Heterogeneity Driven Trapping at the Pore-Network Scale in Edwards Brown Dolomite

Trapping is a key control governing the stability and long-term containment of CO₂ within geological storage formations, with residual trapping at the pore scale being well established and routinely incorporated into reservoir simulation models. In contrast, integrating the effects of capillary trapping arising from spatial variability in capillary entry pressure at the micron to centimeter scale remains a challenge for field-scale models, despite clear evidence of its influence on plume migration. Studying pore-scale heterogeneity allows direct quantification of how heterogeneity in pore connectivity and throat geometry translates into capillary entry pressures and snap-off mechanisms, which ultimately control trapping efficiency and is not often resolved at the continuum scale. In this study, we performed flow experiments with brine and decane under capillary-dominated conditions (C _a = 2.6 × 10^-7) while acquiring time-resolved 3D micro-CT images at 5.6 μm voxel size on a 12 mm by 60 mm rock sample. Fractional-flow drainage and imbibition steps were imaged at steady state. Segmented volumes were analyzed with pore-network analysis and trapped volumes were investigated with ganglia volume and count analysis. The sample contains a downstream low-porosity region that acts as a partial capillary barrier. This region remained brine-saturated even during 100% decane injection, indicating entry pressures above the applied capillary driving force. Pore-network analysis showed limited connectivity where the resolved coordination number is approximately only 2, with more than 30% of pores connected by two or fewer throats. The relationship between local initial and residual saturations shows that, within the barrier region, the two values are nearly equivalent, indicating negligible displacement of the mobile phase demonstrating minimal displacement and enhanced trapping. The ganglia analysis shows that the volume and count of ganglia trapped behind the barrier remained elevated after imbibition. These results show that capillary barriers increase immobilization while reducing accessible pore volume. This has an influence on plume migration pathways and should be captured in upscaled models for storage in heterogeneous formations.