Subsurface gas storage in porous reservoirs: a comparison of working gas capacity, deliverability and production between hydrogen and natural gas

The energy transition is a long-term strategy dedicated to achieving carbon neutrality in Germany by 2045. This process includes the development of renewably generated electricity from wind and solar power. As these energy sources are dependent on weather conditions, imbalances in availability and demand might result in a deficit or excess of renewable energy. In times of excessive availability, hydrogen can be generated and stored in subsurface storage sites from which it can be withdrawn when the demand increases. Using subsurface storage sites is a viable option as those currently store natural gas. Considering the properties of hydrogen, converting existing storage sites poses many challenges, including the total stored energy content and the amount of extractable working gas. Using an exemplary storage formation, a numerical simulation model is set up in an open-source software and used to calculate and compare the stored volumes and energy contents for natural gas and hydrogen. Storage withdrawal and pressure profiles for both cases are developed. A pseudo gas is defined with individually alternating compressibility, density and viscosity behavior to independently assess the influence of these properties on the withdrawal rates. A sensitivity analysis of various storage, bottom-hole flowing pressures and the skin factor on the extraction rates is also performed. The results show that a larger volume of natural gas and a resulting higher energy content can be stored in the storage site compared to hydrogen. Changes in extraction rates occur earlier and pressure decrease is greater for stored hydrogen. The compressibility factor has the largest influence on the extraction behavior of the gas, leading to a steeper decline in hydrogen withdrawal rates and a quicker pressure depletion. The levels of the storage and the bottom-hole flowing pressure impact the slope of the decline and the level of withdrawal rates.