Pore-Scale Effect of Dissolution on Residual Oil Saturation in Carbonate Rock Under Three Wettability Conditions

Dissolution increases the porosity and permeability of carbonate rock, but its effect on residual oil saturation and the dependence of this effect on wettability remain poorly constrained. This work evaluates how dissolution-induced changes in pore space affect oil displacement by water under water-wet, neutral-wet, and oil-wet conditions. Four carbonate cores, composed of 99–100% calcite, were treated with 12% HCl and scanned by X-ray microtomography before and after dissolution at a resolution of 18 μm. Two cubic sub-volumes were extracted from each sample at identical coordinates in both states, giving eight before-and-after pairs. Pore networks were built in pnextract, and two-phase flow was simulated in pnflow at contact angles of 20–30°, 80–90°, and 100–110°. The paired design allowed each sub-volume to serve as its own control, while running the three scenarios on identical networks separated the contribution of structure from that of wettability. Dissolution reshaped the pore space of all eight sub-volumes, enlarging pores, raising network connectivity, reducing tortuosity and increasing absolute permeability, although the magnitude of these changes was uneven, and in one sub-volume permeability did not increase at all. The reduction in residual oil saturation was statistically significant under water-wet and neutral-wet conditions but absent under oil-wet conditions, despite identical structural changes in all three cases. Oil is retained by different mechanisms in water-wet and oil-wet rock, so the same changes in pore space act on displacement in different ways. In these simulations dissolution improved oil displacement only while water remained the wetting phase, which indicates that permeability enhancement alone may not lead to a lower residual oil saturation under oil-wet conditions and that wettability should be considered when evaluating post-acidizing performance.