Coal bed gas reservoir rock physical modeling method and system based on VTI medium

Abstract

The invention discloses a VTI medium-based coalbed methane reservoir rock physical modeling method and system, and belongs to the technical field of unconventional oil and gas resource exploration, and the method comprises the following steps: determining mineral components and content of a coal matrix, and calculating an equivalent stiffness coefficient of the coal matrix by using an anisotropic self-consistent model; adding the adsorbed gas into the coal matrix, and calculating an equivalent stiffness coefficient of the coal matrix containing the adsorbed gas; determining components and contents of brittle minerals, adding the brittle minerals into the adsorption gas-containing coal matrix, and calculating an equivalent stiffness coefficient of a coal matrix dry skeleton; adding pores and fractures into the coal matrix dry skeleton, and calculating an equivalent stiffness coefficient of the pore and fracture-containing coal matrix dry skeleton; free gas is added into the pore-fissure-containing coal matrix dry skeleton, and the equivalent stiffness coefficient of the coal bed gas reservoir is calculated; and calculating the longitudinal wave velocity, the transverse wave velocity, the density and the Thomsen anisotropy parameter of the coalbed methane reservoir rock based on the modeling result. The method can improve the prediction precision, and provides theoretical guidance for seismic inversion.

Description

Technical Field

[0001] The present invention relates to the technical field of unconventional oil and gas resource exploration, and in particular to a coalbed methane reservoir rock physics modeling method and system based on VTI medium, which is used to quantitatively predict the elastic parameters and anisotropic characteristics of the coalbed methane reservoir. Background Art

[0002] As an unconventional energy source, coalbed methane reservoirs have a dual pore system where adsorbed gas and free gas coexist, and exhibit significant anisotropy due to the influence of sedimentation and tectonic action. Existing reservoir rock physics modeling mainly focuses on solving two problems: on the one hand, based on the constructed seismic rock physics model, forward simulation is used to analyze the pore and fracture structure characteristics, brittle characteristics, anisotropy characteristics, and seismic parameter response characteristics of the reservoir; on the other hand, driven by the constructed reservoir rock physics model, the inversion prediction and evaluation of reservoir physical parameters (mineral composition, fluid type, pore and fracture parameters, etc.) are achieved based on seismic observation data. Existing rock physics models are mostly targeted at conventional reservoirs and do not comprehensively consider the complex pore structure and anisotropy characteristics of coalbed methane, resulting in insufficient prediction accuracy. In addition, the existing technology lacks a quantitative description of the Thomsen anisotropy parameter, which restricts the accuracy of seismic inversion. Summary of the Invention

[0003] Technical problems to be solved: In response to the problems faced by coalbed methane reservoir rock physics modeling and parameter prediction in the existing technology, the present invention provides a coalbed methane reservoir rock physics modeling method and system based on VTI medium, which accurately characterizes the anisotropic characteristics of the coalbed methane reservoir through the VTI medium model; integrates the dual pore system of adsorbed gas and free gas to improve prediction accuracy; and provides a quantitative calculation formula for Thomsen parameters to provide theoretical guidance for seismic inversion.

[0004] Technical solution: The present invention provides a method for rock physics modeling of coalbed methane reservoirs based on VTI media, comprising the following steps: Step 1: Determine the mineral composition and content of the coal matrix, and calculate the equivalent stiffness coefficient of the coal matrix using the anisotropic self-consistent model; Step 2: Using the anisotropic differential equivalent medium model, add the adsorbed gas into the coal matrix and calculate the equivalent stiffness coefficient of the coal matrix containing the adsorbed gas; Step 3: Determine the composition and content of brittle minerals, calculate their bulk modulus and shear modulus based on the Voigt-Reuss-Hill boundary theory, and add the brittle minerals to the adsorbed gas coal matrix using the anisotropic DEM model to calculate the equivalent stiffness coefficient of the coal matrix dry skeleton; Step 4: Using the anisotropic DEM model, add pores and cracks to the dry skeleton of the coal matrix and calculate the equivalent stiffness coefficient of the dry skeleton of the coal matrix containing pores and cracks; Step 5: Using the Brown-Korringa anisotropic fluid replacement theory, free gas is added to the dry skeleton of the porous and fractured coal matrix to calculate the equivalent stiffness coefficient of the coalbed methane reservoir; Step 6: Based on the modeling results of steps 1 to 5, calculate the P-wave velocity, S-wave velocity, density and Thomsen anisotropy parameters of the CBM reservoir rock.

[0005] Preferably, the mineral components of the coal matrix in step 1 include organic matter and clay, and the sum of their percentages is 1; the calculation formula of the anisotropic self-consistent model is as follows: ; in, ; Where: is the equivalent stiffness coefficient matrix of coal matrix; 1 is the stiffness coefficient matrix of organic matter; 2 is the stiffness coefficient matrix of clay; v toc and v clay are the percentages of organic matter and clay, respectively; I ijkl is a fourth-order unit tensor; ijkl is the Eshelby tensor related to the pore morphology of the inclusion.

[0006] Preferably, the relative porosity of the adsorbed gas in step 2 is satisfy , is the relative porosity of free gas, is the total porosity; and the differential equation of the anisotropic DEM model is: ; Where: is the equivalent stiffness coefficient matrix of coal matrix containing adsorbed gas; is the stiffness coefficient matrix of adsorbed gas.

[0007] Preferably, the brittle minerals in step 3 include calcite, quartz and pyrite, and their bulk modulus and shear modulus are calculated by the Voigt-Reuss-Hill formula as follows: ; ; Where: K VRH is the bulk modulus of brittle minerals; μ VRHis the shear modulus of brittle minerals; K1, µ1, and v1 are the bulk modulus, shear modulus, and percentage of calcite, respectively; K2, µ2, and v2 are the bulk modulus, shear modulus, and percentage of quartz, respectively; K3, µ3, and v3 are the bulk modulus, shear modulus, and percentage of pyrite, respectively; The calculation formula of the equivalent stiffness coefficient of the coal matrix dry skeleton is as follows: ; Where: is the equivalent stiffness coefficient matrix of the coal matrix dry skeleton; is the percentage of brittle minerals; is the stiffness coefficient matrix of brittle minerals.

[0008] Preferably, the differential equation of the anisotropic DEM model in step 4 is: ; Where: is the equivalent stiffness coefficient matrix of the dry skeleton of the porous and fractured coal matrix; is the stiffness coefficient matrix of the empty crack.

[0009] Preferably, the calculation formula of the equivalent stiffness coefficient matrix of the coalbed methane reservoir in step 5 is as follows: ; in: ; ; Where: is the equivalent flexibility coefficient of the coalbed methane reservoir; 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the dry skeleton of the porous and fractured coal matrix; m 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the coal matrix respectively; β f and β m are the compressibility coefficients of free gas and rock skeleton respectively; K f is the bulk modulus of the free gas.

[0010] Preferably, in step 5, the equivalent flexibility matrix of the coalbed methane reservoir is inverted and converted into the equivalent stiffness matrix of the coalbed methane reservoir: ; in: VTI is the equivalent stiffness matrix of the coalbed methane reservoir, and its specific expression is: ; Where: C 11 、C13 、C 33 、C 44 、C 66 Both are equivalent stiffness coefficients of coalbed methane reservoirs.

[0011] Preferably, in step 6, the five independent equivalent stiffness coefficients of the equivalent stiffness matrix of the coalbed methane reservoir are used to calculate the longitudinal wave and shear wave velocities of the coalbed methane reservoir, and the calculation formula is as follows: ; ; ; Where: is the density of the coalbed methane reservoir; is the vertical velocity of the longitudinal wave, is the horizontal velocity of the longitudinal wave; is the SV wave vertical velocity, is the horizontal velocity of SV wave; is the SH wave vertical velocity, is the horizontal velocity of SH wave; The calculation formula of the density of coalbed methane reservoir is as follows: ; Where: are the percentage and density of organic matter, respectively; are the percentage and density of clay, respectively; are the percentage and density of calcite respectively; are the percentage and density of quartz respectively; are the percentage and density of pyrite respectively; is the adsorbed gas density; is the free gas density.

[0012] Preferably, the calculation formula of the Thomsen anisotropy parameter of the coalbed methane reservoir in step 6 is as follows: ; ; ; Where: ε is the anisotropy parameter of longitudinal waves; γ is the anisotropy parameter of transverse waves; δ is the anisotropy parameter of longitudinal waves and SV waves.

[0013] The present invention also discloses a coalbed methane reservoir rock physics modeling system based on VTI media, comprising: Several groups of computing modules, configured to execute steps 1 to 6 according to any one of claims 1 to 9; Data input module, used to receive mineral composition, porosity and physical property parameters; Output module, used to generate equivalent elastic parameters and anisotropic parameters of coalbed methane reservoirs.

[0014] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention targets the anisotropic characteristics of coalbed methane reservoirs and their unique dual pore system of adsorbed gas and free gas, constructs a seismic rock physics model of the gas reservoir based on VTI media, accurately calculates the equivalent stiffness coefficient, density and Thomsen anisotropy parameters of the coalbed methane reservoir rock physics model, and improves prediction accuracy; and provides theoretical guidance for the forward simulation analysis of the seismic response of coalbed methane reservoirs and the inversion prediction and evaluation of reservoir physical property parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the modeling process of the present invention; Figure 2 1. The graphs of elastic parameters observed by well logging of coalbed methane reservoirs with depth in an embodiment of the present invention ((a) is a curve showing a change in the longitudinal wave velocity; (b) is a curve showing a change in the shear wave velocity; (c) is a curve showing a change in the density; (d) is a curve showing a change in the porosity; (e) is a curve showing a change in the water saturation; and (f) is a curve showing a change in the shale content). Figure 3 The following are curves showing changes in the elastic parameters of the coalbed methane reservoir logging observation data and the predicted results with depth in the embodiment of the present application ((a) is the curve showing changes in the longitudinal wave velocity; (b) is the curve showing changes in the shear wave velocity; and (c) is the curve showing changes in density); Figure 4 The Thomsen anisotropy parameter prediction results of the coalbed methane reservoir in the embodiment of the present invention ((a) is the prediction result of the anisotropy parameter ε; (b) is the prediction result of the anisotropy parameter δ; (c) is the prediction result of the anisotropy parameter γ); Figure 5 This is a scatter plot of the predicted P-wave velocity and the predicted Thomsen anisotropy parameter ε of the coalbed methane reservoir in an embodiment of the present invention; Figure 6 This is a scatter plot of the predicted shear wave velocity and the predicted Thomsen anisotropy parameter γ of the coalbed methane reservoir in an embodiment of the present invention. DETAILED DESCRIPTION

[0016] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the following Figures 1 to 6 The technical solutions of the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field are within the scope of protection of the present invention.

[0017] Example 1: Figure 1 As shown, the present invention discloses a coalbed methane reservoir rock physics modeling method based on VTI medium, comprising the following steps: First, determine the mineral composition and content of the coal matrix, and calculate the equivalent stiffness coefficient of the coal matrix using an anisotropic self-consistent model; The mineral components of the coal matrix are composed of brittle minerals, organic matter and clay, the sum of their percentages is 1. Brittle minerals include calcite, quartz and pyrite.

[0018] Using the anisotropic self-consistent model, organic matter and clay are mixed to obtain a coal matrix. The calculation formula of the anisotropic self-consistent model for calculating the equivalent stiffness coefficient of the coal matrix is as follows: ; in, ; Where: is the equivalent stiffness coefficient matrix of coal matrix; 1 is the stiffness coefficient matrix of organic matter; 2 is the stiffness coefficient matrix of clay; v toc and v clay are the percentages of organic matter and clay, respectively; I ijkl is a fourth-order unit tensor; ijkl is the Eshelby tensor related to the pore morphology of the inclusion.

[0019] In the embodiment of the present invention, the rock physical parameters used are as follows: organic matter bulk modulus K toc =5 GPa, organic matter shear modulus μ toc = 2.5 GPa, clay bulk modulus K clay =25 GPa, clay shear modulus μ clay =9 GPa.

[0020] 2. Using the anisotropic differential equivalent medium model, the adsorbed gas is added to the coal matrix to calculate the equivalent stiffness coefficient of the coal matrix containing the adsorbed gas; the relative porosity of the adsorbed gas satisfy , is the relative porosity of free gas, is the total porosity; and the differential equation of the anisotropic DEM model is: ; Where: is the equivalent stiffness coefficient matrix of coal matrix containing adsorbed gas; is the stiffness coefficient matrix of adsorbed gas.

[0021] In the embodiment of the present invention, the rock physical parameters used are as follows: adsorbed gas bulk modulus K abs =7.5 GPa, adsorbed gas shear modulus μ abs =0 GPa.

[0022] Third, the composition and content of brittle minerals were determined, and their bulk modulus and shear modulus were calculated based on the Voigt-Reuss-Hill boundary theory. The brittle minerals were added to the adsorbed gas coal matrix through the anisotropic DEM model, and the equivalent stiffness coefficient of the dry skeleton of the coal matrix was calculated.

[0023] (1) Brittle minerals include calcite, quartz and pyrite. The sum of the relative contents of calcite, quartz and pyrite is the percentage of brittle minerals. The bulk modulus and shear modulus of brittle minerals are calculated using the Voigt-Reuss-Hill formula as follows: ; ; Where: K VRH is the bulk modulus of brittle minerals; μ VRH is the shear modulus of brittle minerals; K1, µ1, and v1 are the bulk modulus, shear modulus, and percentage of calcite, respectively; K2, µ2, and v2 are the bulk modulus, shear modulus, and percentage of quartz, respectively; K3, µ3, and v3 are the bulk modulus, shear modulus, and percentage of pyrite, respectively.

[0024] In the embodiment of the present invention, the rock physical parameters used are as follows: K1=37 GPa, K2=76.8 GPa, K3=147 GPa, μ1=44 GPa, μ2=32 GPa, μ3=133 GPa.

[0025] (2) The stiffness coefficient matrix of brittle minerals is calculated by the bulk modulus and shear modulus of brittle minerals. The anisotropic DEM model is used to add brittle minerals into the coal matrix containing adsorbed gas. The calculation formula of the equivalent stiffness coefficient of the dry skeleton of the coal matrix is as follows: ; Where: is the equivalent stiffness coefficient matrix of the coal matrix dry skeleton; is the percentage of brittle minerals; is the stiffness coefficient matrix of brittle minerals.

[0026] Fourth, using the anisotropic DEM model, pores and cracks are added to the dry skeleton of the coal matrix to calculate the equivalent stiffness coefficient of the dry skeleton of the coal matrix containing pores and cracks. The differential equation of the anisotropic DEM model is: ; Where: is the equivalent stiffness coefficient matrix of the dry skeleton of the porous and fractured coal matrix; is the stiffness coefficient matrix of the empty crack.

[0027] In the embodiment of the present invention, the rock physical parameters used are as follows: pore and fracture bulk modulus K empty =0 GPa, pore crack shear modulus μ empty =0 GPa.

[0028] 5. Using the Brown-Korringa anisotropic fluid substitution theory, free gas is added to the dry skeleton of the porous fractured coal matrix to calculate the equivalent stiffness coefficient of the coalbed methane reservoir. The calculation formula of the matrix is as follows: ; in: ; ; Where: is the equivalent flexibility coefficient of the coalbed methane reservoir; 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the dry skeleton of the porous and fractured coal matrix; m 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the coal matrix respectively; β f and β m are the compressibility coefficients of free gas and rock skeleton respectively; K f is the bulk modulus of the free gas.

[0029] In the embodiment of the present invention, the rock physical parameters used are as follows: free gas bulk modulus K f =0.003630 GPa, free gas shear modulus μ f =0 GPa.

[0030] The equivalent flexibility matrix of the coalbed methane reservoir is inverted and converted into the equivalent stiffness matrix of the coalbed methane reservoir: ; in: VTI is the equivalent stiffness matrix of the coalbed methane reservoir, and its specific expression is: ; Where: C 11 、C 13 、C 33 、C 44 、C 66 Both are equivalent stiffness coefficients of coalbed methane reservoirs.

[0031] 6. Based on the modeling results obtained above, the P-wave velocity, S-wave velocity, density and Thomsen anisotropy parameters of the CBM reservoir rock are calculated.

[0032] (1) The five independent equivalent stiffness coefficients of the equivalent stiffness matrix of the coalbed methane reservoir are used to calculate the longitudinal and shear wave velocities of the coalbed methane reservoir. The calculation formula is as follows: ; ; ; Where: is the density of the coalbed methane reservoir; is the vertical velocity of the longitudinal wave, is the horizontal velocity of the longitudinal wave; is the SV wave vertical velocity, is the horizontal velocity of SV wave; is the SH wave vertical velocity, is the horizontal velocity of SH wave; The calculation formula of the density of coalbed methane reservoir is as follows: ; Where: are the percentage and density of organic matter, respectively; are the percentage and density of clay, respectively; are the percentage and density of calcite respectively; are the percentage and density of quartz respectively; are the percentage and density of pyrite respectively; is the adsorbed gas density; is the free gas density.

[0033] In the embodiment of the present invention, the rock physical parameters used are as follows: ρ1=2.65 g / cm 3 ,ρ2=2.71g / cm 3 , ρ3=4.93 g / cm 3 , ϕ abs =0.77 g / cm 3 , ρ fre =0.000717 g / cm 3 .

[0034] (3) The calculation formula of the Thomsen anisotropy parameter of the coalbed methane reservoir is as follows: ; ; ; Where: ε is the anisotropy parameter of longitudinal waves; γ is the anisotropy parameter of transverse waves; δ is the anisotropy parameter of longitudinal waves and SV waves.

[0035] Example 2: The present invention also discloses a coalbed methane reservoir rock physics modeling system based on VTI media, comprising: The first calculation module is used to determine the mineral components and content of the coal matrix and calculate the equivalent stiffness coefficient of the coal matrix; The second calculation module is used to add the adsorbed gas into the coal matrix and calculate the equivalent stiffness coefficient of the coal matrix containing the adsorbed gas; The third calculation module is used to calculate the shear modulus and bulk modulus of brittle minerals, add the brittle minerals to the adsorbed gas-containing coal matrix, and calculate the equivalent stiffness coefficient of the dry skeleton of the coal matrix; The fourth calculation module is used to add pores and cracks into the coal matrix dry skeleton and calculate the equivalent stiffness coefficient of the coal matrix dry skeleton containing pores and cracks; The fifth calculation module is used to add free gas to the dry skeleton of the porous and fractured coal matrix to calculate the equivalent stiffness coefficient, density and Thomsen anisotropy parameter of the coalbed methane reservoir rock; Data input module, used to receive mineral composition, porosity and physical property parameters; Output module, used to generate equivalent elastic parameters and anisotropic parameters of coalbed methane reservoirs.

[0036] Figure 2 1 is a curve showing changes in elastic parameters of a coalbed methane reservoir observed by well logging with depth in an embodiment of the present invention ((a) is a curve showing changes in the longitudinal wave velocity; (b) is a curve showing changes in the shear wave velocity; (c) is a curve showing changes in density; (d) is a curve showing changes in porosity; (e) is a curve showing changes in water saturation; and (f) is a curve showing changes in mud content). Figure 3 The following are the curves of the coalbed methane reservoir elastic parameter logging observation data and prediction results with depth in the embodiment of the present application ((a) is the longitudinal wave velocity change curve; (b) is the shear wave velocity change curve; (c) is the density change curve); Figure 3 As shown in (a) and (b), the predicted results of P-wave velocity and S-wave velocity (dashed line) are consistent with the observed data (solid line) (average error < 5%), and the overall trend of change is basically the same. There is a certain deviation in the value, which may be caused by the inaccurate estimation of the bulk modulus and shear modulus of the brittle minerals in the coalbed methane reservoir. Figure 3 As shown in (c), the density prediction results (dashed line) and the observed data (solid line) have consistent changes and are numerically consistent. Figure 4These are the Thomsen anisotropy parameter prediction results of the coalbed methane reservoir in the embodiment of the present invention ((a) is the prediction result of the anisotropy parameter ε; (b) is the prediction result of the anisotropy parameter δ; (c) is the prediction result of the anisotropy parameter γ). The Thomsen parameters ε and γ are between 0.1~0.3 and 0.05~0.015, respectively, which is consistent with the anisotropic characteristics of the coalbed methane reservoir. Figure 5 This is a scatter plot of the predicted P-wave velocity of the coalbed methane reservoir and the predicted Thomsen anisotropy parameter ε in an embodiment of the present invention, indicating that the P-wave velocity of the coalbed methane reservoir and the anisotropy parameter ε have a certain correlation. Figure 6 This is a scatter plot of the predicted shear wave velocity of the coalbed methane reservoir and the predicted Thomsen anisotropy parameter γ in the embodiment of the present invention, indicating that the shear wave velocity of the coalbed methane reservoir and the anisotropy parameter γ also have a certain correlation.

[0037] The present invention targets the anisotropic characteristics of coalbed methane reservoirs and their unique dual pore system of adsorbed gas and free gas, constructs a seismic rock physics model of the gas reservoir based on VTI media, accurately calculates the equivalent stiffness coefficient, density and Thomsen anisotropy parameters of the coalbed methane reservoir rock physics model, and improves prediction accuracy; and provides theoretical guidance for the forward simulation analysis of the seismic response of coalbed methane reservoirs and the inversion prediction and evaluation of reservoir physical property parameters.

[0038] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A coalbed methane reservoir rock physics modeling method based on VTI medium, characterized by: The following steps are involved: Step 1: Determine the mineral composition and content of the coal matrix, and calculate the equivalent stiffness coefficient of the coal matrix using the anisotropic self-consistent model; Step 2: Using the anisotropic differential equivalent medium model, add the adsorbed gas into the coal matrix and calculate the equivalent stiffness coefficient of the coal matrix containing the adsorbed gas; Step 3: Determine the composition and content of brittle minerals, calculate their bulk modulus and shear modulus based on the Voigt-Reuss-Hill boundary theory, and add the brittle minerals to the adsorbed gas coal matrix using the anisotropic DEM model to calculate the equivalent stiffness coefficient of the coal matrix dry skeleton; Step 4: Using the anisotropic DEM model, add pores and cracks to the dry skeleton of the coal matrix and calculate the equivalent stiffness coefficient of the dry skeleton of the coal matrix containing pores and cracks; Step 5: Using the Brown-Korringa anisotropic fluid replacement theory, free gas is added to the dry skeleton of the porous and fractured coal matrix to calculate the equivalent stiffness coefficient of the coalbed methane reservoir; Step 6: Based on the modeling results of steps 1 to 5, calculate the P-wave velocity, S-wave velocity, density and Thomsen anisotropy parameters of the CBM reservoir rock.

2. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 1, characterized in that: The mineral components of the coal matrix in step 1 include organic matter and clay, and the sum of their percentages is 1. The calculation formula of the anisotropic self-consistent model is as follows: ； in, ; Where: is the equivalent stiffness coefficient matrix of coal matrix; 1 is the stiffness coefficient matrix of organic matter; 2 is the stiffness coefficient matrix of clay; v toc and v clay are the percentages of organic matter and clay, respectively; I ijkl is a fourth-order unit tensor; ijkl is the Eshelby tensor related to the pore morphology of the inclusion.

3. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 2, characterized in that: Relative porosity of adsorbed gas in step 2 satisfy , is the relative porosity of free gas, is the total porosity; and the differential equation of the anisotropic DEM model is: ； Where: is the equivalent stiffness coefficient matrix of coal matrix containing adsorbed gas; is the stiffness coefficient matrix of adsorbed gas.

4. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 3, characterized in that: The brittle minerals in step 3 include calcite, quartz and pyrite, and their bulk modulus and shear modulus are calculated using the Voigt-Reuss-Hill formula as follows: ； ； Where: K VRH is the bulk modulus of brittle minerals; μ VRH is the shear modulus of brittle minerals; K1, µ1, and v1 are the bulk modulus, shear modulus, and percentage of calcite, respectively; K2, µ2, and v2 are the bulk modulus, shear modulus, and percentage of quartz, respectively; K3, µ3, and v3 are the bulk modulus, shear modulus, and percentage of pyrite, respectively; The calculation formula of the equivalent stiffness coefficient of the coal matrix dry skeleton is as follows: ； Where: is the equivalent stiffness coefficient matrix of the coal matrix dry skeleton; is the percentage of brittle minerals; is the stiffness coefficient matrix of brittle minerals.

5. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 4, characterized in that: The differential equation of the anisotropic DEM model in step 4 is: ； Where: is the equivalent stiffness coefficient matrix of the dry skeleton of the porous and fractured coal matrix; is the stiffness coefficient matrix of the empty crack.

6. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 5, characterized in that: The calculation formula of the equivalent stiffness coefficient matrix of the coalbed methane reservoir in step 5 is as follows: ； in: ; ； Where: is the equivalent flexibility coefficient of the coalbed methane reservoir; 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the dry skeleton of the porous and fractured coal matrix; m 、 are the equivalent flexibility coefficient and equivalent stiffness matrix of the coal matrix respectively; β f and β m are the compressibility coefficients of free gas and rock skeleton respectively; K f is the bulk modulus of the free gas.

7. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 6, characterized in that: In step 5, the equivalent flexibility matrix of the coalbed methane reservoir is inverted and converted into the equivalent stiffness matrix of the coalbed methane reservoir: ； in: VTI is the equivalent stiffness matrix of the coalbed methane reservoir, and its specific expression is: ； Where: C 11 、C 13 、C 33 、C 44 、C 66 Both are equivalent stiffness coefficients of coalbed methane reservoirs.

8. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 7, characterized in that: In step 6, the five independent equivalent stiffness coefficients of the equivalent stiffness matrix of the coalbed methane reservoir are used to calculate the longitudinal wave and shear wave velocities of the coalbed methane reservoir. The calculation formula is as follows: ； ； ； Where: is the density of the coalbed methane reservoir; is the vertical velocity of the longitudinal wave, is the horizontal velocity of the longitudinal wave; is the SV wave vertical velocity, is the horizontal velocity of SV wave; is the SH wave vertical velocity, is the horizontal velocity of SH wave; The calculation formula of the density of coalbed methane reservoir is as follows: ； Where: are the percentage and density of organic matter, respectively; are the percentage and density of clay, respectively; are the percentage and density of calcite respectively; are the percentage and density of quartz respectively; are the percentage and density of pyrite respectively; is the adsorbed gas density; is the free gas density.

9. The coalbed methane reservoir rock physics modeling method based on VTI medium according to claim 8, characterized in that: The calculation formula of the Thomsen anisotropy parameter of the coalbed methane reservoir in step 6 is as follows: ； ； ； Where: ε is the anisotropy parameter of longitudinal waves; γ is the anisotropy parameter of transverse waves; δ is the anisotropy parameter of longitudinal waves and SV waves.

10. A coalbed methane reservoir rock physics modeling system based on VTI media, characterized in that: include: Several groups of computing modules, configured to execute steps 1 to 6 according to any one of claims 1 to 9; Data input module, used to receive mineral composition, porosity and physical property parameters; Output module, used to generate equivalent elastic parameters and anisotropic parameters of coalbed methane reservoirs.

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