Influence of nitrogen cushion gas in 3-phase surface phenomena for hydrogen storage in gas condensate reservoirs

Recent research has primarily focused on 2-phase systems for hydrogen storage in porous media. However, the impact of cushion gases mixed with hydrocarbon fluids, such as n-octane, in depleted condensate-type reservoirs remains poorly understood. This study addresses the gap by evaluating the effects of pressure (500–3000 psi), temperature (30–70 °C), and NaCl brine salinities (2–20 wt%) by measuring the contact angles (CAs) between n-octane and substrates (for Quartz and Wolf camp shale) in brine, and interfacial tensions (IFTs) between brine and n-octane while injecting pure H₂ gas (base case) and a mixture of 60% H₂ and 40% cushion gas (30% N₂ + 5% CH₄ + 5% CO₂). The CA results revealed that pressure had a relatively minor impact on the system with pure H₂ injection, compared to temperature and salinity. However, when 40% cushion gas was added, the CA increased with pressure and salinity but decreased with rising temperature. It was observed that CAs for pure H₂ gas were lower than those for the H₂ + cushion gas mixture. Also, the equilibrium IFTs showed a consistent reduction with increasing pressure and temperature, while they increased with higher salinity. Comparing the IFTs of the 3-phase (H₂ + cushion)/n-octane/brine system to those of the 2-phase (H₂ + cushion)/brine system revealed that n-octane could effectively reduce the system's IFT by approximately 33%. Overall, this study provides valuable data for hydrogen storage in depleted gas condensate reservoirs containing hydrocarbon fluids like n-octane.