Computational analysis of feasibility of methane displacement by carbon dioxide during enhanced gas recovery from calcite-rich shale

Abstract

Vast reserves of unharnessed shale gas, primarily CH4, present attractive and sustainable energy resources as an alternative to the current conventional petroleum-based energy resources. As in case of petroleum resources, an important question to address while harnessing these resources is the feasibility of their recovery in lean gas wells. Enhanced gas recovery techniques aim at the recovery of CH4 via its displacement by suitable gases. Mineral-gas interactions and the associated adsorption–desorption energetics have been conventionally used to predict the success of gas recovery operations. On the basis of adsorption–desorption energetics, CO2 has been proposed as one of the promising gases by several investigators. However, the mechanism of gas recovery involves the displacement of CH4 by CO2, which is an activated process, and the science behind the activated displacement needs to be explored. In this study, we have utilised van der Waals corrected density functional theory calculations to provide insights into shale-CH4 and shale-CO2 interaction energetics with calcite as the representative shale mineral. We have provided a detailed mechanism behind the displacement of CH4 by CO2 via a comparative sampling to its gas phase versus its displacement via surface diffusion. Our computational investigations conclusively prove the thermodynamic feasibility of the displacement of CH4 by CO2 with the adsorption of CO2 being one and a half times stronger than the adsorption of CH4 over a representative calcite surface. Further, our computational analyses also conclusively prove the kinetic feasibility of the displacement of CH4 by CO2 to its gas phase via crossing of an activation barrier of an acceptable magnitude. Finally, we have provided insights into the science behind the feasibility of successful CO2 sequestration during the enhanced gas recovery process.