Opportunities and challenges in aquifer underground natural gas storage with a CO₂ cushion

Natural gas (NG) is a relatively clean fuel that provides close to 25% of global primary energy. Converting coal-fired plants to NG is a promising intermediate step, as we transition toward net zero emissions. However, greater adoption of NG is hindered by a combination of seasonal demand cycles and uncertain supply. Underground natural gas storage (UNGS) addresses these issues by providing large-scale and cost-effective storage. This chapter presents a detailed discussion of various aspects of UNGS sites and current market status and focusses on aquifers, which are ubiquitous and generally located close to major metropolitan demand centers. A major development in UNGS has been leveraging the supercritical behavior of CO2 as cushion gas to reduce the amount of unrecoverable NG. The benefits and challenges associated with a CO2 cushion along with experimental and numerical approaches for assessing the UNGS performance are outlined. We discuss in detail the NG-CO2 mixing phenomenon during gas injection and production, which can severely impact the quality of produced gas. Recent trends and innovations in UNGS and the adoption of data-driven methods for operational optimization are also presented. Finally, we discuss opportunities associated with the coupling of power-to-gas with UNGS to mitigate the variability of renewable sources.