Organic-rich source rock/H2/brine interactions: Implications for underground hydrogen storage and methane production

There is increasing global interest in the attainment of a carbon dioxide-free global economy by replacement of carbon-based fossil fuels with clean hydrogen. But huge volume of hydrogen needs to be stored to address the imbalance between energy demand and supply due to low volumetric energy content of H2. Underground Hydrogen Storage (UHS) is an integral part of the hydrogen economy value chain and an appealing technology for the attainment of global decarbonization. The Jordan oil shale (Upper Cretaceous, organic-rich source rock sequence) has been proposed as a promising geological storage medium for hydrogen due to its exceptionally high organic content because methane can be produced from the reaction of trapped hydrogen with the kerogen. However, the extraction of methane from kerogen during hydrogen storage in organic-rich shale has not yet been reported. In this study, pressurized hydrogen was injected into high TOC shale samples for 80 days at 1500 psi and 75 degrees C to assess the extent of hydrogen-rock reaction and possible methane production. We also measured H2/ brine and CH4/brine interfacial tensions (IFT), as well as the contact angles of H2/brine/shale and CH4/brine/ shale systems at 75 degrees C and varying pressure (500-1500 psi) to understand the shale-fluid interaction at subsurface conditions. Results indicate some reaction of H2 with the shale after 80 days. No hydrogen sulfide was detected from gas chromatography analysis at the end of the experiment, but traces of methane (0.018 %) were detected. The reaction between hydrogen and organic matter of the shale yielded only a little methane under the conditions investigated in this research, possibly because the experimental temperature and reaction time was insufficient for copious methane generation. Furthermore, contact angle values of CH4/brine were found to be higher than of H2/brine. This means that under geo-storage conditions, the surface becomes fully CH4-wet condition (131o-149o) but remains only intermediate-wet in the presence of the H2/brine system (94o -106o) with increasing pressure, confirming that the interaction of methane with the shale surface is higher than hydrogenshale surface interaction at similar conditions. The opposite trend was observed for IFT values. The hydrogen/ brine IFT was 69.4 mN/m at 500 psi but decreased to 67.9 mN/m at 1500 psi. Likewise, the methane/brine IFT decreased from 69.7 mN/m to 58.3 mN/m with increasing pressure from 500 to 1500 psi. The findings of this study contribute to a better understanding of the adsorption trapping potential of organic-rich source rocks.