Effects of methyl orange on the H2/brine wettability of carbonate rocks: Implications for H2 geo-storage

Nowadays, researchers have shown interest in hydrogen energy because it is regarded as a clean energy and a viable alternative to carbon-based fossil fuels. However, hydrogen has low volumetric energy density and must be stored in large volumes to fulfil global energy demands. Underground hydrogen storage in a geological structure is considered the best choice due to its safety and higher storage capacity than surface storage. In this study, the advancing $\left({\theta }_a\right)$ and receding $\left({\theta }_r\right)$ contact angles of the stearic acid–aged calcite before and after the treatment with different concentrations (10–100 mg/L) of methyl orange (MO) were measured at reservoir conditions (pressure range: 1–15 MPa, temperatures of 25 °C and 50 °C, and constant salinity of 0.3 M). Moreover, atomic force microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy were used to characterize the effects of MO on stearic acid–aged calcite. The findings demonstrated that stearic acid–aged calcite exhibited hydrophobic condition (H₂-wet); however, the wettability of the stearic acid–aged calcite was restored to the original hydrophilic condition (water-wet) upon treatment with optimum MO concentration (100 mg/L). The surface roughness of pure calcite increased from 5.4 nm to 137 nm when modified with stearic acid due to the adsorption of organic acid molecules on the surface. However, the surface roughness further decreased from 137 to 49 nm when treated with MO, showing that MO treatment increased the rock water's wetting tendency. This research highlights the importance of the MO injection into an underground reservoir that could provide a better solution to subside the negative effects of discharging MO into the environment and maximise the storage capacity of H₂.