Modelling stress and strain in coal seams during CO2 injection incorporating the rock–fluid interactions

Abstract

To reduce the adverse effects of global warming, geological sequestration has been suggested, which includes capturing and pumping anthropogenic CO2 into deep underground formations, such as coalbeds. Despite the advantages of coalbed sequestration, its geomechanical aspects are not well studied. In particular, the coupling between geomechanical and reservoir/adsorption behaviours of coal seam has been largely neglected. This paper aims to address this shortcoming by developing a coupled chemo-poro-mechanical model that predicts the geomechanical performance of a coal seam in which CO2 is being injected. Recent studies have shown that the interaction of CO2 and coal results in changes in geomechanical properties of coal, namely elastic modulus and peak strength. In order to investigate the significance of these changes in the geomechanical response of the storage site, an analytical solution was found to predict the distribution of stress within an axisymmetric reservoir, considering the effect of sorption-induced changes on mechanical behaviour. The analytical model was then coupled with a simplified dual porous reservoir simulation tool to study the effect of CO2 injection on reservoir and geomechanical performance of the coal seam. The results of the simulation showed that the changes in geomechanical properties of coal can significantly influence the stress and strain distributions within the formation, and therefore, the permeability distribution. Also, it was found that for the example simulated in this paper the adsorption induced reduction in strength did not influence the extent of the mechanical failure zone of the reservoir.