Temperature- and pressure-dependent gas diffusion in coal particles: Numerical model and experiments

The diffusion coefficient is a key parameter related to gas migration in coalbed methane extraction and CO2 geologic sequestration applications. Isothermal assumptions are usually used in diffusion models and experimental tests, in which the thermal effect and its impact on gas adsorption/desorption are neglected. Coal powder gas diffusion tests are not exactly reflective of an isothermal process, in fact, due to gas-desorption-induced decreases in temperature. In this study, a coupled thermal-diffused-mechanical model for gas desorption-diffusion under non-isothermal conditions is developed in which the effects of gas pressure, temperature, and coal porosity on the gas diffusion coefficient are considered. The effect of gas-desorption-induced temperature variation is also included. Gas desorption-diffusion tests are conducted on coal powder under isothermal and adiabatic conditions, respectively. The developed model is then used to simulate the gas desorption-diffusion process under adiabatic conditions and the results are compared against the test data. The proposed non-isothermal model with varying diffusion coefficient, which can be regarded as an improvement of the classical unipore diffusion model, is shown to replicate the whole desorption-diffusion process and back-calculate a relatively consistent diffusion coefficient value range for all scenarios. The results also show that the assumption of isothermal conditions in a diffusion model significantly overestimates the accumulated gas diffusion content throughout the gas desorption-diffusion process. The assumption of constant diffusion coefficient partially underestimates the gas diffusion rate and accumulated gas diffusion content in the later stages.