CO ₂ Cushion Gas Replacement in U.S. Underground Natural Gas Storage: A Facility-Level Analysis of Sequestration, Cushion Gas Recovery, and Working Gas Storage Expansion

Underground natural gas storage (UNGS) facilities in the U.S. store ∼9.2 Tscf of gas in depleted oil and gas reservoirs, saline aquifers, and solution-mined salt caverns, nearly half of which is immobilized as cushion gas. This study presents the first nationwide, site-level assessment of substituting natural gas cushion with the less expensive CO 2, sourced from anthropogenic emissions or geologic reservoirs. This novel CO 2 -utilization approach both liberates stranded cushion gas and expands working gas storage space within existing infrastructure, driven by CO 2 ’s greater compressibility under typical storage conditions. When sourced from anthropogenic emissions, the approach also contributes to climate change mitigation through CO 2 sequestration. A database of 384 U.S. UNGS facilities was analyzed using a real-gas-law volumetric framework to estimate site-level CO 2 utilization, recoverable cushion gas, and additional working gas storage space, categorized by UNGS type and region, and integrated with data on CO 2 emitters, geologic CO 2 reservoirs, and CO 2 pipelines. Results indicate that full cushion replacement would require ∼0.33 gigatonnes CO 2, roughly two months of U.S. stationary emissions. Individual facility capacities range from nearly 2200 tonnes to 19 million tonnes. In 96% of facilities (370 sites), CO 2 substitution increases working gas storage space by 0.3–344% (average 28%), totaling 1113 Bscf nationwide. Depleted reservoirs and siliciclastic aquifers contribute most, with the Midwest, East, and South Central regions showing highest potential. Even a conservative 20% cushion replacement could utilize ∼67 million tonnes CO 2, free ∼0.9 Tscf of gas, and add ∼223 Bscf of storage space for working gas. Spatial correspondence between UNGS facilities, CO 2 emitters, geologic CO 2 reservoirs, and CO 2 transportation infrastructure indicates potential opportunities for CO 2 sourcing and transport, warranting further site-specific evaluation. These results highlight a scalable strategy to enhance energy storage capacity, monetize stranded natural gas, support climate change mitigation, and create new opportunities for CO 2 utilization.