Low CO₂ relative permeability in Bentheimer sandstone revealed by steady-state measurements and pore-scale imaging

Quantifying pore-scale fluid displacement mechanisms in CO 2 /brine systems is essential for predicting multiphase flow and trapping during CO 2 storage. In this study, we imaged the steady-state flow of brine and CO 2 in a water-wet Bentheimer sandstone sample. We devised an experimental method based on differential imaging to examine the pore-scale behaviour of CO 2 within the system at reservoir conditions during drainage. This allowed us to measure relative permeability and capillary pressure while observing the evolution of the gas ganglia in the pore space at different fractional flows under capillary-dominated conditions. The measured CO 2 relative permeability remained low during the initial stages of drainage across a broad saturation range and only increased modestly to 0.24 at 100% CO 2 injection, with a gas saturation of 0.57. Initially, CO 2 appeared as small and disconnected ganglia occupying the largest pores and progressively invaded smaller pores and throats with increasing gas fractional flow, leading to partial connectivity. The interface exhibited predominantly positive mean curvature, consistent with a water-wet system. Capillary pressure estimated from interfacial curvature showed good agreement with independent porous-plate measurements reported in the literature. Overall, the connectivity of the CO 2 phase, quantified by Euler characteristic, remained limited during the displacement process contributing to the measured low relative permeability. These observations are consistent with a capillary-dominated displacement process with local trapping by Roof snap-off.