Green Hydrogen Storage in Depleted Gas Reservoirs

ABSTRACT: This study explores the potential of repurposing depleted natural gas reservoirs for green hydrogen storage, utilizing the material balance equation (MBE) framework. The presence of an aquifer, a common feature in many depleted reservoirs, is considered, and the Carter and Tracy aquifer model is applied alongside the Peng-Robinson equation of state for hydrogen. A case study is conducted using a reference reservoir geometry with varying hydraulic properties (porosity and permeability) and a range of natural gas production rates. Results indicate that natural gas production rate, formation permeability, and porosity are critical factors not only for gas extraction but also for determining the feasibility of hydrogen storage. The study finds that typical sandstone reservoirs are most suitable for hydrogen storage, while formations with very high or low permeability present challenges. The findings highlight that with appropriate reservoir management, depleted gas reservoirs could serve as an effective hydrogen storage solution, contributing to a more flexible and resilient energy system. Future research should focus on understanding biochemical reactions in these reservoirs when exposed to hydrogen, which remains a relatively unexplored area. 1. INTRODUCTION The management of energy, a critical component in the transition from conventional energy resources to renewables, requires bridging the gap between demand and supply, often through the use of energy storage. One form of storage is hydrogen gas which is an energy carrier that can be used to store, transport, and deliver energy produced from other sources. In this energy transition, green hydrogen which is produced via electrolysis using renewable energy sources could play a vital role (Kourougianni et al., 2024). Although the production of green hydrogen is largely secured from a technological standpoint, the limited storage capacity remains a significant barrier to further development. Current storage technologies can accommodate limited volumes. To store large quantities of hydrogen, subsurface formations like aquifers, caverns, and depleted oil and gas reservoirs are promising candidates. Depleted oil and gas reservoirs are porous media which host fluids within their pore network, which once accommodated hydrocarbons. Together with aquifers, they can store large volumes accommodating variations on a weekly basis with TWh in terms of energy storage capacity (Edlmann et al., 2021).