Influences of moisture on adsorption and desorption of methane on gas shales

Abstract

ABSTRACT The moisture (H2O) within gas shale reservoirs significantly affects methane (CH4) adsorption and desorption on shales. To further address these influences, H2O adsorption on four shales selected from the Lower Silurian Longmaxi Formation located in southern Sichuan Basin of China was investigated. Moreover, CH4 adsorption and desorption behaviors on moist shales were also studied and elucidated. Results verify that the modified Brunauer-Emmet-Teller (BET) model with the assumption of dual adsorption sites can well describe H2O adsorption on shales, the multiple correlation coefficients of which are within the range of 0.9926–0.9979. The monolayer adsorption capacity of H2O on shales derived from the modified Brunauer-Emmet-Teller (BET) model varies between 0.0052 and 0.0087 g/g. For relative pressure (P/P s) less than 0.65–0.80, the oxygenic functional groups accommodate the primary adsorption sites for H2O on shales. Then, H2O clusters formed on the secondary adsorption sites, i.e. the previously adsorbed H2O molecules and the clay minerals, play a dominant role for H2O adsorption on shales at P/P s greater than 0.65–0.80. The elevated moisture content of shales leads to a sharp decrease in CH4 adsorption capacity. Particularly, the decreasing magnitude in maximum CH4 adsorption capacity generated based on the Langmuir model varies between 11.80% and 48.30% for all the moist shale samples. The desorption study indicates that CH4 adsorption/desorption hysteresis is more pronounced for the moist shales than the dry shales. Two possible mechanisms that the pore throat of shales is blocked by the desorbed H2O molecules, and CH4 molecules are readsorbed on the adsorption sites previously occupied by H2O clusters, could explain the influences of moisture on CH4 adsorption/desorption hysteresis on shales. The moist shales at reservoir saturation condition are strongly suggested to perform CH4 adsorption test to accurately estimate the total shale gas resources. Furthermore, aiming to deeply explore the mechanism regarding H2O dependence of CH4 adsorption and desorption within shales, the terahertz waves with superior accuracy of monitoring trace H2O under high-pressure CH4 condition, are recommended into the volumetric or gravimetric method to generate highly reliable data regarding CH4 adsorption and desorption on moist shales.