Entropy and enthalpy changes during adsorption and displacement of shale gas

Abstract

Shale gas is a major component of the hydrocarbon economy, and there are abundant studies in related exploration and exploitation fields using simulations and experiments. In this paper, the kinetic processes of adsorption and displacement during shale gas exploitation are analyzed using enthalpy-driven and entropy-driven processes with molecular dynamics simulations and statistical mechanics analyses. First, the changes in symmetry of CH4 and CO2 adsorbed on the graphene surface are analyzed. The adsorption process is a spontaneous process, where the Gibbs free energy decreases. During this process, the gas molecular entropy and the enthalpy of the system decrease, which indicates an enthalpy-driven process. The changes in entropy and enthalpy of various gas molecules (CH4, CO2, N2, and H2O) during the adsorption process are obtained. Through comparing the entropy and enthalpy changes before and after the displacement of CH4 by the different displacement media (CO2, N2, and H2O). The CO2-injected displacement of CH4 is driven by both enthalpy and entropy, and the N2-injected displacement of CH4 is driven by entropy. However, H2O-injected displacement of CH4 is not a spontaneous process. It plays a certain displacement role by reducing the partial pressure of CH4. To clarify the detailed spontaneous processes, the entropy change and exothermic processes during the supercritical fluid displacement of shale gas are discussed; then, the differences in displacement efficiency of different supercritical fluids are explained. This study is useful for understanding the kinetics mechanism of adsorption and displacement of shale gas.