Core-scale evaluation of CO₂ reinjection feasibility in a carbonate reservoir–caprock system: The S Field case study, Sarawak Basin, Malaysia

This study evaluates the feasibility of reinjecting separated CO₂ into its source carbonate reservoir, the high-temperature, high-pressure S Field in the Sarawak Basin, offshore Malaysia. The reservoir gas comprises ∼30% CH₄ and ∼70% CO₂. We combine mineralogical, petrophysical, and geomechanical analyses to assess reservoir integrity and caprock sealing performance under in-situ conditions. The calcite-dominated gas zone exhibits high porosity (>30%), whereas the underlying aquifer zone is co-dominated by calcite and dolomite with lower porosity (20–25%) and higher strength. The primary caprock (Seal A), a 500 m-thick Miocene mudrock–siltstone unit, has porosities of 2–10% and low permeability, with illite and quartz as dominant minerals. Laboratory experiments exposed reservoir and caprock samples to CO₂-charged brines at reservoir pressure (30 MPa) and temperature (150 °C) for up to six months. Post-reaction analyses revealed only minor changes in mineralogy, porosity, permeability, and mechanical strength. These results indicate limited CO₂–water–rock reactivity and confirm the mechanical and geochemical stability of both reservoir and caprock. The findings support the viability of CO₂ reinjection and long-term geological storage in the S Field, providing a benchmark case for carbonate-hosted CCS systems in Southeast Asia.