Hydraulic fracturing horizon and method for determining same, method for determining hydraulic fracturing position

Abstract

The application provides a hydraulic fracturing horizon and a determination method thereof and a hydraulic fracturing horizon determination method, relates to the field of coal mining, and aims to solve the technical problem of low accuracy in determining the hydraulic fracturing horizon. The hydraulic fracturing horizon determination method comprises the following steps: obtaining rock sample information of a rock sample, wherein the rock sample information comprises rock sample mechanical parameters, ground stress parameters and rock layer parameters; establishing a three-dimensional elastic-plastic numerical model according to the rock sample mechanical parameters, the ground stress parameters and the rock layer parameters, wherein the three-dimensional elastic-plastic numerical model comprises a plurality of rock layers arranged in a stack; performing iterative calculation on the three-dimensional elastic-plastic numerical model to obtain unbalanced force distribution information of the three-dimensional elastic-plastic numerical model; and determining the hydraulic fracturing horizon according to the unbalanced force distribution information, wherein the unbalanced force in a lower region of the hydraulic fracturing horizon is greater than the unbalanced force in an upper region.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of coal mining, and in particular to a hydraulic fracturing horizon and a method for determining the hydraulic fracturing horizon and a method for determining the position of the hydraulic fracturing. BACKGROUND

[0002] Hydraulic fracturing technology has been widely used in coal mining, and its principle is to use high-pressure liquid to fracture rock. When the pressure of the liquid injected into the well exceeds the stress of the stratum and the tensile strength of the rock, the rock will crack and form a fracture, thereby pre-releasing the stress of the mining working face.

[0003] There is a roof formed by sandstone, conglomerate or limestone above the coal seam. Such a roof has a large thickness, high strength, and poor joint fissure development, and has strong integrity and self-bearing capacity. The "joint fissure" refers to a fracture structure of a rock mass along a fracture surface without significant displacement. When the coal seam is mined, the roof will be exposed in the goaf in a large area. When the roof collapses for the first time, the collapse area is large, which will cause a sudden increase in the pressure of the working face, forming the initial roof pressure of the working face. After the initial roof pressure of the working face occurs, pressure conduction will further occur. After a certain conduction time and distance, the roof pressure will reoccur, forming strong periodic pressure, which will cause dynamic phenomenon disasters such as rock burst and mine earthquake. Lightly, it will cause equipment damage, and heavily, it will cause serious accidents endangering personal safety. Through hydraulic fracturing technology, part of the rock stratum can be pre-fractured to avoid excessive pressure on the roof and cause safety accidents. Therefore, determining the appropriate hydraulic fracturing horizon plays a crucial role in controlling the pressure and improving the safety of mining operations. It should be noted that the "goaf" refers to a "cavity" produced by human excavation or natural geological movement under the ground surface.

[0004] However, the collapse of the coal seam roof is a highly complex structural stability problem. In engineering practice, the existing methods for determining the hydraulic fracturing horizon mainly include the rock dilatancy coefficient method, the magnetotelluric method, and the key layer method. However, the above methods have low sensitivity to complex geological conditions and nonlinear effects, and the accuracy of determining the hydraulic fracturing horizon and the position of the hydraulic fracturing is low. SUMMARY

[0005] In view of the above problems, the embodiments of the present application provide a hydraulic fracturing horizon and a method for determining the hydraulic fracturing horizon and a method for determining the position of the hydraulic fracturing, which are used to improve the accuracy of determining the hydraulic fracturing horizon and the position of the hydraulic fracturing, reduce mine pressure disasters, and thereby improve the safety of coal mining operations.

[0006] To achieve the above object, the embodiments of the present application provide the following technical solutions:

[0007] The first aspect of the embodiments of the present application provides a method for determining a hydraulic fracturing horizon, comprising the following steps:

[0008] Obtain rock sample information, including rock sample mechanical parameters, geostress parameters, and rock strata parameters;

[0009] A three-dimensional elastoplastic numerical model is established based on the rock sample mechanical parameters, geostress parameters, and rock strata parameters. The three-dimensional elastoplastic numerical model includes multiple rock strata stacked together.

[0010] Iterative calculations were performed on the three-dimensional elastoplastic numerical model to obtain the unbalanced force distribution information of the three-dimensional elastoplastic numerical model;

[0011] Based on the information on the distribution of unbalanced forces, the hydraulic fracturing layer is determined. The unbalanced force in the lower region of the hydraulic fracturing layer is greater than that in the upper region.

[0012] The beneficial effects of the embodiments of this application are as follows: In the method for determining the hydraulic fracturing layer provided in the embodiments of this application, based on the deformation and reinforcement theory and the calculation of unbalanced forces, a three-dimensional elastoplastic numerical model of the rock sample is established, which can be well applied to the analysis of complex geological conditions and nonlinear structural effects, and accurately judge the disturbance of the rock sample; in the simulation of the rock sample mining process, compared with general numerical simulation methods (such as discrete element, traditional finite element, finite difference method, etc.), the method provided in the embodiments of this application has strong convergence, fast calculation speed, and can accurately show the location of rock layer failure and the rock layer movement process during the mining process; by calculating the distribution information of unbalanced forces during the rock sample mining process, the hydraulic fracturing layer can be accurately determined, thereby controlling the pressure more accurately and stably, and further determining the hydraulic fracturing location, improving the safety of coal mining operations.

[0013] In one possible implementation, the steps for iterative calculation of the three-dimensional elastoplastic numerical model include:

[0014] The stress field σ in the three-dimensional elastoplastic numerical model is calculated using the following formula:

[0015] ∑ e ∫ Ve B T σdV=F Equation (1)

[0016] Where F is the equivalent nodal force vector of the external load, B is the strain matrix of the three-dimensional elastoplastic numerical model, and V is the volume of the three-dimensional elastoplastic numerical model.

[0017] f(σ) is calculated based on the stress field σ. The formula is as follows:

[0018]

[0019] in, I1 = σ1 + σ2 + σ3, σ1, σ2, and σ3 are the principal stresses corresponding to σ. wherein, a is the material friction angle, c is the material cohesion, and f(σ) is used to determine whether the stress exceeds the yield surface;

[0020] calculating the unbalanced force field Δσ p , the calculation formula is:

[0021] Δσ p = nσ ab + pδ ab Equation (3)

[0022] wherein, m = a(3λ + 2μ), I1, J2 and f are obtained by calculation according to Equation (2), E is the Young's modulus of the rock sample, and v is the Poisson's ratio of the rock sample;

[0023] calculating the unbalanced force Q, the calculation formula is:

[0024] Q = ∑ e ∫ Ve B T Δσ p dV Equation (4)

[0025] The unbalanced force Q is the equivalent node force of the unbalanced force field Δσ p .

[0026] In a possible implementation, the three-dimensional elastoplastic numerical model further comprises a thin layer element arranged between each adjacent two rock layers;

[0027] The interlayer between the rock layers is simulated by the thin layer element, and the unbalanced force Q is distributed in each thin layer element.

[0028] In a possible implementation, the step of iteratively calculating the three-dimensional elastoplastic numerical model comprises:

[0029] comparing the unbalanced force Q with a first threshold T1, wherein the first threshold T1 is set as a real number greater than or equal to 0;

[0030] When the unbalanced force Q in the thin layer element is greater than the first threshold T1, the thin layer element is removed.

[0031] In a possible implementation, the rock sample mechanical parameters include an elastic modulus E, a density r, a Poisson's ratio v, a material friction angle a material cohesion c, and a uniaxial tensile strength R.

[0032] In a possible implementation, the in-situ stress parameters include at least one in-situ stress tensor of the rock sample.

[0033] Preferably, the in-situ stress parameters include multiple in-situ stress tensors of the rock sample.

[0034] In a possible implementation, the stratum parameters include a stratum horizon map, a stratum thickness, and a stratum lithology.

[0035] In a possible implementation, the step of determining the hydraulic fracturing horizon according to the unbalanced force distribution information includes:

[0036] In the three-dimensional elastic-plastic numerical model, s mining steps are set, and a scalar of the unbalanced force of the jth stratum at the ith mining step is denoted as Q ij An increment of the unbalanced force of the jth stratum at the tth mining step after the ith mining step is denoted as ΔQ ijt , wherein,

[0037] ΔQ ijt = Q (i+t)j - Q ij Equation (5)

[0038] In the equation, s, i, j, and t are all integers greater than 0.

[0039] All i, j, and t values satisfying the equation (6), the equation (7), and the equation (8) are obtained and denoted as an array A m , wherein A m = (i m , j m , t m ).

[0040] Q ij > 0 Equation (6)

[0041] 0 < ΔQ i(j-1)t < T2 Equation (7)

[0042] ΔQ i(j+1)t > T2 Equation (8)

[0043] In the equation, T2 is a second threshold value, and is a real number greater than or equal to 0.

[0044] In the array A m , when t takes the maximum value, the j value corresponding to the maximum value is the hydraulic fracturing horizon, and the position where the unbalanced force Q penetrates the jth stratum is the hydraulic fracturing position.

[0045] The hydraulic fracturing horizon provided in the second aspect of the embodiment of the present application is obtained by any of the above determination methods.

[0046] The third aspect of the embodiment of the present application provides a method for determining a hydraulic fracturing position, including any of the above determination methods, and the hydraulic fracturing position is the position where the unbalanced force Q penetrates the jth stratum.

[0047] The second and third aspects of the embodiments of the present application provide the water fracturing layer and the water fracturing position, which can guide the positioning of the roof part of the rock stratum in the mining operation in advance, accurately determine the water fracturing position, and make the part of the rock stratum to be fractured in advance to avoid the dynamic phenomenon such as mine shock caused by the excessive pressure of the roof, and greatly avoid the mine pressure disaster. BRIEF DESCRIPTION OF DRAWINGS

[0048] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0049] Figure 1 A three-dimensional elastic-plastic numerical model profile unbalanced force distribution cloud diagram when the mining distance is 90 meters is provided for the embodiments of the present application;

[0050] Figure 2 A three-dimensional elastic-plastic numerical model profile unbalanced force distribution cloud diagram when the mining distance is 100 meters is provided for the embodiments of the present application;

[0051] Figure 3 A three-dimensional elastic-plastic numerical model profile unbalanced force distribution cloud diagram when the mining distance is 110 meters is provided for the embodiments of the present application;

[0052] Figure 4 An unbalanced force development process schematic diagram when the mining distance is 90 meters is provided for the embodiments of the present application;

[0053] Figure 5 An unbalanced force development process schematic diagram when the mining distance is 100 meters is provided for the embodiments of the present application;

[0054] Figure 6 An unbalanced force development process schematic diagram when the mining distance is 110 meters is provided for the embodiments of the present application;

[0055] Figure 7 A flowchart of the method for determining the water fracturing layer provided for the embodiments of the present application is provided;

[0056] Figure 8 A flowchart of the single-step calculation of the unbalanced force in the mining process provided for the embodiments of the present application is provided;

[0057] Figure 9 A flowchart of the method for determining the water fracturing layer and position according to the unbalanced force increment provided for the embodiments of the present application is provided.

[0058] Reference signs:

[0059] 1 - area of influence of unbalanced forces; 2 - location and direction of extraction; 3 - specific horizon; 4 - specific location. DETAILED DESCRIPTION

[0060] As the background art, in the related art, the hydraulic fracturing technology is essential for coal mining, in which the hydraulic fracturing horizon of the rock mass roof is determined to control the coming pressure to play a crucial role in reducing the mine pressure disaster, but the related art has the problems of high technical cost and low accuracy of determining the hydraulic fracturing horizon and the hydraulic fracturing position. The inventor found that the reason for this problem is that in engineering practice, the hydraulic fracturing horizon is mainly determined by methods such as rock dilatancy coefficient method, magnetotelluric method and key layer method, the above-mentioned methods have low sensitivity to complex geological conditions and nonlinear effects, the accuracy of determining the hydraulic fracturing horizon and the hydraulic fracturing position is low, however, different hydraulic fracturing horizons have different effects on the coming pressure control including the initial coming pressure and / or the periodic coming pressure, improving the accuracy of determining the hydraulic fracturing horizon and the hydraulic fracturing position will reduce the peak value of the initial coming pressure and the periodic coming pressure, and thus improve the work safety of the staff.

[0061] In view of the above problem of low accuracy of determining the hydraulic fracturing horizon, the embodiment of the present application provides a method for determining the hydraulic fracturing horizon, and applies the deformation reinforcement theory to it. The deformation reinforcement theory is an extension and expansion of classical elastoplastic mechanics, which is based on non-equilibrium elastoplastic mechanics and takes the minimum plastic residual energy density principle as the theoretical core, and is suitable for dealing with the non-stable elastoplastic zone beyond the yield surface to study the case where the load exceeds the ultimate bearing capacity of the structure. In the deformation reinforcement theory, the difference between the equivalent node force of external load and the equivalent node force of internal force of the structure is called unbalanced force, and after the external load and the overall stability safety of the structure are given, the size and distribution of the unbalanced force can be obtained, so that the size, degree and stability degree of the damage area can be inferred according to the unbalanced force. By using the deformation reinforcement theory, the embodiment of the present application can simulate the possible continuity damage of the rock mass structure according to the distribution of the unbalanced force on the basis of the finite element elastoplastic analysis.

[0062] Furthermore, considering the deformation and failure characteristics of rock materials that differ from those predicted by classical plasticity theory, such as non-associated plastic flow and shear dilatation, deformation-strengthening theory studied the case where solutions do not exist during three-dimensional nonlinear finite element calculations. This showed that the absence of solutions implies that iterative calculations do not converge, and that there are unbalanced forces in the structure that cannot be transferred. The physical meaning of these unbalanced forces is the driving force for structural failure, and the reverse force of these unbalanced forces is the strengthening force required to prevent structural failure. Therefore, determining the distribution of unbalanced forces is crucial for the stability control of geotechnical engineering. The traditional three-dimensional finite element method's requirement for continuity makes it difficult to handle discontinuous failure processes such as roof collapse, often encountering calculation non-convergence problems. However, the deformation-strengthening theory method has the characteristic of first-order unconditional convergence. The unbalanced forces obtained through calculation simulation indicate the possible location and extent of failure. Combining deformation-strengthening theory with nonlinear finite element analysis to determine hydraulic fracturing layers significantly improves the accuracy of hydraulic fracturing layer determination. Moreover, the embodiments of this application lay a theoretical and methodological foundation for the study of roof rock movement and roof control mechanisms in underground caverns and coal mines, possessing high application value.

[0063] To better understand the embodiments of this application, the deformation reinforcement theory is further explained below:

[0064] Deformation strengthening theory studies loads exceeding the ultimate bearing capacity of a structure. Its starting point is to determine the magnitude and distribution of unbalanced forces given an external load and the overall structural stability safety factor. Based on this, the size and extent of the failure zone and overall stability can be inferred. As mentioned earlier, the reverse force of the unbalanced force is the strengthening force required to prevent structural failure. Therefore, the engineering application of deformation strengthening theory aims to determine the strengthening force required to maintain stability when a structure is subjected to loads exceeding its ultimate bearing capacity, thereby achieving stability control in geotechnical engineering. The region where unbalanced forces appear under a given load is the initial failure zone. Under a given load, the structure always tends towards a state of minimizing strengthening force and maximizing self-supporting force, and overall stability can be quantitatively evaluated using the complementary energy norm. Based on the three-dimensional nonlinear finite element method, expressions for unbalanced forces and the plastic complementary energy norm are derived. Thus, by combining the three-dimensional nonlinear finite element method and deformation strengthening theory, deformation stability analysis and safety evaluation of rock masses can be achieved.

[0065] In nonlinear finite element analysis, nonconvergence means that a solution does not exist. For a continuum structure, if a solution exists including both stress and displacement fields, it must simultaneously satisfy equilibrium conditions, deformation compatibility conditions, and constitutive relations. The nonlinear finite element analysis in this application is based on the displacement-based elastoplastic finite element analysis. In this displacement-based elastoplastic finite element analysis, the deformation compatibility conditions are naturally satisfied, and nonconvergence manifests as the stress fields satisfying the equilibrium conditions not fully satisfying the yield condition. For the Gaussian point stress σ1 that satisfies the equilibrium conditions, its equilibrium conditions are expressed in the finite element method as follows:

[0066] ∑ e ∫ Ve B T σ1 dV = F (Equation 1)

[0067] In Equation 1, F is the equivalent node force vector of external load, and B is the strain matrix. It should be noted that the Gauss point stress σ1 in the stress field cannot all satisfy the yield condition, i.e., there are cases where f(σ1) > 0 at some areas or Gauss point stresses σ1, where the mapping relationship satisfied by σ1 is:

[0068]

[0069] wherein, I1 = σ1 + σ2 + σ3, σ1, σ2, σ3 are the principal stresses corresponding to σ, is the material friction angle, and c is the material cohesion.

[0070] Specific to a Gauss point stress σ1, if f(σ1) > 0, the stress σ1 needs to be adjusted to the yield surface σ, and the difference between the stress before and after adjustment is the plastic stress Δσ p = σ1 - σ = D: Δe p where Δe p is the plastic strain increment. In elastic-plastic finite element analysis, the equivalent node force of Δσ p is the unbalanced force,

[0071] Q = ∑ e ∫ Ve B T Δσ p dV (Equation 3)

[0072] Substituting σ1 = σ + Δσ p and Equation (Equation 3) into the balance condition Equation 1 gives:

[0073] ∑ e ∫ Ve B T σdV + Q = F (Equation 4)

[0074] The adjusted stress field σ satisfies the yield condition completely, so its equivalent node force can be regarded as the self-bearing force of the structure. Formula 4 can be understood as the internal force of the structure at the node force level being the sum of the self-bearing force and the unbalanced force, i.e., self-bearing force + unbalanced force = external load. Formula 4 shows that, under the action of the external load F, if the structure cannot be self-stable (i.e., the structure cannot satisfy the yield condition completely), a reinforcing force equal in size and opposite in direction to the unbalanced force Q can be applied to the structure. At this time, the self-bearing force of the structure satisfies the yield condition completely, and the structure is stable. It can also be expressed as (self-bearing force satisfying the yield condition) = external load - unbalanced force. As can be seen from formula 3, the unbalanced force vector Q is a load vector generated by the initial strain Δε p , so the reinforcing force is a self-balanced force system. For a given external load, there are infinitely many combinations of the self-bearing force and the reinforcing force. In order to determine the true self-bearing force and the reinforcing force, for a structure based on ideal elastic-plastic material, there is the minimum plastic complementary energy principle: the true plastic stress field Δσ p must minimize the structural plastic complementary energy ΔE (i.e., the norm of the complementary energy), i.e.,

[0075]

[0076] where C is the compliance tensor. The plastic complementary energy ΔE is the norm of the unbalanced force, so the minimum plastic complementary energy principle requires the unbalanced force to be minimized.

[0077] Further, the relationship between the deformation reinforcement theory and the non-continuous failure of the structure is explained.

[0078] For a given load, the mechanical solution (including the displacement field, stress field, etc.) of the structure must satisfy the balance condition, the deformation compatibility condition, and the constitutive relationship. Exemplarily, the constitutive relationship includes the stress-strain relationship. For an elastic material structure, the mechanical solution of the structure always exists and is unique. However, for an elastic-plastic material structure, the mechanical solution of the structure may not exist or be infinitely many, because different materials have different allowable stresses. When the external force reaches the allowable stress of the material, the material can satisfy the yield condition. Therefore, the reason why the mechanical solution of the structure does not exist is that the yield condition cannot be satisfied. When the mechanical solution of the elastic-plastic material structure exists, the stress field of the structure satisfies the yield condition everywhere. At this time, the structure is stable, and the solution completely satisfies the balance condition, the deformation compatibility condition, and the constitutive relationship. When the mechanical solution of the structure does not exist, the stress field of the structure cannot satisfy the yield condition completely, and the structure is unstable.

[0079] It should be noted that the unbalanced force is an intermediate variable in the elastic-plastic finite element calculation, and is essentially a virtual force, which will not appear in actual engineering. If the unbalanced force appears in the finite element calculation of the structure, the unbalanced force will be released through structural damage, cracking, etc. in actual engineering. Therefore, calculating the unbalanced force of the structure is crucial for simulating and controlling the stability of the engineering structure.

[0080] In the finite element analysis of displacement form, the structure continuity condition and deformation compatibility condition are forced to be satisfied, and the discontinuity and non-coordination caused by local cracking of the structure cannot be reflected, resulting in non-convergent finite element calculation results and unbalanced forces. In actual engineering, if the structure is not reinforced to balance the unbalanced force, the structure can only eliminate the unbalanced force by generating damage cracking (changing the constitutive relation) at the unbalanced force, so as to weaken the forced deformation compatibility condition. Therefore, the unbalanced force is essentially an equivalent expression of the degree of material damage cracking.

[0081] The method of characterizing structural damage by unbalanced force can be analogous to the characterization of damage by effective stress in damage mechanics. In finite element calculation, the method of characterizing structural damage by unbalanced force requires fewer solution parameters, generally only stiffness and strength parameters, without additional damage evolution related parameters. Moreover, this method has the characteristics of first-order unconditional convergence, and can be well adapted to the engineering calculation of complex rock mass.

[0082] The method for determining the hydraulic fracturing horizon provided by the embodiments of the present application includes the following steps:

[0083] Obtain rock sample information of a rock sample, the rock sample information including rock sample mechanical parameters, ground stress parameters and rock layer parameters; establish a three-dimensional elastic-plastic numerical model according to the rock sample mechanical parameters, the ground stress parameters and the rock layer parameters, the three-dimensional elastic-plastic numerical model including a plurality of rock layers arranged in layers; perform iterative calculation on the three-dimensional elastic-plastic numerical model to obtain unbalanced force distribution information of the three-dimensional elastic-plastic numerical model; and determine the hydraulic fracturing horizon according to the unbalanced force distribution information, the unbalanced force in a lower region of the hydraulic fracturing horizon being greater than the unbalanced force in an upper region. In this way, the analysis sensitivity to complex geological conditions and nonlinear effects in complex geology is improved, and the accuracy of determining the hydraulic fracturing horizon and the hydraulic fracturing position is improved. The method has a guiding effect on good control of the weighting in coal mining, and thus the work safety of the workers is improved.

[0084] In some possible embodiments, the method for determining the hydraulic fracturing horizon includes the following steps:

[0085] Rock mechanics tests are carried out on the rock mass samples. For the working face with conditions, the rock core obtained from the geological exploration drilling hole can be directly used as the rock sample. It needs to be explained that the "working face with conditions" refers to the rock mass surface that can directly obtain rock information from the existing geological exploration drilling hole columnar graph; for the general working face, the rock sample can be obtained according to the lithological parameters, such as the color, composition, structure, cementation fabric characteristics, cementation type and whether it is a special mineral of the rock, and then the rock sample is obtained in the roadway around the working face to carry out the rock mechanics test. The rock mechanics test includes uniaxial compression test, triaxial compression test and Brazilian splitting test. The parameters obtained by the rock mechanics test include the elastic modulus E, density r, Poisson's ratio v, internal friction angle cohesion c and uniaxial tensile strength R, so as to provide necessary parameter conditions for the establishment and calculation of the subsequent three-dimensional elastoplastic numerical model.

[0086] It needs to be explained that the "working face" refers to the working area formed within the scope of the direct activity of the coal mining collection work; and the "geological exploration drilling hole columnar graph" is an engineering geological map prepared by describing the layer, thickness, lithology, structural features and contact relationship of the rock layer penetrated by the drilling hole, underground water sampling and test, drilling structure and drilling, etc.

[0087] The in-situ stress test is carried out on the rock sample working face to be tested. The method used can be stress contact method or water pressure cracking method or stress recovery method, so as to obtain at least one initial in-situ stress tensor of the working face to be tested, so as to realize the in-situ stress inversion through the initial in-situ stress tensor;

[0088] Preferably, a plurality of in-situ stress tensors are obtained from the rock sample working face to be tested, so that the in-situ stress information is enriched, the accuracy of the numerical model is improved, and the in-situ stress inversion is more accurate.

[0089] It needs to be explained that the "in-situ stress" refers to the stress existing in the earth's crust caused by the mining of the working face. The "in-situ stress inversion" refers to constructing a three-dimensional geological generalization model according to the actual geographical location of the engineering area, combining with the in-situ stress influencing factors, and constructing the initial in-situ stress field of the engineering area by means of data analysis software through limited measured in-situ stress data.

[0090] A three-dimensional elastoplastic numerical model was established based on rock mass information obtained from geological exploration borehole columnar sections, geostress information obtained from geostress tests, and parameters from rock mechanics tests. The rock mass information obtained from the geological exploration borehole columnar sections includes stratigraphic maps, strata thickness, and lithology. Combined with rock mechanics parameters obtained from rock mechanics tests, a three-dimensional finite element model of the rock mass working face was established using the TFINE finite element software. Then, based on the geostress test results, the lateral pressure coefficient was adjusted segmentally to obtain the initial geostress field, thereby establishing a three-dimensional finite element elastoplastic numerical model including the initial geostress field.

[0091] In some embodiments, thin-layer elements are provided at the interfaces between rock strata in the three-dimensional finite element elastoplastic numerical model, which facilitates the simulation and calculation of delamination between rock strata.

[0092] As will be understood by those skilled in the art, the "lateral pressure coefficient" refers to the ratio of horizontal compressive stress to vertical compressive stress based on the working surface.

[0093] Based on deformation-strengthening theory, a three-dimensional finite element elastoplastic numerical model is used to calculate the rock mass mining process, obtaining the stress-strain state of the rock mass. After the model stabilizes during the mining process, the unbalanced force field Δσ is obtained. p The distribution of its equivalent nodal forces, i.e., unbalanced forces Q, and the calculation process for the single step length are as follows:

[0094] ① Calculate the stress field σ of the three-dimensional elastoplastic numerical model. The calculation formula is:

[0095] ∑ e ∫ Ve B T σdV=F Equation (1)

[0096] Where F is the equivalent nodal force vector of the external load, B is the strain matrix of the three-dimensional elastoplastic numerical model, and V is the volume of the three-dimensional elastoplastic numerical model.

[0097] ② Calculate f(σ) based on the stress field σ. The calculation formula is:

[0098]

[0099] in, I1 = σ1 + σ2 + σ3, σ1, σ2, and σ3 are the principal stresses corresponding to σ. σ is the friction angle of the material, c is the cohesive force of the material, and f(σ) is used to determine whether the stress exceeds the yield surface.

[0100] It should be noted that equation (2) can be any form of yield criterion.

[0101] ③For Gaussian point stress f(σ)>0, calculate the unbalanced force field Δσ p , the calculation formula is:

[0102] Δσ p = nσ ab + pδ ab Formula (3)

[0103] Wherein, m = α(3λ + 2μ), I1, J2 and f are obtained by formula (2), E is the Young's modulus of the rock sample material, and v is the Poisson's ratio of the rock sample material;

[0104] ④Calculate the unbalanced force Q, and the calculation formula is:

[0105] Q = ∑ e ∫ Ve B T Δσ p dV Formula (4)

[0106] The unbalanced force Q is the equivalent node force of the unbalanced force field Δσ p ;

[0107] ⑤Take the unbalanced force Q as a criterion, and iteratively calculate in combination with the interlayer separation situation. If there is an interlayer thin layer element with an unbalanced force Q greater than a threshold value T1, the interlayer thin layer element is removed from the model, the model after removing the interlayer thin layer element is taken as a new model, and the calculation of steps ①-④ is repeated, and the iterative calculation is performed until there is no interlayer thin layer element with an unbalanced force Q greater than the threshold value T1, and the single-step calculation is ended. Wherein, the threshold value T1 is a real number greater than or equal to 0.

[0108] ⑥Increase the mining step length, and repeat the calculation of steps ①-⑤.

[0109] (5) According to the unbalanced force field distribution of the three-dimensional finite element elastoplastic numerical model, the hydraulic fracturing layer position is determined. In the mining process, the hydraulic fracturing layer position determined by the embodiment of the application is the rock layer with unbalanced force aggregation at the lower part and continuous at the upper part, and the hydraulic fracturing position determined by the embodiment of the application is the position of the unbalanced force penetrating the rock layer. Wherein, the determination process of the hydraulic fracturing design layer and the hydraulic fracturing position is as follows:

[0110] Suppose that the model calculation has s equal-step mining processes, use the letter i to represent the i-th mining, and use the letter j to represent the j-th rock layer.

[0111] ①Take the scalar sum of the unbalanced force of the j-th rock layer in the i-th mining step length as Q ij , and take the increment of the unbalanced force of the j-th rock layer after the i-th mining step length to the t-th mining step length as ΔQ ijt , wherein,

[0112] ΔQ ijt =Q (i+t)j -Q ij Equation (5)

[0113] It should be noted that s, i, j, t are all integers greater than 0;

[0114] ② Set a judgment threshold T2, T2 is a real number greater than or equal to 0. Get all i, j, t values, so that the following three formulas are met at the same time:

[0115] Q ij > 0 Equation (6)

[0116] 0 < ΔQ i(j-1)t < T2 Equation (7)

[0117] ΔQ i(j+1)t > T2 Equation (8)

[0118] It should be noted that the i, j, t values that meet the above conditions are denoted as an array A m = (i m , j m , t m ).

[0119] ③ In the array A m , get the maximum value t max of t, and all j values corresponding to the maximum value t max are the hydraulic fracturing design horizons determined by the present application. The position where the unbalanced force distribution area first penetrates the jth layer is the hydraulic fracturing position determined by the present application.

[0120] In this way, the prediction and guidance of the calculation results of the three-dimensional finite element elastoplasticity numerical model are used, and in actual engineering, combined with the unbalanced force cloud map and the site exploration situation, the accurate hydraulic fracturing horizon and fracturing position can be finally determined, the stable control of the roof pressure is realized, and the safety of the mining operation is improved.

[0121] In order to better understand the present application, the following will be combined with Figures 1 to 9 to describe the determination of the hydraulic fracturing horizon and the hydraulic fracturing position, wherein the unbalanced force distribution area can be represented as the failure range of the mining rock mass.

[0122] Combined with Figure 1 and Figure 4 , as the mining range increases, the unbalanced force influence area 1 continuously extends along the mining direction of the rock stratum and above it, the range of the damaged rock stratum continuously expands, further, such as Figure 2 and Figure 5As shown, with the continuous increase of the mining range, a specific horizon 3 appears above the mining stratum, at this time, the specific horizon 3 hinders the extension of the unbalanced force, the unbalanced force influence area 1 no longer extends upwards with the mining process, that is, the unbalanced force influence area 1 only extends in the mining direction, and in the specific horizon 3, the unbalanced force in the upper area is smaller than that in the lower area, that is, the unbalanced force in the upper area of the specific horizon 3 is smaller and the unbalanced force in the lower area of the specific horizon 3 is larger, a concentrated unbalanced force field appears, the lower part of the stratum presents structural discontinuity, and a separation occurs, which is specifically manifested as a long-span separation area appearing in the lower part of the specific horizon 3. For example, as shown, the lower part of the specific horizon 3 is separated, and the upper part remains continuous, at this time, the specific horizon 3 bears the stress of the stratum above the specific horizon 3, at this time, the specific horizon 3 is a stress concentration area, it needs to be noted that in actual mining, the suspended range of the lower part of the roof is long, the stress concentration is significant, and the mechanical properties are relatively weak, at this time, once the stratum is fractured, more energy than the conventional stratum will be released, therefore, it is necessary to perform hydraulic fracturing in advance to make the relatively weak stratum fracture in advance to avoid safety accidents caused by dynamic phenomena such as mine shock caused by excessive pressure bearing of the roof, therefore, the specific horizon 3 is the hydraulic fracturing horizon determined by the embodiments of the present application, further, the specific position 4 through which the unbalanced force penetrates the specific horizon 3 is the hydraulic fracturing position determined by the embodiments of the present application. Figure 5 For example, as shown, the lower part of the specific horizon 3 is separated, and the upper part remains continuous, at this time, the specific horizon 3 bears the stress of the stratum above the specific horizon 3, at this time, the specific horizon 3 is a stress concentration area, it needs to be noted that in actual mining, the suspended range of the lower part of the roof is long, the stress concentration is significant, and the mechanical properties are relatively weak, at this time, once the stratum is fractured, more energy than the conventional stratum will be released, therefore, it is necessary to perform hydraulic fracturing in advance to make the relatively weak stratum fracture in advance to avoid safety accidents caused by dynamic phenomena such as mine shock caused by excessive pressure bearing of the roof, therefore, the specific horizon 3 is the hydraulic fracturing horizon determined by the embodiments of the present application, further, the specific position 4 through which the unbalanced force penetrates the specific horizon 3 is the hydraulic fracturing position determined by the embodiments of the present application.

[0123] In combination with the above-mentioned formula (1) and formula (2), the specific horizon 3 is determined as the hydraulic fracturing horizon, and the specific position 4 through which the unbalanced force penetrates the specific horizon 3 is determined as the hydraulic fracturing position. Figure 3 and Figure 6 When the mining distance increases to 110 meters, the unbalanced force area 1 breaks through the specific horizon 3 and continues to extend upwards in the stratum, that is, the specific horizon 3 cannot continue to hinder the extension of the unbalanced force upwards in the stratum, until the next specific horizon appears with the increase of the mining distance, the extension of the unbalanced force upwards in the stratum is inhibited again.

[0124] In some embodiments, the mining step length is increased, a plurality of hydraulic fracturing horizons and hydraulic fracturing positions are determined, so that the operation continuity and efficiency are improved under the need of controlling the roof pressure during mining.

[0125] It needs to be noted that the formula (6), formula (7) and formula (8) are quantitative criteria for determining the above-mentioned hydraulic fracturing horizon and hydraulic fracturing position, when the criterion formula (6), formula (7) and formula (8) are satisfied and k is taken as the maximum value, it indicates that in the maximum continuous step length, the specific horizon 3 hinders the extension of the unbalanced force upwards in the stratum, at this time, the specific horizon 3 is the hydraulic fracturing horizon determined by the embodiments of the present application, and the specific position 4 through which the unbalanced force penetrates the specific horizon 3 is the hydraulic fracturing position determined by the embodiments of the present application.

[0126] The embodiments or examples in the specification are described in a progressive manner, and each embodiment focuses on the difference from other embodiments. The same or similar parts between embodiments can be mutually referred to.

[0127] It should be noted that the embodiments referred to in the specification as "one embodiment", "an embodiment", "example embodiment", "some embodiments" and the like can include a specific feature, structure or characteristic, but not necessarily every embodiment. In addition, such phrases do not necessarily refer to the same embodiment. In addition, when a specific feature, structure or characteristic is described in conjunction with an embodiment, it is within the knowledge of those skilled in the art to implement such feature, structure or characteristic in conjunction with other embodiments that are explicitly or implicitly described.

[0128] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and such modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for determining the hydraulic fracturing layer, characterized in that, Includes the following steps: Obtain rock sample information, including rock sample mechanical parameters, geostress parameters, and rock stratum parameters; A three-dimensional elastoplastic numerical model is established based on the rock sample mechanical parameters, geostress parameters, and rock strata parameters. The three-dimensional elastoplastic numerical model includes multiple rock strata stacked together. The unbalanced force distribution information of the three-dimensional elastoplastic numerical model is obtained by iterative calculation of the three-dimensional elastoplastic numerical model. Based on the unbalanced force distribution information, the hydraulic fracturing layer is determined, wherein the unbalanced force in the lower region of the hydraulic fracturing layer is greater than the unbalanced force in the upper region. The step of determining the hydraulic fracturing zone based on the unbalanced force distribution information includes: In the three-dimensional elastoplastic numerical model, s mining steps are set, and the scalar sum of the unbalance forces of the j-th rock layer at the i-th mining step is denoted as Q. ij Let ΔQ be the increment of the unbalanced force of the j-th rock layer at the t-th mining step after the i-th mining step. ijt ,in, ΔQ ijt = Q (i+t)j - Q ij Equation (5) In the formula, s, i, j, and t are all integers greater than 0; Obtain all i, j, t values ​​that simultaneously satisfy formulas (6), (7), and (8), and denote them as array A. m , where A m =(i m j m , t m ); Q ij >0 Equation (6) 0<ΔQ i(j-1)t <T2 formula (7) ΔQ i(j+1)t >T2 (8) Where T2 is the second threshold, which is a real number greater than or equal to 0; In the array A m In the above, the value of j corresponding to the maximum value of t is the hydraulic fracturing layer position, and the position where the unbalanced force Q penetrates the j-th layer is the hydraulic fracturing position.

2. The determination method according to claim 1, characterized in that, The steps for iterative calculation of the three-dimensional elastoplastic numerical model include: The stress field σ of the three-dimensional elastoplastic numerical model is calculated using the following formula: ∑ e ∫ Ve B T σdV=F Equation (1) Where F is the equivalent nodal force vector of the external load, B is the strain matrix of the three-dimensional elastoplastic numerical model, and V is the volume of the three-dimensional elastoplastic numerical model. The formula for calculating f(σ) is as follows: in, I1 = σ1 + σ2 + σ3, σ1, σ2, and σ3 are the principal stresses corresponding to σ. σ is the material friction angle, c is the material cohesion, and f(σ) is used to determine whether the stress exceeds the yield surface. Calculate the unbalanced force field Δσ p The calculation formula is: Board p =nσ ab +pδ ab expression(3) in, m=α(3λ+2μ), I1, J2 and f are calculated from the above equation (2), where E is the Young's modulus of the rock sample material and v is the Poisson's ratio of the rock sample material; The formula for calculating the unbalanced force Q is as follows: Q=∑ e ∫ Ve B T Board p dV formula(4) The unbalanced force Q is the unbalanced force field Δσ. p The equivalent nodal force.

3. The determination method according to claim 2, characterized in that, The three-dimensional elastoplastic numerical model also includes thin-layer units set between each two adjacent rock layers; The thin-layer unit simulates the delamination between rock strata, and the unbalanced force Q is distributed in each of the thin-layer units.

4. The determination method according to claim 3, characterized in that, The steps for iterative calculation of the three-dimensional elastoplastic numerical model include: Compare the unbalanced force Q with a first threshold T1, wherein the first threshold T1 is set to a real number greater than or equal to 0; When the unbalanced force Q in the thin-layer unit is greater than the first threshold T1, the thin-layer unit is removed.

5. The determination method according to claim 1, characterized in that, The mechanical parameters of the rock sample include elastic modulus E, density r, Poisson's ratio v, and internal friction angle. Cohesion c and uniaxial tensile strength R.

6. The determination method according to claim 1, characterized in that, The geostress parameters include at least one geostress tensor of the rock sample; The geostress parameters include multiple geostress tensors of the rock sample.

7. The determination method according to claim 1, characterized in that, The rock strata parameters include the strata location map, strata thickness, and strata lithology.

8. A method for determining the location of hydraulic fracturing, characterized in that, The method of determination includes any one of claims 1-7, wherein the hydraulic fracturing position is the position where the unbalanced force Q penetrates the j-th layer.

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