Integrated simulation method and system for coal-series gas yield increase and CO2 geological sequestration

Abstract

The invention discloses a coal-series gas yield increasing and CO2 geological sequestration integrated simulation method and system, and relates to the technical field of gas exploitation of each layer, and the method comprises the following specific steps: building a geological model representing a multi-lithology superposed coal-series reservoir; the coal series reservoir is a heterogeneous reservoir and comprises shale, a coal seam and sandstone, the shale and the coal seam are double-hole double-permeability media, and the sandstone is a single-hole single-permeability medium; based on the geologic model, establishing a multi-physical field coupling model of a coupling temperature field, a seepage field and a stress field; performing discretization solution on the multi-physics field coupling model to obtain a numerical simulation result; constructing a numerical model, and performing inversion correction on model parameters of the numerical model based on experimental data and the numerical simulation result; and utilizing the corrected numerical model to simulate different injection-production working conditions so as to carry out double-target evaluation and scheme optimization of coal-series gas yield increase and CO2 geological sequestration. The method provides a reliable decision basis for engineering scheme design.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of each layer gas production, more particularly to a coalbed gas production and CO2 geological storage integrated simulation method and system. BACKGROUND

[0002] At present, with the continuous growth of global energy demand and the deepening of the "double carbon" strategy, the integrated technology of coalbed gas efficient development and CO2 geological storage has become an important research direction in the field of energy and environmental protection. Coalbed reservoirs have geological characteristics such as multiple lithology superimposition, large hydrocarbon generation potential, coexistence of multiple phase gases, source-reservoir interaction, complex gas-water relationship, and fragile system dynamic balance. The traditional single reservoir numerical simulation method cannot accurately represent the gas migration rule under the mechanism of heat-flow-solid multi-field coupling, resulting in poor coalbed gas production effect and large deviation in CO2 storage capacity evaluation.

[0003] The existing model describes the heat-flow-solid multi-field coupling effect based on simplified assumptions (such as linear constitutive relationship and homogeneous medium), which cannot accurately depict the heterogeneity and dynamic evolution process of coalbed reservoirs, and has certain theoretical model limitations. The interaction mechanism between temperature change (temperature field), gas adsorption / desorption (seepage field) and coal deformation (stress field) is complex, and the dynamic response of key parameters (such as permeability and adsorption expansion coefficient) lacks universal rules, and the sensitivity of numerical simulation parameters is not clear.

[0004] Therefore, how to construct an integrated numerical simulation method that can accurately represent the heat-flow-solid multi-field coupling process in multiple lithology superimposed reservoirs and reveal the synergistic mechanism of CO2 storage and coalbed gas production is a problem that needs to be solved by those skilled in the art. SUMMARY

[0005] Therefore, the present application provides a coalbed gas production and CO2 geological storage integrated simulation method and system, which overcomes the above-mentioned defects.

[0006] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0007] A coalbed gas production and CO2 geological storage integrated simulation method, the specific steps are:

[0008] A geological model representing a multiple lithology superimposed coalbed reservoir is established; the coalbed reservoir is a heterogeneous reservoir, including shale, coal seam and sandstone, wherein the shale and coal seam are double-pore double-permeability media, and the sandstone is single-pore single-permeability media;

[0009] Based on the geological model, a multi-physical field coupling model coupling temperature field, seepage field and stress field is established;

[0010] The multi-physical field coupling model is discretely solved to obtain numerical simulation results;

[0011] constructing a numerical model, and inversely correcting model parameters of the numerical model based on experimental data and simulation results of the numerical model;

[0012] simulating different injection and production conditions by using the corrected numerical model to evaluate and optimize the dual targets of coalbed gas production and CO2 geological storage.

[0013] Optionally, the multi-physics coupling model comprises:

[0014] a binary gas transport control equation in the pore system, which describes competitive adsorption of CH4 and CO2 based on a modified Langmuir equation and describes gas diffusion based on Fick's law;

[0015] a two-phase flow transport equation in the fracture system, which considers gas slip effect, gas-water two-phase seepage, and water vapor transport based on Darcy's law and Klinkenberg effect in porous media;

[0016] a stress field control equation constructed based on elastic variables, gas adsorption / desorption strain, thermal strain, and effective stress change in the coupling process;

[0017] a temperature field control equation considering gas adsorption / desorption thermal effect, thermal convection, and thermal conduction;

[0018] a matrix porosity evolution sub-model based on effective stress, gas adsorption / desorption strain, and thermal strain.

[0019] Optionally, the specific steps of discretization and solution are:

[0020] solving stress simulation parameters in the multi-physics coupling model based on a solid mechanics module;

[0021] solving temperature simulation parameters in the multi-physics coupling model based on a thermal conduction module;

[0022] solving transport and diffusion simulation parameters of gas and water in the multi-physics coupling model using a coefficient form partial differential equation.

[0023] Optionally, the binary gas transport control equation is:

[0024]

[0025] where i is a gas component, 1 is CH4, and 2 is CO2; t is time; M gi is the molar mass of the gas; R is the molar constant of the gas; T is the temperature of the gas; p mgi is the gas pressure in the pore system; V Li is the Langmuir volume constant; b iis the coefficient related to Langmuir pressure constant; p c is the skeleton density; p gsi is the gas density at standard state; c1 and c2 are the gas adsorption heat coefficients; p m is the total gas pressure in the matrix; T ref is the reference temperature; D i is the diffusion coefficient; is the matrix porosity; τ i is the desorption time; p fgi is the gas pressure in the fracture.

[0026] Optionally, the two-phase flow transport equation is:

[0027]

[0028] where k is the absolute permeability; k rg and k rw are the relative permeability of gas and water, respectively; μ gi and μ w are the dynamic viscosity of gas and water, respectively; b k is the Klinkenberg factor; p fw is the water pressure in the fracture; p fgi is the gas pressure in the fracture.

[0029] Optionally, the stress field control equation is:

[0030]

[0031] where G is the shear modulus; u k is the deformation in k direction; v is the Poisson's ratio; a m and a f are the Biot's coefficients of the matrix and the fracture, respectively; p m is the total gas pressure in the matrix; p f is the fluid pressure in the fracture; K is the bulk modulus of elasticity; a T is the thermal expansion coefficient; T is the temperature of the gas; ε s is the strain caused by CH4 and CO2 desorption and adsorption; f k is the body load component; k, l = x, y, z coordinate axes.

[0032] Optionally, the temperature field control equation is:

[0033]

[0034] where (ρC p ) eff is the effective specific heat capacity; η eff is the effective heat convection coefficient; λ effis the effective thermal conductivity; ε V is the volumetric strain; q sti is the isochoric heat of gas adsorption; M gi is the molar mass of the gas; ρ gsi is the gas density at standard conditions; V sgi is the adsorbed gas content; t is time.

[0035] An integrated simulation system for coal measure gas production and CO2 geological storage includes:

[0036] A geological model construction module is configured to establish a geological model representing a multi-lithology superimposed coal measure reservoir; the coal measure reservoir is a heterogeneous reservoir including shale, coal seam and sandstone, wherein the shale and the coal seam are double-pore double-permeability media, and the sandstone is single-pore single-permeability media;

[0037] A coupled model construction module is configured to establish a multi-physical field coupled model coupling a temperature field, a seepage field and a stress field based on the geological model;

[0038] A model solving module is configured to discretely solve the multi-physical field coupled model to obtain a numerical simulation result;

[0039] A parameter correction module is configured to construct a numerical model and correct model parameters of the numerical model based on experimental data and the numerical simulation result;

[0040] A scheme optimization module is configured to simulate different injection and production conditions by using the corrected numerical model to evaluate and optimize a double target of coal measure gas production and CO2 geological storage.

[0041] According to the technical solution, the present application provides a coal measure gas production and CO2 geological storage integrated simulation method and system, which has the following beneficial effects compared with the prior art:

[0042] 1. By establishing a full-coupling mathematical model of heat-flow-solid multi-physical fields, the CH4-CO2 competitive adsorption, coal matrix expansion / contraction effect and Darcy seepage behavior are comprehensively considered, and the thermal effect of gas adsorption / desorption and the heat conduction-convection process in the reservoir are coupled, which can more truly reflect the heterogeneity and dynamic evolution process of the multi-lithology superimposed coal measure reservoir, and improve the accuracy and reliability of the simulation of the whole process of coal measure gas production and CO2 storage.

[0043] 2. Based on the experimental data and the numerical simulation results, the model parameters are corrected by inversion, and the simulation accuracy is improved. Using the calibrated high-precision model, the development effect under different geological conditions and engineering schemes can be predicted and compared, so as to scientifically optimize the injection-production parameters (such as injection pressure, temperature, rate, well pattern layout), realize the maximization of coal measure gas yield increase effect and CO2 storage benefit, and provide reliable decision basis for engineering scheme design. BRIEF DESCRIPTION OF DRAWINGS

[0044] In order to more clearly illustrate the technical solutions in 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. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of the provided drawings.

[0045] Figure 1 The method flowchart provided by the present application is shown in the figure;

[0046] Figure 2 The structure diagram of the numerical model provided by the present application is shown in the figure;

[0047] Fig. 3(a) is the outlet CH4 volume composition under 6MPa injection pressure provided by the present application; Fig. 3(b) is the outlet CO2 volume composition under 6MPa injection pressure provided by the present application;

[0048] Fig. 4(a) is the CH4 content in coal measure strata under CO2 injection provided by the present application; Fig. 4(b) is the CH4 recovery rate in coal measure strata under CO2 injection provided by the present application. DETAILED DESCRIPTION

[0049] The technical solutions in the embodiments of the present application will be described clearly and completely in the following with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0050] The present application discloses a coal measure gas yield increase and CO2 geological storage integrated simulation method, and the specific steps are as follows:

[0051] Step 1, establishing a geological model representing a multi-lithology superimposed coal measure reservoir; the coal measure reservoir is a heterogeneous reservoir, including shale, coal seam and sandstone, wherein the shale and coal seam are double-porosity and double-permeability media, and the sandstone is single-porosity and single-permeability media;

[0052] Step 2: Based on the geological model, establish a multi-physics coupling model that couples the temperature field, seepage field, and stress field;

[0053] Step 3: Discretize and solve the multiphysics coupling model to obtain numerical simulation results;

[0054] Step 4: Construct a numerical model and perform inversion correction on the model parameters of the numerical model based on experimental data and numerical simulation results;

[0055] Step 5: Using the corrected numerical model, simulate different injection and production conditions to conduct a dual-objective evaluation and scheme optimization for coalbed methane production enhancement and CO2 geological sequestration.

[0056] In one embodiment, in step 1, the coal-bearing reservoir undergoes elastic deformation under different stresses.

[0057] In one embodiment, the multiphysics coupling model includes:

[0058] Binary gas transport control equations in porous systems are used to describe the competitive adsorption of CH4 and CO2 based on the modified Langmuir equation and the gas diffusion based on Fick's law.

[0059] Based on Darcy's law and the Klinkenberg effect in porous media, the two-phase flow transport equations for gas slippage, gas-water two-phase flow, and water vapor transport are considered in fractured systems.

[0060] The stress field control equations are constructed based on elastic variables, gas adsorption / desorption strain, thermal strain, and effective stress changes during the coupling process.

[0061] Considering the heat effects of gas adsorption / desorption, heat convection, and heat conduction, the temperature field governing equations are as follows:

[0062] A sub-model for the evolution of matrix porosity was established based on effective stress, gas adsorption / desorption strain, and thermal strain.

[0063] In one embodiment, the steps for constructing the governing equations for the transport of a binary gas in a porous system are as follows:

[0064] When CO2 is injected into a coal-bearing reservoir, the mass conservation in the pore system can be described as follows:

[0065]

[0066] In the formula, i represents the gaseous component (1 represents CH4, 2 represents CO2); m mgi The mass of gas per unit volume of matrix, kg / m³ 3 t represents time, in seconds; D i m is the diffusion coefficient. 2 / s;M giThe value is the molar mass of the gas, expressed in g / mol. p represents matrix porosity. mgi R is the gas pressure in the porous system, Pa; R is the gas molar constant, J / (mol K); T is the gas temperature, K; Q si For source term, kg / m 3 ·s.

[0067] The mass of gas per unit volume of matrix can be expressed as:

[0068]

[0069] In the formula, V sgi m represents the adsorbed gas content. 3 / kg; ρ c Frame density, kg / m³ 3 ;ρ gsi The gas density under standard conditions is kg / m³. 3 ;

[0070] Adsorbed gas content V sgi The modified Langmuir equation is expressed as:

[0071]

[0072] In the formula, V Li Let m be the Langmuir volume constant. 3 / kg; b i b is a coefficient related to the Langmuir pressure constant. i =1 / P Li , where P Li Here, c is the Langmuir pressure constant in Pa; c1 and c2 are both gas adsorption heat coefficients; T ref This is the reference temperature, in K; p m =p mg1 +p mg2 ρ represents the total gas pressure in the matrix, in Pa.

[0073] According to Fick's law, the gas mass exchange process between a porous system and a fractured system can be expressed as:

[0074]

[0075] In the formula, τ i For desorption time, s; p fgi Let be the gas pressure in the fissure, in Pa.

[0076] Based on the above formula, the migration equation of coal-bearing gas in coal-bearing reservoirs can be obtained as follows:

[0077]

[0078] In the formula, i represents the gaseous component (1 is CH4, 2 is CO2); t is time, s; M gi R is the molar mass of the gas, g / mol; R is the molar constant of the gas, J / (mol K); T is the temperature of the gas, K; p mgi V represents the gas pressure in the porous system, in Pa; Li Let m be the Langmuir volume constant. 3 / kg; b i =1 / P Li ;P Li ρ is the Langmuir pressure constant in Pa. c Frame density, kg / m³ 3 ;ρ gsi The gas density under standard conditions is kg / m³. 3 c1 and c2 are the gas adsorption heat coefficients; p m =p mg1 +p mg2 T represents the total gas pressure in the matrix, in Pa; ref For reference temperature, K; D i m is the diffusion coefficient. 2 / s; τ represents matrix porosity. i For desorption time, s; p fgi Let be the gas pressure in the fissure, in Pa.

[0079] In one embodiment, the steps for constructing the governing equations for the two-phase flow in the fractured system are as follows:

[0080] Considering the gas slippage effect, according to the generalized Darcy law for gas-water two-phase flow, the governing equations for the seepage field of coalbed methane and groundwater in fractures are:

[0081]

[0082] In the formula, s g This refers to the gas saturation level. ρ is the fracture porosity. fgi The density of the gas is kg / m³. 3 ;q gi The velocity of the gas is expressed in m / s; s w =1-s g ρ represents water saturation. fgdi =H gi ×ρ fgi The density of the dissolved gas is given by Henry's Law, in kg / m³. 3 , where H gi q is the Herny coefficient for the gas; wLet be the velocity of the water, in m / s.

[0083] Regarding water, considering water vapor in the gas phase within a fractured system, the law of conservation of mass can be written as follows:

[0084]

[0085] In the formula, ρ w The density of water is kg / m³. 3 ;ρ fv =ρ fv0 ×exp(p cgw / (ρ fgi ×R v ×T)) is the density of water vapor based on the Kelvin-Laplace law, in kg / m³. 3 ;ρ fv0 The density of saturated water vapor is kg / m³. 3 ;R v The latent heat of water vapor is expressed in J / (K·kg).

[0086] According to Darcy's law and the Klinkenberg effect in porous media, the transport of two-phase flow can be expressed as:

[0087]

[0088] In the formula, k is the absolute permeability, and m 2 ;k rg and k rw These are the relative permeabilities of air and water, respectively; μ gi and μ w The dynamic viscosity of gas and water, respectively, is given in Pa·s; b k For the Klinkenberg factor, Pa; p fw =p fg -p cgw Let p be the water pressure in the fissure, Pa, where p fg =p fg1 +p fg2 p represents the total gas pressure in the fracture, in Pa. cgw The capillary force is Pa.

[0089] Relative penetration rate is defined as:

[0090]

[0091] In the formula, k rg0 and k rw0 The relative permeability at the endpoints of air and water, respectively; s gr Residual gas saturation; s wr This represents the non-reducible water saturation level.

[0092] In one embodiment, the steps for constructing the stress field governing equations are as follows:

[0093] Considering the coupling process, the total strain can be defined as:

[0094]

[0095] Where, ε kl Strain components; G = E / (2+2ν) is the shear modulus, Pa; K = E / 3(1-2ν) is the bulk modulus, Pa, where E is Young's modulus, Pa; ν is Poisson's ratio; σ kl For stress components, Pa; σ dd For the normal stress component, Pa; δ kl Kronecker coefficient; α m and α f Biot coefficients for the matrix and fractures, respectively; p f =s w p fw +s g p fg α is the fluid pressure in the fracture, Pa; T ε is the coefficient of thermal expansion, 1 / K; s The strain is caused by the desorption and adsorption of CH4 and CO2.

[0096] ε s Defined as:

[0097]

[0098] In the formula, ε L1 and ε L2 denoted as Langmuir strain coefficients for CH4 and CO2.

[0099] Combining the equilibrium equation and the strain-deformation relationship equation, the stress field governing equation is:

[0100]

[0101] In the formula, u k Let m be the deformation in the k-direction; k, l = x, y, z coordinate axes; f k Let N be the volume load component.

[0102] In one embodiment, the steps for constructing the temperature field governing equations are as follows:

[0103] In a coal seam, the coal skeleton, CH4, and water coexist within a single volume system. Temperature changes cause changes in the system's internal energy, and the volumetric strain of the coal mass generates deformation work. The adsorption / desorption of CH4 is accompanied by the release / absorption of heat. Simultaneously, the system exchanges heat with the surrounding environment through thermal convection and heat transfer. By the law of conservation of energy, the governing equation for the temperature field is:

[0104]

[0105] In the formula, (ρC p ) eff For effective specific heat capacity, J / (m 3 ·K); η eff The effective heat convection coefficient is J / (m 2 ·s); λ eff For effective thermal conductivity, W / (m·K); ε V For volumetric strain; q sti The isochoric heat of gas adsorption is expressed in kJ / mol.

[0106] In addition, (ρC p ) eff η eff and λ eff They are defined as follows:

[0107]

[0108] In the formula, C s C gi C w and C v , respectively, represent the specific heat capacity of the rock skeleton, gas, water, and water vapor, in J / (kg·K).

[0109]

[0110] In the formula, λ s , λ mgm , λ fgm and λ fw These are the thermal conductivity coefficients of the rock skeleton, the mixed gas in the matrix, the mixed gas in the fissures, and the water in the fissures, respectively, in W / (m·K).

[0111] The expression for the mixed gas (CH4 and CO2) in the matrix is:

[0112]

[0113] The expression for the mixed gas (CH4, water vapor, and CO2) in the fracture is:

[0114]

[0115] In the formula, x i and λ i These represent the mole fraction and thermal conductivity of the gas, respectively.

[0116] In one embodiment, the steps for constructing a matrix porosity evolution sub-model, including the evolution of dynamic porosity and anisotropic permeability, are as follows:

[0117] Porosity and permeability play key roles in the THM coupled model. In the matrix, porosity can be expressed as follows, based on changes in gas pressure, temperature, and gas adsorption and desorption:

[0118]

[0119] Where, ε e =ε v +p m / K s -α T T-ε s The subscript 0 represents the initial value of the parameter.

[0120] Within a fracture, fracture deformation determines its porosity and permeability. The change in fracture width is defined as:

[0121] Δb=(Δσ n -α f Δp f ) / K n (20);

[0122] In the formula, Δ b For the change in crack width, m; Δσ n Let K be the normal stress of the crack, Pa; n The corresponding normal stiffness is expressed in Pa / m.

[0123] Normal stiffness is a function of the effective normal stress and is defined as:

[0124]

[0125] In the formula, K ni σ' is the initial normal stiffness in Pa / m; n Δvmax is the effective normal stress, Pa; Δvmax is the maximum fracture diameter, m, that approaches infinity when the effective normal stress is applied.

[0126] Since the initial permeability of shale differs in the vertical and horizontal directions, the initial fracture width *b* also differs in the orthogonal directions based on the dual-porosity medium. Therefore, the fracture porosity can be expressed as:

[0127]

[0128] In the formula, j represents the orthogonal direction; bj0 The initial crack width is m; N is the dimension, N=2 for two dimensions and N=3 for three dimensions.

[0129] When calculating permeability in each direction, a cubic relationship between crack width and permeability is used:

[0130]

[0131] In the formula, k j0 This represents the initial penetration rate.

[0132] In summary, the seepage field model considers the competitive adsorption of CH4-CO2, the expansion / contraction effect of the coal matrix, and Darcy seepage behavior; the temperature field model couples the heat effects of gas adsorption / desorption and the heat conduction-convection process in the reservoir; and the stress field model considers the influence of dynamic changes in effective stress on reservoir permeability. Based on the above methods, the competitive adsorption model adopts the extended Langmuir equation and considers the influence of temperature on the adsorption equilibrium; the permeability dynamic model adopts the stress-adsorption coupling equation, combined with the statistical characterization of fracture networks, to describe the anisotropic changes in reservoir permeability.

[0133] In one embodiment, based on the above governing equations, the integrated simulation of coalbed methane production enhancement and CO2 geological sequestration in multi-lithological superimposed coalbed methane is solved using coefficient-form partial differential equations in COMSOL Multiphysics. The specific steps are as follows:

[0134] Solving stress simulation parameters in a multiphysics coupled model based on solid mechanics modules;

[0135] Solving temperature simulation parameters in a multiphysics coupled model based on a heat conduction module;

[0136] The parameters for simulating the transport and diffusion of gas and water in a multiphysics coupled model are solved using partial differential equations in coefficient form.

[0137] Furthermore, the specific steps for discretization and solution are as follows:

[0138] (1) In COMSOL Multiphysics, the Solid Mechanics module describes the deformation and stress distribution of solid materials based on the equilibrium equations and constitutive relations of continuum mechanics. Its core equation is:

[0139] Transient studies in solid mechanics:

[0140]

[0141] In the formula, S represents stress; F V For volume force, Pa;

[0142] Constitutive equation for linear elastic materials (Hooke's Law):

[0143] S = S inel +S el (25);

[0144] ∈ el =∈-∈ inel (26);

[0145] ∈ inel =∈0+∈ ext +∈ th +∈ hs +∈ pl +∈ cr +∈ vp +∈ ve (27);

[0146] S el =C:∈ el (28);

[0147] S inel =S0+S ext +S q (29);

[0148]

[0149] C = C(E,v)(31);

[0150] In the formula, S represents stress; inel S represents the inherent strain generated during a phase transition. el S0 is the thermal expansion strain caused by Joule heating; S0 is the prestress, Pa; S0 is the thermal expansion strain caused by Joule heating. ext External stress, Pa; S q For viscous stress, Pa; ∈ represents elastic strain; ∈0 represents pre-strain; ∈ ext For external strain; ∈ th For thermal strain; ∈ hs For wetting expansion strain; ∈ pl For plastic strain; ∈ cr C represents creep; C is the elasticity matrix.

[0151] (2) In COMSOL Multiphysics, the coefficient-form partial differential equations (PDEs) of the Heat Transfer module are used to describe heat transfer processes (conduction, convection, radiation, etc.), and their expressions are as follows:

[0152]

[0153]

[0154] In the formula, e a d is the quality coefficient; ais the product of density and specific heat capacity, is the damping coefficient; c is the diffusion coefficient; α is the conserved flux correlation coefficient; β is the convection coefficient; γ is the source flux; a is the absorption coefficient; f is the source term.

[0155] (3) In COMSOL Multiphysics, the diffusion process of coal-bearing gases can be described by the mass transfer equation or the PDE. The following is the coefficient form of the PDE for methane diffusion and its parameter settings, applicable to the diffusion behavior of gases in porous media, free space, or chemical reaction systems:

[0156]

[0157] In the formula, pm1 is the coalbed methane pressure in the matrix of the coal-bearing reservoir, Pa; pfg1 is the coalbed methane pressure in the fractures of the coal-bearing reservoir, Pa.

[0158] (4) The migration and diffusion of CO2 in coal-bearing reservoirs is similar to the diffusion process of coal-bearing gas described above, and its coefficient form partial differential equation is expressed as follows:

[0159]

[0160] In the formula, pm2 is the CO2 pressure in the matrix of the coal-bearing reservoir, Pa; pfg2 is the CO2 pressure in the fractures of the coal-bearing reservoir, Pa.

[0161] (5) The coefficient form of the PDE equation for water migration within coal-bearing reservoirs in COMSOL is:

[0162]

[0163] Based on the parameters set for the partial differential equation in coefficient form mentioned above, with a narrow grid resolution of 1, curvature factor of 0.2, maximum cell size of 2m, minimum cell size of 0.05m, and maximum cell growth rate of 1.2, a numerical simulation was performed to solve the integrated coalbed methane production enhancement and CO2 geological sequestration based on multi-lithological superposition.

[0164] In one embodiment, based on experimental data and numerical simulation results obtained by solving partial differential equations in the above coefficient form, the model parameters are inverted and corrected to improve simulation accuracy; a cylindrical numerical model matching the size of the experimental shale sample (100mm×50mm) is established to verify the consistency between numerical simulation and test data under the conditions of confining pressure of 4MPa, vertical stress of 6MPa, and CO2 injection pressure of 6MPa.

[0165] In one embodiment, to study the effect of CO2 injection on coal-bearing gas production enhancement in coal-bearing superimposed polylithic reservoirs, two operating conditions were set up for comparative analysis: natural depressurization (no CO2 injection, CH4 extraction at 0.1 MPa) and CO2 injection at 7 MPa, CH4 extraction at 0.1 MPa. To study the influence mechanism of CO2 injection at different frequencies on CO2 geological sequestration and coal-bearing gas production enhancement in coal-bearing superimposed polylithic reservoirs, two operating conditions were set up for comparative analysis: CO2 injection at 1000-day intervals and CO2 injection at 2500-day intervals (7 MPa CO2 injection, 0.1 MPa CH4 extraction). In addition, to study the influence mechanism of different injection and extraction pressures on CO2 geological sequestration and coal-bearing gas production enhancement in coal-bearing superimposed polylithic reservoirs, nine operating conditions were set up for comparative analysis, combining injection pressures of 6 MPa, 7 MPa, and 8 MPa and extraction pressures of 0.1 MPa, 0.2 MPa, and 0.3 MPa. Based on the above-mentioned thermal-fluid-solid multiphysics coupled numerical simulation mathematical model, the recovery rate and CO2 geological storage of coal-bearing gas under various working conditions such as different injection and production pressures and different CO2 injection pressures are analyzed. Through the dual-objective evaluation of coal-bearing gas production enhancement and CO2 geological storage, the coal-bearing gas production enhancement efficiency and CO2 storage potential of different development schemes are quantified, thereby outputting the optimal coal-bearing gas production enhancement and CO2 storage.

[0166] CO2-CH4 displacement experiments were conducted on coal-bearing shale samples from the Longtan Formation of the Songzao Coalfield in the Sichuan Basin. The cylindrical samples were 100 mm long and 50 mm in diameter. Before testing, the samples were saturated with CH4 at an initial pressure of 3 MPa. The experimental conditions were a confining pressure of 4 MPa, a vertical stress of 6 MPa, and a CO2 injection pressure maintained at 6 MPa. The experimental temperature was 288.15 K, and the outlet pressure was atmospheric pressure (0.1 MPa). For verification purposes, a numerical model matching the size of the experimental shale samples (100 mm × 50 mm) was established, as follows: Figure 2 As shown in the figure. The operating parameters of the numerical model were configured to accurately replicate the experimental conditions, ensuring a direct one-to-one correspondence. Here, the experimental results and simulation results for the proportions of CH4 (as shown in Figure (3a)) and CO2 (as shown in Figure (3b)) at the outlet are consistent, verifying the feasibility and reliability of the established numerical model.

[0167] After 10,000 days of natural pressure drop, the CH4 recovery rate of the coal-bearing reservoir was 31.85%. Under CO2 injection conditions, 47,888.36 kg of CO2 was sequestered, mainly through adsorption (85.28%), and 19,136.82 kg of CH4 was recovered, resulting in a CH4 recovery rate of 40.88%. Under natural pressure drop conditions, the CH4 recovery rate was only 31.85%, indicating that CO2 injection significantly improved the CH4 recovery rate of the coal-bearing reservoir. Furthermore, there were significant differences in the distribution of sequestered CO2 among coal, shale, and sandstone layers, accounting for 44.64%, 54.27%, and 1.09%, respectively. Figures 4(a)-4(b) As shown.

[0168] Another aspect of this embodiment discloses an integrated simulation system for coalbed methane production enhancement and CO2 geological sequestration, including:

[0169] The geological model construction module is used to establish a geological model characterizing multi-lithological superimposed coal-bearing reservoirs. The coal-bearing reservoirs are heterogeneous reservoirs, including shale, coal seams and sandstone. Among them, shale and coal seams are dual-pore dual-permeability media, and sandstone is single-pore single-permeability media.

[0170] The coupling model construction module is used to build a multi-physics coupling model that couples the temperature field, seepage field, and stress field based on the geological model.

[0171] The model solving module is used to discretize and solve multiphysics coupling models to obtain numerical simulation results;

[0172] The parameter calibration module is used to construct the numerical model and perform inversion calibration on the model parameters of the numerical model based on experimental data and numerical simulation results.

[0173] The scheme optimization module is used to simulate different injection and production conditions using a corrected numerical model to conduct dual-objective evaluation and scheme optimization for coalbed methane production enhancement and CO2 geological sequestration.

[0174] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0175] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration, characterized in that, The specific steps are as follows: A geological model is established to characterize a multi-lithological superimposed coal-bearing reservoir. The coal-bearing reservoir is a heterogeneous reservoir, including shale, coal seam and sandstone. Shale and coal seam are dual-pore dual-permeability media, and sandstone is single-pore single-permeability media. Based on the geological model, a multi-physics coupling model is established that couples the temperature field, seepage field, and stress field. The multiphysics coupling model is discretized and solved to obtain numerical simulation results; A numerical model is constructed, and the model parameters of the numerical model are inverted and corrected based on experimental data and the numerical simulation results. Using the corrected numerical model, different injection and production conditions are simulated to conduct a dual-objective evaluation and scheme optimization for coalbed methane production enhancement and CO2 geological sequestration.

2. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 1, characterized in that, The multiphysics coupling model includes: Binary gas transport control equations in porous systems are used to describe the competitive adsorption of CH4 and CO2 based on the modified Langmuir equation and the gas diffusion based on Fick's law. Based on Darcy's law and the Klinkenberg effect in porous media, the two-phase flow transport equations for gas slippage, gas-water two-phase flow, and water vapor transport are considered in fractured systems. The stress field control equations are constructed based on elastic variables, gas adsorption / desorption strain, thermal strain, and effective stress changes during the coupling process. Considering the heat effects of gas adsorption / desorption, heat convection, and heat conduction, the temperature field governing equations are as follows: A matrix porosity evolution sub-model was established based on effective stress, gas adsorption / desorption strain, and thermal strain.

3. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 1, characterized in that, The specific steps for discretization are as follows: The stress simulation parameters in the multiphysics coupling model are solved based on the solid mechanics module; The temperature simulation parameters in the multiphysics coupling model are solved based on the heat conduction module; The migration and diffusion simulation parameters of gas and water in the multiphysics coupling model are solved using partial differential equations in coefficient form.

4. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 2, characterized in that, The governing equation for the transport of a binary gas is: In the formula, i represents the gas component, 1 represents CH4, 2 represents CO2; t represents time; M represents M. gi R is the molar mass of the gas; R is the molar constant of the gas; T is the temperature of the gas; p mgi V represents the gas pressure in the porous system. Li b is the Langmuir volume constant; i The coefficient related to the Langmuir pressure constant; ρ c ρ is the skeleton density. gsi c is the gas density under standard conditions; c1 and c2 are the gas adsorption heat coefficients; p m T represents the total gas pressure in the matrix. ref For reference temperature; D i The diffusion coefficient is denoted as . τ represents matrix porosity. i p is the desorption time. fgi This represents the gas pressure within the fissure.

5. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 2, characterized in that, The two-phase flow transport equation is: In the formula, k is the absolute permeability; k rg and k rw These are the relative permeabilities of air and water, respectively; μ gi and μ w b, representing the dynamic viscosity of gas and water, respectively; k p is the Klinkenberg factor; fw p is the water pressure in the fissure; fgi This represents the gas pressure within the fissure.

6. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 2, characterized in that, The governing equations of the stress field are: In the formula, G is the shear modulus; u k ν is the deformation in the k-direction; ν is Poisson's ratio; α m and α f Biot coefficients for the matrix and fractures, respectively; p m p represents the total gas pressure in the matrix. f The fluid pressure in the fracture is K; the bulk modulus is α. T ε is the coefficient of thermal expansion; T is the temperature of the gas; ε s The strain is caused by the desorption and adsorption of CH4 and CO2; f k For the volume load components; k, l = x, y, z coordinate axes.

7. The integrated simulation method for coalbed methane production enhancement and CO2 geological sequestration according to claim 2, characterized in that, The governing equation for the temperature field is: In the formula, (ρC p ) eff For effective specific heat capacity; η eff λ is the effective thermal convection coefficient. eff For effective thermal conductivity; ε V For volumetric strain; q sti M is the isochoric heat of gas adsorption; gi ρ is the molar mass of the gas. gsi V is the gas density under standard conditions. sgi t represents the adsorbed gas content; t represents time.

8. An integrated simulation system for coalbed methane production enhancement and CO2 geological sequestration, characterized in that, include: The geological model construction module is used to establish a geological model characterizing multi-lithological superimposed coal-bearing reservoirs; the coal-bearing reservoirs are heterogeneous reservoirs, including shale, coal seams and sandstone, wherein the shale and coal seams are dual-pore dual-permeability media, and the sandstone is single-pore single-permeability media; The coupling model construction module is used to establish a multi-physics coupling model that couples the temperature field, seepage field, and stress field based on the geological model. The model solving module is used to discretize and solve the multiphysics coupling model to obtain numerical simulation results; The parameter correction module is used to construct a numerical model and perform inversion correction on the model parameters of the numerical model based on experimental data and the numerical simulation results. The scheme optimization module is used to simulate different injection and production conditions using the corrected numerical model to conduct dual-objective evaluation and scheme optimization for coalbed methane production enhancement and CO2 geological sequestration.

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