Saturation time dependency of liquid and supercritical CO2 permeability of bituminous coals: Implications for carbon storage

Various types of lithospheric geological reservoirs are available for the safe and long-term storage of CO2. The phase of CO2 transitions between liquid and supercritical (mostly supercritical for depths greater than 800 m) when injected into the subsurface. This high-density super-critical phase prevents CO2 molecules from upward diffusion toward the surface. Many questions arise in the context of coal seam sequestration with respect to its capacity, injectivity, plume movement, and leakage assessment. This work examines the effect of various saturation periods on the permeability evolution of porous coal. In this triaxial study, liquid and supercritical CO2, along with N2 were injected into a coal sample from the Jharia coal field (Jharkhand state, India). It was observed that the permeability reduction for liquid CO2 was 25%, 13.5% and 1% for 0-20 h phase, 20-40 h phase and 40-60 h phase, respectively. For supercritical CO2, the reduction was observed to be 42.5%, 38.4% and 11%, respectively, for the three saturation cycles. The findings highlight that while high CO2 sorption in supercritical phase stores more of the adsorbate, the reduction in permeability slows down injection operation and the saturation induced coal expansion threatens the overall stability of the system.