A model of coal–gas interaction under variable temperatures

Although coal–gas interactions have been comprehensively investigated, fewer studies consider the impact of thermal effects. In this study, a fully coupled model of coal deformation, gas transport, and thermal transport is developed and solved using the finite element method. A general model is developed to describe the evolution of coal porosity under the combined influence of gas pressure, thermally induced solid deformation, thermally induced gas adsorption change, and gas-desorption-induced solid deformation. This porosity-evolution relationship is implemented into a fully coupled model for coal deformation, gas transport, and thermal transport using the finite element (FE) model. The FE model represents important nonlinear responses due to the effective stress effects that cannot be recovered where mechanical influences are not rigorously coupled with the gas and the thermal transport systems. The controlling effects of gas pressure, temperature and gas sorption on these nonlinear responses of coal porosity and permeability to gas production are quantified through a series of simulations. It is found that the gas-desorption-induced deformation is the most important factor that controls these nonlinear responses. In this work, among the factors such as thermal expansion of solid and gas, and convective heat flux, in addition to the thermal diffusion, the heat sink due to thermal dilatation of gas is most prominent factor in altering the temperature of coal seam. This conclusion demonstrates that the thermal impact on coal–gas interactions cannot be neglected especially where the temperature is high.

Research Highlights
► A general porosity model under gas pressure, temperature and deformation is proposed.
► A coupled model of coal-gas interaction under variable temperatures is developed.
► The porosity model and the coupled model are implemented with finite element method.
► The coal-gas interaction under variable temperature is numerically quantified.