CO2 response type clean fracturing fluid as well as preparation method and application thereof

Abstract

The invention discloses CO2 response type clean fracturing fluid as well as a preparation method and application thereof, and belongs to the technical field of oil and gas reservoir fracturing chemical agents. The CO2 response type clean fracturing fluid provided by the invention mainly comprises a surfactant composition, wherein the surfactant composition comprises alkyl amide propyl carboxyl betaine, diamine small molecules and sodium phthalate; the surfactant composition can be uniformly dispersed in an aqueous solution and acts with CO2 to initiate crosslinking to form a supramolecular structure type surfactant aggregation system, so that the viscosity of the solution is increased to generate a fracturing fluid system, and the supramolecular structure type aggregation system can endow the fracturing fluid system with higher temperature resistance; after CO2 dissolved in the fracturing fluid system is displaced by inert gas, the supramolecular structure is dissociated, and the viscosity of the solution is reduced, so that the gel breaking effect is achieved, and the fracturing fluid system has the characteristic of being recycled for multiple times.

Description

Technical Field

[0001] The present application belongs to the technical field of oil and gas reservoir fracturing chemicals, and in particular relates to a CO2 responsive clean fracturing fluid and a preparation method and application thereof. Background Art

[0002] With the development of the oil and gas exploration industry, low-permeability tight reservoirs have gradually become the focus of development. Fracturing, as the main means to improve the productivity of oil and gas wells in low-permeability reservoirs, has been widely used in oil and gas production enhancement projects. However, after the conventional fracturing fluid breaks, the molecular structure is likely to undergo irreversible changes / destruction, so that the fracturing return fluid needs to undergo a lot of cumbersome post-processing before it can be reused. In addition, the fracturing return fluid usually contains a large amount of undegraded residues, which can easily pollute the environment if not properly handled. Therefore, designing and preparing recyclable clean fracturing fluid has high practical value.

[0003] At present, in order to achieve the recycling of clean fracturing fluid, CO2-responsive surfactant fracturing fluid has been designed and prepared in the relevant field. For example, the patented technology with publication number CN 107142099 A uses tertiary amine surfactants and additives (sodium salicylate, maleic acid, phthalic acid, sodium dodecyl sulfonate, sodium p-toluene sulfonate, sodium benzoate) to prepare CO2-responsive clean fracturing fluid, which can effectively achieve the recycling of flowback fluid by regulating gel formation and regulating gel breaking by introducing CO2 gas and inert gas.

[0004] However, the applicable temperature of existing CO2-responsive clean fracturing fluids is generally limited to medium-high temperatures (85°C) and below, and the high-temperature performance is poor. In particular, there are few reports on high-temperature resistant (90-120°C) CO2-responsive clean fracturing fluids, which cannot effectively meet the performance requirements of high-temperature fracturing. Summary of the invention

[0005] The present application discloses a CO2 responsive clean fracturing fluid and a preparation method and application thereof, aiming to realize the recycling of the clean fracturing fluid while effectively improving the high temperature resistance of the existing CO2 responsive clean fracturing fluid.

[0006] In order to achieve the above objectives, the technical solution adopted in this application is:

[0007] The first aspect of the present application provides a CO2-responsive clean fracturing fluid. The main component of the CO2-responsive clean fracturing fluid is a surfactant composition, and the surfactant composition comprises alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate;

[0008] The surfactant composition can be uniformly dispersed in an aqueous solution and react with CO2 to initiate cross-linking into a supramolecular structure.

[0009] In a preferred embodiment, the molar ratio of the alkylamidopropyl carboxy betaine, the diamine small molecule and sodium phthalate is 2:1:1.

[0010] In a preferred embodiment, the supramolecular structure has a structural formula comprising formula [i]:

[0011]

[0012] Where R1 is C 15-21 Alkyl; R2 is C 1-2 Alkyl; n is 2 or 3.

[0013] In a preferred embodiment, the alkylamidopropyl carboxy betaine has a chemical structure of formula [ii]:

[0014]

[0015] In a preferred embodiment, the diamine small molecule has a chemical structure of formula [iii]:

[0016]

[0017] In a preferred embodiment, the sodium phthalate is selected from any one of sodium phthalate, sodium isophthalate and sodium terephthalate.

[0018] The second aspect of the present application provides a method for preparing the CO2-responsive clean fracturing fluid of the present application, the preparation method comprising:

[0019] The step of dispersing the raw material components contained in the surfactant composition in an aqueous medium according to a proportion to prepare an aqueous solution;

[0020] And, a step of introducing CO2 gas into the aqueous solution in the previous step to prepare a supramolecular structure.

[0021] In a preferred embodiment, the surfactant composition is dispersed in an aqueous medium at a concentration of 100-500 mmol·mL -1 .

[0022] In a preferred embodiment, the flow rate of CO2 gas introduced into the aqueous solution is 0.3-3 L·min -1 .

[0023] The third aspect of the present application provides the use of the CO2 responsive clean fracturing fluid of the present application in fracturing construction of low permeability oil and gas reservoirs.

[0024] Compared with the prior art, the advantages or beneficial effects of the present application include at least:

[0025] The CO2 responsive clean fracturing fluid provided in the present application is designed with a surfactant composition composed of alkylamide propyl carboxy betaine, diamine small molecules and sodium phthalate. On the one hand, the surfactant composition can be evenly dispersed in the aqueous medium and the diamine small molecules contained therein are protonated with CO2 to be converted into diquaternary ammonium salt small molecules. The diquaternary ammonium salt small molecules then crosslink and cooperate with the betaine surfactant through the electrostatic charge attraction effect. At the same time, the quaternary ammonium group of the betaine surfactant and sodium phthalate also crosslink and cooperate through the electrostatic charge attraction effect, thereby forming a supramolecular structure through mutual crosslinking, causing the solution viscosity to increase to produce a fracturing fluid system, and the supramolecular structure has a relatively high packing density and symmetry, which effectively limits the dissociation change of the supramolecular structure, so that the fracturing fluid system has higher temperature resistance. On the other hand, when the CO2 dissolved in the clean fracturing fluid system is displaced by the introduced inert gas (N2), the formed diquaternary ammonium salt small molecules are deprotonated and restored to diamine small molecules. Since diamine small molecules do not carry a positive charge, they cannot generate electrostatic charge attraction with the betaine surfactant, which significantly reduces the overall cross-linking and coordination of the supramolecular structure. The supramolecular structure is no longer stable and disintegrates, resulting in a decrease in the viscosity of the solution and a degelling effect. The gelling-degelling process can be repeated many times, and has the characteristics of being recyclable many times. On the third aspect, the betaine surfactant and small molecules used in the surfactant composition have the characteristics of low cost, low pollution and high-efficiency reversible cross-linking and coordination, which can effectively reduce environmental pollution while ensuring the rapid construction and degelling of the fracturing fluid system, and effectively improving the construction effect of fracturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0027] Figure 1 The infrared spectrum of octadecylamidopropyl carboxybetaine provided in the examples of the present application;

[0028] Figure 2 The hydrogen nuclear magnetic resonance spectrum of octadecylamidopropyl carboxybetaine provided in the examples of the present application;

[0029] Figure 3 The shear viscosity curves of the clean fracturing fluids VES1-VES6 and DVES1-DVES4 provided in the embodiments of the present application;

[0030] Figure 4A graph showing the temperature resistance and shear resistance characteristics of the clean fracturing fluid VES2 provided in the embodiment of the present application;

[0031] Figure 5 This is a repetitive responsiveness curve of the clean fracturing fluid VES2 provided in an embodiment of the present application. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments described in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0033] In the following description of this specification, the term "and / or" is used to describe the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, B exists alone, and A and B exist at the same time. Among them, A and B can be singular or plural; the symbol " / " means "or".

[0034] In the following description of this specification, the term "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, "at least one of A, B or C", or "at least one of A, B and C" can mean any one of A, B, C, or A+B, or A+C, or B+C, or A+B+C, where A, B, C can be single or multiple, respectively.

[0035] In the following description of this specification, the order of serial numbers does not mean the order of execution. Some or all of the steps can be executed in parallel or one after the other. The execution order of each process should be determined by its function and internal logic, and does not constitute any limitation on the execution process of this embodiment.

[0036] In the following description of this specification, the numerical range should be understood to also specifically disclose each intermediate value between the upper and lower limits of the range. Any stated value or intermediate value in the stated range and any other stated value or each smaller range between the intermediate values ​​in the range are also included in this embodiment, and the upper and lower limits of the smaller range can be independently included or excluded in the range.

[0037] Unless otherwise specified, the technical / scientific terms used in this specification have the meanings commonly understood by those of ordinary skill in the art. Although this specification only describes preferred materials and methods, any similar or equivalent methods and materials may also be used in specific embodiments or test cases. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0038] In a first aspect, an embodiment of the present application provides a CO2-responsive clean fracturing fluid, wherein the main component of the CO2-responsive clean fracturing fluid is a surfactant composition, and the surfactant composition comprises alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate;

[0039] The surfactant composition can be uniformly dispersed in an aqueous solution and react with CO2 to initiate cross-linking into a supramolecular structure.

[0040] The embodiments of the present application design a surfactant composition composed of alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate. On the one hand, the surfactant composition can be evenly dispersed in an aqueous medium to fully exert their respective effects. At the same time, the diamine small molecules in the composition can be protonated with CO2 and converted into positively charged diquaternary ammonium salt small molecules, and the diquaternary ammonium salt small molecules can produce crosslinking and coordination based on electrostatic attraction with the carboxylic acid groups on the alkylamidopropyl carboxy betaine structure in the composition to form a structure with pseudo-gemini surfactant characteristics; at the same time, the quaternary ammonium group of the betaine surfactant also produces crosslinking and coordination with the two carboxylate groups of sodium phthalate through the electrostatic charge mutual attraction effect. The above cross-linking and synergistic effect based on electrostatic attraction can make the surfactant composition form a supramolecular structure type surfactant aggregation system. The viscosity-increasing effect of the supramolecular structure type surfactant aggregation system can increase the viscosity of the solution to form a fracturing fluid system, giving the fracturing fluid CO2 responsiveness; at the same time, the supramolecular structure has a relatively higher packing density and symmetry than a single surfactant. The high packing density and symmetry can effectively limit the dissociation of the supramolecular structure, so that the fracturing fluid system has a relatively higher temperature resistance, and is expected to be used in the fracturing construction of high-temperature reservoirs; on the other hand, when the CO2 dissolved in the clean fracturing fluid is displaced by the introduced inert gas (N2), the diquaternary ammonium salt small molecules are A deprotonation reaction occurs and the diamine small molecules are restored. Since the diamine small molecules do not carry a positive charge, they cannot produce an electrostatic charge attraction with the betaine surfactant, which significantly reduces the overall crosslinking and coordination of the supramolecular structure. The supramolecular structure is no longer stable and disintegrates, causing the viscosity of the solution to decrease and achieve a degelling effect. The gelling-degelling process can be repeated many times and has the characteristic of being recyclable many times. Thirdly, the betaine surfactant and small molecules used in the surfactant composition have the characteristics of low cost, low pollution and high-efficiency reversible crosslinking and coordination, which effectively reduces environmental pollution while ensuring the rapid construction and degelling of the fracturing fluid system, and effectively improves the construction effect of fracturing.

[0041] Based on the above description, the CO2 responsive clean fracturing fluid of the embodiment of the present application can be dispersed in the aqueous medium after the surfactant composition is used, and CO2 gas is passed into the aqueous medium dispersed with the surfactant composition to form a fracturing fluid system by cross-linking reaction. After fracturing, an inert gas (such as N2, Ar, etc.) of deprotonation effect can be passed into the fracturing fluid system, so that the diquaternary ammonium salt small molecules produced by passing CO2 undergo deprotonation reaction and are restored to diamine small molecules, which cannot produce static charge mutual attraction with betaine surfactants due to lack of positive charge, so that the overall cross-linking and coordination of the supramolecular structure is significantly reduced, causing the supramolecular structure to be no longer stable and disintegrate, causing the solution viscosity to decrease and reach the gel breaking effect. The recycling of passing CO2 gas for gelling and passing N2 gas, etc. for gel breaking effectively improves the recycling of clean fracturing fluid, meets the development theme of high efficiency, low pollution and low cost of fracturing fluid, and has great commercial prospects.

[0042] In a possible embodiment, the molar ratio of the alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate is preferably 2:1:1. In particular, the embodiment of the present application controls the molar ratio between the components of the composition and constructs a supramolecular structure type surfactant aggregation system through CO2 induced crosslinking, thereby forming a fracturing fluid system, effectively reducing the free small molecules and betaine surfactants, effectively ensuring the stability of the fracturing fluid system, and making the fracturing fluid system have excellent temperature resistance, viscoelasticity and shear resistance.

[0043] In a possible embodiment, the supramolecular structure has a structural formula including formula [i]:

[0044]

[0045] Where R1 is C 15-21 Alkyl; R2 is C 1-2 Alkyl; n is 2 or 3.

[0046] It should be noted that R1 is specifically selected as C 15 Straight chain alkyl, C 16 Straight chain alkyl, C 17 Straight chain alkyl, C 18 Straight chain alkyl, C 19 Straight chain alkyl, C 20 Straight chain alkyl or C 21Any of the straight-chain alkyl groups. Among them, the embodiments of the present application control the carbon chain length, which not only ensures that the alkyl amidopropyl carboxy betaine containing R1 has good water solubility, so that it can be fully dissolved and dispersed in the aqueous medium, but also makes the alkyl amidopropyl carboxy betaine containing R1 have excellent viscosity-increasing effect, ensuring the production of the fracturing fluid system. At the same time, R2 is specifically selected as methyl or ethyl, and the specific value of n is 2 or 3. Among them, the embodiments of the present application select the carbon chain length and the degree of polymerization of R2 to ensure that the component has a low molecular weight, which can not only make it fully dissolved and dispersed in the aqueous medium, but also effectively improve the cleanliness and degradability of the fracturing fluid.

[0047] According to some embodiments disclosed herein, the alkylamidopropyl carboxy betaine has a chemical structure of formula [ii]:

[0048]

[0049] It should be noted that the specific source of alkylamidopropyl carboxy betaine is not particularly limited in the examples of the present application, and it can be obtained by commercial purchase or self-synthesis. However, since the commercial purchase route is relatively limited, this article takes octadecylamidopropyl carboxy betaine as an example to provide a method for synthesizing alkylamidopropyl carboxy betaine, preferably comprising the following steps:

[0050] Step 1: Octadecanoic acid (0.5 mol) was heated to 170°C under N2 atmosphere, and then 3-dimethylaminopropylamine (0.75 mol) was added within 4 hours, and the mixture was stirred at 170°C for 6 hours. After the reaction, the product was cooled to room temperature and distilled under reduced pressure at 100°C to remove excess 3-dimethylaminopropylamine and water, and a solid product was collected;

[0051] Step 2: The solid product is subjected to a quaternization reaction with sodium chloroacetate at a molar ratio of 1:1.2, and water is used as a solvent (solute content is about 30-40%) during the reaction, and refluxed at 80° C. for 12 hours. After the reaction is completed, the product is spin-dried and vacuum-dried at 60° C. for 12 hours to obtain octadecylamidopropyl carboxybetaine.

[0052] To verify that the method successfully synthesized octadecyl amidopropyl carboxy betaine, the infrared spectrum and nuclear magnetic resonance hydrogen spectrum of octadecyl amidopropyl carboxy betaine are provided below. The results are Figure 1 to Figure 2 As shown. Among them, Figure 1 is the infrared spectrum of octadecylamidopropylcarboxybetaine; Figure 2 This is the hydrogen nuclear magnetic resonance spectrum of octadecylamidopropylcarboxybetaine.

[0053] according to Figure 1 Visible, 3321cm -1and 1066cm -1 The NH stretching vibration peak and CN stretching vibration peak of secondary amine are at 2914cm -1 and 2852cm -1 The two peaks are the antisymmetric stretching vibration peak and the symmetric stretching vibration peak of -CH in -CH2-; 1631cm -1 The C=O stretching vibration peak is at 1463cm -1 -CN + -C- quaternary nitrogen ion characteristic absorption peak; 1335cm -1 The position corresponds to the stretching vibration peak of CO in carboxylic acid; located at 715cm -1 At (CH2) n In-plane rocking vibration absorption peak.

[0054] Figure 2 show: 1 H NMR(600MHz, CD3ODδppm)δ3.84(s,2H),3.67–3.61(m,2H),3.27(s,8H),2.23(q,J=7.8,6.9Hz,2H),1. 97(dd,J=11.7,9.1,5.7Hz,2H),1.62(t,J=7.4Hz,2H),1.32(d,J=20.7Hz,30H),0.92(t,J=7.0Hz,3H).

[0055] Figure 1 to Figure 2 It was confirmed that the example of the present application successfully synthesized octadecylamidopropylcarboxybetaine.

[0056] It should be noted that, in this paper, octadecyl amidopropyl carboxy betaine can be prepared by octadecanoic acid, and of course, corresponding alkyl amidopropyl carboxy betaine can also be prepared by other carboxylic acids, for example, hexadecyl amidopropyl carboxy betaine can be prepared by hexadecanoic acid, etc., which will not be described in detail one by one here.

[0057] According to some embodiments disclosed herein, the diamine small molecule has a chemical structure of formula [iii]:

[0058]

[0059] Among them, according to the chemical structure disclosed above, the diamine small molecules exemplified in this embodiment are specifically N,N,N',N'-tetramethyl-1,3-propylenediamine [iv], N,N,N',N'-tetraethyl-1,3-propylenediamine [v], N,N,N',N'-tetramethyl-1,3-ethylenediamine [vi], N,N,N',N'-tetraethyl-1,3-ethylenediamine [vii].

[0060]

[0061] According to some embodiments disclosed herein, the sodium phthalate is selected from any one including but not limited to sodium phthalate, sodium isophthalate, and sodium terephthalate.

[0062] In a second aspect, the present application also provides a method for preparing the CO2-responsive clean fracturing fluid described above, preferably comprising:

[0063] The step of dispersing the raw material components contained in the surfactant composition in an aqueous medium according to a proportion to prepare an aqueous solution;

[0064] And, a step of introducing CO2 gas into the aqueous solution in the previous step to prepare a supramolecular structure.

[0065] The aqueous medium refers to various waters in the usual sense, including but not limited to tap water, deionized water, distilled water and possible industrial water, etc. Meanwhile, when the surfactant composition is dispersed in the aqueous medium, in order to make the various raw material components disperse and mix evenly, it can be treated by stirring, ultrasound and the like, but not only by other methods. Of course, there are no special requirements for the mixing treatment conditions such as temperature and time.

[0066] In a possible embodiment, the concentration of the surfactant composition dispersed in the aqueous medium is preferably 100-500 mmol·mL -1 , which can be exemplified as 100mmol·mL -1 、150mmol·mL -1 、200mmol·mL -1 、300mmol·mL -1 、400mmol·mL -1 、500mmol·mL -1 Or any one within the above range, those skilled in the art can reasonably select according to actual conditions, as long as a fracturing fluid system that satisfies the viscosity-increasing effect can be obtained.

[0067] In a possible embodiment, the flow rate of CO2 gas introduced into the aqueous solution is 0.3-3 L·min -1 , which can be 0.3L·min -1 、0.5L·min -1 、0.8L·min -1 、1.0L·min -1 、1.5L·min -1 、2.0L·min -1 、2.2L·min -1 、2.5L·min -1 、3.0L·min -1Or any one within the above range, as long as it can promote the surfactant composition to undergo rapid protonation reaction and form a fracturing fluid system, but too fast an introduction rate may cause instability of the solution system.

[0068] On the second aspect, the embodiments of the present application also provide the application of the CO2-responsive clean fracturing fluid described above in the fracturing construction of low permeability oil and gas reservoirs. Among them, the CO2-responsive clean fracturing fluid based on the embodiments of the present application can form gels based on CO2 response and break gels based on inert gas (N2) response, and has the advantages of cleanliness, high efficiency, low cost, good high temperature performance, and wide temperature resistance range. Therefore, after being used for fracturing construction of low permeability oil and gas reservoirs, it can not only realize the multiple recycling of fracturing return fluid, greatly reduce engineering costs and reduce environmental pollution, but also can be applied to the working conditions of high temperature reservoirs (90-120°C), significantly improving the fracturing operation for complex and harsh working conditions.

[0069] The technical solution of the present application will be further described below in conjunction with specific embodiments.

[0070] Example 1

[0071] This embodiment provides a preparation of a clean fracturing fluid VES1, and the specific steps include:

[0072] At room temperature, octadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium terephthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0073] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES1.

[0074] Example 2

[0075] This embodiment provides a preparation of a clean fracturing fluid VES2, and the specific steps include:

[0076] At room temperature, behenylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium terephthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0077] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES2.

[0078] Example 3

[0079] This embodiment provides a preparation of a clean fracturing fluid VES3, and the specific steps include:

[0080] At room temperature, eicosylamidopropyl betaine, N,N,N',N'-tetramethyl-1,3-propylenediamine and sodium isophthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0081] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES3.

[0082] Example 4

[0083] This embodiment provides a preparation of a clean fracturing fluid VES4, and the specific steps include:

[0084] At room temperature, behenylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium phthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0085] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES4.

[0086] Example 5

[0087] This embodiment provides a preparation of a clean fracturing fluid VES5, and the specific steps include:

[0088] At room temperature, octadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-ethylenediamine and sodium phthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0089] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES5.

[0090] Example 6

[0091] This embodiment provides a preparation of a clean fracturing fluid VES6, and the specific steps include:

[0092] At room temperature, hexadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium phthalate were mixed and dissolved in clean water at a molar ratio of 2:1:1, and the total concentration of the composition was 100 mmol·mL -1 , obtaining an aqueous solution of the surfactant composition;

[0093] CO2 gas was introduced into the aqueous solution at a flow rate of 0.3 L / min to convert the solution into a clear and transparent viscoelastic fluid, thereby obtaining the clean fracturing fluid VES6.

[0094] In order to illustrate the effect of the surfactant composition contained in the clean fracturing fluid of the present application, the surfactant composition of Example 2 is taken as an example to explore the effect of the component combination on the performance of the clean fracturing fluid system. Among them, the clean fracturing fluids DVES1-DVES3 were prepared according to the component combination of Table 1, and the total concentration was 100 mmol·mL -1 , the preparation method is the same as Example 2.

[0095] Table 1 - Surfactant composition of clean fracturing fluids DVES1-DVES3

[0096]

[0097] 1. Shear resistance test

[0098] Clean fracturing fluids VES1-VES6 and DVES1-DVES3 were tested for shear resistance respectively. The specific test methods are as follows:

[0099] At room temperature, the test shear rate is 0.01~100s -1 The viscosity changes of clean fracturing fluids VES1-VES6 and DVES1-DVES3 in the range of Figure 3 As shown. Among them, Figure 3 The shear viscosity curves of clean fracturing fluids VES1-VES6 and DVES1-DVES3 are shown.

[0100] according to Figure 3 It can be seen that the maximum shear viscosity of the clean fracturing fluids VES1-VES6 in the examples is 10 3 mPa·s. Among them, the maximum shear viscosity of VES5 and VES6 is relatively low, and the shear viscosity changes regularly with the shear rate, indicating that the fracturing fluid prepared by combining alkyl amide propyl carboxy betaine, diamine small molecules and sodium phthalate has excellent shear resistance. The maximum shear viscosity of the comparative clean fracturing fluid DVES1-DVES3 is 10 2mPa·s, and the shear viscosity changes regularly with the shear rate. Among them, the shear viscosity of clean fracturing fluids DVES2 and DVES3 changes more slowly than that of clean fracturing fluid DVES1, indicating that the addition of diamine small molecules or sodium phthalate can effectively improve the shear resistance of clean fracturing fluids. At the same time, the maximum shear viscosity of clean fracturing fluids VES1-VES6 is significantly higher than that of clean fracturing fluids DVES1-DVES3, indicating that the combination of alkylamidopropyl carboxybetaine, diamine small molecules and sodium phthalate can significantly enhance the viscoelasticity of clean fracturing fluids.

[0101] 2. Temperature resistance and shear resistance test

[0102] Take clean fracturing fluid VES2 as an example to test its temperature resistance and shear resistance. The specific test method is:

[0103] At room temperature, the viscoelastic properties of the clean fracturing fluid VES2 were tested using a HAAKER RS600 rheometer with a CO2 injection time greater than 90 s and a shear rate of 170 s. -1 Under the condition of , the temperature was set to rise from 25℃ and continuous shear was performed for 7000s to investigate the temperature resistance and shear resistance of the clean fracturing fluid system. The results were as follows: Figure 4 As shown. Among them, Figure 4 This is a curve diagram of the temperature and shear resistance of the clean fracturing fluid VES2.

[0104] according to Figure 4 It can be seen that at a constant shear rate of 170s -1 Under the above conditions, as the temperature rises, the apparent viscosity of the clean fracturing fluid VES2 drops sharply and shows a constant viscosity characteristic at 115°C. The final viscosity of the clean fracturing fluid VES2 is about 55 mPa·s, indicating that the clean fracturing fluid VES2 has excellent temperature and shear resistance and can meet the application requirements of high-temperature reservoir sites.

[0105] 3. Cyclic reversibility test

[0106] The reversible change of zero shear viscosity indicates a good viscosity retention rate, which is one of the ways to achieve recycling of clean fracturing fluid. Therefore, the present application repeatedly introduced N2 to break the gel and CO2 to increase the viscosity of the clean fracturing fluid VES2 at room temperature, and tested the change of zero shear viscosity of the clean fracturing fluid VES2 with CO2 / N2 introduction conditions. The results are as follows: Figure 5 As shown. Among them, Figure 5 This is the repetitive response curve of the clean fracturing fluid VES2.

[0107] according to Figure 5It can be seen that at room temperature, when CO2 was introduced into the aqueous solution of the surfactant composition of Example 2 for about 2 minutes, the viscosity of the aqueous solution system rapidly rose to 248 mPa·s, the solution became clear and the zero shear viscosity remained basically unchanged; then N2 was introduced into the system solution, and the solution viscosity began to decrease slowly, but as the N2 introduction time increased, the solution viscosity decreased rapidly and returned to the initial viscosity. This operation was repeated 3 times, and the phenomenon that occurred was basically the same as the first time, indicating that the clean fracturing fluid VES2 has good cyclic reversibility.

[0108] Based on the above tests, the present application designs a surfactant composition composed of alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate. On the one hand, the surfactant composition can be evenly dispersed in the aqueous medium to fully exert their respective effects. At the same time, the amine groups on the diamine small molecule structure in the composition can be protonated with CO2 and converted into positively charged diquaternary ammonium salt small molecules, and the diquaternary ammonium salt small molecules can produce crosslinking and coordination based on electrostatic attraction with the carboxylic acid groups on the alkylamidopropyl carboxy betaine structure in the composition to form a structure with pseudo-gemini surfactant characteristics; at the same time, the quaternary ammonium group of the betaine surfactant and the two carboxylate groups of sodium phthalate produce crosslinking and coordination through the electrostatic charge mutual attraction effect. The above cross-linking and synergistic effect based on electrostatic attraction can make the surfactant composition form a supramolecular structure type surfactant aggregation system. The viscosity-increasing effect of the supramolecular structure type surfactant aggregation system can increase the viscosity of the solution to form a fracturing fluid system, giving the fracturing fluid the characteristics of CO2 response; at the same time, the supramolecular structure has a higher packing density and symmetry than the single surfactant aggregate. The high packing density and symmetry can effectively limit the dissociation of the supramolecular structure, so that the fracturing fluid system has a relatively higher temperature resistance, and is expected to be used in the fracturing construction of high-temperature reservoirs; on the other hand, when the CO2 dissolved in the clean fracturing fluid is displaced by the introduced inert gas (N2), the diquaternary ammonium salt The small molecules undergo a deprotonation reaction and revert to diamine small molecules. Since diamine small molecules do not carry a positive charge, they cannot produce electrostatic charge attraction with the betaine surfactant, which significantly reduces the overall crosslinking and coordination of the supramolecular structure. The disintegration of the supramolecular structure causes the viscosity of the solution to decrease, thereby achieving a degelling effect. The gelling-degelling process can be repeated many times, and has the characteristic of being recyclable many times. Thirdly, the betaine surfactant and small molecules used in the surfactant composition have the characteristics of low cost, low pollution and high-efficiency reversible crosslinking and coordination, which effectively reduces environmental pollution while ensuring the rapid construction and degelling of the fracturing fluid system, and effectively improves the construction effect of fracturing.

[0109] The various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

[0110] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit the present application. Although the present application has been described in detail with reference to the aforementioned embodiments, a person of ordinary skill in the art should understand that the technical solutions recorded in the aforementioned embodiments may still be modified, or some or all of the technical features therein may be replaced by equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present application.

Claims

1. A CO2-responsive clean fracturing fluid, characterized in that: The main component is a surfactant composition, and the surfactant composition comprises alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate; The surfactant composition can be uniformly dispersed in an aqueous solution and react with CO2 to initiate cross-linking into a supramolecular structure.

2. The CO2-responsive clean fracturing fluid according to claim 1, characterized in that: The molar ratio of the alkylamidopropyl carboxy betaine, the diamine small molecule and sodium phthalate is 2:1:

1.

3. The clean fracturing fluid according to claim 2, characterized in that: The supramolecular structure has a structural formula including formula [i]: Where R1 is C 15-21 Alkyl; R2 is C 1-2 Alkyl; n is 2 or 3.

4. The CO2-responsive clean fracturing fluid according to claim 3, characterized in that: The alkylamidopropyl carboxy betaine has a chemical structure of formula [ii]:

5. The CO2-responsive clean fracturing fluid according to claim 3, characterized in that: The diamine small molecule has a chemical structure of formula [iii]:

6. The CO2-responsive clean fracturing fluid according to claim 3, characterized in that: The sodium phthalate is selected from any one of sodium phthalate, sodium isophthalate and sodium terephthalate.

7. A method for preparing the CO2 responsive clean fracturing fluid according to any one of claims 1 to 6, characterized in that: Mainly include: The step of dispersing the raw material components contained in the surfactant composition in an aqueous medium according to a proportion to prepare an aqueous solution; And, a step of introducing CO2 gas into the aqueous solution in the previous step to prepare a supramolecular structure.

8. The preparation method according to claim 7, characterized in that: The surfactant composition is dispersed in the aqueous medium at a concentration of 100-500 mmol·mL -1 .

9. The preparation method according to claim 7, characterized in that: The flow rate of CO2 gas introduced into the aqueous solution is 0.3-3 L·min -1 .

10. Use of the CO2 responsive clean fracturing fluid according to any one of claims 1 to 6 in fracturing construction of low permeability oil and gas reservoirs.

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