CO2 rich cushion gas for hydrogen storage in depleted gas reservoirs: Insight on contact angle and surface tension

Abstract

Hydrogen storage in porous media (aquifers and depleted reservoirs) is an effective means of achieving large-scale inter-seasonal demand for energy. In particular, the depleted gas reservoir provides substantial storage capacity and additional economic incentives due to the preexistence of the storage infrastructure as well as the requisite reservoir integrity. However, the interfacial phenomenon between the injected hydrogen, the native gas (cushion gas), the formation brine, and the host rocks require further investigation for efficient gas containment. In this study, we conducted a comprehensive examination of the application of CO2 as a cushion gas. Our investigation involved performing contact angle and surface tension experiments using the drop shape analyzer (DSA) 100 equipment. We examined various gas mixtures (H2–CO2 – CH4–N2) under different pressure ranges (500–3000 psi), temperature levels (30, 40, 50, 60, 70 °C), and salinities (2, 5, 10, 15, and 20 wt%).Results indicate that the wettability behavior of the studied gas-mixture compositions remains relatively consistent unless there are changes in the initial wetting state of the rock. The contact angles observed ranged from 29 to 51°, independent of reservoir pressure, temperature, and salinity. The contact angle values increased with higher pressure but decreased with higher temperature. Furthermore, the contact angle was lower for a gas mixture with a lower CO2 fraction compared to a gas mixture with a higher CO2 fraction. Conversely, the gas mixture brine surface tension exhibited a decrease with increasing pressure, temperature, and CO2 fraction. However, it increased with increasing salinity for all gas mixtures. Based on the results obtained from the contact angle and surface tension analysis, mixture 4 emerged as the ideal fraction for designing CO2 cushion gas in terms of both hydrogen storage and withdrawal. These findings present precise and valuable input parameters and data that can be utilized in reservoir-scale simulations to optimize geo-storage in depleted natural gas reservoirs.