The Effect of Wettability on Waterflood Remaining Oil Saturation and Endpoint Relative Permeabilities in Carbonate and Sandstone Rocks

Abstract

We systematically examine how wettability governs waterflood remaining oil saturation and endpoint relative permeabilities in carbonate and sandstone rocks using capillary-dominated coreflooding experiments. Water-wet and mixed-wet conditions (intermediate- and strongly oil-wet) are imposed by chemical aging of core plugs and validated by contact-angle measurements. Each experiment comprises primary oil drainage to establish the initial water saturation, an optional aging step, and subsequent waterflooding with 20–25 pore volumes (PV) of brine. In carbonate plugs, endpoint oil relative permeability is highest under water-wet conditions and decreases as oil-wetness increases. Across all wettability conditions, samples with larger average pore sizes exhibit higher endpoint oil relative permeability. Water-wet carbonates reach a recovery plateau after 1–2 PV, whereas intermediate- and oil-wet systems continue to recover oil over substantially larger injected volumes. Consistent with trends widely reported for sandstones, the carbonate cores achieve maximum recovery at near-intermediate wettability over the 20–25 PV window studied. We further infer that strongly oil-wet plugs may ultimately attain the lowest remaining oil saturations at much larger injected volumes due to persistent, hydraulically conductive oil layers, although this lies beyond the PV range tested. Endpoint water relative permeability increases as wettability shifts away from water-wet conditions; under water-wet states, it increases slightly as average pore size decreases. Sandstone experiments corroborate prior observations that near-neutral wettability yields the highest recovery after 20–25 PV of brine injection.