A method for calculating geostress caused by hydraulic fracturing in deep low permeability reservoirs

Abstract

The present invention provides a method for calculating hydraulic fracturing geostress for deep low-permeability reservoirs, which relates to the field of geological measurement technology. The present invention collects rock samples and records relevant geological data; secondly, the porosity of the sample is determined by the drainage method, and pressure is applied to determine the compressive strength and elastic modulus, and a microscopic model is established to generate a microscopic stress state; then, based on the microscopic stress state and geological data, a mesoscopic model is constructed by discrete element software, and water pressure is applied to the fracture surface, and the initial stress state and the stress change caused by the application of water pressure are calculated, thereby generating a mesoscopic stress state; finally, a regional macroscopic model is established by combining geological modeling software with mesoscopic stress data, and a critical fracture width is generated and the maximum horizontal principal stress is calculated, which is the hydraulic fracturing geostress. This method combines microscopic, mesoscopic and macroscopic modeling techniques to provide an effective geostress analysis tool for hydraulic fracturing of deep low-permeability reservoirs.

Description

Technical Field

[0001] The invention relates to the technical field of geological measurement, and in particular to a method for calculating hydraulic fracturing ground stress for deep low-permeability reservoirs. Background Art

[0002] The safe and efficient development of oil, gas and geothermal resources in deep low-permeability reservoirs faces many technical challenges. Due to the low permeability and high stress environment of deep reservoirs, traditional hydraulic fracturing technology often has difficulty in effectively breaking through the rock structure, resulting in insufficient crack formation, which in turn affects the fluid permeability and economic benefits of the reservoir. In addition, existing geostress calculation methods mostly rely on macro data and lack comprehensive consideration of rock structure. Therefore, they have problems of insufficient accuracy and poor applicability in practical applications, which leads to inaccurate prediction of the geostress state during hydraulic fracturing, resulting in insufficient crack expansion and stability, which seriously affects the efficient exploitation of deep low-permeability reservoir resources.

[0003] The existing calculation methods of hydraulic fracturing geostress based on theoretical derivation still have some problems in practical application, including: the geological conditions of underground rock formations are usually complex and changeable, including different rock types, bedding structures, faults and cracks, which affect the derivation of hydraulic fracturing geostress; and the traditional calculation methods ignore the flow characteristics of fluids in cracks and their influence on crack expansion, resulting in inaccurate predictions of crack propagation speed, direction and final shape, thereby affecting mining or water injection efficiency; ignoring the influence of environmental factors. During hydraulic fracturing, environmental factors such as temperature, original geostress, and fluid properties will affect the formation and expansion of cracks. If the influence of these factors is ignored, it will lead to underestimation or overestimation of crack expansion behavior, thereby affecting the effect and safety of hydraulic fracturing.

[0004] The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not constitute the prior art that is already known to one of ordinary skill in the art. Summary of the invention

[0005] The object of the present invention is to provide a method for calculating hydraulic fracturing in-situ stress in deep low permeability reservoirs to solve the problems raised in the above-mentioned background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for calculating hydraulic fracturing ground stress for deep low permeability reservoirs, comprising the following specific steps:

[0008] Step 1: Collect rock samples in the target deep low-permeability reservoir and record the geological data of the rock samples, wherein the geological data include rock type, rock density, collection depth, Poisson's ratio and stiffness coefficient;

[0009] Step 2: Determine the porosity of the rock sample based on the drainage method, apply pressure to the rock sample, determine the compressive strength and elastic modulus of the rock, establish a microscopic model, and generate a microscopic stress state;

[0010] Step 3: Based on the microscopic stress state and geological data of the rock sample, a mesoscopic model is established by discrete element software to generate an initial stress state, and water pressure is applied to the fracture surface of the model to generate a mesoscopic stress state based on the initial stress state, the stress generated by the applied water pressure, and the resistance of the fracture;

[0011] Step 4: Based on the mesoscopic stress state, geological data and geological structure of the rock samples, a regional macro model of the deep low-permeability reservoir is established through geological modeling software, and a critical fracture width is generated according to the constructed regional macro model. The critical fracture width represents the fracture width when hydraulic fracturing occurs in the rock sample, and the maximum horizontal principal stress is generated as the hydraulic fracturing geostress.

[0012] Furthermore, the formula for determining the porosity of rock samples based on the drainage method is:

[0013] ;

[0014] ;

[0015] in, represents the porosity of the rock sample, represents the volume of pores in the rock sample, represents the total volume of the rock sample, represents the weight of the dry rock sample, Indicates the weight of the rock sample after it is saturated with water. Represents the density of water.

[0016] Furthermore, the principles for determining the compressive strength and elastic modulus of rock are as follows:

[0017] The principle of determining the compressive strength of rock samples is:

[0018] The rock sample is cut into regular cubes, and the pressure of the test machine is applied to the cube rock sample with gradually increasing pressure until the sample is destroyed. The deformation of the cube rock sample under different pressures is recorded, and the pressure-deformation curve is drawn. The pressure corresponding to the first inflection point in the pressure-deformation curve is taken as the maximum applied pressure. The calculation formula of the compressive strength is:

[0019] ;

[0020] in, Indicates compressive strength, Indicates the maximum applied pressure, Indicates the contact area between the pressure-applying part of the testing machine and the sample.

[0021] The principle on which the elastic modulus is determined is:

[0022] The elastic modulus of the sample is generated from the average slope of the pressure-deformation curve according to the formula:

[0023] ;

[0024] in, represents the elastic modulus, Indicates pressure, represents the pressure of a cubic rock sample The deformation caused by Indicates the maximum applied pressure.

[0025] Furthermore, the formula for generating the microscopic stress state is:

[0026] ;

[0027] in, represents the micro stress state, represents the elastic modulus, represents Poisson's ratio, represents the maximum applied pressure on a cubic rock sample The deformation caused by Deformation The component in the x direction, Deformation Component in the y direction.

[0028] Furthermore, the principle underlying the generation of the mesoscopic stress state is:

[0029] The formula for generating initial geostress is:

[0030] ;

[0031] in, represents the initial ground stress, represents the density of the rock sample, represents the acceleration due to gravity, Indicates the depth at which the rock sample was collected.

[0032] The formula used to generate the stress due to applied water pressure is:

[0033] ;

[0034] in, represents the stress caused by the application of water pressure, Indicates the applied water pressure, Represents the stiffness coefficient of rock.

[0035] Furthermore, the formula for generating the critical crack width is:

[0036] ;

[0037] in, represents the critical crack width, is the crack growth rate constant, Indicates compressive strength, express t The fracture water pressure at the moment, represents the time from the application of water pressure to the critical crack width. Represents the elastic modulus.

[0038] Furthermore, the formula for generating the maximum horizontal principal stress is:

[0039] ;

[0040] in, represents the maximum horizontal principal stress, represents the initial ground stress, Indicates the resistance when the critical crack width is reached, Represents the influence coefficient of crack width on ground stress.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] The present invention sequentially models and calculates geostress from three levels: micro, meso and macro. In the micro model, the mechanical properties of rock and geological data are combined to model and generate a micro stress state, so that the established micro model can more comprehensively reflect the actual state of the rock. A meso model is established based on the data in the micro model, and the water pressure applied force is combined with the crack resistance in the meso model to provide a more comprehensive stress state evaluation. By accurately simulating the crack expansion process, the hydraulic fracturing design is optimized, the crack formation and expansion effects are improved, and resource waste and potential risks are reduced.

[0043] The present invention also establishes a regional macro model of the deep low permeability reservoir through the meso-stress state of the meso-model, comprehensively evaluates the behavior of the deep low permeability reservoir, generates a critical crack width according to the established regional macro model, ensures that the cracks can effectively expand during the hydraulic fracturing process, and provides specific numerical guidance for the actual hydraulic fracturing operation, which helps to optimize the measurement of water pressure application and reduce unnecessary waste of resources; the present invention combines micro-models, meso-models and macro-models, and conducts modeling layer by layer, from the geological data of rock samples, to simulating the complex stress state that may occur in the rock during the hydraulic fracturing process, and then to generating the response of the geological structure to hydraulic fracturing from the perspective of the entire deep low permeability reservoir, and understands the complex process in layers, effectively reducing the uncertainty of the model and improving the accuracy of hydraulic fracturing ground stress calculation. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is a schematic diagram of a method flow of an embodiment of the present invention;

[0045] Figure 2 A schematic diagram of a process flow for establishing a microscopic stress state for an embodiment of the present invention;

[0046] Figure 3 A schematic diagram of a mesoscopic stress state process for an embodiment of the present invention;

[0047] Figure 4 A schematic diagram of a macroscopic stress state process for an embodiment of the present invention. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments.

[0049] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the present invention should be understood by people with ordinary skills in the field to which the present invention belongs. The words "first", "second" and similar words used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprise" and similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and similar words are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0050] Example:

[0051] See also Figure 1, the present invention provides a technical solution:

[0052] A method for calculating hydraulic fracturing ground stress for deep low permeability reservoirs, comprising the following specific steps:

[0053] Step 1: Collect rock samples in the target deep low-permeability reservoir and record the geological data of the rock samples, wherein the geological data include rock type, rock density, collection depth, Poisson's ratio and stiffness coefficient;

[0054] In this embodiment, when collecting rock samples, different depths are selected in the target deep low-permeability reservoir and rock samples are collected separately to avoid the particularity of a single position affecting the results. In subsequent steps, rock samples at a depth position are analyzed, and when building a model, all rock samples collected at different depths are integrated for modeling; rock samples are collected by directional drilling technology, and the core length is between 150mm and 300mm. The core length within this range can better reflect the uniformity of the reservoir and avoid the impact of local unevenness on the test results. The core diameter is between 25mm and 100mm. If the core diameter is too small, the stress concentration effect on the sample during the test will increase, affecting the accuracy of the test results. If the core diameter is too large, the sample processing and experimental operation may become complicated.

[0055] Step 2: Determine the porosity of the rock sample based on the drainage method, apply pressure to the rock sample, determine the compressive strength and elastic modulus of the rock sample, establish a microscopic model, and generate a microscopic stress state;

[0056] In this embodiment, the formula for determining the porosity of a rock sample based on the drainage method is:

[0057] ;

[0058] ;

[0059] in, represents the porosity of the rock sample, represents the volume of pores in the rock sample, represents the total volume of the rock sample, represents the weight of the dry rock sample, represents the weight of the rock sample after saturation, Represents the density of water.

[0060] Place the rock sample in an oven and dry it at 105°C to 110°C for 24 hours. Record the initial volume of water. Then immerse the rock sample completely in water and record the final volume of water. The difference between the final volume of water and the initial volume is the total volume of the sample. , weigh the dry sample mass Then, immerse the sample in water at room temperature for 24 hours to ensure that the pores inside the sample are completely filled with water, and weigh the sample in a saturated state. .

[0061] The principles for determining the compressive strength and elastic modulus of rock samples are based on:

[0062] The principle of determining the compressive strength of rock samples is:

[0063] The rock sample is cut into regular cubes, and the pressure of the test machine is applied to the cube rock sample with gradually increasing pressure until the sample is destroyed. The deformation of the cube rock sample under different pressures is recorded, and the pressure-deformation curve is drawn. The pressure corresponding to the first inflection point in the pressure-deformation curve is taken as the maximum applied pressure. The calculation formula of the compressive strength is:

[0064] ;

[0065] in, Indicates compressive strength, Indicates the maximum applied pressure, Indicates the contact area between the pressure-applying part of the testing machine and the sample.

[0066] The compressive strength reflects the strength characteristics of rock under pressure, that is, the ability to resist fracture. The higher the compressive strength, the greater the stress the rock can withstand.

[0067] The principle on which the elastic modulus is determined is:

[0068] The elastic modulus of the sample is generated from the average slope of the pressure-deformation curve according to the formula:

[0069] ;

[0070] in, represents the elastic modulus, Indicates pressure, represents the pressure of a cubic rock sample The deformation caused by Indicates the maximum applied pressure.

[0071] The elastic modulus reflects the rigidity of the rock sample and the degree of deformation under mechanical load. The higher the elastic modulus, the smaller the deformation of the rock when subjected to stress.

[0072] The formula for generating the micro stress state is:

[0073] ;

[0074] in, represents the micro stress state, represents the elastic modulus, represents Poisson's ratio, represents the maximum applied pressure on a cubic rock sample The deformation caused by Deformation The component in the x direction, Deformation Component in the y direction.

[0075] The microscopic stress state reflects the stress distribution inside the rock sample. As an adjustment factor, the elastic modulus and Poisson's ratio of the rock sample are combined to reflect the resistance of the rock material to deformation. The higher the elastic modulus, the stronger the resistance to deformation. This shows the influence of the rigidity and Poisson's ratio of the sample on the microscopic stress state. The influence of Poisson's ratio on the microscopic stress state is demonstrated, which means that when stress is applied in one direction, not only the total strain caused by the applied stress but also the strains in other directions should be considered.

[0076] The microscopic stress state provides the stress distribution characteristics of rock samples at a smaller scale, including the stress state of microscopic cracks, pores and their interactions, revealing the transmission and distribution of stress inside the rock.

[0077] Step 3: Based on the microscopic stress state and geological data of the rock sample, a mesoscopic model is established using discrete element software to generate an initial stress state. Water pressure is applied to the fracture surface of the model to generate a mesoscopic stress state based on the initial stress state, the stress generated by the applied water pressure, and the resistance of the fracture.

[0078] In this embodiment, the discrete element simulation software PFC is selected to establish the mesoscopic model. The geometric shape and size of the mesoscopic model are determined in the software according to the geological data of the rock sample, and a discrete particle model is generated in the software according to the porosity and particle characteristics of the rock, and physical properties are assigned to each particle to reflect the physical properties of the actual rock.

[0079] In this embodiment, the principle for generating the mesoscopic stress state is:

[0080] The formula for generating initial geostress is:

[0081] ;

[0082] in, represents the initial ground stress, represents the density of the rock sample, represents the acceleration due to gravity, Indicates the depth at which the rock sample was collected.

[0083] The initial geostress reflects the vertical geostress formed when the gravity of the upper rock exerts pressure on the lower rock as the depth of the stratum increases. The stress field in the rock is analogous to the hydrostatic pressure. The pressure generated at the corresponding depth based on the depth, gravitational acceleration and rock density is regarded as the initial geostress when no other forces are applied.

[0084] The formula used to generate the stress due to applied water pressure is:

[0085] ;

[0086] in, represents the stress caused by the application of water pressure, Indicates the applied water pressure, Represents the stiffness coefficient of rock.

[0087] The stress generated by applying water pressure indicates the change of stress in the rock after the water pressure is applied, reflecting the influence of water pressure on effective stress. Rock will show linear strain response when a small external pressure is applied. The magnitude of water pressure directly affects the change of stress in the rock. The greater the water pressure, the greater the pore pressure of the rock, and thus the stress increases. The stiffness coefficient of the rock reflects the ability of the rock to resist deformation when subjected to external force. The calculation formula of the stiffness coefficient is:

[0088] ;

[0089] in, represents the stiffness coefficient of rock, represents the elastic modulus, represents Poisson's ratio.

[0090] The stiffness coefficient determines the amplitude of the stress converted when the pressure is applied. The greater the stiffness, the smaller the stress change when the same water pressure is applied; the smaller the stiffness, the greater the stress change when the same water pressure is applied.

[0091] The formula for generating the mesoscopic stress state is:

[0092] ;

[0093] in, represents the mesoscopic stress state, Indicates the resistance to cracks.

[0094] The mesoscopic stress state reflects the stress state corresponding to the expansion of the crack during the application of water pressure, taking into account the influence of water pressure and the physical properties of the rock; the resistance of the crack represents the energy that needs to be overcome for the crack to expand, which is calculated based on the Mohr-Coulomb strength criterion, and the formula is:

[0095] ;

[0096] ;

[0097] in, Indicates the resistance to cracks, represents the internal friction, represents the normal stress, represents the friction angle.

[0098] The internal friction and friction angle of the rock are determined by direct shear tests.

[0099] Step 4: Based on the mesoscopic stress state, geological data and geological structure of the rock samples, a regional macro model of the deep low-permeability reservoir is established through geological modeling software, and a critical fracture width is generated according to the constructed regional macro model. The critical fracture width represents the fracture width when hydraulic fracturing occurs in the rock sample, and the maximum horizontal principal stress is generated as the hydraulic fracturing geostress.

[0100] In this embodiment, a three-dimensional grid is established through the geological modeling software GMS to define the spatial range of the reservoir, and the stratigraphic units are divided according to the rock type and the acquisition depth to ensure that each stratigraphic unit can represent the corresponding lithological characteristics and physical properties. The stratigraphic units are assigned geological attributes including density, porosity, Poisson's ratio, stiffness coefficient and compressive strength according to the data of the micro-model and the meso-model. The meso-stress state reflects the change of stress magnitude with the water pressure magnitude. The meso-stress state and the initial geostress generated in step 3 are input into the macro-model to establish the initial stress field, and water pressure is applied to the fracture surface of the model to simulate the hydraulic fracturing.

[0101] In this embodiment, the formula for generating the critical crack width is:

[0102] ;

[0103] in, represents the critical crack width, is the crack growth rate constant, Indicates compressive strength, represents the fracturing water pressure at time t, represents the time from the application of water pressure to the critical crack width. Represents the elastic modulus.

[0104] Based on known hydraulic fracturing data, fracture width, geological characteristics, and applied water pressure data, the fracture propagation rate constant is calculated based on regression analysis. k .

[0105] During the hydraulic fracturing process, water pressure will cause changes in the stress distribution inside the rock. When the applied water pressure reaches a certain value, the rock's compressive resistance and initial geostress will determine the expansion of the crack. The increase in crack width will lead to local stress concentration, which will help further promote the expansion of the crack. Therefore, maximizing the crack width means finding an optimal state so that the stress reaches the critical point required for crack expansion. At this time, the corresponding geostress is the desired hydraulic fracturing geostress.

[0106] When the applied water pressure is greater than the compressive strength of the rock, cracks begin to expand in the rock. The difference between the two indicates the ability of water pressure to drive crack expansion. The elastic modulus of rock affects the deformation capacity of rock under water pressure. The larger the elastic modulus, the smaller the increase in crack width. The existence of the elastic modulus makes the crack width proportional to the difference between the compressive strength and the water pressure. The duration of applied water pressure affects the cumulative effect of crack width. The longer the time, the wider the crack.

[0107] The formula used to generate the maximum horizontal principal stress is:

[0108] ;

[0109] in, represents the maximum horizontal principal stress, represents the initial ground stress, Indicates the resistance when the critical crack width is reached, Represents the influence coefficient of crack width on ground stress.

[0110] The maximum horizontal principal stress indicates the maximum pressure applied on the horizontal plane, reflecting the critical stress level of rock fracturing under water pressure. It is proportional to the initial ground stress and water pressure, inversely proportional to the energy of the rock to resist crack expansion, and proportional to the width of the crack and the influence coefficient of the crack width on the ground stress. The larger the crack width, the greater the ground stress required to overcome the resistance of the rock, thus the higher the maximum horizontal principal stress, and the influence coefficient of the crack width on the ground stress. It is used to quantify the influence of crack width on ground stress. The larger the value is, the more significant the effect of crack width on ground stress is, resulting in a greater increase in the fracture-causing ground stress. The smaller the crack width is, the weaker the effect of crack width on ground stress is, resulting in a smaller increase in fracture-causing ground stress. To optimize the calculation formula of the maximum horizontal principal stress to meet the actual calculation needs.

[0111] Calculation formula based on resistance:

[0112] ;

[0113] ;

[0114] When hydraulic fracturing occurs in a rock sample, the corresponding water pressure Substituting into the calculation formula of resistance, we get the critical resistance:

[0115] ;

[0116] in, Indicates critical resistance.

[0117] The above formulas are all dimensionless and numerical calculations. The formula is a formula for the most recent real situation obtained by collecting a large amount of data and performing software simulation. The preset parameters in the formula are set by technicians in this field according to actual conditions.

[0118] The above embodiments may be implemented in whole or in part by software, hardware, firmware or any other combination thereof. When implemented by software, the above embodiments may be implemented in whole or in part in the form of a computer program product. Those skilled in the art may appreciate that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein may be implemented by electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed by hardware or software methods depends on the specific application and design constraints of the technical solution.

[0119] The units described as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, and may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0120] The above description is only a specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application.

Claims

1. A method for calculating hydraulic fracturing ground stress for deep low permeability reservoirs, characterized in that: The specific steps include: Step 1: Collect rock samples in the target deep low-permeability reservoir and record the geological data of the rock samples, wherein the geological data include rock type, rock density, collection depth, Poisson's ratio and stiffness coefficient; Step 2: Determine the porosity of the rock sample based on the drainage method, apply pressure to the rock sample, determine the compressive strength and elastic modulus of the rock sample, establish a microscopic model, and generate a microscopic stress state; Step 3: Based on the microscopic stress state and geological data of the rock sample, a mesoscopic model is established by discrete element software to generate an initial stress state, and water pressure is applied to the fracture surface of the model to generate a mesoscopic stress state based on the initial stress state, the stress generated by the applied water pressure, and the resistance of the fracture; Step 4: Based on the mesoscopic stress state, geological data and geological structure of the rock samples, a regional macro model of the deep low-permeability reservoir is established through geological modeling software, and a critical fracture width is generated according to the constructed regional macro model. The critical fracture width represents the fracture width when hydraulic fracturing occurs in the rock sample, and the maximum horizontal principal stress is generated as the hydraulic fracturing geostress.

2. A method for calculating hydraulic fracturing in-situ stress for deep low permeability reservoirs according to claim 1, characterized in that: The formula for determining the porosity of the rock sample based on the drainage method in step 2 is: ; ； in, represents the porosity of the rock sample, represents the volume of pores in the rock sample, represents the total volume of the rock sample, represents the weight of the dry rock sample, Indicates the weight of the rock sample after it is saturated with water. Represents the density of water.

3. The method for calculating hydraulic fracturing ground stress for deep low permeability reservoirs according to claim 1, characterized in that: The principle for determining the compressive strength and elastic modulus of rock in step 2 is: The principle of determining the compressive strength of rock samples is: The rock sample is cut into regular cubes, and the pressure of the test machine is applied to the cube rock sample with gradually increasing pressure until the sample is destroyed. The deformation of the cube rock sample under different pressures is recorded, and the pressure-deformation curve is drawn. The pressure corresponding to the first inflection point in the pressure-deformation curve is taken as the maximum applied pressure. The calculation formula of the compressive strength is: ； in, Indicates compressive strength, Indicates the maximum applied pressure, Indicates the contact area between the pressure-applying part of the testing machine and the sample; The principle on which the elastic modulus is determined is: The elastic modulus of the sample is generated from the average slope of the pressure-deformation curve according to the formula: ； in, represents the elastic modulus, Indicates pressure, represents the pressure of a cubic rock sample The deformation caused by Indicates the maximum applied pressure.

4. The method for calculating hydraulic fracturing ground stress for deep low permeability reservoirs according to claim 1, characterized in that: The formula for generating the micro stress state in step 2 is: ； in, represents the micro stress state, represents the elastic modulus, represents Poisson's ratio, represents the maximum applied pressure on a cubic rock sample The deformation caused by Deformation The component in the x direction, Deformation Component in the y direction.

5. The method for calculating hydraulic fracturing in-situ stress for deep low permeability reservoirs according to claim 1, characterized in that: The principle for generating the mesoscopic stress state in step 3 is: The formula for generating initial geostress is: ； in, represents the initial ground stress, represents the density of the rock sample, represents the acceleration due to gravity, Indicates the depth from which the rock sample was collected; The formula used to generate the stress due to applied water pressure is: ； in, represents the stress caused by the application of water pressure, Indicates the applied water pressure, represents the stiffness coefficient of rock; The formula for generating the mesoscopic stress state is: ； in, represents the mesoscopic stress state, Indicates the resistance to cracks.

6. A method for calculating hydraulic fracturing in-situ stress for deep low permeability reservoirs according to claim 1, characterized in that: The formula for generating the critical crack width in step 4 is: ； in, represents the critical crack width, is the crack growth rate constant, Indicates compressive strength, express t The fracture water pressure at the moment, represents the time from the application of water pressure to the critical crack width. Represents the elastic modulus.

7. A method for calculating hydraulic fracturing in-situ stress for deep low permeability reservoirs according to claim 6, characterized in that: The formula used to generate the maximum horizontal principal stress is: ； in, represents the maximum horizontal principal stress, represents the initial ground stress, Indicates the resistance when the critical crack width is reached, Represents the influence coefficient of crack width on ground stress.

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