Assessment of Geochemical Reactivity in Geologic CO2 Utilization and Storage Systems

Abstract

Increasing atmospheric carbon dioxide concentration is a consequence of increasing energy consumption. As a result, developing ways to manage excess CO2 emissions has become a vital concern to avert the exacerbating effects of global warming. This has given rise to ideas for the utilization of greenhouse gases like CO2 as cushion gas for energy storage systems and subsurface sequestration of excess captured greenhouse gases. The potential geochemical reactivity of CO2 with brine in porous aquifers and corresponding implications on the operational system is uncertain. Additionally, the rate and extent of trapping, including how much of the injected CO2 will be permanently stored as minerals, is not well understood. To enhance understanding of geochemical reactions in these systems with varying flow conditions and aquifer properties, this study will 1) use micro-continuum scale numerical simulations to assess the potential of utilizing CO2 as a cushion gas for compressed energy storage and 2) leverage field-scale numerical simulations to evaluate the optimum aquifer properties for maximizing CO2 sequestration in porous saline aquifers. Core scale reactive transport simulations were used to evaluate the geochemical reactions that occur during injection and extraction flow cycles for a compressed energy system in a porous saline formation using CO2 as cushion gas. The results of the cyclic flow regime simulation for the energy storage scenario are compared with a model that simulates CO2 sequestration by considering an injection-only flow regime. Results show that in the injection-only flow regime, larger extents of dissolution occur. The dissolution extent is limited during the continuous cyclic flow of acidified brine. This implies that CO2 is a viable choice of cushion gas. Further investigation of the CO2 potential as a cushion involves determining the impact of the operational schedule of a compressed energy system on the geochemical reaction pathway in the system. For this study, the operational schedule which comprises injection, withdrawal, and reservoir closure was used to simulate the periodic operational schedule. This operational schedule was compared to the continuous injection and extraction cycling operational schedule to understand the impact of the storage duration. The geochemical reaction results show that the operational schedule does not have a significant effect on the geochemical evolution of the formation used for compressed energy storage. In addition, field scale reactive transport simulation is leveraged to enhance the understanding the influence of aquifer properties (porosity, permeability, depth, and carbonate mineralogy) on the overall geochemical reactivity in the reservoir and CO2 trapping potential. Here, the simulations reveal that the considered aquifer properties impact the sequestration efficiency, defined as the rate at which the CO2 injected into the aquifer is converted to aqueous or mineralized CO2. Based on the studied properties, the impact of aquifer properties on CO2 evolution depends on the stage of the sequestration project. The final study on the heterogeneity of aquifer shows that the potential to accurately estimate the sequestration efficiency of a formation will present more uncertainty during the injection phase which will reduce as the simulation timeline increases.