Abstract
The substitution of fossil fuel with clean hydrogen (H2) has been identified as a promising route to achieve net zero carbon emissions by this century. However, enough H2 must be stored underground at an industrial scale to achieve this objective due to the low volumetric energy density of H2. In underground H2 storage, cushion gases, such as methane (CH4), are required to maintain a safe operational formation pressure during the withdrawal or injection of H2. The wetting characteristics of geological formations in the presence of H2, cushion gas, and the resultant gas mixture in the mixing zone between them are essential for determining storage capacities. Therefore, the present work measured the contact angles of four Jordanian oil shale rocks with H2, CH4, and H2- CH4/brine mixture systems and their interfacial tension (IFT) in geological storage (geo-storage) conditions (pressures of 0.1 to 1600 psi and temperature at 323 K) to evaluate the residual and structural trapping potential and efficiency of CH4 as a cushion gas. Various analytical methods were employed to comprehend the bulk mineralogy, elemental composition, topographic characterization, functional groups, and surface properties of the Jordanian oil shale rocks. The total organic carbon (TOC) effect on wettability was demonstrated and compared with previous studies. The Jordanian oil shale samples with high to ultrahigh TOC of 13 % to 18 % exhibited high brine advancing/receding contact angles. The rock samples became hydrophobic at the highest experimental pressure and temperature conditions (1600 psi and 323 K). The rock/CH4/brine contact angles were higher than the rock/H2/brine contact angles, and the H2-CH4/brine mixture contact angles remained in between those for pure gases. Moreover, the IFT displayed the inverse trend, where the H2/brine IFT measured higher than the CH4/brine IFT. The results suggest that the H2 geo-storage in the tested organic-rich source rocks could be favorable when CH4 is used as a cushion gas, consistent with previous studies using synthetically acid -aged shale samples. For the first time, the present work used organic-rich rocks from Jordanian oil shale to present a more realistic situation and evaluate the influence of missing organic material and gas on the H2/brine/ rock wettability during H2 geo-storage.