Optimizing Performance of Hydrogen Underground Storage in Aquifers by Evaluating the Impact of Key Operational Design Parameters

Seasonal storage of hydrogen (H2) in saline aquifers is a viable technology for large-scale hydrogen energy storage. Numerical simulation of hydrogen underground storage (HUS) in porous media is a promising technique for evaluating and designing such process. The main objective of this study is to optimize the performance of the HUS process by investigating the impact of key design parameters, such as cushion gas (CH4, N2 and CO2) requirements and utilization; H2 injection and production cycles; and well completions and placement, on storage capacity, reservoir pressure, produced H2 purity and hydrogen recovery efficiency. Multiple H2 gas injectors/producers were placed at the crest of the structure to utilize most of the available pore volume. Various operational schemes of the HUS process were selected, which started with injection of cushion gas followed by multiple cycles of hydrogen injection and production. Several sensitivity cases were developed considering injection of three different types of cushion gas (CH4, N2 and CO2) in different volumes and different lengths of H2 injection and production periods with multiple well configuration options. The impact on key HUS design parameters, such as storage capacity, reservoir pressure, produced H2 purity and hydrogen recovery efficiency, were evaluated. Results showed that injection of cushion gas, prior to H2 injection/production cycles, is needed to build reservoir pressure, which ensures H2 deliverability. Regardless of the cushion gas type, H2 recovery efficiency increases with increasing the cushion gas-hydrogen ratio (CGR); however, beyond CGR of 1.5, there is no benefit in this case. Lighter cushion gas (N2 and CH4) injection provided better recovery efficiency than CO2 due to less gas segregation and viscous fingering. Hydrogen wells placed updip and completed in the top part of the reservoir resulted in better performance in terms of H2 recovery and purity in all types of cushion gas scenarios due to reduced cushion gas back production and less water encroachment. An operational scheme, including H2 injection and withdrawal rates and length of injection/production period, should be optimized to achieve higher H2 recovery efficiency. This study provides not only the comprehensive evaluation of cushion gas requirements but also highlights the importance of operational design parameters, which are essential in designing and optimizing performance of HUS projects.