A CO2-responsive clean fracturing fluid and its preparation method and application

Abstract

The present application discloses a CO2-responsive clean fracturing fluid and its preparation method and application, which belongs to the technical field of oil and gas reservoir fracturing chemicals. The main component of the CO2-responsive clean fracturing fluid of the present application is a surfactant composition, which contains alkylamidopropyl carboxy betaine, diamine small molecules and sodium phthalate; the surfactant composition can be evenly dispersed in an aqueous solution and react with CO2 to induce cross-linking into a supramolecular structure-type surfactant aggregation system, causing the solution viscosity to increase and produce a fracturing fluid system, and the aggregation system of the supramolecular structure can give the fracturing fluid system higher temperature resistance; after the CO2 dissolved in the fracturing fluid system is displaced by an inert gas, the supramolecular structure dissociates and causes the solution viscosity to decrease, thereby achieving a gel-breaking effect, so that the fracturing fluid system has the characteristics of multiple recycling.

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 a development priority. Fracturing, as the primary method for improving the productivity of oil and gas wells in low-permeability reservoirs, has been widely used in oil and gas production enhancement projects. However, conventional fracturing fluids are prone to irreversible molecular structure changes and destruction after gel breakage, requiring extensive and tedious post-processing before reuse. Furthermore, fracturing flowback fluids often contain large amounts of undegraded residues, which can easily pollute the environment if not properly handled. Therefore, the design and preparation of recyclable, clean fracturing fluids are of great practical value.

[0003] To achieve the recyclability of clean fracturing fluids, CO2-responsive surfactant fracturing fluids have been designed and prepared in the relevant field. For example, patented technology with publication number CN 107142099 A utilizes a tertiary amine surfactant and additives (sodium salicylate, maleic acid, phthalic acid, sodium dodecylsulfonate, sodium p-toluenesulfonate, and sodium benzoate) to prepare a CO2-responsive clean fracturing fluid. This fluid can be controlled by introducing CO2 gas to form a gel and inert gas to break the gel, effectively achieving the recycling of flowback fluids.

[0004] However, the applicable temperature of existing CO2-responsive clean fracturing fluids is generally limited to medium-high temperatures (85°C) and below, and their 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, a preparation method thereof, and an 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 solutions adopted in this application are:

[0007] The first aspect of the present application provides a CO2-responsive clean fracturing fluid. The CO2-responsive clean fracturing fluid is mainly composed of a surfactant composition, and the surfactant composition includes alkylamidopropyl carboxybetaine, a diamine small molecule, and sodium phthalate;

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

[0009] In a preferred embodiment, the molar ratio of the alkylamidopropyl carboxybetaine, 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 carboxybetaine 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 concentration of the surfactant composition dispersed in the aqueous medium is 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 this application include at least:

[0025] The CO2-responsive clean fracturing fluid provided in the present application is designed to have a surfactant composition composed of alkylamidopropyl carboxyl 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 mutual attraction effect. At the same time, the quaternary ammonium groups of the betaine surfactant and sodium phthalate also crosslink and cooperate through the electrostatic charge mutual 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 changes 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 positive charges, they cannot generate electrostatic charge attraction with betaine surfactants, which significantly reduces the overall cross-linking and coordination effect 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 gel-breaking effect. The gel-breaking process can be repeated many times and has the characteristics of multiple recycling. On the third aspect, the betaine surfactants 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 gel-breaking of the fracturing fluid system, effectively improving the fracturing construction effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] To more clearly illustrate the technical solutions of the embodiments of this application, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some of the embodiments described in this application. For those skilled in the art, other drawings can be derived from these drawings without inventive effort.

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

[0028] Figure 2 This is the hydrogen nuclear magnetic resonance spectrum of octadecylamidopropylcarboxybetaine provided in the examples of the present application;

[0029] Figure 3 This is a shear viscosity curve diagram of the clean fracturing fluids VES1-VES6 and DVES1-DVES4 provided in the examples of the present application;

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

[0031] Figure 5 This is the repeatability response curve of the clean fracturing fluid VES2 provided in the examples of the present application. DETAILED DESCRIPTION

[0032] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, not all of the embodiments. Based on the embodiments described in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0033] In the following description of this specification, the term "and / or" is used to describe the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, B exists alone, and A and B exist simultaneously. 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, or C, or A+B, or A+C, or B+C, or A+B+C, where A, B, and C can each be single or plural.

[0035] In the following description of this specification, the order of serial numbers does not mean the order of execution. Some or all steps can be executed in parallel or one after another. 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, numerical ranges should be understood to also specifically disclose each intermediate value between the upper and lower limits of the range. Each smaller range between any stated value or intermediate value in a stated range and any other stated value or intermediate value in the stated range is also included in this embodiment, and the upper and lower limits of the smaller ranges may independently be included or excluded in the range.

[0037] Unless otherwise indicated, the technical / scientific terms used in this specification have the meanings commonly understood by those skilled in the art. Although this specification describes only preferred materials and methods, any similar or equivalent methods and materials may be used in the 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 carboxybetaine, a diamine small molecule, and sodium phthalate;

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

[0040] The embodiments of the present application are designed to have a surfactant composition composed of an alkylamidopropyl carboxybetaine, a diamine small molecule, and sodium phthalate. On the one hand, the surfactant composition can be uniformly dispersed in an aqueous medium to fully exert its respective effects. At the same time, the diamine small molecule in the composition can be protonated with CO2 and converted into a positively charged diquaternary ammonium salt small molecule. The diquaternary ammonium salt small molecule can produce a crosslinking and coordination interaction based on electrostatic attraction with the carboxylic acid groups on the alkylamidopropyl carboxybetaine 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 a crosslinking and coordination interaction with the two carboxylate groups of sodium phthalate through the electrostatic charge mutual attraction effect. The above cross-linking and coordination effects based on electrostatic attraction can make the surfactant composition form a supramolecular structure surfactant aggregation system. The viscosity-increasing effect of the supramolecular structure 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 for 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 generate electrostatic charge attraction with the betaine surfactant, which significantly reduces the overall cross-linking and coordination effect of the supramolecular structure. The supramolecular structure is no longer stable and disintegrates, causing the solution viscosity to decrease and achieving a gel-breaking effect. The gel-breaking process can be repeated many times and has the characteristics of multiple recycling. Thirdly, 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 effectively reduces environmental pollution while ensuring the rapid construction and gel-breaking of the fracturing fluid system, effectively improving the construction effect of fracturing.

[0041] Based on the above description, the CO of the present application embodimentResponse type clean fracturing fluid in use, can be dispersed in aqueous medium after surfactant composition, into the aqueous medium dispersed with surfactant composition, pass into COGas to carry out crosslinking reaction and form fracturing fluid system, after fracturing, the inert gas (such as N2, Ar etc.) of deprotonation effect can be passed into fracturing fluid system, make to pass into COThe diquaternary ammonium salt small molecule generation deprotonation reaction and revert to diamine small molecule, diamine small molecule cannot produce static charge mutual attraction effect with betaine surfactant because of non-positive charge, so that the overall crosslinking cooperation of supramolecular structure is significantly reduced, causing supramolecular structure to be no longer stable and disintegrate, causing solution viscosity to reduce and reach gel breaking effect.So pass into COGas carries out gelling and passes into NGas etc. carry out the recycling of gel breaking and effectively improves the recycling of clean fracturing fluid, meets the development theme of high efficiency, low pollution and low cost of fracturing fluid, has great commercial prospect.

[0042] In a possible embodiment, the molar ratio of the alkylamidopropyl carboxy betaine, diamine small molecule, and sodium phthalate is preferably 2:1:1. In this embodiment, by controlling the molar ratios of the components of the composition and constructing a supramolecular structure-type surfactant aggregation system through CO2-induced crosslinking, the fracturing fluid system is formed while effectively reducing the free distribution of 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 embodiment of the present application ensures that the alkyl amidopropyl carboxy betaine containing R1 has good water solubility by controlling its carbon chain length, so that it can be fully dissolved and dispersed in the aqueous medium, and the alkyl amidopropyl carboxy betaine containing R1 has excellent viscosity-increasing effect, thereby 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 embodiment of the present application ensures that the component has a low molecular weight by selecting the carbon chain length and degree of polymerization of R2, so that it can be fully dissolved and dispersed in the aqueous medium, and can effectively improve the cleanliness and degradability of the fracturing fluid.

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

[0048]

[0049] It should be noted that the specific source of alkylamidopropyl carboxybetaine is not particularly limited in the examples of this application, and it can be obtained by commercial purchase or self-synthesis. However, due to the limited commercial access, this article provides a method for synthesizing alkylamidopropyl carboxybetaine using octadecylamidopropyl carboxybetaine as an example, preferably comprising the following steps:

[0050] Step 1: Octadecanoic acid (0.5 mol) was heated to 170°C under a nitrogen atmosphere, followed by the addition of 3-dimethylaminopropylamine (0.75 mol) over 4 hours. 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, yielding a solid product.

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

[0052] To verify that the method was successful in synthesizing octadecyl amidopropyl carboxy betaine, the infrared spectrum and nuclear magnetic resonance hydrogen spectrum of octadecyl amidopropyl carboxy betaine are provided below. The results are Figures 1 to 2 As shown. Among them, Figure 1 is the infrared spectrum of octadecylamidopropylcarboxybetaine; Figure 2 This is the H NMR 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 respectively at 2914cm -1 and 2852cm -1 The peaks at 1631 cm are the antisymmetric stretching vibration peak and the symmetric stretching vibration peak of -CH in -CH2- respectively; -1 The C=O stretching vibration peak is at 1463 cm -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 (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] Figures 1 to 2 It was confirmed that the example of the present application successfully synthesized octadecylamidopropylcarboxybetaine.

[0056] It should be noted that, herein, octadecyl amidopropyl carboxy betaine can be prepared from octadecanoic acid. Of course, corresponding alkyl amidopropyl carboxy betaine can also be prepared from other carboxylic acids, for example, hexadecyl amidopropyl carboxy betaine can be prepared from hexadecanoic acid, etc., which will not be described in detail 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], and 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 invention also provides a method for preparing the CO2-responsive clean fracturing fluid described above, which preferably includes:

[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 types of water in the conventional sense, including but not limited to tap water, deionized water, distilled water, and possibly industrial water. Furthermore, when dispersing the surfactant composition in the aqueous medium, stirring, ultrasonic treatment, and the like may be employed to uniformly disperse and mix the various raw material components, but this is not limited to other methods. There are no particular requirements for the mixing 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 meets 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 exemplified as 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 rapid protonation reaction of the surfactant composition and form a fracturing fluid system, but too fast an injection speed 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 for use in 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 a wide temperature resistance range. Therefore, after being used for fracturing construction of low-permeability oil and gas reservoirs, it can not only realize multiple recycling of fracturing flowback fluids, greatly reduce engineering costs and reduce environmental pollution, but also be applicable to high-temperature reservoir (90-120°C) working conditions, significantly improving fracturing operations 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 method for a clean fracturing fluid VES1, which includes the following steps:

[0072] At room temperature, octadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine, and sodium terephthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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 method for a clean fracturing fluid VES2, which includes the following steps:

[0076] At room temperature, behenylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium terephthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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 method for a clean fracturing fluid VES3, which includes the following steps:

[0080] At room temperature, eicosylamidopropyl betaine, N,N,N',N'-tetramethyl-1,3-propylenediamine, and sodium isophthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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 method for a clean fracturing fluid VES4, which includes the following steps:

[0084] At room temperature, behenylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine and sodium phthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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 method for a clean fracturing fluid VES5, which includes the following steps:

[0088] At room temperature, octadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-ethylenediamine, and sodium phthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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 method for a clean fracturing fluid VES6, which includes the following steps:

[0092] At room temperature, hexadecylamidopropyl betaine, N,N,N',N'-tetraethyl-1,3-propylenediamine, and sodium phthalate were mixed in a molar ratio of 2:1:1 and dissolved in clean water to a total concentration of 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] 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 used as an example to explore the effect of the component combination on the performance of the resulting clean fracturing fluid system. Among them, the clean fracturing fluids DVES1-DVES3 were prepared according to the component combination of Table 1, with a total concentration of 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. 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 within 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 alkylamidopropyl carboxybetaine, 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 showed a regular change with shear rate. The shear viscosity of clean fracturing fluids DVES2 and DVES3 changed 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. Furthermore, the maximum shear viscosity of clean fracturing fluids VES1-VES6 was 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] Taking clean fracturing fluid VES2 as an example, its temperature resistance and shear resistance were tested. The specific test method is as follows:

[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 90s and a shear rate of 170s. -1 Under the conditions of 25℃, the temperature was set to rise from 25℃ and continuous shear was performed for 7000s to investigate the heat 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 these conditions, the apparent viscosity of the clean fracturing fluid VES2 decreases sharply with increasing temperature, and exhibits a constant viscosity at 115°C. The final viscosity of the clean fracturing fluid VES2 is approximately 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 good viscosity retention rate, which is one of the ways to achieve recycling of clean fracturing fluid. Therefore, the present invention repeatedly introduces N2 into the clean fracturing fluid VES2 at room temperature to break the gel and CO2 into the viscosifier. During this period, the zero shear viscosity of the clean fracturing fluid VES2 was tested as a function of CO2 / N2 introduction conditions. The results are as follows: Figure 5 As shown. Among them, Figure 5 This is the repeatability 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; subsequently, N2 was introduced into the system solution, and the solution viscosity began to slowly decrease, but as the N2 introduction time increased, the solution viscosity decreased rapidly and returned to the initial viscosity. This operation was repeated three 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 carboxybetaine, diamine small molecules and sodium phthalate. On the one hand, the surfactant composition can be evenly dispersed in the aqueous medium to fully exert its respective effects. At the same time, the amino 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 carboxybetaine structure in the composition and 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 coordination effects based on electrostatic attraction can make the surfactant composition form a supramolecular structure surfactant aggregation system. The viscosity-increasing effect of the supramolecular structure 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 for 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 gel-breaking effect. The gel-breaking process can be repeated many times and has the characteristics of multiple recycling. 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 gel-breaking of the fracturing fluid system, effectively improving the construction effect of fracturing.

[0109] The various embodiments in this specification are described in a progressive manner. 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 skilled in the art should understand that the technical solutions described in the aforementioned embodiments can still be modified, or some or all of the technical features therein can 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 includes alkyl amidopropyl carboxy betaine, diamine small molecules and sodium phthalate; wherein the surfactant composition can be uniformly dispersed in an aqueous solution and react with CO2 to induce cross-linking to form a supramolecular structure; the molar ratio of the alkyl amidopropyl carboxy betaine, diamine small molecules and sodium phthalate is 2:1:1; The alkylamidopropyl carboxybetaine has the chemical structure of formula [ii]: [ii]; [ii], R1 is C 15-21 Alkyl; The diamine small molecule has a chemical structure of formula [iii]: [iii] [iii], R2 is C 1-2 Alkyl; n is 2 or 3.

2. The CO2-responsive clean fracturing fluid according to claim 1, characterized in that: The supramolecular structure has a structural formula including formula [i]: [i]。 3. The CO2-responsive clean fracturing fluid according to claim 1, characterized in that: The sodium phthalate is selected from any one of sodium phthalate, sodium isophthalate and sodium terephthalate.

4. A method for preparing the CO2-responsive clean fracturing fluid according to any one of claims 1 to 3, characterized in that: Mainly includes: 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.

5. The preparation method according to claim 4, characterized in that The concentration of the surfactant composition dispersed in the aqueous medium is 100-500 mmol·mL -1 .

6. The preparation method according to claim 4, characterized in that The flow rate of CO2 gas introduced into the aqueous solution is 0.3-3 L·min -1 .

7. Use of the CO2-responsive clean fracturing fluid according to any one of claims 1 to 3 in fracturing construction of low permeability oil and gas reservoirs.

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