Role of Brine Composition in Wormhole Formation and Carbonate Reactivity: Insights into the Role of Magnesium in Accelerated Dissolution and Dolomitization Potential

Abstract

Deep saline aquifers are considered highly promising candidates for carbon sequestration due to their widespread availability and significant storage potential. However, the injection of CO₂ into these geological formations can significantly alter the physical and chemical properties of the reservoir rocks. These changes, driven by interactions between CO₂, brine, and the rock matrix, can influence critical factors such as porosity, permeability, and mineral stability. As such, a comprehensive understanding of the impacts of CO₂ injection on rock properties is essential to ensure the long-term stability, safety, and effectiveness of carbon sequestration efforts. This study explores the effects of CO₂-saturated brine injection on limestone, focusing on the role of brine composition in wormhole formation. Six limestone samples, each measuring 1.5 in. in diameter and 3 in. in length, were used, with an average porosity of 18% and permeability of 99 mD. Core flooding experiments were conducted under reservoir-relevant conditions, including a 1 cm³/min injection rate, 60 °C temperature, and 0.6 M brine salinity. Results demonstrate that the enhanced reactivity of MgCl₂ brine, driven by ion exchange effects, promotes extensive calcite dissolution. MgCl₂ consistently exhibited the highest wormhole volume-to-bulk volume ratio, enabling rapid development of interconnected wormhole networks. Additionally, HCl-modified MgCl₂ solutions showed increased total inorganic carbon content compared to HCl-modified CaCl₂ solutions, further highlighting magnesium’s superior reactivity with calcite formations. These findings underscore the critical influence of brine composition on carbonate reactivity and permeability, providing insights for optimizing reservoir selection and brine management strategies in CO₂ sequestration projects. Furthermore, these observations may provide insights into the long-standing “dolomite problem”, whereby magnesium-rich brines have the ability to react faster with limestone and release extensive Ca²⁺ ions into the porewaters, promoting and accelerating dolomitization processes potentially even at ambient temperature.