Three-phase relative permeability of water-wet, oil-wet, and mixed-wet sandpacks

We study three-phase flow in water-wet, oil-wet, mixed-wet, and fractionally wet sandpacks. We make the mixed-wet pack by invading a water-filled water-wet pack with crude oil and aging it for a week. This process mimics wettability changes in reservoir settings, leading to a realistic arrangement of wettability at the pore scale. We characterize the wettability of each sand pack by measuring the capillary pressure curves. We obtain the oil and water relative permeabilites during three-phase gravity drainage, by measuring the saturation in situ using computerized tomography scanning. In an analog experiment, we measure pressure gradients in the gas phase to obtain the gas relative permeability. Thus we determine all three relative permeabilities as a function of saturation for each wettability. We find that under uniform wetting, the relative permeabilities of the most-wetting phase (water in a water-wet pack, oil in an oil-wet pack) are similar. However, the relative permeabilities of the intermediate-wet phase (oil in a water-wet pack, water in a oil-wet pack) are very different at low saturations, with spreading oils showing a characteristic layer drainage regime. The mixed-wet pack also shows the layer drainage regime. We also find that the gas relative permeability is smaller in an oil-wet medium than in a water-wet medium. We explain the observations in terms of wetting, spreading, and the pore scale configurations of fluid.