The structure of residual oil as a function of wettability alteration using pore-scale network modelling

In the water flooding of mixed-wet systems, oil may drain down to relatively low residual saturations and various studies have indicated that such low saturations can only be reached when oil layers in pore corners are included in the pore-scale modelling. Recently, the authors have developed thermodynamic criteria for oil layer existence in pores with non-uniform wettability which takes as input geometrically and topologically representative networks, to calculate realistic Sor values for mixed-wet and oil-wet sandstones. This work has recently been updated to include (i) the visualisation of the 3D structure of this residual oil, and (ii) a statistical analysis of this “residual/remaining” oil under a wide range of wettability conditions, which is reported for the first time in this paper.

This paper shows the qualitative trends in the structure of residual oil that are predicted as a function of wettability. Indeed, we are aware that mico CT studies are reaching the point where they can visualise the structure of (at least part of) the residual oil and we would like to publish these results before we “see the answer” for mixed-wet systems. The structure of residual oil for strongly water wet systems is well known (where residual = remaining oil) and our model agrees with this but this structure changes radically for mixed wet systems (where residual ? remaining what do you mean with this inequality???) and this has not yet been visualised experimentally. The predictions in this paper can therefore be verified or refuted by appropriate experiments in the future.

We find that for more water-wet systems high final residual oil saturations are reached at relatively small amounts of water injected and this oil is present in the pores as bulk oil. On the other hand, for more oil-wet systems we find a slow decrease of the amount of remaining oil with increasing amounts of injected water. During the process, the remaining connectivity of the oil phase is increasingly provided by oil layers only, hence the slow drainage. The final residual oil saturation, only reached in the theoretical limit of an infinite amount of injected water, is almost entirely contained in large number of (relatively low volume) oil layers, which are present in pores of most radius sizes.