Geochemical interactions of supercritical CO₂-brine-rock under varying injection strategies: implications for mechanical integrity in aquifers

Carbon dioxide (CO₂) interacts with rock minerals due to a series of geochemical reactions affecting rock geomechanical properties. These changes in mineralogy, mechanical, and elastic properties weaken the reservoir rock and caprocks. These effects, as consequences, affect the deep saline formation’s mechanical integrity, storage capacity, storage efficiency, and permanence or safety of Carbon Capture and Sequestration (CCS). Generally, CO₂ is injected into the rock continuously in supercritical conditions. Different injection schemes and strategies have been proposed to manage these effects; however, the dynamics of CO₂ interaction when subjected to these strategies are unknown. This paper provides a comprehensive analysis of the geochemical and geomechanical impacts of supercritical CO₂ (scCO₂) on rock formations, employing three distinct injection strategies: Continuous scCO₂ Injection (CCI), Water- Alternating Gas (scCO₂) Injection (WAG), and Simultaneous Water and scCO₂ Aquifer Injection (SAI). Through experimental approaches, the study utilizes core samples from Gray Berea sandstone and Indiana limestone to examine short-term and long-term effects on rock elasticity, strength, and mineralogy. This research assesses the alterations in elastic and mechanical properties by employing a suite of coupled experimental approaches, including core flooding, uniaxial compression testing, and X-Ray Diffraction (XRD). Results demonstrate that different scCO₂ injection strategies significantly affect the rock mechanical integrity and mineral stability due to acidification, geochemical reaction, sequences of the dissolution and precipitation processes, cyclic effect, and mineral hardness. CCI and WAG demonstrate a more favorable impact on the geomechanical properties of both rock types. Conversely, the SAI strategy proves less beneficial, adversely affecting both elastic and mechanical properties despite occurring geochemical reactions. The study highlights the interplay between mineral dissolution and precipitation processes under varying injection conditions, providing insights into optimizing injection schemes to maximize CO₂ storage while maintaining the structural integrity of the geological formations.