A dual responsive dual network fracturing fluid thickener and a preparation method thereof

Abstract

The application discloses a dual-response double-network fracturing fluid thickening agent and a preparation method thereof, relates to the technical field of oil field chemistry and fine chemistry, and comprises the following steps: cross-linking and polymerizing non-ionic monomers and a cross-linking agent under the action of an initiator to form a first network structure, and utilizing a carboxyl-containing polymer filled in the first network structure to form a second network structure. The non-ionic monomers are at least one of acrylamide and modified acrylamide. The double-network fracturing fluid thickening agent is ionically cross-linked with high-valence cations under high salinity, forms an "egg box" structure, causes molecular chain aggregation and gelation. Under acidic conditions, the carboxyl groups are protonated, the charge density of the molecular chains is reduced, the electrostatic repulsion is weakened, inter-chain aggregation and precipitation are caused. In addition, the protonated carboxyl groups form intramolecular and intermolecular hydrogen bonds with hydroxyl groups, further promote inter-chain aggregation. Therefore, the prepared double-network thickening agent has the characteristics of high density, salt resistance and acid resistance, and remarkably improves the stimulation effect of unconventional oil and gas reservoirs.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oilfield chemistry and fine chemistry, in particular to a dual-response double-network fracturing fluid thickening agent and a preparation method thereof. BACKGROUND

[0002] With the continuous growth of global energy demand, unconventional oil and gas reservoirs with low porosity and low permeability have become the focus of development. However, such reservoirs have complex geological conditions, generally characterized by high temperature, high salinity, acidic environment, and complex fracture system, which leads to the rapid deterioration of the performance of traditional fracturing fluid systems under extreme conditions, severely restricting the economic and effective development of unconventional oil and gas resources. As the core treatment agent of fracturing fluid, the development of thickening agent has experienced a transition from natural polymers (such as guar gum and cellulose derivatives) to synthetic polymers (such as polyacrylamide). However, natural thickening agents have problems such as high residue, easy biodegradation, and poor temperature resistance and salt tolerance. Although synthetic polymers have improved drag reduction and sand carrying performance, they still cannot fully meet the stability requirements under various extreme conditions.

[0003] Current polymer fracturing fluids usually form gels by adding organic zirconium, organic titanium, or organic boron cross-linking agents, which have the advantages of low drag, good sand carrying performance, low residue, and low formation damage. However, they still have significant defects: in high salinity environments, metal ions (such as Ca 2+ , Mg 2+ ) can cause polymer chain curling and cross-linking point damage, resulting in rapid viscosity reduction and loss of sand carrying capacity; under acidic conditions, polymer chains are prone to hydrolysis or oxidative degradation, leading to molecular chain breakage and significant reduction in thickening capacity; some cross-linking agents are toxic and do not meet environmental protection requirements, and the cost of freshwater preparation is high and the operation is difficult in space-limited scenarios such as offshore platforms. Although some research has attempted to improve certain performance through chemical modification or composite polymers, it still cannot balance salt tolerance under high salinity and stable thickening capacity under acidic conditions, etc. Therefore, there is an urgent need to develop an innovative thickening agent system with multiple response characteristics, stable structure, and strong environmental adaptability to improve the effectiveness of unconventional oil and gas reservoir modification and reduce the overall development cost. SUMMARY

[0004] The present application relates to the technical field of oilfield chemistry and fine chemistry, in particular to a dual-response double-network fracturing fluid thickening agent and a preparation method thereof.

[0005] Embodiments of the present application are implemented as follows:

[0006] 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 network structure; wherein the non-ionic monomers are at least one of acrylamide and modified acrylamide.

[0007] A preparation method of the above-mentioned double-network fracturing fluid thickening agent, comprising:

[0008] Mixing the non-ionic monomers, the carboxyl-containing polymer and the cross-linking agent in an inert atmosphere to obtain a raw material mixture;

[0009] Mixing the raw material mixture with the initiator and performing cross-linking polymerization reaction in an inert atmosphere.

[0010] The beneficial effects of the embodiments of the present application are:

[0011] The embodiments of the present application provide a dual-response double-network fracturing fluid thickening agent and a preparation method thereof, which forms 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 network structure. Wherein the non-ionic monomers are at least one of acrylamide and modified acrylamide. The double-network fracturing fluid thickening agent has ion cross-linking between the carboxyl (-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 (-COO - ) is protonated to -COOH, the charge density of the molecular chain is reduced, the electrostatic repulsion is weakened, resulting in inter-chain aggregation and precipitation. In addition, the protonated carboxyl and the hydroxyl form intramolecular and intermolecular hydrogen bonds, further promoting inter-chain 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. The double-network fracturing fluid thickening agent is simple to prepare, has excellent performance, and has good practical value. BRIEF DESCRIPTION OF DRAWINGS

[0012] 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.

[0013] Figure 1 A schematic diagram of the double-network fracturing fluid thickening agent provided in Embodiment 1 of the present application;

[0014] Figure 2 Viscosity curves of the double-network fracturing fluid thickening agent provided in the test example 1 of the present application under different concentrations of non-ionic monomers;

[0015] 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 carboxyl-containing polymers;

[0016] 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 cross-linking agents;

[0017] 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 initiators;

[0018] 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;

[0019] 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;

[0020] Figure 8 Viscosity curves of each fracturing fluid thickening agent 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);

[0021] Figure 9 Viscosity curves of each fracturing fluid thickening agent 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

[0022] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. If specific conditions are not indicated in the embodiments, conventional conditions or the conditions suggested by the manufacturers are adopted. If the reagents or instruments used are not indicated by the manufacturers, they are all conventional products that can be purchased in the market.

[0023] A double-responsive double-network fracturing fluid thickening agent, a preparation method and applications thereof will be described in detail below.

[0024] 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 network structure; wherein the non-ionic monomers are at least one of acrylamide and modified acrylamide.

[0025] The double-network fracturing fluid thickening agent has carboxyl groups (-COO - ) and high-valence cations (such as Ca 2+ , Mg 2+ ) ionically cross-linked under high salinity, forming an "egg box" structure, resulting in molecular chain aggregation and gelation. Under acidic conditions, the carboxyl groups (-COO - ) are 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 groups form intramolecular and intermolecular hydrogen bonds with hydroxyl groups, further promoting inter-chain 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.

[0026] The carboxyl-containing polymer includes at least one of sodium alginate, carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, and pectin. The above polymers have relatively high carboxyl density, which can meet the performance requirements of the present application. At the same time, they are all mature products that can be directly obtained commercially, and the raw material sources are more extensive.

[0027] Optionally, the cross-linking agent is N,N'-methylenebisacrylamide. The cross-linking 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.

[0028] The present application also provides a preparation method of the above-mentioned double-network fracturing fluid thickening agent, which comprises:

[0029] S1. Mix the non-ionic monomers, carboxyl-containing polymers, and cross-linking agents in an inert atmosphere to obtain a raw material mixture.

[0030] S2. Mix the raw material mixture with the initiator and perform cross-linking polymerization reaction in an inert atmosphere.

[0031] 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, and the oxygen is exhausted.

[0032] Optionally, the concentration of the nonionic monomer in the dual-network fracturing fluid thickening agent is 2.5wt%-5wt%. When the concentration of the nonionic monomer is low, the degree of polymerization is low or even no polymerization occurs; when the concentration of the nonionic 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 terms of viscosity, fluidity, solubility, etc.

[0033] Further, the concentration of the carboxyl-containing polymer in the dual-network fracturing fluid thickening agent is 0.2wt%-0.7wt%. When the concentration of the carboxyl-containing polymer is too low, it cannot meet the requirement of filling to form a dual-network structure, and a part of the nonionic monomer self-polymerizes to form a single network. When the concentration of the carboxyl-containing polymer is too high, the dual-network system structure formed is too dense, and the molecular chains cannot freely extend, resulting in reduced viscosity.

[0034] Optionally, the mass ratio of the nonionic monomer, the crosslinking agent and the initiator is 1:(0.0004-0.002):(0.02-0.06). The crosslinking agent forms a network structure by crosslinking linear polymer chain segments, and has a great influence on the viscosity of the system. 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 wound, resulting in loss of fluidity 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 and chain free radicals decomposed from the initiator undergo termination reaction, the polymer chain segments terminate quickly, the polymer chain segments are too short, the molecular weight is low, and the reaction is difficult to control. Through condition screening, when the ratio is in the above range, the product formed has moderate viscosity and better overall performance.

[0035] The temperature for mixing the nonionic 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 use. Under this condition, the substances can be dissolved and mixed to be fully dispersed and more uniform.

[0036] The crosslinking polymerization reaction is carried out at 30-50℃ for 5-10h. The length of the polymerization reaction time directly affects the polymerization process. If the reaction time is too long, the amide groups in the polymer will be crosslinked to a certain extent between or within the molecules; 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.

[0037] The features and performance of the present application are further described in detail below in combination with examples.

[0038] Example 1

[0039] The embodiment provides a dual-network fracturing fluid thickening agent with double response, and a preparation method thereof is as follows:

[0040] S1. In a reaction kettle, deionized water, sodium alginate, acrylamide and N,N'-methylene bisacrylamide were sequentially added to obtain solution A, which was deoxygenated by nitrogen for 20 minutes and then reserved; deionized water and ammonium persulfate were mixed to obtain solution B, which was deoxygenated by nitrogen for 20 minutes after being stirred until being uniformly mixed and then reserved.

[0041] S2. Solution A and solution B were mixed, and then gently stirred by using a glass rod and placed in a water bath, and the temperature was increased to 45 DEG C, and the reaction was continued for 6 hours under a nitrogen atmosphere to obtain the dual-network fracturing fluid thickening agent.

[0042] The concentration of acrylamide is 3.5 wt%, and the concentration of sodium alginate is 0.35 wt%. The mass ratio of acrylamide, N,N'-methylene bisacrylamide and ammonium persulfate is 1:0.0008:0.04.

[0043] The viscosity of the dual-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the dual-network fracturing fluid thickening agent is 540 mPa s, and the morphology is as shown in Figure 1 .

[0044] Embodiment 2

[0045] The embodiment provides a dual-network fracturing fluid thickening agent with double response, and a preparation method thereof is as follows:

[0046] S1. In a reaction kettle, deionized water, carboxymethyl cellulose, acrylamide and N,N'-methylene bisacrylamide were sequentially added to obtain solution A, which was deoxygenated by nitrogen for 15 minutes and then reserved; deionized water and ammonium persulfate were mixed to obtain solution B, which was deoxygenated by nitrogen for 15 minutes after being stirred until being uniformly mixed and then reserved.

[0047] S2. Solution A and solution B were mixed, and then gently stirred by using a glass rod and placed in a water bath, and the temperature was increased to 50 DEG C, and the reaction was continued for 5 hours under a nitrogen atmosphere to obtain the dual-network fracturing fluid thickening agent.

[0048] The concentration of acrylamide is 2.5 wt%, and the concentration of carboxymethyl cellulose is 0.5 wt%. The mass ratio of acrylamide, N,N'-methylene bisacrylamide and ammonium persulfate is 1:0.002:0.06.

[0049] The viscosity of the dual-network fracturing fluid thickening agent is measured by using a six-speed viscometer, and the viscosity of the dual-network fracturing fluid thickening agent is 463 mPa s.

[0050] Embodiment 3

[0051] The embodiment provides a dual-network fracturing fluid thickening agent with double response, and a preparation method thereof is as follows:

[0052] S1. Deionized water, polyacrylic acid, acrylamide, and N,N'-methylenebisacrylamide are added sequentially to the reactor to obtain solution A. Nitrogen gas is purged for 30 minutes to remove oxygen before use. Separately, deionized water and ammonium persulfate are mixed to obtain solution B. After stirring until homogeneous, nitrogen gas is purged for 30 minutes to remove oxygen before use.

[0053] S2. Mix solution A and solution B, stir gently with a glass rod, place in a water bath, heat to 30°C, and continue the reaction for 10 hours under a nitrogen atmosphere to obtain the dual-network fracturing fluid thickener.

[0054] The acrylamide concentration is 5 wt%, and the polyacrylic acid concentration is 0.7 wt%. The mass ratio of acrylamide, N,N'-methylenebisacrylamide, and ammonium persulfate is 1:0.0004:0.02.

[0055] The viscosity of the dual-network fracturing fluid thickener was measured using a six-speed viscometer and found to be 931 mPa·s.

[0056] Example 4

[0057] This embodiment provides a dual-response dual-network fracturing fluid thickener, the preparation method of which is as follows:

[0058] S1. Deionized water, polymethacrylic acid, acrylamide, and N,N'-methylenebisacrylamide are added sequentially to a reaction vessel to obtain solution A. Nitrogen gas is purged for 20 minutes to remove oxygen before use. Separately, deionized water and ammonium persulfate are mixed to obtain solution B. After stirring until homogeneous, nitrogen gas is purged for 20 minutes to remove oxygen before use.

[0059] S2. Mix solution A and solution B, stir gently with a glass rod, place in a water bath, heat to 45°C, and continue the reaction for 6 hours under a nitrogen atmosphere to obtain the dual-network fracturing fluid thickener.

[0060] The acrylamide concentration is 4 wt%, and the polymethacrylic acid concentration is 0.2 wt%. The mass ratio of acrylamide, N,N'-methylenebisacrylamide, and ammonium persulfate is 1:0.001:0.04.

[0061] The viscosity of the dual-network fracturing fluid thickener was measured using a six-speed viscometer and found to be 768 mPa·s.

[0062] Comparative Example 1

[0063] This comparative example provides a dual-network fracturing fluid thickener, the preparation method of which is basically the same as that of Example 1, except that sodium alginate is replaced with an equal amount of isotridecyl alcohol polyoxyethylene ether.

[0064] Comparative Example 2

[0065] This comparative example provides a fracturing fluid thickener, the preparation method of which is basically the same as that of Example 1, except that sodium alginate is not added.

[0066] Experimental Example 1

[0067] This experimental example uses the preparation method of Example 1 to test the effect of acrylamide concentration on the viscosity of the thickener in the dual-network fracturing fluid. The specific method is as follows:

[0068] Five samples were prepared with acrylamide concentrations of 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, and 4.5 wt%, respectively. Based on the mass ratio of acrylamide, sodium alginate was added at 10 wt%, N,N'-methylenebisacrylamide at 0.08 wt%, ammonium persulfate at 4 wt%, and the remainder was deionized water. The reaction temperature was 50℃, and the reaction time was 9 h. The viscosity of each group was measured using a six-speed viscometer.

[0069] Test results are as follows Figure 2 As shown. From Figure 2 It can be seen that when the monomer concentration is below 3.5 wt%, the acrylamide monomer concentration is low, the interaction between polyacrylamide and sodium alginate is weak, and fewer double-network structures are formed. The system viscosity increases slowly with increasing monomer dosage. When the monomer concentration is above 3.5 wt%, the interaction between polyacrylamide and sodium alginate gradually strengthens with increasing acrylamide monomer dosage, resulting in more and stronger double-network structures. The system viscosity increases rapidly with increasing monomer dosage. Therefore, the optimal monomer concentration selected in this invention is 3.5%.

[0070] Experimental Example 2

[0071] This experimental example uses the preparation method of Example 1 to test the effect of sodium alginate concentration on the viscosity of the thickener in the dual-network fracturing fluid. The specific method is as follows:

[0072] Five samples were prepared with sodium alginate concentrations of 0.23 wt%, 0.29 wt%, 0.35 wt%, 0.44 wt%, and 0.7 wt%, and acrylamide concentration of 3.5 wt%. Based on the mass ratio of acrylamide, 0.08 wt% N,N'-methylenebisacrylamide and 4 wt% ammonium persulfate were added, with the remainder being deionized water. The reaction temperature was 50℃, and the reaction time was 9 hours. The viscosity of each group was measured using a six-speed viscometer.

[0073] Test results are as follows Figure 3 As shown. From Figure 3It can be seen that when the concentration of sodium alginate is less than 0.29wt% (sodium alginate addition accounts for 1 / 12 of the monomer concentration), the interaction between polyacrylamide and sodium alginate gradually increases with the increase of sodium alginate, and the viscosity of the system slightly increases with the increase of the concentration of sodium alginate.

[0074] When the concentration of sodium alginate is between 0.29wt% and 0.35wt%, the viscosity of the double network fracturing fluid thickening agent rapidly increases with the increase of the concentration, and reaches the maximum when the concentration is about 0.35wt%, and the viscosity of the system is 576mPa·s. In this stage, with the increase of the concentration of sodium alginate, the double network formed by the double network fracturing fluid thickening agent increases, the hydrogen bond interaction enhances, and at the same time, the excessive gelation of polyacrylamide becomes less, and the viscosity of the system increases.

[0075] 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 increase 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 decreases, and the viscosity of the system with a concentration of 0.44wt% is 450 mPa·s. Therefore, the optimal sodium alginate addition is 0.35wt%.

[0076] Test Example 3

[0077] In this test example, the preparation method of Example 1 is used to test the influence of the concentration of crosslinking agent on the viscosity of the double network fracturing fluid thickening agent, and the specific method is as follows:

[0078] Six samples are prepared, the concentration of acrylamide is 3.5wt%, the addition of sodium alginate is 0.35wt%, the addition 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 of ammonium persulfate is 4wt%, and the rest is deionized water, the reaction temperature is 50℃, and the reaction time is 9h. The viscosity of each group is tested by using six-speed viscometer.

[0079] The test results are shown in Figure 4 It can be seen from Figure 4 that the viscosity of the double network fracturing fluid thickening agent increases first and then decreases with the increase of the addition of N,N'-methylene bisacrylamide, and when the addition is 0.07wt%, the viscosity of the system reaches the maximum value of 612mPa·s. It is speculated that after the concentration is greater than 0.07wt%, the molecular chain segments in the polymerization system are crosslinked excessively, which leads to the decrease of the viscosity of the system.

[0080] Test Example 4

[0081] This experimental example uses the preparation method of Example 1 to test the effect of initiator concentration on the viscosity of the thickener in the dual-network fracturing fluid. The specific method is as follows:

[0082] Five samples were prepared with the following concentrations: acrylamide 3.5 wt%, sodium alginate 0.35 wt%, N,N'-methylenebisacrylamide 0.07 wt% (calculated by mass ratio of acrylamide), and ammonium persulfate 2 wt%, 3 wt%, 4 wt%, 5 wt%, and 6 wt%, respectively. The remainder was deionized water. The reaction temperature was 50℃, and the reaction time was 9 h. The viscosity of each group was measured using a six-speed viscometer.

[0083] Test results are as follows Figure 5 As shown. From Figure 5 It can be seen that the viscosity of the dual-network fracturing fluid thickener first increases and then decreases with increasing ammonium persulfate concentration, reaching a maximum of 630 mPa·s at 5 wt%. It is speculated that after the concentration exceeds 5 wt%, the number of free radicals increases, the polymerization rate increases, the molecular weight decreases, and the polymer properties deteriorate.

[0084] Experimental Example 5

[0085] This experimental example uses the preparation method of Example 1 to test the effect of reaction temperature on the viscosity of the thickener in the dual-network fracturing fluid. The specific method is as follows:

[0086] Five samples were prepared with acrylamide concentration of 3.5 wt%, sodium alginate concentration of 0.35 wt%, N,N'-methylenebisacrylamide concentration of 0.07 wt% (calculated by mass ratio of acrylamide), ammonium persulfate concentration of 5 wt%, and the remainder being deionized water. The reaction temperatures were 35℃, 40℃, 45℃, 50℃, 55℃, and 60℃, respectively, and the reaction time was 9 hours. The viscosity of each group was measured using a six-speed viscometer.

[0087] Test results are as follows Figure 6 As shown. From Figure 6 It can be seen that between 35℃ and 60℃, the viscosity of the sodium alginate / polyacrylamide system first increases and then decreases with increasing temperature, reaching a maximum of 630 mPa·s at 45℃. Free radical polymerization is an exothermic reaction, and theoretically, low temperatures are beneficial for polymerization. However, excessively low temperatures lead to a low initiator decomposition rate and low conversion rate, ultimately resulting in a low viscosity polymer. Conversely, excessively high temperatures reduce the initiator half-life, generating a large number of free radicals in a short time, accelerating the polymerization reaction and accelerating polymer chain termination. This results in a small molecular weight, high brittleness, and similarly reduced performance.

[0088] Experimental Example 6

[0089] The test example adopts the preparation method of Example 1, and tests the influence of reaction time on the viscosity of the double-network fracturing fluid thickening agent. The specific method is as follows:

[0090] Five samples were configured, the acrylamide concentration was 3.5wt%, the sodium alginate addition amount was 0.35wt%, the N,N'-methylene bisacrylamide addition amount was 0.07wt% calculated according to the mass ratio of acrylamide, the ammonium persulfate addition amount was 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.

[0091] The test results are shown in Figure 7 From Figure 7 it can be seen that, with the increase of the synthesis time, the viscosity of the sodium alginate / polyacrylamide system gradually increases and finally tends to be stable. When the synthesis time is 6h, the system viscosity reaches the maximum value of 609mPa·s. After increasing the synthesis time, the monomer and initiator consumption tends to be complete, and at this time, the viscosity of the polymer is basically unchanged by prolonging the reaction time.

[0092] Test Example 7

[0093] The fracturing fluid thickening agent provided by Example 1, Comparative Example 1 and Comparative Example 2 was diluted to a non-ionic monomer concentration of 1wt%. Anhydrous calcium chloride was used to adjust the system salinity, and the salinity range was 0-200000 mg / L. The viscosity of the fracturing fluid thickening agent under different salinities was tested. The test results are shown in Figure 8 .

[0094] From Figure 8 it can be seen that the viscosity of the fracturing fluid thickening agent provided by Example 1 is 88mPa·s when the salinity is 0, and then the viscosity increases first and then decreases with the increase of the salinity. When the salinity increases to 25000 mg / L, the system viscosity rapidly rises to the maximum value of 142mPa·s. When the salinity is 200000mg / L, the system viscosity is 78mPa·s, and the viscosity retention rate is 88%.

[0095] Comparative Example 1 uses isotridecanol polyoxyethylene ether to form the 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 100000mg / L, and then the viscosity decreases to tend to be stable. When the salinity is 200000mg / L, the system viscosity is 71mPa·s, and the viscosity retention rate also reaches 88%.

[0096] Comparative Example 2 is a single network system of polyacrylamide, the initial viscosity of which is 75 mPa·s, and the concentration continuously decreases with the increase of the salinity, and finally the viscosity of the system is 43 mPa·s and the viscosity retention rate is 57% when the salinity is 200,000 mg / L.

[0097] It can be seen that both the carboxyl-containing polymer of the embodiment of the present application and the double-network fracturing fluid thickener without carboxyl-containing polymer in the prior art have 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.

[0098] Test Example 8

[0099] The fracturing fluid thickeners provided in Example 1, Comparative Example 1 and Comparative Example 2 are diluted to a non-ionic 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 thickener under different pH conditions. The test results are shown in Table 2. Figure 9 .

[0100] From Figure 9 It can be seen that the viscosity of the fracturing fluid thickener provided in Example 1 of the present application changes greatly under acidic conditions, and the viscosity of the system first increases and then decreases with the addition of hydrochloric acid. The initial viscosity is 86 mPa·s when the pH is 7.5. 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%.

[0101] In comparison, the double-network fracturing fluid thickener provided in Comparative Example 1 has an initial viscosity of 81 mPa·s when the pH is 7.5. With the decrease of the pH value, the viscosity of the system also gradually decreases, and the viscosity of the system is 22 mPa·s when the pH is 0.25, and the viscosity retention rate of the system is 27%. It can be seen that the existing double-network fracturing fluid thickener without carboxyl-containing polymer does not have acid resistance.

[0102] Similarly, the single network fracturing fluid thickener provided in Comparative Example 2 has an initial viscosity of 78 mPa·s when the pH is 7.5. With the decrease of the pH value, the viscosity of the system also gradually decreases, and the viscosity of the system is 12 mPa·s when the pH is 0.25, and the viscosity retention rate of the system is 15%. Also without acid resistance.

[0103] In summary, the embodiment of the present application provides a dual-response double-network fracturing fluid thickening agent and a preparation method thereof, which forms a first network structure by cross-linking polymerization of non-ionic monomers and a cross-linking agent under the action of an initiator, and forms a second network structure by using a carboxyl-containing polymer filled in the first network structure. - ) 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, 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, significantly improving 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.

[0104] The above merely describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A dual-network fracturing fluid thickener with dual response, characterized in that, The invention includes a first network structure formed by crosslinking polymerization of a nonionic monomer 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 nonionic monomer is at least one of acrylamide and modified acrylamide. In the dual-network fracturing fluid thickener, the concentration of the nonionic monomer is 2.5wt%~5wt%; the concentration of the carboxyl-containing polymer is 0.2wt%~0.7wt%. The mass ratio of the nonionic monomer, the crosslinking agent, and the initiator is 1:(0.0004~0.002):(0.02~0.06); The crosslinking polymerization reaction is carried out at 30~50℃ for 5~10h.

2. The dual-network fracturing fluid thickener according to claim 1, characterized in that, The carboxyl-containing polymer includes at least one of sodium alginate, carboxymethyl cellulose, polyacrylic acid, polymethacrylic acid, and pectin.

3. The dual-network fracturing fluid thickener according to claim 1, characterized in that, The crosslinking agent is N,N'-methylenebisacrylamide.

4. The dual-network fracturing fluid thickener according to claim 1, characterized in that, The initiator is at least one of ammonium persulfate, potassium persulfate, and sodium percarbonate.

5. A method for preparing a dual-network fracturing fluid thickener as described in any one of claims 1 to 4, characterized in that, include: The nonionic monomer, the carboxyl-containing polymer, and the crosslinking agent are mixed under an inert atmosphere to obtain a raw material mixture; The raw material mixture is mixed with the initiator, and a crosslinking polymerization reaction is carried out under an inert atmosphere.

6. The preparation method according to claim 5, characterized in that, The nonionic monomer, the carboxyl-containing polymer, and the crosslinking agent are mixed at a temperature of 50-60°C for a duration of 0.5-2 hours. After mixing, the mixture is cooled to room temperature before use.

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