Dual-response dual-network fracturing fluid thickening agent and preparation method thereof

Abstract

The invention discloses a dual-response dual-network fracturing fluid thickening agent and a preparation method thereof, and relates to the technical field of oilfield chemistry and fine chemistry, a nonionic monomer and a cross-linking agent are subjected to cross-linking polymerization under the action of an initiator to form a first network structure, and a carboxyl-containing polymer filled in the first network structure is utilized to form a second network structure. Wherein the nonionic monomer is at least one of acrylamide and modified acrylamide. According to the dual-network fracturing fluid thickening agent, carboxyl and high-valence cations are subjected to ionic crosslinking under high salinity to form an egg box structure, so that molecular chain aggregation and gelation are caused. Under the acidic condition, the charge density of carboxyl protonated molecular chains is reduced, the electrostatic repulsive force is weakened, and inter-chain aggregation and precipitation are caused. In addition, protonated carboxyl and hydroxyl form intramolecular and intermolecular hydrogen bonds, so that interchain aggregation is further promoted. Therefore, the prepared dual-network thickening agent has the characteristics of high density, salt resistance and acid resistance, and the yield increase and transformation effect of an unconventional oil and gas reservoir is remarkably improved.

Description

Technical Field

[0001] This invention relates to the fields of oilfield chemistry and fine chemistry, and more specifically, to a dual-response dual-network fracturing fluid thickener and its preparation method. Background Technology

[0002] With the continued growth of global energy demand, unconventional oil and gas reservoirs with low porosity and low permeability have become a development focus. However, these reservoirs have complex geological conditions, generally characterized by high temperature, high salinity, acidic environment, and complex fracture systems. This leads to a sharp deterioration in the performance of traditional fracturing fluid systems under extreme conditions, severely restricting the economical and effective development of unconventional oil and gas resources. As the core processing agent of fracturing fluids, thickeners have undergone a transformation from natural polymers (such as guar gum and cellulose derivatives) to synthetic polymers (such as polyacrylamide). Natural thickener products have problems such as high residue, easy biodegradability, and poor temperature and salt resistance. Although synthetic polymers have improved drag reduction and proppant carrying performance, they still cannot fully meet the stability requirements under various extreme conditions.

[0003] Current polymer fracturing fluids typically form gels by adding crosslinking agents such as organozirconium, organotitanium, or organoboron, offering advantages such as low friction, good proppant carrying capacity, low residue, and minimal formation damage. However, they still have significant drawbacks: in high-salinity environments, metal ions (such as Ca) can cause problems. 2+ Mg 2+ These crosslinking agents are prone to polymer chain curling and crosslinking point destruction, resulting in a sharp drop in viscosity and loss of sand-carrying capacity. Under acidic conditions, polymer chains are easily hydrolyzed or oxidatively degraded, leading to molecular chain breakage and a significant reduction in thickening capacity. Some crosslinking agents are toxic, do not meet environmental protection requirements, and are costly and difficult to prepare with fresh water in space-constrained scenarios such as offshore platforms. Although some studies have attempted to improve certain properties through chemical modification or composite polymers, it is still impossible to simultaneously achieve salt resistance under high salinity and stable thickening capacity under acidic conditions. Therefore, there is an urgent need to develop an innovative thickener system with multiple response characteristics, structural stability, and strong environmental adaptability to improve the effect of unconventional oil and gas reservoir stimulation and reduce overall development costs. Summary of the Invention

[0004] The purpose of this invention is to provide a dual-response dual-network fracturing fluid thickener and its preparation method. This dual-network fracturing fluid thickener is simple and convenient to prepare and exhibits good adaptability under high salinity and acidic conditions.

[0005] The embodiments of the present invention are implemented as follows: A dual-response double-network fracturing fluid thickening agent, comprising a first network structure formed by cross-linking polymerization of non-ionic monomers and a cross-linking agent under the action of an initiator, and a second network structure composed of a carboxyl-containing polymer filled in the first grid structure; wherein the non-ionic monomer is at least one of acrylamide and modified acrylamide.

[0006] A preparation method of the above-mentioned double-network fracturing fluid thickening agent, comprising: Mixing the non-ionic monomer, the carboxyl-containing polymer and the cross-linking agent in an inert atmosphere to obtain a raw material mixture; Mixing the raw material mixture with the initiator and performing cross-linking polymerization reaction in an inert atmosphere.

[0007] The beneficial effects of the embodiments of the present application are: The embodiments of the present application provide a dual-response double-network fracturing fluid thickening agent and a preparation method thereof, which form a first network structure by cross-linking polymerization of non-ionic monomers and a cross-linking agent under the action of an initiator, and a second network structure composed of a carboxyl-containing polymer filled in the first grid structure. Wherein the non-ionic monomer is at least one of acrylamide and modified acrylamide. The double-network fracturing fluid thickening agent prepared by the method has the characteristics of high density, salt resistance and acid resistance, and significantly improves the stimulation effect of unconventional oil and gas reservoirs. - ) and high-valence cations (such as Ca 2+ , Mg 2+ ) to form an "egg box" structure, resulting in molecular chain aggregation and gelation. Under acidic conditions, the carboxyl group (-COO - ) is protonated to -COOH, the charge density of the molecular chain is reduced, and the electrostatic repulsion is weakened, resulting in inter-chain aggregation and precipitation. In addition, the protonated carboxyl group forms intramolecular and intermolecular hydrogen bonds with the hydroxyl group, further promoting inter-chain aggregation. Thus, the prepared double-network thickening agent has the characteristics of high density, salt resistance and acid resistance, and significantly improves the stimulation effect of unconventional oil and gas reservoirs. The double-network fracturing fluid thickening agent is simple to prepare, has excellent performance, and has good practical value. BRIEF DESCRIPTION OF DRAWINGS

[0008] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0009] Figure 1 A schematic diagram of the double-network fracturing fluid thickening agent provided in Embodiment 1 of the present application; Figure 2 A viscosity curve of the double-network fracturing fluid thickening agent provided in Test Example 1 of the present application under different concentrations of non-ionic monomers; Figure 3 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 2 of the present application under different concentrations of the carboxyl-containing polymer; Figure 4 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 3 of the present application under different concentrations of the crosslinking agent; Figure 5 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 4 of the present application under different concentrations of the initiator; Figure 6 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 5 of the present application under different synthesis temperatures; Figure 7 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 6 of the present application under different synthesis times; Figure 8 Viscosity curve comparison of the fracturing fluid thickening agents provided in the test example 7 of the present application under different salinities, wherein the vertical coordinate represents viscosity (unit: mPa·s) and the horizontal coordinate represents salinity (unit: ×10 4 mg / L); Figure 9 Viscosity curve comparison of the fracturing fluid thickening agents provided in the test example 8 of the present application under different pH values, wherein the vertical coordinate represents viscosity (unit: mPa·s) and the horizontal coordinate represents pH value. DETAILED DESCRIPTION

[0010] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described below. If the specific conditions are not specified in the embodiments, the conventional conditions or the conditions suggested by the manufacturers are adopted. If the reagents or instruments used are not specified by the manufacturers, they are all conventional products that can be purchased in the market.

[0011] The double-responsive double-network fracturing fluid thickening agent, the preparation method and the application thereof will be specifically described below.

[0012] The double-responsive double-network fracturing fluid thickening agent comprises a first network structure formed by crosslinking polymerization of non-ionic monomers and a crosslinking agent under the action of an initiator, and a second network structure composed of a carboxyl-containing polymer filled in the first network structure; wherein the non-ionic monomer is at least one of acrylamide and modified acrylamide.

[0013] The double-network fracturing fluid thickening agent has a carboxyl group (-COO - ) and a high-valence cation (such as Ca 2+ , Mg 2+) ion crosslinking occurs, forming a "egg box" structure, leading to molecular chain aggregation and gelation. In acidic conditions, the carboxyl group (-COO - ) is protonated to -COOH, the charge density of the molecular chain is reduced, the electrostatic repulsion is weakened, leading to interchain aggregation and precipitation. In addition, the protonated carboxyl group forms intramolecular and intermolecular hydrogen bonds with the hydroxyl group, further promoting interchain aggregation. Thus, the prepared double network thickening agent has the characteristics of high density, salt resistance and acid resistance, significantly improving the stimulation effect of unconventional oil and gas reservoirs.

[0014] Among them, the carboxyl-containing polymer includes at least one of sodium alginate, carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, and pectin. The carboxyl density of the above polymers is relatively high, which can meet the performance requirements of the present application. At the same time, they are all mature products that can be directly obtained in the market, and the raw material sources are more extensive.

[0015] Optionally, the crosslinking agent is N,N'-methylenebisacrylamide. The crosslinking agent can form covalent bridges between polymer chains to form a network structure. The initiator is at least one of ammonium persulfate, potassium persulfate, and sodium percarbonate. It is used to initiate the free radical reaction.

[0016] The present application also provides a preparation method of the above-mentioned double network fracturing fluid thickening agent, which comprises: S1. Mix the non-ionic monomer, carboxyl-containing polymer, and crosslinking agent in an inert atmosphere to obtain a raw material mixture.

[0017] S2. Mix the raw material mixture with the initiator and perform a crosslinking polymerization reaction in an inert atmosphere.

[0018] Further, the inert atmosphere can be obtained by using inert gases such as nitrogen, helium, and argon. The inert gas is directly introduced into the reaction container to exhaust oxygen.

[0019] Optionally, in the double network fracturing fluid thickening agent, the concentration of the non-ionic monomer is 2.5wt%-5wt%. When the concentration of the non-ionic monomer is low, the polymerization degree is low or even no polymerization occurs; when the concentration of the non-ionic monomer is too high, the system forms a solid gel with poor fluidity. The inventors have found through tests that when the concentration is in the range of 2.5wt%-5wt%, the product has better comprehensive performance in viscosity, fluidity, and solubility.

[0020] Further, in the double network fracturing fluid thickening agent, the concentration of the carboxyl-containing polymer is 0.2wt%-0.7wt%. When the concentration of the carboxyl-containing polymer is too low, it cannot meet the requirement of filling and forming a double network structure, and part of the non-ionic monomer self-polymerizes to form a single network. When the concentration of the carboxyl-containing polymer is too high, the double network system structure is too dense, and the molecular chain cannot freely extend, resulting in reduced viscosity.

[0021] Optionally, the mass ratio of the non-ionic monomer, the crosslinking agent and the initiator is 1:(0.0004-0.002):(0.02-0.06). The crosslinking agent has a great influence on the viscosity of the system by crosslinking the linear polymer chain segments to form a network structure. When the amount of the crosslinking agent is too small, the network structure formed is sparse and unstable; when the amount of the crosslinking agent is too large, the network structure is tightly intertwined, resulting in loss of flowability of the system and complete solidification. As for the initiator, when the amount of the initiator is too low, the reaction rate of the system is slow, the initiation efficiency is low, the polymerization reaction is difficult to proceed, and the product conversion rate is low. When the amount of the initiator is too high, the primary free radicals decomposed from the initiator and the chain free radicals undergo termination reaction, the molecular chain segments are terminated quickly, the polymer molecular chain segments are too short, the molecular weight is low, and the reaction is difficult to control. Through condition screening, within the above ratio range, the product formed has moderate viscosity and good overall performance.

[0022] The mixing temperature of the non-ionic monomer, the carboxyl-containing polymer and the crosslinking agent is 50-60℃, the mixing time is 0.5-2h, and after mixing is completed, the mixture is cooled to room temperature for standby. Under this condition, the substances can be dissolved and mixed to make the substances fully dispersed and the mixture more uniform.

[0023] The crosslinking polymerization reaction is carried out at 30-50℃ for 5-10h. The polymerization reaction time directly affects the polymerization process. If the reaction time is too long, the amide groups in the polymer will undergo a certain degree of intermolecular or intramolecular crosslinking; if the reaction time is too short, the polymerization will be uneven and incomplete, resulting in poor product performance. Under the above temperature and reaction time, the reaction has better effect.

[0024] The features and performance of the present application are further described in detail below in combination with examples.

[0025] Example 1

[0026] The present example provides a dual-response double-network fracturing fluid thickening agent, and a preparation method thereof is as follows: S1. Deionized water, sodium alginate, acrylamide and N,N'-methylenebisacrylamide are sequentially added to a reaction kettle to obtain solution A, which is deoxygenated by nitrogen for 20min for standby; and deionized water and ammonium persulfate are mixed to obtain solution B, which is deoxygenated by nitrogen for 20min after stirring until uniform for standby.

[0027] S2. Solution A and solution B are mixed, gently stirred with a glass rod, and then placed in a water bath kettle, heated to 45℃, and continuously reacted under a nitrogen atmosphere for 6h to obtain the double-network fracturing fluid thickening agent.

[0028] The concentration of acrylamide is 3.5wt%, and the concentration of sodium alginate is 0.35wt%. The mass ratio of acrylamide, N,N'-methylene bisacrylamide and ammonium persulfate is 1:0.0008:0.04.

[0029] The viscosity of the double-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the double-network fracturing fluid thickening agent is 540 mPa·s, and the morphology thereof is as shown in Figure 1

[0030] Example 2

[0031] The double-network fracturing fluid thickening agent provided in the embodiment has a dual response, and the preparation method thereof is as follows: S1. Deionized water, carboxymethyl cellulose, acrylamide and N,N'-methylene bisacrylamide are sequentially added into a reaction kettle to obtain solution A, which is deoxygenated by nitrogen for 15 minutes and then reserved; deionized water and ammonium persulfate are mixed to obtain solution B, which is deoxygenated by nitrogen for 15 minutes after being stirred until uniform and then reserved.

[0032] S2. Solution A and solution B are mixed, and then gently stirred by using a glass rod and placed in a water bath kettle, which is heated to 50℃, and then the reaction is continued for 5 hours under a nitrogen atmosphere to obtain the double-network fracturing fluid thickening agent.

[0033] The concentration of acrylamide is 2.5wt%, and the concentration of carboxymethyl cellulose is 0.5wt%. The mass ratio of acrylamide, N,N'-methylene bisacrylamide and ammonium persulfate is 1:0.002:0.06.

[0034] The viscosity of the double-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the double-network fracturing fluid thickening agent is 463 mPa·s.

[0035] Example 3

[0036] The double-network fracturing fluid thickening agent provided in the embodiment has a dual response, and the preparation method thereof is as follows: S1. Deionized water, polyacrylic acid, acrylamide and N,N'-methylene bisacrylamide are sequentially added into a reaction kettle to obtain solution A, which is deoxygenated by nitrogen for 30 minutes and then reserved; deionized water and ammonium persulfate are mixed to obtain solution B, which is deoxygenated by nitrogen for 30 minutes after being stirred until uniform and then reserved.

[0037] S2. Solution A and solution B are mixed, and then gently stirred by using a glass rod and placed in a water bath kettle, which is heated to 30℃, and then the reaction is continued for 10 hours under a nitrogen atmosphere to obtain the double-network fracturing fluid thickening agent.

[0038] ​The concentration of acrylamide is 5wt%, the concentration of polyacrylic acid is 0.7wt%. The mass ratio of acrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 1:0.0004:0.02.

[0039] The viscosity of the double-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the double-network fracturing fluid thickening agent is 931 mPa·s.

[0040] Example 4

[0041] The embodiment provides a double-response double-network fracturing fluid thickening agent, and a preparation method thereof. S1. Deionized water, polymethylacrylic acid, acrylamide and N,N'-methylenebisacrylamide are sequentially added into a reaction kettle to obtain solution A, and the solution A is deoxygenated by nitrogen for 20 minutes and then reserved; and deionized water and ammonium persulfate are mixed to obtain solution B, and the solution B is deoxygenated by nitrogen for 20 minutes and then reserved.

[0042] S2. The solution A and the solution B are mixed, and then gently stirred by using a glass rod, and then placed into a water bath kettle, and heated to 45 DEG C, and then continuously reacted for 6 hours under a nitrogen atmosphere to obtain the double-network fracturing fluid thickening agent.

[0043] The concentration of acrylamide is 4wt%, the concentration of polymethylacrylic acid is 0.2wt%. The mass ratio of acrylamide, N,N'-methylenebisacrylamide and ammonium persulfate is 1:0.001:0.04.

[0044] The viscosity of the double-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the double-network fracturing fluid thickening agent is 768 mPa·s.

[0045] Comparative Example 1 The comparative example provides a double-network fracturing fluid thickening agent, and a preparation method thereof is basically same as that of the example 1, and the difference lies in that the sodium alginate is replaced by an equal amount of isotridecanol polyoxyethylene ether.

[0046] Comparative Example 2 The comparative example provides a fracturing fluid thickening agent, and a preparation method thereof is basically same as that of the example 1, and the difference lies in that the sodium alginate is not added.

[0047] Test Example 1 The test example adopts the preparation method of the example 1, and the influence of the concentration of acrylamide on the viscosity of the double-network fracturing fluid thickening agent is tested, and the specific method is as follows. Five samples were configured, and the acrylamide concentrations were 2.5wt%, 3wt%, 3.5wt%, 4wt% and 4.5wt% respectively. According to the mass ratio of acrylamide, the amount of sodium alginate was 10wt%, the amount of N,N'-methylene bisacrylamide was 0.08wt%, the amount of ammonium persulfate was 4wt%, and the rest was deionized water, the reaction temperature was 50℃, and the reaction time was 9h. A six-speed viscometer was used to test the viscosity of each group.

[0048] The test results are shown in Table 2. Figure 2 As can be seen from Table 2, Figure 2 when the monomer concentration is less than 3.5wt%, the acrylamide monomer concentration is low, the interaction between polyacrylamide and sodium alginate is weak, the double network structure formed is small, and the system viscosity slowly increases with the increase of monomer amount. When the monomer concentration is greater than 3.5wt%, with the increase of acrylamide monomer amount, the interaction between polyacrylamide and sodium alginate gradually increases, the double network structure increases and the strength increases, and the system viscosity rapidly increases with the increase of monomer amount. Therefore, the optimal monomer concentration selected by the present application is 3.5%.

[0049] Test Example 2 In this test example, the preparation method of Example 1 was used to test the influence of sodium alginate concentration on the viscosity of the double network fracturing fluid thickening agent. The specific method is as follows: Five samples were configured, and the sodium alginate concentrations were 0.23wt%, 0.29wt%, 0.35wt%, 0.44wt% and 0.7wt% respectively, and the acrylamide concentration was 3.5wt%. According to the mass ratio of acrylamide, the amount of N,N'-methylene bisacrylamide was 0.08wt%, the amount of ammonium persulfate was 4wt%, and the rest was deionized water, the reaction temperature was 50℃, and the reaction time was 9h. A six-speed viscometer was used to test the viscosity of each group.

[0050] The test results are shown in Table 2. Figure 3 As can be seen from Table 2, Figure 3 when the sodium alginate concentration is less than 0.29wt% (the amount of sodium alginate accounts for 1 / 12 of the monomer concentration), with the increase of sodium alginate, the interaction between polyacrylamide and sodium alginate gradually increases, and the system viscosity increases with the increase of sodium alginate concentration.

[0051] When the sodium alginate concentration is between 0.29wt% and 0.35wt%, the viscosity of the double network fracturing fluid thickening agent rapidly increases with the increase of concentration, and the viscosity reaches the maximum at about 0.35wt%, and the system viscosity is 576mPa·s. In this stage, with the increase of sodium alginate concentration, the double network formed by the double network fracturing fluid thickening agent increases, the hydrogen bond interaction increases, and at the same time, the excessive gelation of polyacrylamide decreases, and the system viscosity increases.

[0052] When the concentration of sodium alginate is greater than 0.35wt%, the viscosity of the double network fracturing fluid thickening agent decreases with the increase of the concentration of sodium alginate. The reason may be that when the concentration of sodium alginate is greater than 0.35wt%, the polymerization reaction speed of the double network fracturing fluid thickening agent is too fast, the temperature rising rate in the system is greater than the heat dissipation rate, the heat accumulates in the reaction process, the temperature in the system is too high, the molecular weight of the polymerization product is low, the solution viscosity is reduced, and the viscosity of the system with a concentration of 0.44wt% is 450 mPa·s. Therefore, the optimal sodium alginate addition amount is 0.35wt%.

[0053] Test Example 3 In this test example, the preparation method of Example 1 is used to test the influence of crosslinker concentration on the viscosity of the double network fracturing fluid thickening agent. The specific method is as follows: Six samples are prepared, the concentration of acrylamide is 3.5wt%, the addition amount of sodium alginate is 0.35wt%, the addition amount of N,N'-methylene bisacrylamide is 0.04wt%, 0.07wt%, 0.08wt%, 0.12wt%, 0.16wt% and 0.2wt% respectively according to the mass ratio of acrylamide, the addition amount of ammonium persulfate is 4wt%, and the rest is deionized water, the reaction temperature is 50℃, and the reaction time is 9h. The six-speed viscometer is used to test the viscosity of each group.

[0054] The test results are shown in Figure 4 From Figure 4 it can be seen that the viscosity of the double network fracturing fluid thickening agent increases first and then decreases with the increase of the addition amount of N,N'-methylene bisacrylamide, and when the concentration is 0.07wt%, the viscosity of the system reaches the maximum value of 612mPa·s. It is speculated that when the concentration is greater than 0.07wt%, the molecular chain segments in the polymerization system are crosslinked too much, resulting in the decrease of the viscosity of the system.

[0055] Test Example 4 In this test example, the preparation method of Example 1 is used to test the influence of initiator concentration on the viscosity of the double network fracturing fluid thickening agent. The specific method is as follows: Five samples are prepared, the concentration of acrylamide is 3.5wt%, the addition amount of sodium alginate is 0.35wt%, the addition amount of N,N'-methylene bisacrylamide is 0.07wt% according to the mass ratio of acrylamide, and the addition amount of ammonium persulfate is 2wt%, 3wt%, 4wt%, 5wt% and 6wt% respectively, and the rest is deionized water, the reaction temperature is 50℃, and the reaction time is 9h. The six-speed viscometer is used to test the viscosity of each group.

[0056] The test results are shown in Figure 5 From Figure 5It can be seen that the viscosity of the double network fracturing fluid thickening agent increases first and then decreases with the increase of ammonium persulfate concentration, and the viscosity of the system reaches a maximum of 630 mPa·s at 5wt%. It is speculated that when the concentration is greater than 5wt%, the number of free radicals increases, the polymerization rate increases, the molecular weight decreases, and the polymer performance is poor.

[0057] Test Example 5 This test example uses the preparation method of Example 1 to test the effect of reaction temperature on the viscosity of the double network fracturing fluid thickening agent. The specific method is as follows: Five samples were prepared, with acrylamide concentration of 3.5wt%, sodium alginate addition of 0.35wt%, N,N'-methylene bisacrylamide addition of 0.07wt% calculated by the mass ratio of acrylamide, ammonium persulfate addition of 5wt%, and the rest was deionized water. The reaction temperature was 35℃, 40℃, 45℃, 50℃, 55℃, and 60℃, respectively, and the reaction time was 9h. The six-speed viscometer was used to test the viscosity of each group.

[0058] The test results are shown in Figure 6 It can be seen from Figure 6 that between 35℃ and 60℃, the viscosity of the sodium alginate / polyacrylamide system increases first and then decreases with the increase of temperature. And it reaches a maximum of 630 mPa·s at 45℃. Free radical polymerization is an exothermic reaction, which is theoretically beneficial to polymerization at low temperature. However, low temperature will lead to low decomposition rate of initiator and low conversion rate, resulting in low viscosity of synthesized polymer. While high temperature will lead to short half-life of initiator, which will produce a large number of free radicals in a short time, accelerate the polymerization reaction process, and accelerate the termination of synthesized polymer chain. At this time, the synthesized molecular weight is small, the brittleness is high, and the performance is also reduced.

[0059] Test Example 6 This test example uses the preparation method of Example 1 to test the effect of reaction time on the viscosity of the double network fracturing fluid thickening agent. The specific method is as follows: Five samples were prepared, with acrylamide concentration of 3.5wt%, sodium alginate addition of 0.35wt%, N,N'-methylene bisacrylamide addition of 0.07wt% calculated by the mass ratio of acrylamide, ammonium persulfate addition of 5wt%, and the rest was deionized water. The reaction temperature was 45℃, and the reaction time was 3~10h. The six-speed viscometer was used to test the viscosity of each group.

[0060] The test results are shown in Figure 7 It can be seen from Figure 7It can be seen that the viscosity of the sodium alginate / polyacrylamide system gradually increases and finally stabilizes with the increase of the synthesis time. When the synthesis time is 6h, the viscosity of the system reaches the maximum value of 609mPa·s. After increasing the synthesis time, the consumption of monomers and initiators tends to be complete, and at this time, the viscosity of the polymer is basically unchanged with the extension of the reaction time.

[0061] Test Example 7 The fracturing fluid thickening agents provided by Example 1, Comparative Example 1 and Comparative Example 2 are diluted to a non-ionic monomer concentration of 1wt%. Anhydrous calcium chloride is used to adjust the salinity of the system, and the salinity ranges from 0 to 200000 mg / L. The viscosity of the fracturing fluid thickening agent under different salinities is tested. The test results are shown in Table 1. Figure 8

[0062] From Figure 8 It can be seen that the viscosity of the fracturing fluid thickening agent provided by Example 1 of the present application is 88mPa·s when the salinity is 0, and then the viscosity increases and then decreases with the increase of the salinity. When the salinity increases to 25000 mg / L, the viscosity of the system rapidly increases and reaches the maximum value of 142mPa·s. When the salinity is 200000 mg / L, the viscosity of the system is 78mPa·s, and the viscosity retention rate is 88%.

[0063] Comparative Example 1 uses isotridecanol polyoxyethylene ether to form a second network structure, and the initial viscosity is 81mPa·s. The viscosity also presents the trend of first increasing and then decreasing with the increase of the salinity, but the timing of the appearance of the viscosity peak is different. The viscosity of the fracturing fluid thickening agent of Comparative Example 1 suddenly increases to 112mPa·s when the salinity reaches 100000 mg / L, and then decreases to stabilize. When the salinity is 200000 mg / L, the viscosity of the system is 71mPa·s, and the viscosity retention rate is also 88%.

[0064] Comparative Example 2 is a single network system of polyacrylamide, and the initial viscosity is 75mPa·s. With the increase of the salinity, the concentration continuously decreases, and finally when the salinity is 200000 mg / L, the viscosity of the system is 43mPa·s, and the viscosity retention rate is 57%.

[0065] It can be seen that whether the carboxyl-containing polymer of the present application is used or the double network fracturing fluid thickening agent without carboxyl in the prior art is used, both of them show good salt resistance. In comparison, the single network system of polyacrylamide has poor salt resistance, and the viscosity decreases obviously with the increase of the salinity.

[0066] Test Example 8 ​The fracturing fluid thickening agent provided by Example 1, Comparative Example 1 and Comparative Example 2 is diluted to a nonionic monomer concentration of 1 wt%. Hydrochloric acid is used to adjust the pH value of the system, and a six-speed viscometer is used to test the viscosity of the sodium alginate / polyacrylamide thickening agent under different pH conditions. The test results are shown in Table 1. Figure 9 .

[0067] From Figure 9 it can be seen that the viscosity of the fracturing fluid thickening agent provided by Example 1 changes greatly under acidic conditions, and the viscosity of the system first increases and then decreases with the addition of hydrochloric acid. When the pH is 7.5, the initial viscosity is 86 mPa·s. With the addition of hydrochloric acid, the viscosity of the system rapidly increases. When the pH is 4, the viscosity of the system increases to a maximum value, and the viscosity is 132 mPa·s. Compared with the initial solution, the viscosity is increased by 52%. When the pH of the system is less than 4, the viscosity of the system gradually decreases with the addition of hydrochloric acid. However, when the pH is 0.25, the viscosity of the system is 76 mPa·s, and the viscosity retention rate of the system is 87%.

[0068] By contrast, the double-network fracturing fluid thickening agent provided by Comparative Example 1 has an initial viscosity of 81 mPa·s at a pH of 7.5. As the pH value decreases, the viscosity of the system also gradually decreases, and when the pH is 0.25, the viscosity of the system is 22 mPa·s, and the viscosity retention rate of the system is 27%. It can be seen that the existing double-network fracturing fluid thickening agent without carboxyl-containing polymer does not have acid resistance.

[0069] Similarly, the single-network fracturing fluid thickening agent provided by Comparative Example 2 has an initial viscosity of 78 mPa·s at a pH of 7.5. As the pH value decreases, the viscosity of the system also gradually decreases, and when the pH is 0.25, the viscosity of the system is 12 mPa·s, and the viscosity retention rate of the system is 15%. Also without acid resistance.

[0070] In summary, the example of the present application provides a double-response double-network fracturing fluid thickening agent and a preparation method thereof. The nonionic monomer, crosslinking agent is crosslinked and polymerized under the action of an initiator to form a first network structure, and a carboxyl-containing polymer filled in the first grid structure forms a second network structure. Among them, the nonionic monomer is at least one of acrylamide and modified acrylamide. The double-network fracturing fluid thickening agent has ion crosslinking between the carboxyl groups (-COO - ) and high-valence cations (such as Ca 2 + , Mg 2+ ) under high salinity, forming an "egg box" structure, resulting in molecular chain aggregation and gelation. Under acidic conditions, the carboxyl groups (-COO -The protonation is -COOH, the molecular chain charge density is reduced, the electrostatic repulsion is weakened, the interchain aggregation and precipitation are caused. In addition, the protonated carboxyl and hydroxyl form intramolecular and intermolecular hydrogen bonds, further promote interchain aggregation. Thus, the prepared double network thickening agent has the characteristics of high density, salt resistance and acid resistance, and significantly improves the stimulation effect of unconventional oil and gas reservoirs. The double network fracturing fluid thickening agent is simple to prepare, has excellent performance and has better practical value.

[0071] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A dual responsive dual network fracturing fluid viscosifier, characterized in that, The first network structure is formed by cross-linking and polymerization of non-ionic monomers and cross-linking agents under the action of an initiator, and the second network structure is formed by a carboxyl-containing polymer filled in the first network structure; wherein the non-ionic monomers are at least one of acrylamide and modified acrylamide; The concentration of the non-ionic monomers in the double-network fracturing fluid thickening agent is 2.5wt%-5wt%, and the concentration of the carboxyl-containing polymer is 0.2wt%-0.7wt%.

2. The dual network frac fluid viscosifier of claim 1, wherein, The carboxyl-containing polymer includes at least one of sodium alginate, carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, and pectin.

3. The dual network frac fluid viscosifier of claim 1, wherein, The cross-linking agent is N,N'-methylene bisacrylamide.

4. The dual network frac fluid viscosifier of claim 1, wherein, The initiator is at least one of ammonium persulfate, potassium persulfate, and sodium percarbonate.

5. A method of preparing a dual network fracturing fluid viscosifier according to any one of claims 1 to 4, characterized in that, The method comprises: Mixing the non-ionic monomers, the carboxyl-containing polymer, and the cross-linking agent in an inert atmosphere to obtain a raw material mixture; Mixing the raw material mixture with the initiator and performing cross-linking polymerization in an inert atmosphere.

6. The production method according to claim 5, wherein The mass ratio of the non-ionic monomers, the cross-linking agent, and the initiator is 1:(0.0004-0.002):(0.02-0.06).

7. The production method according to claim 6, characterized by, The temperature for mixing the non-ionic monomers, the carboxyl-containing polymer, and the cross-linking agent is 50-60℃, the mixing time is 0.5-2h, and after mixing is completed, the mixture is cooled to room temperature for use.

8. The method of claim 7, wherein, The cross-linking polymerization is performed at 30-50℃ for 5-10h.

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