Assessment of hydrogen storage potential in depleted gas fields and power-to-hydrogen conversion efficiency: A northern California case study

Abstract

This study assessed the potential for storing hydrogen underground in depleted gas fields using Northern California as a case study. We examined how much hydrogen California could produce from the electrolysis of curtailed solar and wind energy. Afterward, we assessed the geological and reservoir properties of fields that could potentially support safe and secure underground hydrogen storage. We applied two stages of a three-stage process to select prospective hydrogen storage sites in depleted gas fields. The first stage of the screening process involved integrating geoscience and environmental factors in order to identify the fields that will not be considered suitable for hydrogen storage. Among 182 depleted and underground gas storage fields in Northern California, 147 were disqualified in the first stage of the assessment. The remaining 35 fields were scored and ranked in stage two based on their potential to maximize hydrogen storage and withdrawal. High-scoring sites for underground hydrogen storage and production included reservoirs with dips between 5° and 15°, reservoir porosity above 20%, reservoir flow capacity above 5000 mDm, and reservoir depths between 430 m and 2400 m. We determined that a total of 203.5 million tonnes of hydrogen could be stored at the ten high-scoring sites. We estimated the potential hydrogen recovery from a hypothetical depleted field in California and evaluated the efficiency of converting the curtailed renewable electricity to hydrogen via electrolysis, storing it in the subsurface, and converting produced hydrogen back to electricity. Based on the results, we determined that depleted gas fields have sufficient underground storage capacity to store hydrogen produced from Northern California's curtailed renewable energy sources. Hydrogen recovery efficiency was estimated to be larger than 75% and was limited by the bottom hole pressure of the withdrawal well, hydrogen mixing with in-situ gas, and hydrogen spreading laterally. This study found that power-to-hydrogen-to-power round-trip efficiency maxed out at 36%. Improvements in hydrogen recovery could increase the round-trip efficiency by 8%, while improvements in electrolyzer efficiency could increase the round-trip efficiency by 33%.