Methane Displacement Desorption in Coal by CO2 Injection: Numerical Modelling of Multi-Component Gas Diffusion in Coal Matrix

Abstract

Laboratory sorption isotherm measurements for pure gases have demonstrated that coal can adsorb at least twice as much CO2 by volume as methane. This suggests that large volumes of CO2 could be stored in deep unminable coal seams worldwide. The injection of CO2 or flue gas in coalbeds has an added advantage that it could enhance coalbed methane recovery. Coal seams may be characterized by two distinctive porosity systems: a well-defined and almost uniformly distributed network of natural fractures (cleats) and matrix blocks containing a highly heterogeneous porous structure between the cleats. The pore structure of coal is highly heterogeneous, with the pore size varying from a few Angstroms to frequently over a micrometer. Three mechanisms have been identified for diffusion of an adsorbing gas in the macropores: molecular diffusion, Knudsen diffusion, and surface diffusion. Although diffusion of methane and other gases in coal has been extensively investigated in the past, research into CO2-CH4 counter-diffusion and competitive adsorption/desorption in coal has been very limited. A numerical model for competitive displacement of adsorbed methane by CO2 injection has been presented in this chapter. 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 analyze 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.