Carbonate-rich Sandstone Reactivity to Supercritical CO₂ and Brine: A Case Study from the Guadalquivir Basin, Spain

This study investigates mineralogical and geochemical alterations at the matrix scale in carbonate-rich sandstone exposed to supercritical CO₂ (SC-CO₂) and formation brine. Batch experiments were conducted under reservoir conditions (≈8 MPa, 333ºK) to simulate the early stages of CO₂ injection in a deep sedimentary formation of the Guadalquivir Basin (southern Spain).Rock samples were analysed before and after exposure using scanning electron microscopy (SEM) with microanalysis, X-ray fluorescence (XRF), and X-ray diffraction (XRD). Complementarily, chemical analyses of the brine before and after the experiments were performed. The interaction with CO₂-rich brine caused a marked pH decrease, leading to carbonate dissolution and minor alteration of clay minerals. The Ca concentration in the brine increased by about 300%, confirms active carbonate dissolution driven by CO₂-induced acidification. These reactions, together with particle detachment and micro-scale pore modification, indicate dynamic fluid-rock interactions within the calcarenite matrix.The results show up that the studied reservoir rocks maintain overall structural integrity under CO₂-rich conditions while undergoing measurable geochemical alteration. This experimental framework provides a reproducible approach to evaluate mineral reactivity and textural evolution in carbonate-rich sandstone reservoirs, offering relevant insights to the design and assessment of CO₂ sequestration projects in comparable geological settings.This research was conducted within the UNDERGY Project (Ref. MIG-20211018), funded by the Programa Misiones CDTI 2021 of the Spanish Ministry of Science and Innovation and the Next Generation EU Fund.