Imaging of oil layers, curvature and contact angle in a mixed‐wet and a water‐wet carbonate rock

We have investigated the effect of wettability of carbonate rocks on the morphologies of remaining oil after sequential oil and brine injection in a capillary‐dominated flow regime at elevated pressure. The wettability of Ketton limestone was altered in situ using an oil phase doped with fatty acid which produced mixed‐wet conditions (the contact angle where oil contacted the solid surface, measured directly from the images, θ=180°, while brine‐filled regions remained water‐wet), whereas the untreated rock (without doped oil) was weakly water‐wet (θ=47 ± 9°). Using X‐ray micro‐tomography, we show that the brine displaces oil in larger pores during brine injection in the mixed‐wet system, leaving oil layers in the pore corners or sandwiched between two brine interfaces. These oil layers, with an average thickness of 47 ± 17 µm, may provide a conductive flow path for slow oil drainage. In contrast, the oil fragments into isolated oil clusters/ganglia during brine injection under water‐wet conditions. Although the remaining oil saturation in a water‐wet system is about a factor of two larger than that obtained in the mixed‐wet rock, the measured brine‐oil interfacial area of the disconnected ganglia is a factor of three smaller than that of oil layers.