Coal Swelling and Permeability Evolutions with Injecting Sub- and Super-Critical Co2 in Sub-Bituminous and Bituminous Coal: An Experimental Study

Abstract

CO2 sequestration in coalbed is currently identified as one of the promising solutions to reduce CO2 emissions, especially in deep un-minable coal seams, which can easily provide a supercritical condition for CO2. Under in-situ stress conditions, the volumetric expansion of coal mass due to CO2 injection was an important factor to affect CO2 sequestration. Although previous research has investigated the effect of coal swelling on permeability, little attention has been paid to distinguish effects of coal rank on the permeability, especially with respect to coal deformation due to supercritical CO2 injection. Therefore, the main objective of this study is to discover how super-critical CO2 permeability and the volumetric strain vary with CO2 injection in sub-bituminous and bituminous coal under stress-constrained confinements. A series of super-critical CO2 permeability tests were conducted under a range of injection pressures (10-15 MPa) and confinements (16-20MPa) at 50℃. Results indicate that a non-linear increase of volumetric strain was observed at 16 MPa confinement in both sub-bituminous coal and bituminous coal, which resulted in an unexpected drop in permeability at initial supercritical CO2 injection. This is mainly attributed to the coupling effects of adsorption-induced swelling and reduction of compressive strength. After that, the coupling effects due to CO2 saturation fade away, and the volumetric strain increased with the progressive injection pressure mainly due to the decrease of effective stress. Thus, a linear relationship between the volumetric strain and the injection pressure was obtained. Due to difference of coal rank, more dominant variation of volumetric strain and permeability occurred in bituminous coal than sub-bituminous coal after supercritical CO2 injection.