The goal of this study was to assess the potential for storing hydrogen underground in depleted gas fields in Northern California. We considered the potential amount of hydrogen generated from the electrolysis of California's curtailed solar and wind energy. We then determined the fields with the best geological and reservoir properties to support secure underground hydrogen storage.
We developed a three-stage set of criteria for selecting potential hydrogen storage sites. In stage 1, our screening approach combines integrated geoscience and environmental factors to identify the fields to exclude from consideration for hydrogen storage. In stage 2, we applied a numerical simulation-based site selection criteria to the fields that passed the stage 1 screening criteria. We started the screening with 182 depleted and underground storage fields in Northern California, of which 147 fields were disqualified in the first stage. We scored and ranked the remaining 35 fields based on their potential to maximize storage and withdrawal of hydrogen using the numerical simulation-based site selection criteria. The top-ten high scoring sites for underground hydrogen storage and production were reservoirs with dips between 5 and 15, reservoir porosity above 20%, reservoir flow capacity above 5000 mDm, and reservoirs at depths between 430 m to 2400 m. The total estimated hydrogen storage capacity for the ten high-scoring sites was 203.5 million tonnes of hydrogen. Our set of site selection criteria has a stage 3 that requires detailed site characterization. With stage 3, we gather additional rock and fluid properties of high-scoring sites that enable detailed modeling of the processes related to hydrogen storage and withdrawal. We did not cover stage 3 in this paper.
We estimated the potential hydrogen recovery from a hypothetical depleted field in California and evaluated the efficiency of converting the renewable energy to hydrogen and back to power. The results show that depleted gas fields in Northern California have sufficient storage capacity to support the seasonal underground storage of hydrogen derived from renewable energy electrolysis. However, recovery is limited to the amount of fluid that can be injected, the mixing between hydrogen and the in-situ gas, and the lateral spread of hydrogen. The round-trip efficiency of power to hydrogen to power conversion maxed at 36% for the system under study.