: Inject the simulated formation water with a salinity of 30000 mg / L into the fractured core until the water cut at the outlet end is higher than 98%, and then stop. Record the injection pressure, recovery rate, water cut, and other data during the experiment.

[0059] Step 6: Inject 4 cm 3 of the salt-responsive gel microsphere aqueous solution into the fractured core, and shut down the injection equipment for 5 h.

[0060] Step 7: Inject the simulated formation water into the fractured core. The water cut at the outlet end continuously increases. When the water cut increases to 98%, switch to injecting 4 cm 3 of tap water, and shut down the injection equipment for 0.5 h. Record the injection pressure, recovery rate, water cut, and other data during the experiment.

[0061] Step 8: Inject 4 cm 3 of the simulated formation water into the fractured core, and shut down the injection equipment for 5 h.

[0062] Subsequently, repeat Step 7 and Step 8 for a total of three rounds, and the dynamic plugging of the entire fractured core can be achieved. During the three-round salt-responsive dynamic plugging process, the injection pressure at the inlet end of the core and the recovery rate and water cut at the outlet end are as Figure 2 shown. After three rounds of dynamic plugging, the final recovery rate is 38%, which is 31% higher than that of the primary water flooding. During the dynamic plugging process, the injection pressure only increases by 80%.

[0063] Example 2 (salinity of reservoir formation water ≥ 40000 mg / L)

[0064] I. Preparation of salt-responsive gel microspheres

[0065] Step 1: Add 2.5 g of acrylamide, 3 g of 2-acrylamido-2-methylpropanesulfonic acid, 3 g of acryloyloxyethyltrimethylammonium chloride, 0.5 g of nano-silica particles, 0.3 g of N,N'-methylenebisacrylamide, and 0.02 g of persulfate into 90.7 g of tap water. Stir well until homogeneous, and then adjust the pH value to neutral with 1% NaOH solution.

[0066] Step 2: Add 0.5 g of Tween-80 and 0.5 g of Span-80 into 99 mL of white oil.

[0067] Step 3: Mix the aqueous solution in Step 1 with the white oil in Step 2, and perform high-speed shear stirring for 25 min. Fill with pure N 2 Deoxygenate for 25 min, then raise the temperature to 50 °C and react for 6 h.

[0068] Step 4: Demulsify and separate the reaction product in Step 3, and prepare aqueous solutions of gel microspheres with a concentration of 15% using tap water and simulated formation water with a salinity of 60000 mg / L respectively.

[0069] Figure 3 Results of particle size and strength tests of salt-responsive gel microspheres involved in Example 2. In tap water, the average particle size of the gel microspheres is 260 μm and the strength is 26 Pa. In simulated formation water with a salinity of 60000 mg / L, the average particle size of the gel microspheres is 470 μm and the strength is 62 Pa.

[0070] II. Dynamic water plugging method for fractured reservoirs based on salt response

[0071] Step 1: Prepare a 15% salt-responsive gel microsphere solution with tap water.

[0072] Step 2: Select an artificial cubic rock core with a length of 100 cm, a width of 4.5 cm, and a height of 4.5 cm. The porosity of the rock core is 9.88% (the pore volume of the rock core is 200 cm 3 ), the water permeability of the rock core measured by water is 10.3×10 -3 μm 2 , and the oil saturation is 61.66%.

[0073] Step 3: Cut the artificial cubic rock core into two equal parts along the length direction from the middle (the size of each part is length 100 cm, width 2.25 cm, and height 4.5 cm). Then, place a rubber band with a thickness of 260 μm along the length edge on the surface of one rock core, and fix the rubber band with epoxy resin. Then cover the other rock core on the rubber band. After aligning the two rock cores, a fractured rock core with a fracture aperture of 260 μm can be formed to simulate the actual fractured reservoir.

[0074] Step 4: Place the fabricated fractured core into a matching core holder, and then connect the displacement experimental device according to the standard GB / T 28912-2012 "Determination Method for Relative Permeability of Two-Phase Fluids in Rock" (including three intermediate containers with a volume of 500 cm 3 , which store the aqueous solution of gel microspheres with a concentration of 15%, tap water, and simulated formation water with a salinity of 60,000 mg / L respectively);

[0075] Step 5: Inject the simulated formation water with a salinity of 60,000 mg / L into the fractured core until the water cut at the outlet end is higher than 98%, and then stop. Record the injection pressure, recovery rate, water cut and other data during the experiment;

[0076] Step 6: Inject 3 cm 3 of the salt-responsive gel microsphere aqueous solution into the fractured core, and shut down the injection equipment for 5 h;

[0077] Step 7: Inject the simulated formation water into the fractured core. The water cut at the outlet end continuously increases. When the water cut increases to 98%, switch to injecting 3 cm 3 of tap water, and shut down the injection equipment for 0.5 h. Record the injection pressure, recovery rate, water cut and other data during the experiment;

[0078] Step 8: Inject 3 cm 3 of the simulated formation water into the fractured core, and shut down the injection equipment for 5 h;

[0079] Subsequently, repeat Step 7 and Step 8 for a total of four rounds to achieve dynamic plugging of the entire fractured core. During the four-round salt-responsive dynamic plugging process, the injection pressure at the inlet end of the core and the recovery rate and water cut at the outlet end are as Figure 4 shown. After four rounds of dynamic plugging, the final recovery rate is 35%, which is 29% higher than that of the primary water flooding. During the dynamic plugging process, the injection pressure only increases by 120%.

Claims

1. A dynamic water plugging method for fractured reservoirs based on salt response, characterized in that, it includes the following steps: Step 1. Mix the components with a mass fraction ratio of 1% - 4% acrylamide, 1% - 4% anionic vinyl monomer, 1% - 4% cationic vinyl monomer, 0.1% - 0.6% nanoparticles, 0.1% - 0.5% N,N'-methylenebisacrylamide, 0.01 - 0.03% persulfate, and the rest being water. Stir evenly and add NaOH dropwise to adjust the pH value to neutral. Then add white oil dissolved with Tween-80 and Span-80, stir, and fill with pure N 2 Remove oxygen, raise the temperature to initiate the polymerization reaction, and after a period of reaction, demulsify to obtain a salt-responsive gel microsphere product; Step 2. Prepare the salt-responsive gel microsphere product obtained in Step 1 into a salt-responsive gel microsphere solution with a concentration range of 5-15%, and use an injection device to inject into the fractured reservoir a salt-responsive gel microsphere solution with a volume times that of the fracture volume. Shut down the injection device. As the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes; and n≥2. For a reservoir with a permeability ≤50×10 -3 μm 2 , n = 4-6; for a reservoir with a permeability ≥50×10 -3 μm 2 , n = 3-4; Step 3. Use an injection device to inject simulated formation water into the fractured reservoir. When the water cut at the outlet end increases to 98%, change to inject low salinity brine, and shut down the injection device. As the low salinity brine gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously decreases and the strength continuously decreases until it smoothly migrates forward in the fracture; among them, when 25000 mg / L ≤ the salinity of the simulated formation water ≤ 40000 mg / L, the anionic vinyl monomer used is acrylic acid; when the salinity of the simulated formation water ≥ 40000 mg / L, the anionic vinyl monomer used is 2-acrylamido-2-methylpropanesulfonic acid; Step 4. Inject simulated formation water with a volume times that of the fracture volume into the fractured reservoir, shut down the injection equipment. As the formation water in the fractured reservoir gradually invades the salt-responsive gel microsphere solution, the particle size of the gel microspheres continuously increases and the strength continuously improves until it stabilizes; Step 5. Repeat Step 3 and Step 4 for a total of n times to achieve dynamic plugging of the entire fractured reservoir.

2. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 1, the cationic vinyl monomer is dimethyldiallylammonium chloride or acryloyloxyethyltrimethylammonium chloride, and the nanoparticles are metal-based nanoparticles, inorganic-based nanoparticles or organic-based nanoparticles; the mass fraction ratio of Tween-80 in white oil is 0.5%, and the mass fraction ratio of Span-80 is 0.5%. The ratio of white oil to water is 1-2:

1.

3. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 1, stir and fill with pure N 2 The specific conditions for deoxygenation, heating up and polymerization reaction are high-speed shearing and stirring for 10 - 30 min, filling with pure N 2 for 15 - 20 min, heating up to 45 - 60 °C, and reacting for 5 - 6 h.

4. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 2, the salt-responsive gel microsphere solution with a concentration range of 5-15% is prepared using tap water, and the time to shut down the injection device is 5-6 h; and the fracture aperture of the fractured reservoir is 100-1000 μm.

5. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 3, the salinity of the low salinity brine ≤ 5000 mg / L, and the salinity of the simulated formation water ≥ 25000 mg / L; when the water cut increases to 98%, the volume of the low salinity brine injected is 1-2 times that of the salt-responsive gel microsphere solution, and the time to shut down the injection device is 0.5-1 h.

6. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 4, the time to shut down the injection device is 5-6 h.

7. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 1, characterized in that: In Step 1, the particle size of the synthesized salt-responsive gel microspheres in low salinity brine is 100-800 μm, and the strength is 10-25 Pa; the particle size of the synthesized salt-responsive gel microspheres in simulated formation water is 250-2000 μm, and the strength is 45-70 Pa.

8. The dynamic water plugging method for fractured reservoirs based on salt response according to claim 7, characterized in that: For reservoirs with a permeability ≤ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water ≥ 2 times the fracture aperture of the reservoir and the strength ≥ 55 Pa; for reservoirs with a permeability ≥ 50×10 -3 μm 2 , the particle size of the salt-responsive gel microspheres in low salinity brine ≤ the fracture aperture of the reservoir, and the particle size of the salt-responsive gel microspheres in simulated formation water ≥ 1.6 times the fracture aperture of the reservoir and the strength ≥ 45 Pa.

9. Use of any one of claims 1 to 8 in dynamic water shutoff in fractured reservoirs. During the entire dynamic plugging process, only the section where the salt-responsive gel microsphere solution is located in the entire fracture is plugged, and the remaining section still provides a flow channel for the fluid.

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