A method and system for fdem simulation of mineral-scale shale hydrofracturing

Abstract

This invention belongs to the field of oil and gas production enhancement and stimulation technology. It provides a FDEM simulation method and system for mineral-scale shale hydration-induced fracturing. The method includes the following steps: obtaining target reservoir stress, rock porosity, and permeability; preparing rock samples from the target stratigraphic layer and obtaining the mineral composition and content through X-ray diffraction experiments; constructing a polycrystalline geometric model of the minerals using random Voronoi mosaic technology; conducting core immersion hydration experiments to obtain the relationship between core water content and time; setting initial and boundary conditions for the model and establishing a numerical model of clay mineral hydration expansion-induced fracturing based on the finite discrete element method; and conducting numerical experiments on shale hydration-induced fracturing to obtain the micro-fracture propagation morphology induced by clay mineral hydration expansion. This invention considers the non-uniform mosaic distribution of multiple minerals in the reservoir rock, simulates the non-uniform stress distribution induced by clay mineral water absorption and expansion, and realizes numerical simulation of the mineral-scale shale hydration-induced fracturing process.

Description

Technical Field

[0001] This invention relates to the field of oil and gas production enhancement and modification technology, and in particular to an FDEM simulation method and system for mineral-scale shale hydration-induced fracturing. Background Technology

[0002] Hydraulic fracturing is a key technology for developing shale oil and gas resources. It involves injecting high-pressure fluid into the formation, generating numerous hydraulic fractures under water pressure, thereby enhancing the overall permeability of the formation. During fracturing and post-fracturing well shut-in, the fluid seeps into the rock matrix along the main fracture, causing changes in the water cut within the matrix, with the largest increase in water cut occurring around the main fracture. The shale mineral composition in the formation is complex and diverse, containing various clay minerals, primarily water-sensitive minerals such as illite. These minerals exhibit a non-uniform mosaic distribution. When the water cut changes, water-sensitive minerals absorb water and swell, generating uneven expansion stress in localized areas of the formation. Superimposed on the original formation stress field, this further creates a non-uniform stress concentration effect. When the stress in a localized area exceeds the tensile strength of the rock, it induces micro-fractures, forming unevenly distributed hydration micro-fractures. These micro-fractures effectively expand the seepage area and shorten the seepage distance, further enhancing the overall permeability of the formation and thus increasing single-well production.

[0003] The development of hydration microfractures is a key factor affecting the mechanical properties and permeability of shale. Numerous studies have analyzed the propagation process of hydration fractures through experimental methods, primarily including scanning electron microscopy, CT, and digital image correlation. Research indicates that fracture propagation is most rapid in the early stages of hydration. However, experimental methods struggle to accurately and in real-time describe the global propagation of microfractures, necessitating numerical simulations. Current numerical methods for hydration fracture propagation only model some mechanisms, equating clay minerals with circular or elongated shapes, without considering the non-uniform mosaic distribution of minerals within the formation. Therefore, they cannot accurately describe the propagation patterns of hydration microfractures under actual formation conditions. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides an FDEM simulation method and system for mineral-scale shale hydration-induced fracturing. This method can consider the non-uniform mosaic structure of multiple minerals in reservoir rocks and simulate the non-uniform stress distribution induced by water absorption and swelling of clay minerals, providing theoretical support for analyzing the permeability enhancement mechanism of shale hydration-induced fracturing.

[0005] To achieve the above objectives, this invention provides an FDEM simulation method for hydration-induced fracturing of shale at the mineral scale, comprising the following steps:

[0006] Step S1: Obtain the target reservoir stress, rock porosity, and permeability;

[0007] Step S2: Prepare rock samples from the target stratum and obtain the mineral composition and content of the rocks by X-ray diffraction experiments;

[0008] Step S3: Construct a polycrystalline geometric model of the mineral using random Voronoi mosaic technique;

[0009] Step S4: Conduct core immersion hydration experiments to obtain the relationship between core water content and time;

[0010] Step S5: Set the initial and boundary conditions of the model, and establish a numerical model of clay mineral hydration expansion-induced cracking based on the finite discrete element method;

[0011] Step S6: Conduct numerical tests on shale hydration-induced cracking to obtain the morphology of microcrack propagation induced by clay mineral hydration expansion.

[0012] Optionally, in step S3, the step of constructing the polycrystalline numerical model of the mineral includes:

[0013] N seed points are randomly generated within the solution domain;

[0014] Based on the seed point location, a polygrain model is generated using Voronoi mosaic technology, the grains are numbered, and the area of ​​each grain is calculated.

[0015] The entire solution domain is discretized into a large number of triangular element meshes using the Delaunay triangulation method;

[0016] The grains are classified according to the rock mineral composition and content obtained in step S2.

[0017] Optionally, in step S4, the core immersion hydration experiment includes:

[0018] Several standard core columns were prepared, dried at low temperature, and the mass m0 of the dried rock samples was measured.

[0019] Soak the dried rock sample in deionized water;

[0020] After soaking for different time periods, the rock samples were removed, cleaned, and dried. The mass m of the water-bearing rock samples after different soaking times was measured. (t) ;

[0021] Obtain the water content w of rock samples under different soaking times (t) .

[0022] Optionally, the water content w of the rock sample (t The method for obtaining it is as follows:

[0023] .

[0024] Optionally, the soaking time includes 3h, 9h, 24h, 48h, and 72h.

[0025] Optionally, in step S5, setting the initial conditions of the model includes assigning values ​​to mineral mechanical parameters, assigning values ​​to the mechanical properties of all grains according to the mineral type, and the mechanical properties include Young's modulus, Poisson's ratio, cohesion, internal friction angle, tensile strength, and density.

[0026] Optionally, in step S5, the model boundary conditions are set, including mechanical boundary conditions and water content boundary conditions. The mechanical boundary conditions are set according to the reservoir stress obtained in S1, and the confining pressure is simulated by applying surface forces to the left and right boundaries of the model. The water content boundary conditions are set according to the water content-time relationship obtained experimentally in S4, and water content variation boundary conditions are set on the upper boundary of the model.

[0027] Optionally, in step S5, the solution domain is divided into a large number of triangular elements and joint elements in the FDEM. The stress-strain response of the triangular elements is used to characterize the solid deformation, and the fracture of the joint elements is used to characterize the crack initiation and propagation.

[0028] Optionally, in step S5, the clay mineral hydration swelling-induced cracking model mainly includes the water diffusion equation and the clay swelling control equation. Under the FDEM calculation framework, the water content distribution in the multi-mineral rock is calculated based on the water diffusion equation:

[0029]

[0030] Due to changes in the water content of the rock, clay minerals expand, resulting in expansion strain. The expansion strain can be calculated using the following formula:

[0031]

[0032] The stress increment caused by clay expansion is calculated based on the expansion strain. The calculation method is as follows:

[0033]

[0034]

[0035] In the formula, For expansion strain; α is the expansion stress increment; A is the expansion coefficient; A is the area of ​​the triangular element; x i w is the distance along the i-th direction; w is the water content; w0 is the initial water content of the rock; w (t) The water content of the rock after water absorption time t; is the calculation coefficient; K* is the modulus; K is the bulk modulus; G is the shear modulus.

[0036] Optionally, in step S6, based on the established polycrystalline geometric model of the mineral, numerical tests on shale hydration-induced fracturing are carried out, and fracture morphology maps, displacement field distribution cloud maps, and stress field distribution cloud maps are output.

[0037] Optionally, the morphological statistics of the crack morphology map include the type, number, length, area, and maximum crack width of the microcracks.

[0038] This invention also provides an FDEM simulation system for mineral-scale shale hydration-induced fracturing, comprising:

[0039] The data acquisition unit is used to acquire the changes in target reservoir stress, rock porosity and permeability, mineral composition, content and water content over time.

[0040] The geometric model building unit is used to construct a polycrystalline geometric model of a mineral using experimentally obtained mineral composition and content parameters and random Voronoi mosaic technique.

[0041] The numerical modeling unit is used to set the initial and boundary conditions of the model and to establish a numerical model of clay mineral hydration expansion and cracking based on the finite discrete element method.

[0042] The numerical simulation unit is used to conduct numerical tests on shale hydration-induced cracking and to obtain the propagation morphology of microcracks induced by the hydration expansion of clay minerals.

[0043] The present invention has the following beneficial technical effects:

[0044] This invention provides a mineral-scale FDEM simulation method and system for shale hydration-induced fracturing. By acquiring shale mineral composition, porosity, and permeability, a polycrystalline geometric model of minerals is constructed based on random Voronoi mosaic technology to simulate the non-uniform mosaic distribution of various minerals in underground rocks. Immersion hydration experiments are conducted to obtain the relationship between core water content and time, characterizing the change in water content in the rock matrix near the main fracture wall, and serving as the boundary condition for the hydration-induced fracturing numerical model. Considering the in-situ stress conditions of the underground rock, the model boundary conditions are set, and a numerical model of clay mineral hydration expansion fracturing is established based on the finite discrete element method. Numerical simulation reveals the propagation morphology of hydration microcracks. The method provided by this invention overcomes the shortcomings of existing experimental methods and numerical simulation methods by considering the non-uniform distribution of clay minerals in the rock and simulating the hydration-induced microcracks caused by the non-uniformly distributed expansion stress generated by clay expansion, thus more completely reflecting the shale hydration-induced fracturing process. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 A flowchart illustrating the method of this invention;

[0047] Figure 2 This is a flowchart illustrating the construction process of a mineral polycrystalline geometric model using the random Voronoi mosaic technique provided in this embodiment of the invention.

[0048] Figure 3 This is a schematic diagram of a numerical model of clay mineral hydration expansion and cracking based on the finite discrete element method provided in an embodiment of the present invention.

[0049] Figure 4 This is a graph showing the relationship between water content and water time based on an immersion experiment provided in an embodiment of the present invention.

[0050] Figure 5 This is an example diagram of the morphology of hydration-induced microcracks provided in an embodiment of the present invention;

[0051] Figure 6 This is a statistical curve of hydration-induced microcracks provided in an embodiment of the present invention;

[0052] Figure 7 This is a schematic diagram of the unit structure of the FDEM simulation system for mineral-scale shale hydration-induced fracturing provided in an embodiment of the present invention. Detailed Implementation

[0053] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples. Those skilled in the art will then fully understand how the present invention uses technical means to solve technical problems and achieve technical effects, and will be able to implement the present invention specifically based on the above-described implementation process. It should be noted that, as long as there is no conflict, the various embodiments and features of the present invention can be combined with each other, and the resulting technical solutions are all within the protection scope of the present invention.

[0054] During hydraulic fracturing and post-fracturing well shut-in, a large amount of fluid seeps into the rock matrix, causing clay mineral hydration and inducing numerous microfractures. This enhances the overall permeability of the formation after fracturing, thereby increasing single-well production. Existing technologies mostly employ experimental methods to characterize the propagation of hydration fractures. These methods are cumbersome and cannot intuitively describe the deformation of underground rocks and the global propagation of microfractures. Numerical simulation is an effective means of describing the propagation of hydration microfractures in reservoirs. However, existing numerical simulation techniques only conduct some mechanistic modeling, equating clay minerals with circular or elongated shapes, and lack consideration for the non-uniform mosaic distribution among various minerals in the underground rock.

[0055] In view of this, to solve the above problems, the present invention provides a mineral-scale FDEM (Finite-discrete element method) simulation method and system for hydration-induced cracking in shale. Taking multi-mineral rocks as the research object, based on the FDEM numerical simulation method and combined with indoor experiments, a mineral-scale shale hydration crack propagation model is established. This model can consider the non-uniform distribution of clay minerals in the rock and simulate the hydration-induced microcracks caused by the non-uniformly distributed expansion stress generated by clay expansion, thus reflecting the shale hydration-induced cracking process more completely.

[0056] Figure 1 The diagram illustrates a flowchart of an FDEM simulation method for hydration-induced fracturing of shale at the mineral scale, provided by an embodiment of the present invention, including the following steps:

[0057] Step S1: Collect relevant reservoir geomechanical parameters to obtain target reservoir stress, rock porosity and permeability;

[0058] Step S2: Prepare rock samples from the target stratum and obtain the mineral composition and content of the rocks by X-ray diffraction experiments;

[0059] Optionally, the core samples taken from the target stratum are ground and crushed into rock powder with a particle size of less than 200 mesh. Three parallel experiments are set up, with each group of rock powder samples weighing approximately 50g. The experimental instrument is an X-ray diffractometer (XRD) to conduct whole-rock mineral analysis and obtain the mineral composition and content of the target reservoir rocks.

[0060] Step S3: Construct a polycrystalline geometric model of the mineral using random Voronoi mosaic technique;

[0061] The specific steps for constructing a numerical model of a polycrystalline mineral are as follows: Figure 2As shown. First, N seed points are randomly generated within the solution domain; based on the positions of the seed points, a polygrain model is generated using Voronoi tessellation technique, the grains are numbered, and the area of ​​each grain is counted; the entire solution is discretized into a large number of triangular element meshes using the Delaunay triangulation method; based on the rock mineral composition and content obtained in step S2, the grains are classified, and the area ratio of all grains of the same mineral reflects the volume content of that mineral.

[0062] Step S4: Conduct core immersion hydration experiments to obtain the relationship between core water content and time;

[0063] The specific steps of the core immersion hydration experiment include: preparing a standard core column, drying it at low temperature, and measuring the mass m0 of the dried rock sample; immersing the dried rock sample in deionized water; removing the rock sample after immersion for 3h, 9h, 24h, 48h, and 72h, respectively, cleaning and drying the surface, and measuring the mass m of the water-bearing rock sample after different immersion times. (t) The water content w of the rock samples under different soaking times was calculated using the following formula. (t) :

[0064]

[0065] Step 5: Set the initial and boundary conditions of the model, and establish a numerical model of clay mineral hydration expansion-induced cracking based on the finite discrete element method;

[0066] Based on the polycrystalline geometric model of the mineral constructed in step S3, initial and boundary conditions are set. The initial conditions include assigning values ​​to mineral mechanical parameters, assigning values ​​to the mechanical property parameters of all grains according to the mineral type. These mechanical property parameters include Young's modulus, Poisson's ratio, cohesion, internal friction angle, tensile strength, and density. The boundary conditions include mechanical boundary conditions and water content boundary conditions. The mechanical boundary conditions are set based on the reservoir in-situ stress obtained in step S1, simulating confining pressure by applying surface forces to the left and right boundaries of the model. The water content boundary conditions are set based on the water content-time relationship obtained experimentally in step S4, with a water content boundary condition set on the upper boundary of the model.

[0067] A numerical model of cracking caused by hydration expansion of clay minerals was established based on FDEM. In FDEM, the solution domain was divided into numerous triangular and joint elements. The stress-strain response of the triangular elements characterized solid deformation, while the fracture of the joint elements characterized crack initiation and propagation. The cracking model mainly includes the water diffusion equation and the clay expansion control equation. Within the FDEM computational framework, the water content distribution in multi-mineral rocks was calculated based on the following water diffusion equation:

[0068]

[0069] Due to changes in the water content of the rock, clay minerals expand, resulting in expansion strain. The expansion strain can be calculated using the following formula:

[0070]

[0071] The stress increment caused by clay expansion can be calculated based on the expansion strain, using the following formula:

[0072]

[0073]

[0074] In the formula, For expansion strain; α is the expansion stress increment; A is the expansion coefficient; A is the area of ​​the triangular element; x i w is the distance along the i-th direction; w is the water content; w0 is the initial water content of the rock; w (t) The water content of the rock after water absorption time t; is the calculation coefficient; K* is the modulus; K is the bulk modulus; G is the shear modulus.

[0075] Step S6: Conduct numerical tests on shale hydration-induced cracking to obtain the morphology of microcrack propagation induced by clay mineral hydration expansion.

[0076] Based on the numerical model of clay mineral hydration expansion and fracturing established in step S5, numerical experiments on shale hydration-induced fracturing were conducted. The numerical experiments can output in real time the fracture morphology map, displacement field distribution cloud map, and stress field distribution cloud map corresponding to each time step during the hydration process. In addition, statistical parameters of the propagation morphology of hydration microcracks can be obtained, including the type, number, length, area, and maximum crack width of the microcracks.

[0077] Application Examples

[0078] Taking a shale reservoir in a block of the Sichuan Basin as an example, the minimum horizontal principal stress of the target stratum is 59-63 MPa, with an average of 61.2 MPa; the formation pressure is 46.1 MPa; the average porosity is 5.7%; and the permeability is 0.21 mD. Test rock samples were prepared from cores of the target stratum, and whole-rock mineral analysis was conducted. The average value of the corresponding parameters from multiple core samples was taken as the target parameter value. The mineral composition and content of the target shale are shown in Table 1. Based on the mineral composition and content, a polycrystalline geometric model of the minerals was constructed using random Voronoi mosaic technology. The grains were classified according to the rock mineral composition and content, and the area ratio of all grains of the same mineral reflected the volume content of that mineral.

[0079] Table 1 Geomechanical and physical property parameters of the target layer

[0080]

[0081] Three standard core columns were prepared using cores from the target stratum, and immersion experiments were conducted at hydration times of 3h, 9h, 24h, 48h, and 72h. The water content of the rock samples at different immersion times was calculated by obtaining the mass of the water-bearing rock sample after immersion and the mass of the dried rock sample before immersion. The relationship between different immersion times and water content was obtained by conducting three parallel experiments and taking the average value. Figure 4 As shown.

[0082] Based on the constructed polycrystalline mineral geometric model, initial and boundary conditions are set, such as... Figure 4 As shown in Table 2, the initial conditions to be set include assigning values ​​to mineral mechanical parameters. Based on the mineral type, values ​​are assigned to the mechanical properties of all grains, including Young's modulus, Poisson's ratio, cohesion, internal friction angle, tensile strength, and density. Specific input parameters are shown in Table 2. The boundary conditions to be set include mechanical boundary conditions and water content boundary conditions. The mechanical boundary conditions are set based on the effective stress of the target reservoir, which is the difference between the minimum horizontal principal stress and the formation pressure. This effective stress is simulated by applying surface forces of 15 MPa to the left and right boundaries of the model. The water content boundary conditions are set based on the experimentally obtained water content-soaking time relationship, with a water content boundary condition set on the upper boundary of the model. Based on the above settings, a numerical model for hydration expansion-induced fracturing of clay minerals is established.

[0083] Table 2. Analog Basic Input Parameters

[0084]

[0085] Numerical experiments on shale hydration-induced cracking were conducted based on FDEM to simulate the uneven stress distribution caused by clay expansion during hydration, which leads to the initiation and propagation of microcracks. Figure 5 The calculated results include crack morphology maps, displacement field distribution cloud maps, and stress field distribution cloud maps, where red represents tensile joints, green represents shear joints, and blue represents tension-shear joints. Figure 5 It is evident that during hydration, water flows in from the upper boundary. Since fluids flow more easily along mineral interfaces, the humidity field diagram shows an uneven advancement of the humidity front. Simultaneously, the expansion stress generated by the clay minerals absorbing water leads to an uneven distribution of the stress field. The crack morphology reveals that hydration is more pronounced near the humidity boundary, resulting in greater expansion stress from clay expansion. Consequently, hydration cracks near the humidity boundary are more numerous, randomly distributed, but shorter, and also generate numerous shear cracks at the mineral boundaries. Conversely, hydration cracks farther from the humidity boundary are longer and more directional, extending along the direction perpendicular to the minimum horizontal principal stress.

[0086] Figure 6Statistical parameters related to hydration microcracks under different expansion coefficients are presented. The results show that with increasing expansion coefficient, the number of cracks, crack length, crack area, and maximum crack width all increase. This is because a larger expansion coefficient results in greater expansion stress generated by the clay absorbing water and expanding after the water content increases, leading to more hydration microcracks and a more significant hydration expansion-induced cracking phenomenon.

[0087] Based on the same inventive concept, and corresponding to the above-described application function implementation method embodiments, the present invention also provides an FDEM simulation system for mineral-scale shale hydration-induced fracturing and corresponding embodiments.

[0088] See Figure 7 , Figure 7 A schematic diagram of the module structure of an FDEM simulation system for mineral-scale shale hydration-induced fracturing. The system includes:

[0089] The data acquisition unit is used to acquire the changes in target reservoir stress, rock porosity and permeability, mineral composition, content and water content over time.

[0090] The geometric model building unit is used to construct a polycrystalline geometric model of a mineral using experimentally obtained mineral composition and content parameters and random Voronoi mosaic technique.

[0091] The numerical modeling unit is used to set the initial and boundary conditions of the model and to establish a numerical model of clay mineral hydration expansion and cracking based on the finite discrete element method.

[0092] The numerical simulation unit is used to conduct numerical tests on shale hydration-induced cracking and to obtain the propagation morphology of microcracks induced by the hydration expansion of clay minerals.

[0093] Regarding the system in the above embodiments, the specific manner in which each unit module performs operations has been described in detail in the embodiments related to the method, and will not be repeated here.

[0094] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A FDEM simulation method for hydration-induced fracturing in mineral-scale shale, characterized in that, Includes the following steps: Step S1: Obtain the target reservoir stress, rock porosity, and permeability; Step S2: Prepare rock samples from the target stratum and obtain the mineral composition and content of the rocks by X-ray diffraction experiments; Step S3: Construct a polycrystalline geometric model of the mineral using random Voronoi mosaic technique; Step S4: Conduct core immersion hydration experiments to obtain the relationship between core water content and time; Step S5: Set the initial and boundary conditions of the model, and establish a numerical model of clay mineral hydration expansion-induced cracking based on the finite discrete element method; Step S6: Conduct numerical tests on shale hydration-induced cracking to obtain the morphology of microcrack propagation induced by clay mineral hydration expansion.

2. The FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 1, wherein step S3, the step of constructing the polycrystalline mineral numerical model, includes: N seed points are randomly generated within the solution domain; Based on the seed point location, a polygrain model is generated using Voronoi mosaic technology, the grains are numbered, and the area of ​​each grain is calculated. The entire solution domain is discretized into a large number of triangular element meshes using the Delaunay triangulation method; The grains are classified according to the rock mineral composition and content obtained in step S2.

3. The FDEM simulation method for mineral-scale shale hydration-induced fracturing according to claim 1, wherein step S4, the core immersion hydration experiment includes: Several standard core columns were prepared, dried at low temperature, and the mass m0 of the dried rock samples was measured. Soak the dried rock sample in deionized water; The rock samples were removed after different periods of immersion, cleaned and dried, and the mass of the water-containing rock samples was measured after different periods of immersion m (t) ; Obtain the water content w of rock samples under different soaking times (t) ; Rock sample water content w (t The method for obtaining it is as follows: 。 4. In the FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 1, step S5 includes setting the initial conditions of the model, which includes assigning values ​​to mineral mechanical parameters and assigning values ​​to the mechanical properties of all grains according to the mineral type. The mechanical properties include Young's modulus, Poisson's ratio, cohesion, internal friction angle, tensile strength, and density.

5. The FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 1, in step S5, the model boundary conditions are set including mechanical boundary conditions and water content boundary conditions. The mechanical boundary conditions are set according to the reservoir in-situ stress obtained in S1, and the confining pressure is simulated by applying surface forces to the left and right boundaries of the model. The water content boundary conditions are set according to the water content-time relationship obtained experimentally in S4, and water content variation boundary conditions are set on the upper boundary of the model.

6. In the FDEM simulation method for hydration-induced cracking of shale at the mineral scale according to claim 1, in step S5, the solution domain is divided into a large number of triangular elements and joint elements in the FDEM. The stress-strain response of the triangular elements is used to characterize solid deformation, and the fracture of the joint elements is used to characterize crack initiation and propagation.

7. In the FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 1, in step S5, the clay mineral hydration-swelling fracturing model mainly includes the water diffusion equation and the clay swelling control equation. Under the FDEM calculation framework, the water content distribution in multi-mineral rocks is calculated according to the water diffusion equation: Due to changes in the water content of the rock, clay minerals expand, resulting in expansion strain. The expansion strain can be calculated using the following formula: The stress increment caused by clay expansion is calculated based on the expansion strain. The calculation method is as follows: In the formula, For expansion strain; α is the expansion stress increment; A is the expansion coefficient; A is the area of ​​the triangular element; x i w is the distance along the i-th direction; w is the water content; w0 is the initial water content of the rock; w (t) The water content of the rock after water absorption time t; is the calculation coefficient; K* is the modulus; K is the bulk modulus; G is the shear modulus.

8. In the FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 1, in step S6, numerical tests on shale hydration-induced fracturing are carried out based on the established polycrystalline mineral geometric model, and the fracture morphology map, displacement field distribution cloud map, and stress field distribution cloud map are output.

9. The FDEM simulation method for hydration-induced fracturing of shale at the mineral scale according to claim 8, wherein the morphological statistical parameters of the fracture morphology map include the type, number, length, area, and maximum fracture width of the microcracks.

10. An FDEM simulation system for mineral-scale shale hydration-induced fracturing, characterized in that, include: The data acquisition unit is used to acquire the changes in target reservoir stress, rock porosity and permeability, mineral composition, content and water content over time. The geometric model building unit is used to construct a polycrystalline geometric model of a mineral using experimentally obtained mineral composition and content parameters and random Voronoi mosaic technique. The numerical modeling unit is used to set the initial and boundary conditions of the model and to establish a numerical model of clay mineral hydration expansion and cracking based on the finite discrete element method. The numerical simulation unit is used to conduct numerical tests on shale hydration-induced cracking and to obtain the propagation morphology of microcracks induced by the hydration expansion of clay minerals.

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