Geochemical Interactions and Reservoir Integrity in CO2 Storage: Insights from the San Andres Formation

Abstract Geological storage of CO2 in deep saline aquifers is a critical component of global decarbonization strategies, yet its long-term success depends on understanding CO2–brine–rock interactions and their influence on reservoir integrity, injectivity, and storage security. This study investigates the core-scale geochemical evolution of the San Andres Formation, a carbonate interval in the Permian Basin, using an integrated workflow that combines mineralogical characterization, laboratory petrophysics, and reactive transport simulations. Baseline mineralogy was established through X-ray diffraction, scanning electron microscopy, and thin-section petrography, revealing a calcite-dominated carbonate with subordinate dolomite and minor quartz (73.4% calcite, 22% dolomite, 4% quartz). These data were used to constrain numerical simulations of CO2 injection under reservoir conditions to quantify how dissolution, precipitation, and brine chemistry evolve over time. The results show that CO2 dissolution into brine induces rapid acidification near the injector, initiating early-stage dissolution of calcite and dolomite. However, ion accumulation in the near-wellbore region drives localized supersaturation and secondary carbonate precipitation, producing slight reductions in porosity and approximately 1% permeability loss. In contrast, mid-distance regions experience sustained undersaturation, continuous carbonate dissolution, and modest increases in porosity and permeability. Far-field and vertically offset blocks show negligible geochemical alteration, demonstrating strong spatial heterogeneity governed by CO2 transport pathways. These findings reveal a dual-zone reactive system in San Andres carbonates: a precipitation-controlled near-wellbore zone with minor injectivity reduction, and a dissolution-controlled distal zone that modestly enhances transmissivity. Although the magnitude of permeability change is small, the spatial patterns provide important insight into geochemical trapping, flow evolution, and reservoir integrity under CO2 storage conditions. This formation-specific assessment advances understanding of CO2 reactivity in Permian carbonates and supports the development of optimized injection strategies that minimize injectivity loss and enhance long-term storage security.