A Laboratory Investigation of Permeability of Coal to Supercritical CO₂

Abstract

The kinetics of gas–coal interaction during coalbed methane (CBM) recovery and/or carbon dioxide sequestration in coal has been subject of investigation over past few years. Swelling of coal matrix due to gaseous phase CO₂ injection is now well established through laboratory experiments and field validations. Further, fluid exchange or flow in coal alters effective stresses in the underground environment. These significantly affect the permeability characteristics of coal and therefore influence the gas recovery/injection projects. Most research works on coal seam sequestration have been carried out for gas or liquid phase CO₂. Considering the pressure–temperature conditions of deep seated coal, studies are now being done on supercritical CO₂ flow and adsorption in coal. A newly developed experimental set up was utilized to replicate the underground conditions in laboratory to investigate the (1) initial N₂ permeability of coal, (2) supercritical CO₂ permeability of coal, (3) effects of CO₂ sorption on N₂ permeability of coal. The temperature of the set up was maintained at 33 °C while CO₂ injection pressures were varied between 11 and 15 MPa at a range of 16–24 MPa confinements, and N₂ was used as a relatively neutral medium to estimate the loss in permeability due to supercritical CO₂ flow. The results indicate that high adsorption of supercritical CO₂ in coal led to significant reduction in permeability. On increasing the confining pressure, further decline in the permeability was recorded. CO₂ becomes liquid-like with increasing confining pressures and the coal–fluid interactions change, causing high sorption and matrix swelling, leading to reduced permeability in coal. This explains why substantial decline in the injection rate of CO₂ was observed progressively in most CO₂ sinks.