GEOCHEMICAL STUDY OF UTILIZING CO2 AS A CUSHION GAS IN POROUS AQUIFER COMPRESSED ENERGY STORAGE SYSTEMS

Increasing CO₂ concentration in the atmosphere has resulted in the need for increasing the renewable energy portfolio. However, the intermittency associated with renewable energy production limits increased reliance on renewable energy. To solve this problem requires advancements in energy storage technology. Compressed Energy Storage (CES) of air, CO₂, or H₂ in porous formations is a promising technology for long term, large capacity energy storage. CES operation involves using excess energy to inject gas in the storage medium and extract it during periods of excess demand to drive turbines. Using saline aquifers as storage mediums with CO₂ as a working or cushion gas has operational advantages over typical air storage in caverns. Also, it provides the possibility to sequester excess CO₂ produced through traditional energy production technologies. However, the geochemical interaction of CO₂ and saline brine during CES operation has not been considered. This work utilizes reactive transport simulations to evaluate the CO₂-brine-mineral geochemical reactions that occur during energy storage in a porous aquifer and compares them to reactions that would occur if the same system were used for CO₂ sequestration. A CES plant with continuous and periodic operational schedules is considered. The continuous schedule consists of a 12-hour injection and extraction cycle while the periodic schedule consists of a 6-hour shut-in period between 7 hours of injection and 11 hours of extraction. As expected, CO₂ creates conditions favorable for dissolution of carbonate and aluminosilicate minerals. However, the cyclic flow regime during compressed energy storage limits the dissolution extent significantly after the initial stages of injection and extraction such that CO₂ is a viable choice of working gas. In the injection-only flow regime, synonymous with CO₂ sequestration, larger extents of dissolution occur as the fluid continues to be undersaturated with respect to formation minerals throughout the study period and porosity increased uniformly from 24.84% to 33.6% throughout the simulation domain. For the cyclic flow conditions, porosity increases non-uniformly to 31.1% and 25.8% closest and furthest from the injection well, respectively.