Rethinking sorption mechanisms in shales with varying petrophysical characteristics - An investigation of hydrogen and carbon dioxide

Sorption in shales is a key process controlling their gas storage capacity. It is, however, a complex phenomenon due to the broad and variable petrophysical characteristics of shales. This complexity is further compounded by the simultaneous occurrence of gas adsorption on mineral and organic surfaces and absorption processes within sedimentary organic matter. In this study, gravimetric sorption experiments were conducted on three geologically distinct shales at 40 °C and pressures ranging from 2 to 15 MPa. We examined both pristine powdered shale samples and their solvent-extracted counterparts to investigate the dominant controls and sorption mechanisms of hydrogen (H 2 ) and carbon dioxide (CO 2 ) - two gases central to energy transition. The sorption capacity of H 2 is shown to be independent of the total organic carbon (TOC) content and is rather governed by the available pore volume in the shale, with thermal maturity exerting a positive influence on porosity development, particularly in samples exhibiting contrasting maturities. Conversely, the CO 2 sorption capacity is controlled by both TOC content and available pore volume. An increase in either parameter enhances the sorption capacity. Comparisons of H 2 and CO 2 sorption capacities among samples possessing relatively similar textural properties and contrasting TOC content indicate that the contribution of H 2 absorption in organic matter becomes increasingly significant at elevated pressures, while CO 2 absorption is shown to increase the total sorption capacity at all investigated pressures. This study provides the first experimental evidence on the role of H 2 and CO 2 absorption in increasing the sorption capacity of immature organic-rich shales. • H 2 retention occurs via adsorption, governed mainly by textural properties. • At high pressure and in TOC-rich shales, absorption enhances H 2 sorption. • CO 2 sorption is enhanced by larger textural properties and/or higher TOC content.