A predictive method for CH4 production and CO2 sequestration in a CO2-ECBM process

Abstract

A prediction method for CH4 production and CO2 sequestration in a CO2-ECBM process belongs to the field of natural hydrogen resource exploration, and the technical points are as follows: a multiphase and multi-component coupling model for describing the migration process of water, methane and carbon dioxide of a coalbed gas reservoir is constructed; a geological model of the coalbed gas reservoir is constructed, the geological model is meshed to obtain a plurality of grid units of different spatial distribution positions of the coalbed gas reservoir, and initial conditions and boundary conditions of the geological model are set; according to the multiphase and multi-component coupling model, the time-space evolution characteristics of the pressure and concentration of water, methane and carbon dioxide of each grid unit of the geological model are solved to obtain the distribution in time and space. The present application fully considers the influence of the existence of groundwater in the coalbed gas on the migration behavior of methane and carbon dioxide, overcomes the limitation that the deviation of the prediction result is caused by the neglect of the groundwater effect in the existing research, and realizes the accurate prediction of the methane production and carbon sequestration in the CO2-ECBM process.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane exploration and development, and specifically discloses a method for predicting CH4 production and CO2 sequestration during the CO2-ECBM process. Background Technology

[0002] CO2-displaced coalbed methane (CO2-ECBM) technology is an important approach to achieving the synergistic development of coalbed methane and carbon emission reduction. This technology injects carbon dioxide into the coal seam, where it is preferentially adsorbed within the coal matrix, thereby displacing the adsorbed methane and achieving efficient coalbed methane extraction. Compared to traditional extraction methods, CO2 displacement not only significantly improves coalbed methane recovery but also enables long-term geological sequestration of carbon dioxide, which is of great significance for mitigating greenhouse gas emissions and promoting low-carbon energy development. Simultaneously, the adsorption, diffusion, and competitive adsorption processes of CO2 in the coal seam, as well as its competition with methane, can provide new scientific evidence for studying gas migration patterns and adsorption characteristics in coal reservoirs. Therefore, in-depth research into the multi-field coupling mechanism and dynamic evolution characteristics of CO2-displaced coalbed methane has significant theoretical and engineering value for improving coalbed methane resource utilization efficiency, promoting carbon recycling, and achieving the "dual carbon" goal.

[0003] Current research on CO2-displaced coalbed methane (CO2-ECBM) generally focuses only on the adsorption-desorption and diffusion-percolation process between CO2 and CH4, while neglecting the role of the water phase prevalent in coal seams. In reality, water occupies a large amount of pore space in coal seams, significantly affecting the effective percolation channels for gas. Furthermore, CO2 and CH4 interact with water, leading to gas dissolution and diffusion into the water, further impacting the adsorption characteristics and storage stability of the coal seam. Therefore, ignoring the presence of the water phase may lead to biased understandings of gas transport patterns, methane displacement efficiency, and carbon dioxide storage safety, making it difficult to effectively reflect the CO2 displacement mechanism in real coal seams. Therefore, it is necessary to introduce the water phase flow process into the CO2-CH4 displacement model to systematically reveal the transport characteristics under the coexistence of CO2, CH4, and water, thereby improving the accuracy of predictions regarding coalbed methane recovery and carbon dioxide storage reliability. Summary of the Invention

[0004] This invention aims to predict the dynamic migration of underground methane and carbon dioxide during CO2-ECBM processes, enabling quantitative assessment of storage and recovery rates. A method for predicting CH4 production and CO2 storage during CO2-ECBM processes is proposed. Based on the prediction method for CH4 production and CO2 storage during CO2-ECBM processes in some embodiments of this application, the method includes the following steps:

[0005] Construct a multiphase, multicomponent coupled model to describe the migration processes of water, methane, and carbon dioxide in coalbed methane reservoirs;

[0006] A geological model of a coalbed methane reservoir is constructed, and the geological model is divided into grids to obtain several grid cells that reflect different spatial distribution locations of the coalbed methane reservoir. The initial conditions and boundary conditions of the geological model are then set.

[0007] Based on the multiphase, multicomponent coupled model, the spatiotemporal evolution characteristics of the pressure and concentration of water, methane, and carbon dioxide in each grid unit of the geological model are solved to obtain their distribution in time and space.

[0008] The prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process according to some embodiments of this application also includes calculating the methane production and CO2 sequestration amount based on the spatiotemporal evolution characteristics of the pressure and concentration of methane and carbon dioxide.

[0009] Based on the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the multiphase and multicomponent coupled model of the coalbed methane reservoir is constructed according to the mass conservation equation of the gas-liquid two-phase fluid, the mass conservation equation of the gas phase components, and the Langmuir gas adsorption equation.

[0010] Based on the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the geological model of the coalbed methane reservoir is constructed based on exploration data.

[0011] Based on the prediction methods for CH4 production and CO2 sequestration during the CO2-ECBM process in some embodiments of this application, a multiphase, multi-component coupled model describing the water, methane, and carbon dioxide transport processes in coalbed methane reservoirs is constructed, including:

[0012] Among them, the mass conservation equations for the gas and liquid phases are constructed as follows:

[0013]

[0014]

[0015] In the formula, Porosity; The density of water; Water saturation; Indicates time; The velocity of the water flow; The density of the gas; This refers to the gas saturation level. This refers to the gas flow rate; For the mass source term of gas adsorption;

[0016] The flow velocities of the gas and liquid phases are calculated using Darcy's law:

[0017]

[0018]

[0019] In the formula, Formation permeability; The relative permeability of the aqueous phase; The viscosity of the aqueous phase; The pressure in the aqueous phase; It is the acceleration due to gravity; This refers to the relative permeability of the gas phase. This refers to the gas phase pressure. This refers to the gas phase density. This refers to the viscosity of the gas phase.

[0020] in:

[0021]

[0022]

[0023] In the formula, The density of methane; This represents the mass fraction of methane in the gas phase. The density of carbon dioxide; This represents the mass fraction of carbon dioxide in the gas phase. The viscosity of methane; The viscosity of carbon dioxide;

[0024] Among them, the mass conservation equations for the gas phase components are constructed as follows:

[0025]

[0026]

[0027] In the formula, The rate of methane mass reduction due to adsorption; Indicates the mass of methane adsorbed; The rate of decrease in carbon dioxide mass due to adsorption; The mass of carbon dioxide adsorbed;

[0028] Among them, the Langmuir gas adsorption equation is constructed:

[0029]

[0030]

[0031] In the formula, The mass of methane adsorbed; This represents the saturated adsorption volume of methane. Methane pressure; The pressure of methane after reaching saturation adsorption capacity; The mass of carbon dioxide adsorbed; This represents the saturated adsorption volume of carbon dioxide. Carbon dioxide pressure; The pressure of carbon dioxide after reaching saturation adsorption capacity.

[0032] According to the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the exploration data includes the height and horizontal width range of the coalbed methane reservoir.

[0033] According to the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the initial conditions of the geological model of the coalbed methane reservoir include the initial gas phase saturation, the initial coalbed methane reservoir pressure, and the initial coalbed methane reservoir temperature.

[0034] According to the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the boundary conditions of the geological model of the coalbed methane reservoir include upper boundary pressure, lower boundary pressure, methane production well pressure, upper boundary temperature, lower boundary temperature, and carbon dioxide injection well pressure.

[0035] According to the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the grid division method includes free triangular grid division, wherein the size of the largest grid cell is 3 times the size of the smallest grid cell.

[0036] According to the prediction method for CH4 production and CO2 sequestration in the CO2-ECBM process in some embodiments of this application, the spatiotemporal evolution characteristics of concentration are characterized by the spatiotemporal evolution characteristics of gas mass fraction.

[0037] Based on the spatiotemporal evolution characteristics of gas mass fraction, spatial integration is performed on the spatial range of the geological model of the coalbed methane reservoir to obtain methane production and carbon dioxide sequestration.

[0038] The beneficial effects of this invention are as follows: This invention proposes a method for predicting CH4 production and CO2 sequestration during the CO2-ECBM process. This method reveals the mass migration characteristics of methane and carbon dioxide during CO2-ECBM and quantifies long-term carbon dioxide sequestration and methane production. Compared with existing technologies, this invention fully considers the influence of groundwater in coalbed methane on the migration behavior of methane and carbon dioxide in the model, overcoming the limitation of existing studies that neglect the role of groundwater and thus lead to biased prediction results. This achieves accurate prediction of methane production and carbon sequestration during the CO2-ECBM process. Attached Figure Description

[0039] Figure 1 This is a schematic flowchart illustrating a method for predicting CH4 production and CO2 sequestration during a CO2-ECBM process according to an embodiment of the present invention.

[0040] Figure 2 This is a schematic diagram of the geological model of the coalbed methane reservoir used in the embodiments of the present invention.

[0041] Figure 3 (a) is a schematic diagram of the evolution of methane pressure over 20 years calculated in an embodiment of the present invention; (b) is a schematic diagram of the evolution of carbon dioxide pressure over 20 years calculated in an embodiment of the present invention; (c) is a schematic diagram of the evolution of methane mass fraction over 20 years calculated in an embodiment of the present invention; (d) is a schematic diagram of the evolution of carbon dioxide mass fraction over 20 years calculated in an embodiment of the present invention.

[0042] Figure 4 This is a schematic diagram showing the calculated methane extraction and carbon dioxide sequestration amounts over 20 years in an embodiment of the present invention. Detailed Implementation

[0043] The 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 should not be construed as limiting the scope of the invention.

[0044] like Figure 1 As shown, this embodiment of the invention provides a method for predicting CH4 production and CO2 sequestration in a CO2-ECBM process, specifically a method for predicting CH4 production and CO2 sequestration in a CO2-ECBM process, comprising the following steps:

[0045] S1. Based on the mass conservation equations for gas-liquid two-phase fluids, gas component mass conservation equations, and the Langmuir gas adsorption equation, establish a multiphase, multicomponent coupled model describing the transport processes of water, methane, and CO2, including:

[0046] Construct the mass conservation equations for the gas and liquid phases:

[0047]

[0048]

[0049] in, Porosity; The density of water; Water saturation; The velocity of the water flow; The density of the gas; This refers to the gas saturation level. This refers to the gas flow rate; This is a gas mass source term.

[0050] The flow velocities of the gas and liquid phases are calculated using Darcy's law:

[0051]

[0052]

[0053] in, Formation permeability; The relative permeability of the aqueous phase; The pressure in the aqueous phase; It is the acceleration due to gravity; The viscosity of the aqueous phase; This refers to the relative permeability of the gas phase. This refers to the viscosity of the gas phase. This refers to the gas phase pressure. This represents the viscosity of the gas phase.

[0054] The methods for calculating gas phase density and viscosity are as follows:

[0055]

[0056]

[0057] in, The density of methane; This represents the mass fraction of methane in the gas phase. The density of carbon dioxide; This represents the mass fraction of carbon dioxide in the gas phase. The viscosity of methane; The viscosity is that of carbon dioxide.

[0058] The mass conservation equations for the gaseous components (methane and carbon dioxide) are constructed as follows:

[0059]

[0060]

[0061] in, The rate of methane mass reduction due to adsorption; The rate at which carbon dioxide mass decreases due to adsorption.

[0062] Constructing the Langmuir gas adsorption equation:

[0063]

[0064]

[0065] in, The mass of methane adsorbed; The saturated adsorption volume of methane; Methane pressure; The pressure of methane after reaching saturation adsorption capacity; The mass of carbon dioxide adsorbed; This represents the saturated adsorption volume of carbon dioxide. Carbon dioxide pressure; The pressure of carbon dioxide after reaching saturation adsorption capacity.

[0066] At any given time, the volumes of water, methane, and carbon dioxide present in the coalbed methane reservoir are:

[0067]

[0068]

[0069]

[0070] In the formula, , as well as These represent the volumes of water, CH4, and CO2 in the formation, respectively. It is the spatial integral operator for coalbed methane reservoirs.

[0071] S2. Based on the field exploration data, establish a geological model of the coalbed methane reservoir and divide it into grids, setting initial and boundary conditions for the reservoir geological model;

[0072] Specifically, setting the initial conditions for the coalbed methane reservoir geological model includes: setting the initial gas phase saturation, initial coal seam pressure, and initial coal seam temperature. Setting the boundary conditions for the coalbed methane reservoir geological model includes: setting the upper boundary pressure, lower boundary pressure, methane production well pressure, upper boundary temperature, lower boundary temperature, and carbon dioxide injection well pressure. For example... Figure 2As shown, the simulation domain has a width of 150 m and a height of 150 m. Initially, the methane saturation is 0.4, the initial pressure is 5 MPa, and the initial temperature is 300 K. The lower boundary pressure is 5 MPa, the upper boundary pressure is 5 MPa, and the lower boundary temperature is 300 K. The pressure of the methane production well is 0.5 MPa, and the pressure of the carbon dioxide injection well is 8 MPa.

[0073] S3. Using the constructed multiphase, multicomponent coupled model, the pressure and concentration distribution of CO2 and CH4 within each grid cell of the geometric model (geological model) are solved, and the CO2 sequestration and CH4 production are calculated, achieving effective prediction of CH4 production and CO2 sequestration processes during CO2-ECBM. Specifically, in step S3, the multiphase, multicomponent coupled model constructed in step S1 is input into the coalbed methane reservoir geological model constructed in step S2 to solve for the pressure and mass fraction of methane and carbon dioxide within each grid cell of the reservoir geological model. After obtaining the mass fractions of methane and carbon dioxide, spatial integration is performed over the entire spatial range of the reservoir geological model to finally obtain the carbon dioxide sequestration and coalbed methane production in the entire reservoir geological model.

[0074] In this embodiment, as Figure 3 As shown, (a) is a schematic diagram of the calculated methane pressure evolution over 20 years; (b) is a schematic diagram of the calculated carbon dioxide pressure evolution over 20 years; (c) is a schematic diagram of the calculated methane mass fraction evolution over 20 years; and (d) is a schematic diagram of the calculated carbon dioxide mass fraction evolution over 20 years, which reveals the transport characteristics under the condition of three-phase coexistence of CO2, CH4 and water. Figure 4 This is a schematic diagram illustrating the calculated methane extraction and carbon dioxide sequestration amounts over 20 years in an embodiment of the present invention. Figure 3 As shown, injecting CO2 into coalbed methane significantly reduces the mass fraction of methane and its component pressure, while the mass fraction and component pressure of CO2 continuously increase. Correspondingly, as... Figure 4 As shown, methane production reached 1.96 × 10⁻⁶ after 20 years. 6 m 3 The CO2 sequestration capacity also reached 1.1 × 10⁻⁶. 7 m 3 .

[0075] This invention belongs to the field of natural hydrogen resource exploration and proposes a method for predicting CH4 production and CO2 sequestration during the CO2-ECBM process. The method includes establishing a multiphase, multi-component coupled model describing the migration processes of water, methane, and CO2 based on the mass conservation equations for gas-liquid two-phase fluids, gas components, and the Langmuir gas adsorption equation; establishing a geological model of the coalbed methane reservoir based on field exploration data and dividing it into grids, setting initial and boundary conditions for the reservoir geological model; and using the constructed multiphase, multi-component coupled model to solve for the pressure and concentration distribution of CO2 and CH4 within each grid cell of the geometric model, calculating CO2 sequestration and CH4 production, thus achieving effective prediction of CH4 production and CO2 sequestration during the CO2-ECBM process. This invention fully considers the influence of groundwater in coalbed methane on the migration behavior of methane and carbon dioxide, overcoming the limitations of existing studies that neglect the role of groundwater, leading to biased prediction results, and achieving accurate prediction of methane production and carbon sequestration during the CO2-ECBM process.

[0076] The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for a particular purpose.

Claims

1. A method for predicting CH4 production and CO2 sequestration in a CO2-ECBM process, characterized in that, Includes the following steps: Construct a multiphase, multicomponent coupled model to describe the migration processes of water, methane, and carbon dioxide in coalbed methane reservoirs; A geological model of a coalbed methane reservoir is constructed, and the geological model is divided into grids to obtain several grid cells that reflect the different spatial distribution locations of the coalbed methane reservoir. The initial conditions and boundary conditions of the geological model are then set. Based on the multiphase, multicomponent coupled model, the spatiotemporal evolution characteristics of pressure and concentration of water, methane, and carbon dioxide in each grid unit of the geological model are solved to obtain their distribution in time and space. Among them, constructing a multiphase, multi-component coupled model for describing the migration processes of water, methane, and carbon dioxide in coalbed methane reservoirs includes: Among them, the mass conservation equations for the gas and liquid phases are constructed as follows: In the formula, Porosity; The density of water; Water saturation; Indicates time; The velocity of the water flow; The density of the gas; This refers to the gas saturation level. This refers to the gas flow rate; For the mass source term of gas adsorption; The flow velocities of the gas and liquid phases are calculated using Darcy's law: In the formula, Formation permeability; The relative permeability of the aqueous phase; The viscosity of the aqueous phase; The pressure in the aqueous phase; It is the acceleration due to gravity; This refers to the relative permeability of the gas phase. This refers to the gas phase pressure. This refers to the gas phase density. This refers to the viscosity of the gas phase. in: In the formula, The density of methane; This represents the mass fraction of methane in the gas phase. The density of carbon dioxide; This represents the mass fraction of carbon dioxide in the gas phase. The viscosity of methane; The viscosity of carbon dioxide; Among them, the mass conservation equations for the gas phase components are constructed as follows: In the formula, The rate of methane mass reduction due to adsorption; Indicates the mass of methane adsorbed; The rate of decrease in carbon dioxide mass due to adsorption; The mass of carbon dioxide adsorbed; Among them, the Langmuir gas adsorption equation is constructed: In the formula, The mass of methane adsorbed; This represents the saturated adsorption volume of methane. Methane pressure; The pressure of methane after reaching saturation adsorption capacity; The mass of carbon dioxide adsorbed; This represents the saturated adsorption volume of carbon dioxide. Carbon dioxide pressure; The pressure of carbon dioxide after reaching saturation adsorption capacity; The geological model of the coalbed methane reservoir is constructed based on the exploration data, which includes the altitude and horizontal width range of the coalbed methane reservoir. The initial conditions of the geological model of the coalbed methane reservoir include the initial gas phase saturation, the initial coalbed methane reservoir pressure, and the initial coalbed methane reservoir temperature. The boundary conditions of the geological model of the coalbed methane reservoir include the upper boundary pressure, lower boundary pressure, methane production well pressure, upper boundary temperature, lower boundary temperature, and carbon dioxide injection well pressure.

2. The method for predicting CH4 production and CO2 sequestration in the CO2-ECBM process according to claim 1, characterized in that, It also includes calculating methane production and carbon dioxide sequestration based on the spatiotemporal evolution characteristics of methane and carbon dioxide pressure and concentration.

3. The method for predicting CH4 production and CO2 sequestration in the CO2-ECBM process according to claim 1, characterized in that, in, The meshing method includes free triangular meshing, wherein the size of the largest mesh cell is three times the size of the smallest mesh cell.

4. The method for predicting CH4 production and CO2 sequestration in the CO2-ECBM process according to claim 2, characterized in that, in, The spatiotemporal evolution of concentration is characterized by the spatiotemporal evolution of gas mass fraction; Based on the spatiotemporal evolution characteristics of gas mass fraction, spatial integration is performed on the spatial range of the geological model of the coalbed methane reservoir to obtain methane production and carbon dioxide sequestration.

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