Numerical Simulation of CO2-ECBM Based on Multi-Physical Field Coupling Model

Abstract

In this paper, heat injection and CO2 injection are combined, and the influence of coal seam parameters on CO2-ECBM is analyzed to improve the production of CH₄ and CO₂ reserves and the effective control of both greenhouse gases. A multi-physical field coupling model of CO2-ECBM was established based on Darcy’s law, Fick’s law of diffusion, the extended Langmuir model for adsorption, and the equation of state. Numerical simulation of CO2-ECBM under different coal seam parameters was carried out by COMSOL Multiphysics. The results show that increasing the injection pressure of the CO2 injection well and the initial pressure of the coal seam can effectively increase the gas pressure and concentration gradient, which has a positive effect on improving the extraction concentration of CH₄ and the sequestration concentration of CO₂ in the coal seam. The increase of the initial temperature of the coal seam will promote the desorption and diffusion of the binary elemental gas, resulting in a decrease in the concentration of coalbed methane and a decrease in the displacement effect. In the process of displacement, the greater the initial permeability, the greater the fracture opening of the coal seam, which is more conducive to the seepage transport of the gas. The closer to the position of the injection well, the better the displacement effect and the lower the permeability rate ratio.

Recent Advances and Perspectives of CO2-Enhanced Coalbed Methane: Experimental, Modeling, and Technological Development

“…Therefore, the increase of the injection pressure will shorten the breakthrough time of CO 2 . 60 Within the same period, CO 2 -ECBM is more effective with lower injection pressure. 59 Liu et al 62 established an improved diffusion model considering the lost gas and the initial desorption time of adsorbed gas.…”

“…The results found that displacing efficiency increases with the CO 2 injection pressure, which domains the adsorption capacity on coal. , CO 2 has a stronger adsorption affinity, resulting in a longer breakthrough time than other gases. Therefore, the increase of the injection pressure will shorten the breakthrough time of CO 2 . Zhang et al conducted CO 2 -ECBM experiments with high-grade bituminous coal cores at injection pressures ranging fron 6 to 10 MPa.…”

Coupling Mechanism of THMC Fields in Crushed Soft Coal with Low Permeability after CO2 Injection and Its Application in CO2-ECBM Technology

“…Internal energy Thermal convection Heat conduction Strain energy Gas adsorption heat (22) The five terms on the left of eq 22 represent the changes of internal energy, heat transfer, and thermal conversion, as well as the changes of coal skeleton strain energy and gas adsorption energy, respectively. where…”

“…During the CO 2 -ECBM process, fluid migration is dominated by a two-phase flow. Considering the Klinkenberg effect in porous media, the flow rate of fluid in fracture follows the generalized Darcy’s law: { true q g i = − k k r g μ g i true( 1 + b k P f g i true) ∇ P f g i q w = − k k r w μ w ∇ P f w where k is the absolute permeability, m 2 ; k rw is the relative permeability of the water phase; k rg is the relative permeability of the gas phase; u w is the hydrodynamic viscosity, Pa · s; u gi is the dynamic viscosity of i, Pa · s; and b k stands for the Klinkenberg factor.…”

Wettability Characteristics of Low-Rank Coals Under the Coupling Effect of High Mineralization and Surfactants

“…Experimental studies aimed at improving the wettability of coal seams and gas extraction efficiency concentrated on water injection and gas injection displacement. Research on water injection focused on the use of surfactants. − Surfactants are the main wetting agents in improving wettability, and surface tension is a key parameter for their effectiveness. − Jiang et al found that OP-10 surfactant solutions had the best effect on improving coal wettability based on surface tension and contact angle measurements of different wetting agents. , Several studies have explored the effect of wetting agents in different proportions or combined with other materials to improve the wettability of coal seams. ,− For instance, Zou et al added water-based SiO 2 nanofluids to the wetting agent solution and proposed a water injection method for coal seam improvement. , Molecular simulation software has been employed by some scholars to investigate the mechanism of surfactant effect on enhancing coal seam wettability. ,, In addition, the COMSOL simulation software was used to develop a coupled damage model during water injection to increase gas extraction rates. , Studies on gas injection displacement for improving gas extraction rates can be categorized into two types: using gases with strong adsorption capacity or gases that can break the dynamic balance of coal seam gas. − CO 2 -enhanced coalbed methane (CO 2 -ECBM) technology is essential to ensure energy security and achieve the “dual carbon” goal. This technology is crucial for achieving the low-carbon, clean, and efficient utilization of coal-based fossil energy.…”