An evaluation method for the fracture promotion ability of lacustrine shale fracturing fluid

Abstract

The present invention provides a method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid, which belongs to the field of oil and gas exploration and development. The method first obtains the invasion depth of the fracturing fluid to be evaluated on the core, the gas flow rate through the core before and after the action of the fracturing fluid to be evaluated, and the cumulative length of the cracks on the end face of the core before and after the action of the fracturing fluid to be evaluated through experiments, and then uses the invasion depth, gas flow rate, and cumulative length of cracks to obtain the fracture-promoting index of the fracturing fluid to be evaluated, and evaluates its fracture-promoting ability based on the fracture-promoting index of the fracturing fluid to be evaluated. The present invention obtains the fracture-promoting index by comparing the number of cracks in the shale core before and after the action of the fracturing fluid, the depth of action and other parameters, and uses the fracture-promoting index to quantitatively evaluate the fracture-promoting effect of the shale core fracturing fluid. The method of the present invention has a simple operating procedure, takes into account the changes in the surface density of the cracks on the end face of the core and the depth of action of the fracturing fluid, and provides an indoor experimental data basis for the design of on-site fracturing fluid solutions.

Description

Technical Field

[0001] The present invention belongs to the field of oil and gas exploration and development, and specifically relates to a method for evaluating the fracture promotion ability of lacustrine shale fracturing fluid, which is used for the shale fracturing fluid fracture promotion index and its evaluation in oil and gas tight reservoir core analysis. Background Art

[0002] With the successful development of shale gas resources in North America, my country has drawn on this experience and achieved remarkable results in marine shale gas development. Building on the success of marine shale gas reservoirs, research on lacustrine shales has received increasing attention. Currently, industrial gas flows have been obtained in exploration projects in Yuanba, Fuling, and Jiannan, with some wells testing high-yield gas flows. However, under current technical and economic conditions, economically viable development is difficult to achieve.

[0003] Compared to marine shale, lacustrine shale has a high clay mineral content and a low content of brittle silica minerals such as quartz. This results in lower brittleness, less complex fractures after induced fractures, greater water sensitivity, and a rapid decline in fracture conductivity, leading to less than ideal fracturing effects. Therefore, during fracturing, the high clay content of lacustrine shale matrix, characterized by its strong water absorption capacity and the large differences in hydration expansion capacity of different minerals, is exploited to promote the initiation and development of microcracks at the edges of different mineral particles within the lacustrine shale matrix. This, in turn, promotes the formation of complex fractures after large-scale fracturing, thereby improving the fracturing effect.

[0004] At present, the slick water systems commonly used in shale fracturing mainly include low-viscosity, medium-viscosity and high-viscosity, powders, emulsions, acidic and oxidizing slick water, etc., which are mainly composed of drag reducers, anti-swelling agents, surfactants and clean water. Different slick water systems have different hydration and fracture-promoting abilities for lacustrine shales. Fracturing fluids with strong fracture-promoting abilities are not only conducive to the production of complex fracture networks during fracturing and reducing the pressure of fracturing construction, but can also release the hydration and expansion potential of the shale matrix in advance, reducing damage to the conductivity of fractures after fracturing, thereby significantly increasing gas well production. Therefore, a lacustrine shale fracturing fluid fracture-promoting index is proposed to evaluate the fracture-promoting abilities of different fracturing fluids on lacustrine shales, and to select a fracturing fluid system with suitable performance, which is of great significance for the design and application of fracturing of plastic lacustrine shales with medium and high clay contents.

[0005] Chinese patent publication CN108645596B discloses an experimental method for evaluating the ability of fluids to create fractures in rocks, which comprises the following steps: testing the brittleness coefficient of dry core and wet core, wave velocity-stress sensitivity coefficient, number of acoustic emission events and natural fracture density respectively, and then non-dimensionalizing the relative change values ​​of the above parameters, establishing a mathematical model of the fracture-forming ability index of different fluids for rocks, and calculating and comparing their sizes. This method can quickly obtain the fracture-forming ability index of different fluids for rocks, and can also evaluate the fracture-forming ability index of the same fluid for different rock samples, which can easily guide the preferred fracturing fluid for hydraulic fracturing construction to promote the formation of artificial complex fracture networks during fracturing. However, the calculation steps of its method are too cumbersome, and the difficulty of the fluid entering the rock is not taken into account, and it is not suitable for evaluating the fracture-forming ability of different fracturing fluids for lacustrine shale. Summary of the Invention

[0006] The purpose of the present invention is to solve the difficulties existing in the above-mentioned prior art and provide a method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid. By establishing a quantitative evaluation method for the fracture-promoting effect of fracturing fluid in hydraulic fracturing of lacustrine shale reservoirs, the fracture-promoting ability of fracturing fluid on lacustrine shale is effectively evaluated, and then a fracturing fluid system with suitable performance is optimized, laying the foundation for subsequent on-site fracturing fluid optimization and fracturing design scheme optimization of lacustrine shale reservoirs.

[0007] The present invention is achieved through the following technical solutions:

[0008] A method for evaluating the fracture promotion ability of a lacustrine shale fracturing fluid is disclosed. The method first obtains, through experiments, the invasion depth of the fracturing fluid to be evaluated on a rock core, the gas flow rate through the rock core before and after the action of the fracturing fluid to be evaluated, and the cumulative length of cracks on the end face of the core before and after the action of the fracturing fluid to be evaluated. The invasion depth, gas flow rate, and cumulative crack length are then used to obtain a fracture promotion index of the fracturing fluid to be evaluated, and the fracture promotion ability of the fracturing fluid to be evaluated is evaluated based on the fracture promotion index.

[0009] A further improvement of the present invention is that the method comprises:

[0010] Step 1: Acquire parameters of the target area, including in situ stress, temperature, and formation water salinity;

[0011] Step 2: Drilling the core and drying the core;

[0012] Step 3: Processing the core into a core column;

[0013] Step 4: Conduct experiments using the fracturing fluid to be evaluated and the core column to obtain experimental data, and use the experimental data to obtain a fracture promotion index of the fracturing fluid to be evaluated.

[0014] The operation of step 2 includes:

[0015] Multiple cores were drilled at the same location on a lacustrine shale matrix;

[0016] The core is dried.

[0017] The operation of step 4 includes:

[0018] (41) Measure the diameter D of the core column;

[0019] (42) The core column is installed in a closed container for displacement, and the initial gas flow rate Q1 through the core column is measured under certain conditions;

[0020] (43) Record the lengths of all cracks on the self-priming end face of the core column and calculate the cumulative length of the initial cracks L1;

[0021] (44) Under the condition of maintaining the formation temperature and confining pressure, the core column is subjected to a self-imbibition experiment using the fracturing fluid to be evaluated;

[0022] (45) Record the lengths of all cracks on the self-priming end face of the core column after the self-priming experiment, and calculate the cumulative length L2 of the cracks after self-priming;

[0023] (46) Record the deepest penetration depth d of the fracturing fluid on the core column, then install the core column in a closed container for displacement, and measure the post-self-priming gas flow rate Q2 through the core column under certain conditions;

[0024] (47) The fracture promotion index of the fracturing fluid to be evaluated is calculated.

[0025] A further improvement of the present invention is that the sealed container in steps (42) and (46) is a core holder.

[0026] A further improvement of the present invention is that the certain conditions in steps (42) and (46) refer to applying a set confining pressure to the core column using the core holder, and applying a set back pressure at the outlet end of the core holder.

[0027] A further improvement of the present invention is that the operation of measuring the initial gas flow rate Q1 through the core column under certain conditions in step (42) includes: after the flow rate of gas through the core column is stabilized, measuring the gas flow rate per unit time at the outlet end of the core holder, that is, the initial gas flow rate Q1.

[0028] The operation of measuring the gas flow rate Q2 after self-imbibition through the core column under certain conditions in step (46) includes: after the flow rate of gas through the core column stabilizes, measuring the gas flow rate per unit time at the outlet end of the core holder, that is, the gas flow rate Q2 after self-imbibition.

[0029] A further improvement of the present invention is that the operation of step (47) includes:

[0030] The seam promotion index α is calculated using the following formula:

[0031]

[0032]

[0033] Among them, D fs is the surface density of cracks on the core end face.

[0034] A further improvement of the present invention is that after step 4, the method further comprises:

[0035] Step 5: Use distilled water and formation water as the fracturing fluid to be evaluated, and repeat step 4 to obtain the fracture promotion index α of distilled water. max , formation water fracture promotion index α min ;

[0036] Step 6: normalize the fracture promotion index of the fracturing fluid to be evaluated obtained in step 4 using the following formula to obtain a normalized fracture promotion index α':

[0037]

[0038] Step 7: Use the normalized fracture promotion index to evaluate the fracture promotion ability of the fracturing fluid to be evaluated.

[0039] A further improvement of the present invention is that the operation of step 7 includes:

[0040] If 0<α'<0.3, it is determined that the fracture promotion ability of the fracturing fluid to be evaluated is weak;

[0041] If 0.3≤α'<0.6, the fracture promotion ability of the fracturing fluid to be evaluated is judged to be medium;

[0042] If 0.6≤α'<1, it is determined that the fracturing fluid to be evaluated has a strong fracture promotion ability.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] A method for evaluating the fracture promotion ability of a lacustrine shale fracturing fluid is disclosed. The method first obtains, through experiments, the invasion depth of the fracturing fluid to be evaluated on a rock core, the gas flow rate through the rock core before and after the action of the fracturing fluid to be evaluated, and the cumulative length of cracks on the end face of the core before and after the action of the fracturing fluid to be evaluated. The invasion depth, gas flow rate, and cumulative crack length are then used to obtain a fracture promotion index of the fracturing fluid to be evaluated, and the fracture promotion ability of the fracturing fluid to be evaluated is evaluated based on the fracture promotion index.

[0045] (1) Forming a quantitative index for the fracture promotion effect of the fracturing fluid injected into the formation. The present invention obtains a fracture promotion index by comparing the number of fractures in the shale core before and after the action of the fracturing fluid, the depth of the action, and other parameters. The fracture promotion index is used to quantitatively evaluate the fracture promotion effect of the fracturing fluid in the shale core.

[0046] (2) Fracturing fluids can be quickly and efficiently screened based on their fracture-promoting capabilities. The optimal selection of fracture-promoting fracturing fluids for shale cores is performed indoors to obtain the working fluid with the best fracturing effect for use in shale reservoir fracturing.

[0047] (3) The method of the present invention has a simple operation process, and takes into account the surface density change of the core end face cracks and the depth of action of the fracturing fluid, providing an indoor experimental data basis for the design of the on-site fracturing fluid program. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A block diagram of the steps of the method of the present invention. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below with reference to the accompanying drawings:

[0050] The present invention provides a method for evaluating the fracture promotion ability of lacustrine shale fracturing fluid. The method mainly compares the penetration depth of the fracturing fluid, the gas flow rate through the core before and after the action of the fracturing fluid, and the cumulative length increase of the cracks on the core end face before and after the action of the fracturing fluid in the same time, calculates the fracture promotion index of the fracturing fluid on the matrix core, comprehensively evaluates the difficulty of the fracturing fluid entering the shale and the fracture promotion ability of the fracturing fluid, and uses the fracture promotion indices corresponding to distilled water and formation water to normalize the fracture promotion index of the fracturing fluid. Then, the fracture promotion ability of the fracturing fluid is graded according to the fracture promotion index, so that the fracture promotion ability of the fracturing fluid can be intuitively evaluated.

[0051] This method can be used to select a fracturing fluid system with suitable performance, which is of great significance for the design and application of fracturing in plastic lacustrine shales with medium and high clay contents, and is suitable for lacustrine shale gas reservoirs.

[0052] like Figure 1 As shown, the method of the present invention comprises:

[0053] Step 1: Obtain parameters of the target area, including in situ stress, temperature, and formation water salinity. These parameters of the target area can be obtained by consulting data;

[0054] Step 2: Drill multiple cores: Select lacustrine shale matrix cores with similar porosity and permeability parameters, dry them, and number them;

[0055] Specifically, multiple cores were drilled at the same location on the lacustrine shale matrix, and the longer blocks were split according to the specifications of the standard cores, so as to ensure that each experimental core was in the same layer, without faults, and with similar physical properties.

[0056] Step 3: Process each core into a core column of a certain height;

[0057] Step 4: Conduct experiments using the fracturing fluid to be evaluated and the core column to obtain experimental data, and use the experimental data to obtain a fracture promotion index of the fracturing fluid to be evaluated.

[0058] The operation of step 4 includes:

[0059] (41) Measure the diameter D of the core column;

[0060] (42) The core column is installed in a closed container for displacement, and the gas flow rate Q1 through the core column is measured under certain conditions;

[0061] Specifically, the sealed container adopts a core holder.

[0062] The certain conditions refer to applying a set confining pressure to the core column by using the core holder, and applying a set back pressure at the outlet end of the core holder.

[0063] The operation of measuring the gas flow Q1 through the core column under certain conditions includes: after the flow through the core column is stabilized, measuring the flow at the outlet end of the core holder, which is the initial gas flow Q1, that is, the amount of gas passing through the core per unit time.

[0064] The confining pressure is designed based on the in-situ stress of the target area, and the back pressure is also determined based on the actual in-situ stress. The purpose of back pressure is to improve pressure transmission efficiency. According to literature research, a back pressure of 1 MPa can facilitate shale flow measurement. However, the specific back pressure should be determined based on the specific conditions of the core. The specific values ​​of the back pressure and confining pressure can be determined based on actual conditions.

[0065] (43) The self-priming end face of the core column was observed using microscopic methods such as CT scanning, and the lengths of all cracks on the original shale end face were recorded. The initial cumulative length of the cracks, L1, was calculated by adding the lengths of all cracks in sequence to obtain the initial cumulative length of the cracks, L1. The self-priming end face refers to the circular end face of the core column near the inlet end of the core holder.

[0066] The microscopic methods used include: scanning electron microscopy observation, thin section analysis, CT scanning and other existing microscopic observation methods that can observe end face cracks.

[0067] (44) A self-priming experiment on the end face of a shale core is conducted in a core holder using the fracturing fluid to be evaluated, with the inlet end of the core holder being used as the self-priming end (fracturing fluid is injected into the pipeline at the inlet end of the core holder, and a pressure equal to the back pressure is applied. The fracturing fluid penetrates from the self-priming end of the core column along its axis to the other end, and finally flows out from the outlet end of the core holder.). During the experiment, a confining pressure equal to the in situ stress is applied to the core column, and a back pressure equal to the inlet pressure is applied to the outlet end.

[0068] During the self-priming experiment, the temperature inside the core holder was maintained at the formation temperature. Confining pressure was applied through the core holder, and the temperature was maintained constant using the reactor. Specifically, the core column was clamped in the core holder, which was then placed in the reactor. Both the core holder and the reactor are existing products, and their structures and usage will not be described in detail here.

[0069] (45) The lengths of all cracks on the end face of the core column after self-imbibition are recorded by microscopic means such as CT scanning, and the cumulative length of cracks after self-imbibition, L2, is calculated; new cracks are generated after self-imbibition, resulting in an increase in the total length;

[0070] (46) After the self-priming time is set, the deepest intrusion depth d of the fracturing fluid on the core column is recorded. Specifically, the color of the core column will change after being infiltrated. The depth of the color change on the core column can be measured using a measuring tool such as a vernier caliper. The largest depth is the intrusion depth d.

[0071] Then, the core column is installed in the core holder, and under certain conditions (i.e., the same confining pressure and back pressure as in step (42)), the gas flow rate Q2 after self-priming through the core is measured after the flow rate through the core stabilizes;

[0072] (47) Calculate the fracture promotion index of the fracturing fluid to be evaluated:

[0073] Substitute L1, L2, and D into formula (1), and then fs , Q1, Q2, d are substituted into formula (2) to obtain the fracture promotion index of the fracturing fluid to be evaluated. The larger the fracture promotion index, the better the fracture promotion effect.

[0074]

[0075]

[0076] Where: α is the crack promotion index, dimensionless; D fs is the surface density of cracks on the core end face, in mm -1; L1 is the cumulative length of all cracks on the original shale end face, that is, the cumulative length of initial cracks, in mm; L2 is the cumulative length of all cracks on the core end face after self-imbibition, that is, the cumulative length of cracks after self-imbibition, in mm; Q1 is the initial flow rate through the core, that is, the initial gas flow rate, in cm 3 / s; d is the depth of fluid intrusion into the core, i.e., immersion depth, in mm; Q2 is the gas flow through the core after self-priming fracturing fluid, i.e., gas flow after self-priming, in cm 3 / s; D is the diameter of the core, in mm;

[0077] In order to normalize the seam-promoting index, the method of the present invention further comprises:

[0078] Step 5: Distilled water and formation water (configured according to the formation water salinity input in step 1, and the experimental conditions simulate the actual reservoir conditions as much as possible) are used as the fracturing fluid to be evaluated. The core column obtained in step 3 is repeated in step 4 to obtain the corresponding fracture promotion index α of distilled water. max , fracture promotion index α corresponding to formation water min In step 3, multiple cores with similar physical properties and located in the same layer are obtained. When testing the fracturing fluid, distilled water, and formation water to be evaluated, these core columns can be used to ensure that the same core is tested.

[0079] Step 6: normalize the fracture promotion index of the fracturing fluid to be evaluated obtained in step 4 using the following formula to obtain a normalized fracture promotion index;

[0080]

[0081] Among them, α' is a number between 0 and 1. The closer it is to 1, the stronger the fracture-promoting ability of the fracturing fluid.

[0082] Step 7: Use the normalized fracture promotion index to evaluate the fracture promotion ability of the fracturing fluid to be evaluated, as follows:

[0083] If 0<α'<0.3, it is determined that the fracture promotion ability of the fracturing fluid to be evaluated is weak;

[0084] If 0.3≤α'<0.6, the fracture promotion ability of the fracturing fluid to be evaluated is judged to be medium;

[0085] If 0.6≤α'<1, it is determined that the fracturing fluid to be evaluated has a strong fracture promotion ability.

[0086] For cores from different locations, the above steps are used to obtain α', and then the fracture promotion ability is determined according to its numerical range, thus realizing the fracture promotion ability evaluation. The fracture promotion ability of cores from different regions can be compared by using a fixed formula for evaluation.

[0087] In order to explain the technical features, objectives and beneficial effects of the present invention more clearly, the content and features of the present invention are further described in detail with reference to specific parameters and embodiments:

[0088] [Example 1]

[0089] Step 1: Select matrix cores with similar porosity and permeability parameters of Jurassic lacustrine shale in the Sichuan Basin, dry them in an oven at 65°C for 24 hours, and number them (A-1, A-2, A-3, A-4, A-5, A-6);

[0090] Step 2: Cut the core into a core column with a height of 5.00 cm and a diameter of about 2.50 cm, and measure its diameter D; apply a back pressure of 1 MPa and a confining pressure of 3 MPa at the outlet of the core holder, and measure the gas flow rate (Q1) after the flow through the core column stabilizes;

[0091] Step 3: Observe the shale end surface using microscopic methods such as CT and record the cumulative length of all cracks on the original shale end surface (L1);

[0092] Step 4: Conducting a self-imbibition experiment in a sealed container with an end-face self-imbibition time of 2 hours using an acidic slickwater fracturing fluid for core columns A-1 and A-2, an oxidizing slickwater fracturing fluid for core columns A-3 and A-4, and a conventional slickwater fracturing fluid for core columns A-5 and A-6, respectively. During the experiment, a confining pressure equal to the in situ stress was applied to the core columns, and the temperature in the container was maintained at the formation temperature.

[0093] Step 5: Use microscopic methods such as CT scanning to observe whether the fracturing fluid to be evaluated can enter the pores of the shale matrix, compare the changes before and after the shale end face, and record the cumulative length of all cracks on the core end face after self-imbibition (L2);

[0094] Step 6. After the core column is imbibed with different imbibing fluids for the same time, record its invasion depth (d), place it in a core holder, apply a back pressure of 1 MPa and a confining pressure of 3 MPa at the outlet of the core holder, and measure the gas flow rate (Q2) after the flow through the core is stable. Substitute L1, L2, and D into formula (1), and then replace D with fs Substituting Q1, Q2 and d into formula (2), we can get the fracture promotion index of different fracturing fluids on matrix cores. The larger the fracture promotion index, the better the fracture promotion effect:

[0095]

[0096]

[0097] Where: α is the crack promotion index, dimensionless; D fs is the surface density of cracks on the core end face, in mm -1; L1 is the cumulative length of all cracks on the original shale end face, that is, the cumulative length of initial cracks, in mm; L2 is the cumulative length of all cracks on the core end face after self-imbibition, that is, the cumulative length of cracks after self-imbibition, in mm; Q1 is the initial flow rate through the core, that is, the initial gas flow rate, in cm 3 / s; d is the depth of fluid intrusion into the core, i.e., immersion depth, in mm; Q2 is the gas flow through the core after self-priming fracturing fluid, i.e., gas flow after self-priming, in cm 3 / s; D is the diameter of the core, in mm;

[0098] Step 7: Calculate the fracture promotion index α corresponding to distilled water and formation water respectively according to the method described in step 6. max and α min ;

[0099] Step 8: Normalize the fracture promotion index; α' is a number between 0 and 1, and the closer it is to 1, the stronger the fracture promotion ability of the fracturing fluid; 0<α'<0.3, the fracture promotion ability is weak; 0.3≤α'<0.6, the fracture promotion ability is medium; 0.6≤α'<1, the fracture promotion ability is strong.

[0100]

[0101] According to the present invention, the fracture promotion index of self-priming fracturing fluid is determined through experiments, and different fracturing fluid systems are optimized based on the fracture promotion index. This provides an indoor experimental data basis for the optimization design of field fracturing schemes and explores new ideas for achieving fracture promotion and production increase in lacustrine shale gas.

[0102] The present invention is to observe whether the fracturing fluid can enter the shale matrix pores under in situ conditions by self-imbibing different fracturing fluids in a closed container, compare the changes in the shale core before and after self-imbibing different fracturing fluids, determine the surface density of the core end face cracks and the fracturing fluid invasion depth, evaluate the crack-promoting effect of different fracturing fluids on the matrix core according to the size of the crack-promoting index, and preferably select the fracturing fluid with the best crack-promoting effect. The present invention is simple to operate, can effectively evaluate the crack-promoting ability of different fracturing fluids on lake shale, preferably a fracturing fluid system with suitable performance, and is of great significance for the design and application of medium and high clay content plastic lake shale fracturing. The fracturing fluid crack-promoting index provided by the present invention can effectively evaluate the crack-promoting ability of fracturing fluid on lake shale, and then preferably a fracturing fluid system with suitable performance, which is of great significance for the design and application of medium and high clay content plastic lake shale fracturing.

[0103] The above technical solution is only one embodiment of the present invention. For those skilled in the art, it is easy to make various types of improvements or modifications based on the application methods and principles disclosed in the present invention, and it is not limited to the method described in the above specific embodiment of the present invention. Therefore, the method described above is only preferred and does not have a restrictive meaning.

Claims

1. A method for evaluating the fracture-promoting ability of a lacustrine shale fracturing fluid, characterized by: The method first obtains, through experiments, the penetration depth of the fracturing fluid to be evaluated on the rock core, the gas flow rate through the rock core before and after the action of the fracturing fluid to be evaluated, and the cumulative length of the cracks on the end face of the core before and after the action of the fracturing fluid to be evaluated. Then, the penetration depth, gas flow rate, and cumulative length of the cracks are used to obtain the fracture promotion index of the fracturing fluid to be evaluated, and the fracture promotion ability of the fracturing fluid to be evaluated is evaluated based on the fracture promotion index. The method comprises: Step 1: Acquire parameters of the target area, including in situ stress, temperature, and formation water salinity; Step 2: Drilling the core and drying the core; Step 3: Processing the core into a core column; Step 4: Conducting experiments using the fracturing fluid to be evaluated and the core column to obtain experimental data, and using the experimental data to obtain a fracture promotion index of the fracturing fluid to be evaluated; The operation of step 4 includes: (41) Measure the diameter D of the core column; (42) The core column is installed in a closed container for displacement, and the initial gas flow rate Q1 through the core column is measured under certain conditions; (43) Record the lengths of all cracks on the self-priming end face of the core column and calculate the cumulative length L1 of the initial cracks; (44) Under the condition of maintaining the formation temperature and confining pressure, the core column is subjected to a self-imbibition experiment using the fracturing fluid to be evaluated; (45) Record the lengths of all cracks on the self-priming end face of the core column after the self-priming experiment, and calculate the cumulative length L2 of the cracks after self-priming; (46) Record the deepest penetration depth d of the fracturing fluid on the core column, then install the core column in a closed container for displacement, and measure the post-self-priming gas flow rate Q2 through the core column under certain conditions; (47) Calculate and obtain the fracture promotion index of the fracturing fluid to be evaluated; The operation of step (47) includes: The seam promotion index α is calculated using the following formula: Among them, D fs is the surface density of cracks on the core end face.

2. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 1, characterized in that: The operation of step 2 includes: Multiple cores were drilled at the same location on a lacustrine shale matrix; The core is dried.

3. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 1, characterized in that: The sealed container in steps (42) and (46) is a core holder.

4. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 1, characterized in that: The certain conditions in steps (42) and (46) refer to applying a set confining pressure to the core column using the core holder and applying a set back pressure at the outlet end of the core holder.

5. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 1, characterized in that: The operation of measuring the initial gas flow rate Q1 through the core column under certain conditions in step (42) includes: after the flow rate of the gas through the core column stabilizes, measuring the gas flow rate per unit time at the outlet end of the core holder, i.e., the initial gas flow rate Q1; The operation of measuring the gas flow rate Q2 after self-imbibition through the core column under certain conditions in step (46) includes: after the flow rate of gas through the core column stabilizes, measuring the gas flow rate per unit time at the outlet end of the core holder, that is, the gas flow rate Q2 after self-imbibition.

6. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 1, characterized in that: After step 4, the method further includes: Step 5: Use distilled water and formation water as the fracturing fluid to be evaluated, and repeat step 4 to obtain the fracture promotion index α of distilled water. max , formation water fracture promotion index α min ; Step 6: normalize the fracture promotion index of the fracturing fluid to be evaluated obtained in step 4 using the following formula to obtain a normalized fracture promotion index α': Step 7: Use the normalized fracture promotion index to evaluate the fracture promotion ability of the fracturing fluid to be evaluated.

7. The method for evaluating the fracture-promoting ability of lacustrine shale fracturing fluid according to claim 6, characterized in that: The operation of step 7 includes: If 0<α'<0.3, it is determined that the fracture promotion ability of the fracturing fluid to be evaluated is weak; If 0.3≤α'<0.6, the fracture promotion ability of the fracturing fluid to be evaluated is judged to be medium; If 0.6≤α'<1, it is determined that the fracturing fluid to be evaluated has a strong fracture promotion ability.

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