Complex evolution of coal permeability during CO₂ injection under variable temperatures

Although the influence of cross couplings between coal deformation, gas flow and thermal transport has been widely recognized, their impacts on the evolution of coal permeability are still not well understood. CO₂ may be injected at -40°C, 60°C lower than that of the targeted coal seams for sequestration. Under these injection conditions, coal matrix may swell due to the thermal expansion and shrink due to the change in adsorption capacity. This uncertainty of swelling/shrinking complicates the prediction of coal permeability. In this study, a fully coupled coal deformation, gas flow and transport, and thermal transport model is developed to evaluate the complex evolution of coal permeability under the combined influence of variable gas pressure and temperature. These combined effects are evaluated through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking and fracture aperture alteration, and translations of these interactions to the evolution of coal permeability. The fully coupled model is applied to evaluate why coal permeability changes instantaneously from reduction to enhancement under the free swelling condition as widely reported in the literature. Our results have revealed the transition of coal matrix swelling from local swelling to macro-swelling as a novel mechanism for the simultaneous switching of coal permeability from the initial reduction to the late recovery. At the initial stage of CO₂ injection under variable temperatures, matrix swelling due to gas sorption, thermal expansion and the change in adsorption capacity is localized within the vicinity of the fracture compartment. As the injection continues, the swelling zone is widening further into the matrix and the swelling becomes macro-swelling. When the swelling is localized, coal permeability is controlled by the internal fracture boundary condition and behaves volumetrically; when the swelling becomes macro-swelling, coal permeability is controlled by the external boundary condition.