Thermodynamic behavior of water vapor adsorption in shale and its dependence on organic matter and clay minerals

Abstract

Isothermal adsorption experiments of water vapor in shale samples with differing TOC (total organic carbon content) and differing clay content were conducted at three set temperatures to reveal the adsorption mechanism, and to determine the thermodynamic dependence of primary and secondary adsorption processes on the content of organic matter and clay minerals. The adsorption of water vapor in shale is spontaneous with the Gibbs free energy change is minus ranging from −0.28 - −5.15 KJ/mol, the adsorption changes from entropy driven to enthalpy driven as the adsorption amount increases (enthalpy change turns from plus to minus ranging from −1.25–––6.72 KJ/mol, and the entropy change is opposite ranging from −0.0031–––0.035 KJ/mol), and the primary adsorption amount increases with higher experimental temperature but secondary adsorption amount decreases. Fitting results and their relying theories indicate that micropore filling and monolayer adsorption occur in primary adsorption mainly though strong hydrogen bonding and electrostatic forces between water vapor and the pore surface, and multilayer adsorption and capillary condensation dominate secondary adsorption mainly through weak hydrogen bonding among water vapor. There are positive correlations of adsorption amount versus both clay content and TOC. Clay minerals are strongly hydrophilic because of their exchangeable cations and polar groups, providing primary and secondary adsorption sites, and enhancing the spontaneity of adsorption. OFGs (oxygen functional groups) in organic matter are hydrophilic, other pore surfaces are hydrophobic or weak hydrophilic, which also develop many adsorption sites. Results from this work provide new insights about the influence of organic matter and clay minerals on the adsorption thermodynamic behavior of water vapor in shale.