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Darcy-scale multiphase flow in geological formations is significantly influenced by the wettability of the fluid-solid system. So far it has not been understood how wettability impacts the pore-scale flow regimes within rocks, which were in most cases regarded as an alteration from the base case of strongly water-wet conditions by adjustment of contact angles. In this study, we directly image the pore-scale flow regime in a carbonate altered to a mixed-wet condition by aging with crude oil to represent the natural configuration in an oil reservoir with fast synchrotron-based X-ray computed tomography. We find that the pore-scale flow regime is dominated by ganglion dynamics in which the pore space is intermittently filled with oil and brine. The frequency and size of these fluctuations are greater than in water-wet rock such that their impact on the overall flow and relative permeability cannot be neglected in modeling approaches.

Plain Language Summary In geological systems, in particular in oil reservoirs, the wetting condition of rock, the preference of a fluid to be in contact with a surface in the presence of another fluid, has a significant impact on multiphase flow. Often a simplified picture based on static, wettability-dependent fluid configurations is used as a basis for modeling where the fluids are assumed to flow through the porous rock within definite connected pathways. Our research, which is based on a time series of 3-D images obtained during multiphase flow showing the pore-scale fluid configurations of the brine and oil, demonstrates that this picture is too simplistic. In reality the flow paths change. In systems in which one phase is strongly wetting those changes are fast, small, and rare. However, oil reservoirs are mostly mixed-wet as surface active components contained in crude oil alter the rock surface. In such mixed-wet situations, we observe that the movement is slower (minutes instead of seconds), is more frequent, and involves larger fluid volumes. This indicates a different flow regime that cannot be estimated from an extrapolation from strongly wetting rock. This has consequences for the way how multiphase flow in mixed-wet rock is described in models.