Experimental Assessment of CO ₂ –Water Relative Permeability in the Tight Brenha Limestone Formation, Lusitanian Basin, Portugal

ABSTRACT Accurate relative permeability data are essential for evaluating the performance of geological reservoirs. Characterizing multiphase flow in these geological reservoirs, particularly in low‐permeability formations (with an average permeability of 0.165 mD), such as the Brenha limestone, where capillary effects are significant, requires an analytical technique that can capture the dynamic interactions between capillary forces and saturation profiles. This study presents experimental measurements of CO 2 –water relative permeability in tight limestone samples from the Brenha Formation, addressing critical parameters for the feasibility assessment of carbon capture and storage (CCS). Unsteady‐state core flooding experiments were performed using these rock samples under reservoir conditions at a constant flow rate to determine multiphase flow properties. These multiphase flow properties values were calculated using the Ramakrishnan–Cappiello method, which accurately captures the realistic flow behavior of heterogeneous core samples, particularly where capillary pressure dominates viscous forces, resulting in a more accurate residual fluid saturation compared to conventional unsteady‐state methods. Results indicate that the Brenha Limestone exhibits typical limestone relative permeability behavior, with CO 2 breakthrough occurring at water saturations ranging from 0.25 to 0.82. The CO 2 relative permeability ranges from 0.021 to 0.876, whereas the water relative permeability varies from 0.014 to 0.672 across the saturation range. Significant hysteresis effects were observed, with a 5%–16% reduction in relative permeability between imbibition and drainage cycles. The tight nature of the Brenha Formation limestone from Barreiro 3 well with average permeability of 0.165 mD, significantly influences multiphase flow characteristics, necessitating the use of specialized experimental and analytical techniques. These findings provide critical input parameters for assessing CO 2 storage capacity, optimizing injection strategies, and assessing containment integrity in the Brenha Limestone formation.