A numerical simulation method for enhanced gas extraction using hydraulic perforation in coal seams

Abstract

This invention provides a numerical simulation method for enhanced gas drainage in coal seams using hydraulic perforation, belonging to the field of coal seam gas drainage simulation technology. The method first derives the permeability evolution equation during coal body damage. Then, based on coal mine geological conditions, it uses Flac3D software to establish coal seam and hydraulic perforation models, and defines the permeability parameter evolution equation after coal body damage using Fish language, assigning the permeability after coal body damage back to the coal seam and hydraulic perforation models. For the coal seam and hydraulic perforation models, the permeability around the hydraulic perforation borehole is extracted. A Flac3D-COMSOL coupled hydraulic perforation coal body damage permeability model is established. Finally, a Flac3D-COMSOL coupled hydraulic perforation gas drainage model is established and solved to obtain gas pressure and gas content. This method combines the advantages of both Flac3D and COMSOL software, realizing numerical simulation of enhanced gas drainage using hydraulic perforation.

Description

Technical Field

[0001] This invention relates to the field of coal seam gas extraction simulation technology, and in particular to a numerical simulation method for enhanced coal seam gas extraction using hydraulic perforation. Background Technology

[0002] With the depletion of shallow coal resources, deep mining has become an inevitable trend. Deep coal mining faces a series of challenges, including high ground stress, poor coal seam permeability, and high gas content. The mining environment in deep coal mines is more complex, leading to increasingly severe coal and gas outbursts. Gas drainage is a crucial means of preventing and controlling coal and gas outbursts. However, for some low-permeability coal seams, conventional drilling cannot achieve gas pre-drainage. Therefore, hydraulic perforation to enhance gas drainage technology has been proposed. The principle of hydraulic perforation is that it alters the stress path of the coal seam surrounding the tunnel, causing damage and increasing the permeability of the coal seam. This increases the velocity of gas migration within the coal seam, thereby improving gas drainage efficiency. Numerical simulation of hydraulic perforation-enhanced gas drainage can reproduce the construction and gas drainage processes in coal mines, allowing for the optimization of hydraulic perforation and gas drainage techniques, which is particularly important for the prevention and control of coal and gas outbursts. Flac3D software has the advantages of being able to calculate large deformation processes and offering a large open space, but its drawbacks include the inability to perform gas-water two-phase flow calculations and difficulties in multiphysics coupling simulations. COMSOL software can perform multiphysics coupling simulations, but it struggles with large solid deformation calculations. Calculating the fracturing process of coal and rock using damage analysis requires combining it with MATLAB software, making the process very complex. Therefore, this invention combines the advantages of both software programs and proposes a numerical simulation method for enhanced gas extraction using hydraulic perforation in coal seams. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a numerical simulation method for enhanced gas extraction by hydraulic perforation in coal seams, which addresses the shortcomings of the prior art and realizes Flac3D-COMSOL coupled simulation of enhanced gas extraction by hydraulic perforation.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by this invention is: a numerical simulation method for enhanced gas drainage in coal seams using hydraulic perforation, deriving the permeability evolution equation during coal body damage, constructing a hydraulic perforation damage permeability model and a gas drainage solution model based on Flac3D and COMSOL software, and coupling the two to achieve enhanced gas drainage; specifically including the following steps:

[0005] Step 1: Establish the evolution equation of permeability parameters during coal body damage;

[0006] Step 1.1: Define the coal damage equation;

[0007] Based on damage mechanics, the relationship between stress and strain in coal during the damage softening process is determined as shown in the following formula:

[0008] σ=(1-D)E0ε (1)

[0009] In the formula: σ is the stress of the coal body, MPa; D is the damage variable of the coal body; E0 is the initial elastic modulus, MPa; ε is the strain of the coal body;

[0010] Based on plasticity theory, plastic strain is generated when coal yields and fails; when coal is damaged, its volume expands; coal failure modes can be divided into shear failure and tensile failure, where shear failure causes plastic shear strain and tensile failure causes plastic tensile strain, the sum of which is called plastic strain; therefore, coal damage D is defined as:

[0011]

[0012] Where: ε t For plastic tensile strain; ε s θ is the plastic shear strain; θ is the shear dilatation angle.

[0013] Step 1.2: Establish the evolution equation of permeability parameters during coal body damage;

[0014] Coal body damage leads to increased permeability. Based on the definition of damage, the relationship between coal body damage and permeability is determined as shown in the following formula:

[0015] k=k0 exp(αD) (3)

[0016] In the formula: k is the permeability of the coal body after damage, m 2 k0 is the initial permeability, m 2 α is the fit index;

[0017] Substituting equation (2) into equation (3) yields the relationship between plastic strain and permeability, and the evolution equation of permeability parameter is constructed:

[0018]

[0019] Step 2: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models, and define the evolution equation of permeability parameters after coal body damage based on Fish language, and assign the permeability after coal body damage back to the coal seam and hydraulic perforation models.

[0020] Step 2.1: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models;

[0021] Step 2.2: Define the permeability function k() after coal body damage based on Fish language;

[0022] Step 2.3: Extract the plastic tensile strain ε from the coal seam and hydraulic perforation models by calling the built-in function zone.prop() in Flac3D software. t Plastic shear strain ε s and the shear dilatation angle parameter θ;

[0023] Step 2.4: Define a global variable t = zone.prop(zone, ε) t ), s = zone.prop(zone, ε s ), j = zone.prop(zone,θ) and initial permeability k0, where zone represents a cell;

[0024] Step 2.5: Use Fish language to write the permeability parameter evolution equation from Step 1 as follows: math.abs(k0*math.exp(α*(t+s*math.sin(j) / 1+t+s*math.sin(j))));

[0025] Step 2.6: Use the built-in back-call function zone.fluid.prop() in Flac3D software to assign the permeability after coal body damage back to the coal seam and hydraulic perforation model;

[0026] Step 3: Extract the permeability around the hydraulic perforation borehole for the coal seam and hydraulic perforation model;

[0027] Step 3.1: Set up a strain softening constitutive model for the constructed coal seam and hydraulic perforation model, setting parameters such as cohesion, internal friction angle, elastic modulus, shear modulus, and permeability. Apply overlying strata load to the upper part of the coal seam and hydraulic perforation model, use normal displacement constraints around the model, and set fixed boundary constraints at the bottom of the model. Set the geostress and gravity for the coal seam and hydraulic perforation model, and perform static equilibrium solution. After solving for equilibrium, clear the displacement of the model to zero, set the hydraulic perforation part as a null constitutive model, simulate hydraulic perforation, call the permeability function after coal damage defined in Step 2, and solve to obtain the permeability of the surrounding coal after hydraulic perforation.

[0028] Step 3.2: Use the built-in back call function zone.fluid.prop() of Flac3D software to extract the permeability of the hydraulically perforated coal body after damage, and save the grid node coordinates and corresponding permeability of the coal seam and hydraulic perforation model as a txt file;

[0029] Step 4: Establish a Flac3D-COMSOL coupled hydraulic perforation coal body damage permeability model;

[0030] Step 4.1: Use COMSOL software to create a hydraulic perforation mesh model of the same size as the coal seam and hydraulic perforation model created in Flac3D software;

[0031] Step 4.2: Import the txt file saved in Step 3.3 into the hydraulic perforation mesh model created in COMSOL software. The txt file contains the mesh node coordinates x, z and the corresponding permeability. In COMSOL software, set variables, import the txt file, and name it the function int1(x, z). Use the interpolation function in COMSOL software to assign the function int1 to the hydraulic perforation mesh model to update the permeability, thus realizing the coupling between FLAC3D and COMSOL.

[0032] Step 5: Establish a Flac3D-COMSOL coupled hydraulic perforation gas extraction model and solve the coupled hydraulic perforation coal body damage permeability model to propose gas pressure and gas content;

[0033] Step 5.1: Based on the hydraulic perforation mesh model established in COMSOL software, set the gas parameters and add physical fields, including the matrix gas seepage equation and the coal seam fracture water and gas diffusion equation; set the extraction boundary conditions and set the extraction negative pressure on the hydraulic perforation surface to realize the simulation of enhanced gas extraction by hydraulic perforation.

[0034] Step 5.2: Solve the coupled hydraulic permeation coal body damage permeability model from Step 4 according to the conditions set in Step 5.1 to obtain the gas pressure and gas content.

[0035] The beneficial effects of adopting the above technical solution are as follows: The numerical simulation method for enhanced gas extraction using hydraulic perforation in coal seams provided by this invention leverages the advantages of Flac3D software, which features large deformation and a large open space, but has the drawback of being unable to simulate two-phase flow and multiphysics coupling simulation. COMSOL software can achieve multiphysics coupling, but it cannot handle large deformations, and damage calculation requires integration with MATLAB, making the operation complex. Therefore, the method of this invention combines the advantages of both software programs, realizing the numerical simulation of enhanced gas extraction using hydraulic perforation. Attached Figure Description

[0036] Figure 1 A flowchart of a numerical simulation method for enhanced gas extraction using hydraulic perforation in coal seams, provided for the implementation of this invention;

[0037] Figure 2 Damage calculation distribution cloud map provided for the implementation of this invention;

[0038] Figure 3 The post-damage permeability int1(x, z) function is provided for the implementation of this invention;

[0039] Figure 4 Permeability damage distribution cloud map provided for the implementation of this invention;

[0040] Figure 5 Gas pressure cloud map for hydraulic perforation enhanced gas extraction provided for the implementation of this invention;

[0041] Figure 6 A cloud map of gas content in hydraulic perforation enhanced gas extraction provided for the implementation of this invention;

[0042] Figure 7 The gas pressure distribution curve after hydraulic perforation-enhanced gas extraction provided for the implementation of this invention;

[0043] Figure 8 The gas content distribution curve after hydraulic perforation enhanced gas extraction provided for the implementation of this invention;

[0044] Figure 9 A comparison curve of gas pressure between hydraulically perforated enhanced gas and conventionally extracted gas, provided for the implementation of this invention. Detailed Implementation

[0045] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0046] This embodiment takes the enhanced gas extraction through hydraulic perforation in a coal seam as an example. The numerical simulation method of enhanced gas extraction through hydraulic perforation in coal seam of the present invention is used to realize enhanced gas extraction. The gas pressure and gas content extracted after hydraulic perforation are extracted, providing technical guidance for the subsequent hydraulic perforation hole spacing.

[0047] In this embodiment, a numerical simulation method for enhanced gas drainage using hydraulic perforation in coal seams is proposed. The method derives the evolution equation of permeability parameters during coal seam damage, constructs a hydraulic perforation damage permeability model and a gas drainage solution model based on Flac3D and COMSOL software, and couples these two models to achieve enhanced gas drainage. Figure 1 As shown, the specific steps include:

[0048] Step 1: Establish the evolution equation of permeability parameters during coal body damage;

[0049] Step 1.1: Define the coal damage equation;

[0050] Based on damage mechanics, the relationship between stress and strain in coal during the damage softening process is determined as shown in the following formula:

[0051] σ=(1-D)E0ε (1)

[0052] In the formula: σ is the stress of the coal body, MPa; D is the damage variable of the coal body; E0 is the initial elastic modulus, MPa; ε is the strain of the coal body;

[0053] Based on plasticity theory, plastic strain is generated when coal yields and fails; when coal is damaged, its volume expands; coal failure modes can be divided into shear failure and tensile failure, where shear failure causes plastic shear strain and tensile failure causes plastic tensile strain, the sum of which is called plastic strain; therefore, coal damage D is defined as:

[0054]

[0055] Where: ε t For plastic tensile strain; ε s θ is the plastic shear strain; θ is the shear dilatation angle.

[0056] Step 1.2: Establish the evolution equation of permeability parameters during coal body damage;

[0057] Coal body damage leads to increased permeability. Based on the definition of damage, the relationship between coal body damage and permeability is determined as shown in the following formula:

[0058] k=k0 exp(αD) (3)

[0059] In the formula: k is the permeability of the coal body after damage, m 2 k0 is the initial permeability, m 2 α is the fit index obtained from the experiment;

[0060] Substituting equation (2) into equation (3) yields the relationship between plastic strain and permeability, and the evolution equation of permeability parameter is constructed:

[0061]

[0062] Step 2: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models, and define the evolution equation of permeability parameters after coal body damage based on Fish language, and assign the permeability after coal body damage back to the coal seam and hydraulic perforation models.

[0063] Step 2.1: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models;

[0064] Step 2.2: Define the permeability function k() after coal body damage based on Fish language;

[0065] Step 2.3: Extract the plastic tensile strain ε from the coal seam and hydraulic perforation models by calling the built-in function zone.prop() in Flac3D software. t Plastic shear strain εs and the shear dilatation angle parameter θ;

[0066] Step 2.4: Define a global variable t = zone.prop(zone, ε) t ), s = zone.prop(zone, ε s ), j = zone.prop(zone,θ) and initial permeability k0, where zone represents a cell;

[0067] Step 2.5: Use Fish language to write the permeability parameter evolution equation from Step 1 as follows: math.abs(k0*math.exp(α*(t+s*math.sin(j) / 1+t+s*math.sin(j))));

[0068] Step 2.6: Use the built-in back-call function zone.fluid.prop() in Flac3D software to assign the permeability after coal body damage back to the coal seam and hydraulic perforation model;

[0069] In this embodiment, the parameter settings for establishing the coal seam and hydraulic perforation model using Flac3D software are shown in Table 1.

[0070] Table 1. Parameter settings for coal seam and hydraulic perforation models in Flac3D software.

[0071] Parameter name Numerical <![CDATA[Density / kg·cm -3 > 1400 Shear modulus / GPa 3.2 Elastic modulus / GPa 2.2 Cohesion / MPa 1.9 Friction angle / (°) 32 Shear expansion angle / (°) 10 <![CDATA[Permeability / m 2 > <![CDATA[8e -19 ]]>

[0072] Step 3: Extract the permeability around the hydraulic perforation borehole for the coal seam and hydraulic perforation model;

[0073] Step 3.1: Set up a strain softening constitutive model for the constructed coal seam and hydraulic perforation model, setting parameters such as cohesion, internal friction angle, elastic modulus, shear modulus, and permeability. Apply overlying strata load to the upper part of the coal seam and hydraulic perforation model, use normal displacement constraints around the model, and set fixed boundary constraints at the bottom of the model. Set the geostress and gravity for the coal seam and hydraulic perforation model, and perform static equilibrium solution. After solving for equilibrium, clear the displacement of the model to zero, set the hydraulic perforation part as a null constitutive model, simulate hydraulic perforation, call the permeability function after coal damage defined in Step 2, and solve to obtain the permeability of the surrounding coal after hydraulic perforation.

[0074] Step 3.2: Use the built-in back call function zone.fluid.prop() of Flac3D software to extract the permeability of the hydraulically perforated coal body after damage, and save the grid node coordinates and corresponding permeability of the coal seam and hydraulic perforation model as a txt file;

[0075] In this embodiment, the hydraulic perforation section is excavated, and the damage is calculated. The hydraulic perforation damage distribution calculated by FLAC3D software is as follows: Figure 2 As shown. Step 4: Establish a Flac3D-COMSOL coupled hydraulic perforation coal body damage permeability model;

[0076] Step 4.1: Use COMSOL software to create a hydraulic perforation mesh model of the same size as the coal seam and hydraulic perforation model created in Flac3D software;

[0077] Step 4.2: Import the txt file saved in Step 3.3 into the hydraulic perforation mesh model created in COMSOL software. The txt file contains the mesh node coordinates x, z and the corresponding permeability. In COMSOL software, set variables, import the txt file, and name it the function int1(x, z). Use the interpolation function in COMSOL software to assign the function int1 to the hydraulic perforation mesh model to update the permeability, thus realizing the coupling between FLAC3D and COMSOL.

[0078] In this embodiment, the function int1(x, y) is as follows: Figure 3 shown

[0079] Step 5: Establish a Flac3D-COMSOL coupled hydraulic perforation gas extraction model and solve the coupled hydraulic perforation coal body damage permeability model to propose gas pressure and gas content;

[0080] Step 5.1: Based on the hydraulic perforation mesh model established in COMSOL software, set the gas parameters and add physical fields, including the matrix gas seepage equation and the coal seam fracture water and gas diffusion equation; set the extraction boundary conditions and set the extraction negative pressure on the hydraulic perforation surface to realize the simulation of enhanced gas extraction by hydraulic perforation.

[0081] Step 5.2: Solve the coupled hydraulic permeation coal body damage permeability model from Step 4 according to the conditions set in Step 5.1 to obtain the gas pressure and gas content.

[0082] In this embodiment, a Flac3D-COMSOL coupled hydraulic perforation extraction model is established, using the same constitutive model as the coal seam and hydraulic perforation model established in FLAC3D software. The model parameters are consistent with those in the coal seam and hydraulic perforation model established in FLAC3D software. The permeability distribution of the coal body after damage in the COMSOL software is shown below. Figure 4 As shown in Table 2, the gas field settings parameters in COMSOL software are as follows.

[0083] Table 2. Parameter settings for COMOSL software numerical simulation

[0084]

[0085]

[0086] In this embodiment, based on the established hydraulic perforation mesh model, a solid mechanics module and the matrix gas seepage equation are added to construct a Flac3D-COMSOL coupled hydraulic perforation extraction model. A negative pressure is set on the hydraulic perforation surface, a mesh is generated, and gas extraction is solved to obtain a cloud map of gas pressure and extraction time after enhanced gas extraction via hydraulic perforation, as shown below. Figure 5 As shown in the diagram. The relationship between gas content and extraction time is illustrated in the cloud map. Figure 6 As shown.

[0087] In the Flac3D-COMSOL coupled hydraulic perforation extraction model, survey lines A(-3, 0)-B(3, 0) are arranged to extract gas pressure distribution data, such as... Figure 7 As shown. The distribution of extracted gas content is as follows. Figure 8 As shown. Figure 9 The numerical simulation results of gas extraction using the hydraulic perforation enhanced permeability method of this invention and the gas extraction effect of normal borehole drilling are demonstrated respectively. At position 1.5, the gas pressure of gas extraction using the hydraulic perforation enhanced permeability method of this invention is reduced by 42.6 times compared with that of gas extraction using normal borehole drilling.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the claims of the present invention.

Claims

1. A numerical simulation method for enhanced gas extraction using hydraulic perforation in coal seams, characterized in that: The permeability evolution equation during coal seam damage was derived. A hydraulic permeation damage permeability model and a gas drainage solution model were constructed based on Flac3D and COMSOL software, and the two were coupled to achieve enhanced gas drainage. The specific steps include: Step 1: Establish the evolution equation of permeability parameters during coal body damage; Step 1.1: Define the coal damage equation; Based on damage mechanics, the relationship between stress and strain in coal during the damage softening process is determined as shown in the following formula: （1）； In the formula: σ is the stress of the coal body, MPa; D is the damage variable of the coal body; E0 is the initial elastic modulus, MPa; ε is the strain of the coal body; Based on plasticity theory, plastic strain is generated when coal yields and fails; when coal is damaged, its volume expands; coal failure modes can be divided into shear failure and tensile failure, where shear failure causes plastic shear strain and tensile failure causes plastic tensile strain, the sum of which is called plastic strain; therefore, coal damage D is defined as: （2）； Where: ε t For plastic tensile strain; ε s θ is the plastic shear strain; θ is the shear dilatation angle. Step 1.2: Establish the evolution equation of permeability parameters during coal body damage; Coal body damage leads to increased permeability. Based on the definition of damage, the relationship between coal body damage and permeability is determined as shown in the following formula: （3）； In the formula: k is the permeability of the coal body after damage, m 2 k0 is the initial permeability, m 2 α is the fit index; Substituting equation (2) into equation (3) yields the relationship between plastic strain and permeability, and the evolution equation of permeability parameter is constructed: （4）； Step 2: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models, and define the evolution equation of permeability parameters after coal body damage based on Fish language, and assign the permeability after coal body damage back to the coal seam and hydraulic perforation models. Step 2.1: Based on the geological conditions of the coal mine, use Flac3D software to establish coal seam and hydraulic perforation models; Step 2.2: Define the permeability function k() after coal body damage based on Fish language; Step 2.3: Extract the plastic tensile strain ε in the coal seam and hydraulic perforation models by calling the built-in function zone.prop() in Flac3D software. t Plastic shear strain ε s and the shear dilatation angle parameter θ; Step 2.4: Define a global variable t = zone.prop(zone, ε) t ), s = zone.prop(zone, ε s ), j=zone.prop(zone, θ) and initial permeability k0, where zone represents a cell; Step 2.5: Use Fish language to write the permeability parameter evolution equation from Step 1 as: math.abs(k0*math.exp(α*(t+s*math.sin(j) / 1+t+s*math.sin(j)))); Step 2.6: Use the built-in back-call function zone.fluid.prop() in Flac3D software to assign the permeability after coal body damage back to the coal seam and hydraulic perforation model; Step 3: Extract the permeability around the hydraulic perforation borehole for the coal seam and hydraulic perforation model; Step 4: Establish a Flac3D-COMSOL coupled hydraulic perforation coal body damage permeability model; Step 5: Establish a Flac3D-COMSOL coupled hydraulic perforation gas extraction model and solve the coupled hydraulic perforation coal body damage permeability model to propose gas pressure and gas content.

2. The numerical simulation method for enhanced gas extraction in coal seams using hydraulic perforation as described in claim 1, characterized in that: The specific method for step 3 is as follows: Step 3.1: Set up a strain softening constitutive model for the constructed coal seam and hydraulic perforation model, setting parameters such as cohesion, internal friction angle, elastic modulus, shear modulus, and permeability. Apply overlying strata load to the upper part of the coal seam and hydraulic perforation model, use normal displacement constraints around the model, and set fixed boundary constraints at the bottom of the model. Set the geostress and gravity for the coal seam and hydraulic perforation model, and perform static equilibrium solution. After solving for equilibrium, clear the displacement of the model to zero, set the hydraulic perforation part as a null constitutive model, simulate hydraulic perforation, call the permeability function after coal damage defined in Step 2, and solve to obtain the permeability of the surrounding coal after hydraulic perforation. Step 3.2: Use the built-in back call function zone.fluid.prop() in Flac3D software to extract the permeability of the hydraulically perforated coal body after damage, and save the grid node coordinates and corresponding permeability of the coal seam and hydraulic perforation model as a txt file.

3. The numerical simulation method for enhanced gas extraction in coal seams using hydraulic perforation as described in claim 2, characterized in that: The specific method for step 4 is as follows: Step 4.1: Use COMSOL software to create a hydraulic perforation mesh model of the same size as the coal seam and hydraulic perforation model created in Flac3D software; Step 4.2: Import the txt file saved in Step 3.2 into the hydraulic perforation mesh model created in COMSOL software. The txt file contains the mesh node coordinates x, z and the corresponding permeability. In COMSOL software, set variables, import the txt file, and name it the function int1(x, z). Use the interpolation function in COMSOL software to assign the function int1 to the hydraulic perforation mesh model to update the permeability, thus realizing the coupling between FLAC3D and COMSOL.

4. The numerical simulation method for enhanced gas extraction in coal seams using hydraulic perforation as described in claim 3, characterized in that: The specific method for step 5 is as follows: Step 5.1: Based on the hydraulic perforation mesh model established in COMSOL software, set the gas parameters and add physical fields, including the matrix gas seepage equation and the coal seam fracture water and gas diffusion equation. Set extraction boundary conditions and set extraction negative pressure on the surface of hydraulic perforation to realize the simulation of enhanced gas extraction by hydraulic perforation. Step 5.2: Solve the coupled hydraulic permeation coal body damage permeability model from Step 4 according to the conditions set in Step 5.1 to obtain the gas pressure and gas content.

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