Methane displacement desorption in coal by CO2 injection: Numerical modelling of multi-component gas diffusion in coal matrix

A numerical model for competitive displacement of adsorbed methane by CO2 injection is presented. A key feature of the model is the use of a bidisperse pore-diffusion model that accounts for both macropore and micropore diffusion in the coal matrix. The model was applied to analyse the performance of a laboratory core flush test and an excellent match to the test data was achieved. The results of history matching indicate that the overall sorption rate is controlled by the micropore diffusion. Furthermore, the micropore diffusivity is not a constant but varies inversely with the total sorbate concentration in the coal sample.