The sedimentary structure and wettability of clastic rocks at the laminaset scale may have an important influence on oil recovery by waterflooding. Previous work from this laboratory reported results for oil displacements in water-wet laminated systems. In this paper, results from a full cycle of drainage-imbibition floods in a cross-laminated rock slab are reported where the wettability of the slab was changed by ageing the system with a "synthetic crude oil". The distribution of fluid saturation was monitored using CT scanning techniques. The main purpose of this work is to investigate the effects of both the laminaset heterogeneity and wettability alteration on the displacement efficiency and oil trapping mechanisms for such systems. Results from from these floods are compared with previous findings for water-wet laminated system.
In these altered wettability experiments, the recovery efficiency significantly increased, due to the slab becoming more oil-wet. In contrast to the strongly water-wet case, an analysis of the remaining oil saturation shows that (a) no significant oil trapping occurred behind low permeability laminae in this flood, and (b) there was a higher remaining oil in the low permeability regions. Numerical simulations were performed to help our understanding and interpretation of the wettability changes. An excellent match of experiment with numerical simulation was obtained for a particular model case with the lower permeability laminae being more oil-wet than the higher permeability ones in general. We have termed this a heterogeneously-wet (or het-wet) system. To our knowledge, this is the first time that such behaviour has been demonstrated and analysed in detail.