Impact of CO₂ exposure on chalk: Surface chemistry and precipitation kinetics

The long-term integrity of CO₂ storage sites in depleted chalk reservoirs depends on the interactions between CO₂, brine and mineral surfaces, which can significantly alter the rock properties. This study investigates the effects of CO₂ exposure on the surface chemical properties of chalk, with a focus on calcite precipitation kinetics and the role of organic surface coatings. Chalk samples from both oil-bearing and water-saturated intervals of reservoirs were treated with dichloromethane (DCM), carbonated water (CW) and supercritical CO₂ to assess changes in surface reactivity. Surface characterization methods in combination with particle size distribution and surface area measurements were used to evaluate the mineral surface chemistry and physical properties of the chalk material before and after treatment, while calcite precipitation tests were used to quantify reaction kinetics. Results show that organic coatings, particularly hydrocarbon components and polysaccharides, inhibit calcite nucleation, leading to reduced precipitation rates. The most effective treatment for enhancing calcite precipitation was the organic solvent DCM, which removed a significant proportion of organic matter and doubled the precipitation rate in oil-containing samples. Treatment with CW and supercritical CO₂ resulted in a moderate increase in chalk reactivity, primarily due to mobilization and redistribution of organic compounds rather than complete removal. These results highlight the enhanced reactivity of chalk surfaces after CO₂ exposure, a process that may be of importance for post-injection seismic monitoring techniques, which rely on accurate estimates of the rock frame properties.