Due to efforts to reduce CO₂ emissions to the atmosphere, renewable energy sources have made up an increasing portion of the total supply mix. In 2020, for example, renewable energy sources that include wind, hydroelectric, solar, biomass, and geothermal energy produced nearly 21% of all the electricity generated in the United States. One of the most significant challenges to this change is dealing with intermittent supply and varying demand. Managing these challenges requires a means to store large amounts of energy so that it can be readily available when demand is high. One method to overcome this challenge is to convert energy and/or other fuels into hydrogen (H₂) gas. H₂ as a fuel source has long been identified as a critical step towards transitioning into a low-carbon, and eventually a zero-carbon energy society. H₂ use has numerous benefits and can be generated by well established and emerging technologies and it can be used in a variety of end uses and transport processes. Large-scale geological H₂ storage (UHS) may offer the needed capacity to balance inter-seasonal supply/demand discrepancies, de-couple energy generation from demand, and decarbonize heating and transportation; thus, supporting decarbonization of the entire energy system.

To advance the H₂ economy, there is a critical need to increase its availability across the United States. A significant gap is the ability to economically and safely store large amounts of H₂, much like how natural gas (NG) is stored today. Underground storage of H₂ is not a new concept and has been commercially demonstrated at scale in underground salt caverns and international projects have shown this as a feasible option for existing underground natural gas storage (UGS) fields in porous and permeable reservoirs. While the gas storage concept is not new, the impacts to reservoirs, H₂ leakage risks, and flow behavior of H₂ and blended mixtures are not well understood. The new demand for widely available H₂ sources and opportunity to use H₂ blended with NG will require reservoirs distributed across the United States.

NG has been stored in North America for over a hundred years in salt caverns, aquifers, and depleted oil and gas fields. Table 1 summarizes typical cushion-gas percentages, injection period, withdrawal period of natural gas, and estimates of design capacity of underground working NG storage in saline aquifer, depleted oil/gas reservoirs, and salt caverns.

Table 1. Summary of cushion-gas percentages, injection period, withdrawal period of natural gas, and estimates of design capacity of underground working NG storage for the three types of UGS