Hydrogen storage in depleted gas reservoirs using nitrogen cushion gas: A contact angle and surface tension study

The utilization of depleted natural gas reservoirs is a significant consideration for large-scale hydrogen (H₂) storage and production. However, accurate knowledge of cushion gas compositions such as nitrogen (N₂), methane (CH₄), and carbon dioxide (CO₂) is crucial, as they are critical components that affect the rock-fluid interfacial phenomenon, thereby impacting deployment feasibility. Moreover, there are limited reported studies on rock/brine/gas-mixture wettability and gas-mixture/brine surface tension for this type of reservoir. To address this gap, we explore the possibility of using N₂ as a cushion gas (in the presence of CH₄ and CO₂) on a pristine quartz for H₂ storage across various pressure (500–3000) psi, temperature (30–70) °C, and salinity (2–20) wt.%. We conducted extensive contact angle (CA) and surface tension (ST) experiments for different gas mixtures (H₂–N₂–CH₄–CO₂) to generate relevant data for H₂ storage in depleted natural gas reservoirs using drop shape analyzer (DSA 100) equipment. Our findings suggest that the wettability behavior of the gas-mixture compositions studied is likely to remain consistent unless the rock's initial wetting state is altered. The CAs were observed to range between 28 and 46°, regardless of reservoir pressure, temperature, and salinity. It also increased with increasing pressure but decreased with increasing temperature. In addition, CA was lower at a lower N₂ fraction (Mix 1–80% H₂ – 10% N₂ – 5% CH₄ –5% CO₂) compared to a higher N₂ fraction (Mix 4–20% H₂ – 70% N₂ – 5% CH₄ –5% CO₂). The ST of the gas-mixture/brine systems declined with increasing (i) pressure, (ii) temperature, and (iii) N₂ fraction but increased with increasing salinity for every gas mixture. Based on our investigation, mixtures 2 and 3 with H₂ (40–60%) and N₂ (30–50%) fractions (at constant 5% CH₄ and 5% CO₂) were identified as the optimal cushion gas for H₂ storage and withdrawal under the studied conditions. The results of this study provide accurate and valuable input parameters and data for reservoir-scale simulations utilized in optimizing geo-storage in depleted natural gas reservoirs.