Brine Composition as a Key Factor in Limestone-CO2 Reaction Dynamics

Abstract Deep saline aquifers represent one of the most viable geological options for carbon dioxide (CO2) storage due to their extensive global presence and substantial capacity. However, the injection of CO2 into such formations initiates complex geochemical interactions among the CO2, the formation brine, and the host rock, which can lead to significant alterations in rock porosity, permeability, and mineral composition. Understanding how these interactions affect reservoir properties is vital for ensuring the long-term integrity and performance of CO2 sequestration operations. This research investigates how the composition of CO2-saturated brines influences wormhole formation in carbonate rocks, particularly limestone. Six cylindrical limestone cores (1.5-inch diameter, 3-inch length) with an average porosity of 18% and permeability of 99 millidarcies were subjected to core-flooding tests at 60 °C, using brines of 0.6 M salinity and an injection rate of 1 cm3/min under simulated reservoir conditions. The experimental results revealed that brines containing magnesium chloride (MgCl2) induced significantly higher levels of calcite dissolution, primarily due to enhanced ion exchange processes. Among all tested fluids, MgCl2 consistently resulted in the largest ratio of wormhole volume to bulk rock volume, leading to the rapid formation of well-connected wormhole structures. When hydrochloric acid was added, the MgCl2-based brine exhibited a higher concentration of total inorganic carbon (TIC) in the effluent compared to its CaCl2 counterpart, further emphasizing magnesium's heightened chemical reactivity with carbonate formations. These outcomes clearly demonstrate that the ionic composition of the injected brine plays a pivotal role in governing both the extent of rock dissolution and the development of permeability-enhancing wormholes. In addition to informing CO2 storage design, the observed behavior of magnesium-rich fluids may also contribute to addressing the long-standing geological question of dolomitization. The accelerated reactivity of Mg2+ with limestone observed in this study supports the hypothesis that such fluids could promote dolomite formation even under low-temperature conditions. Overall, these insights are valuable for optimizing brine selection and managing geochemical interactions in CO2 sequestration projects, with broader implications for carbonate reservoir characterization and the geochemical evolution of subsurface formations.