Surface thermodynamics of hydrocarbon vapors and carbon dioxide adsorption on shales

Abstract

Understanding hydrocarbon vapors and carbon dioxide adsorption mechanism on shales lays the foundation for in situ hydrocarbon resource estimation and enhanced hydrocarbon recovery via carbon dioxide injection. However, surface thermodynamic potentials of hydrocarbon vapor and carbon dioxide adsorption on shales have rarely been reported. This work develops a rigorous framework for direct description of hydrocarbon vapors and carbon dioxide adsorption isotherms on shales and for straightforward calculation of the intrinsic thermodynamic potentials by considering non-ideal gas behavior. On the basis of the Langmuir adsorption model, the maximum adsorption capacity of methane, ethane, propane, n-butane, iso-butane and carbon dioxide adsorption on shales positively correlates to each gas’ molecular mass. Carbon dioxide adsorption capacity is higher than methane and ethane but is lower than propane, n-butane and iso-butane. According to the generalized multilayer adsorption model, the monolayer adsorption capacity of n-hexane is slightly higher than that of n-heptane due to the small molecular diameter of n-hexane. The temperature-dependent behavior of isosteric enthalpy and entropy for these vapors is attributed to their non-ideal gas behavior and the temperature-dependent adsorption uptakes. Isosteric enthalpy and entropy in general positively correlate to the molecular mass of vapors. Isosteric enthalpy and entropy of carbon dioxide and propane are almost identical in behavior given that their molecular masses are very close. Isosteric enthalpy and entropy of iso-butane are lower than that of n-butane due to their molecule polarity difference. The shale selectivity of propane, n-butane and iso-butane is higher than carbon dioxide while the shale selectivity of methane and ethane is lower than carbon dioxide. These surface thermodynamic characteristics therefore provide new perspectives on understanding the interaction of hydrocarbon vapors/carbon dioxide and shales for enhanced hydrocarbon recovery via carbon dioxide injection.