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) degrees 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 degrees, 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.(c) 2023 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.