In-situ disposal of CO2: Liquid and supercritical CO2 permeability in coal at multiple down-hole stress conditions

•Liquid and supercritical CO2 permeability in coal at deep, in-situ conditions.•High reduction in permeability using supercritical CO2 over liquid CO2.•Empirical relations for exponential decline in permeability with effective stresses.•Early data on subcritical and supercritical CO2 flow in Indian bituminous coals.•N2 effectively reduces loss in coal permeability with very low adsorption.

Geological CO2 sequestration is one of the most effective methods to counter global climate change. Coal matrix shrinkage, swelling, gas diffusion and permeability are the key phenomena associated with geological CO2 disposal in coal. In this study, permeability experiments were approached using the supercritical and liquid phases of CO2 (less understood; most likely insitu phases) for naturally fractured bituminous coal. Experiments were performed under triaxial conditions using four sets of various confinement conditions corresponding to variable depths. Injection pressure was varied gradually and the permeability changes were calculated using the Darcy's equation for subcritical and supercritical CO2 exclusively. Changes in CO2 phases were obtained by changing the system temperature from 26° C for liquid CO2 to 34 °C for supercritical CO2. N2 was alternatively injected at the start and between the injections of CO2 to analyze the changes in the permeability from a relatively very less sorptive medium's perspective. It was observed that the supercritical CO2 flow reduced the permeability significantly and this behavior was greatly attributed to the highly viscous nature of the supercritical phase and the high volumetric deformation or the swelling of coal under supercritical CO2 as compared to liquid CO2. The injection pressures were observed to reduce the effective stress behavior, which in turn pushed the permeability evolution at each confinement to a positive trend. However, the permeability of CO2 reduced exponentially with increasing effective stresses. Two different empirical equations were proposed for permeability of both the phases of CO2 with effective stresses.