Osmosis as Mechanism for Low Salinity EOR

We believe that osmosis has been overlooked as a possible mechanism for observed low salinity EOR effects. Osmosis can occur in an oil/water/rock system when injecting low salinity water, as the system is full of an excellent semi-permeable membrane, namely the oil itself. In the present work, water transport through oil films was visualized both in 2D micromodels as well as in sandstone cores imaged in a micro-CT. After treating these model systems with Hexamethyldisilazane (HMDS) to render them more oil-wet, water became discontinuous, and it was possible to establish osmotic gradients. Either expansion or contraction of the connate water was observed, depending on the direction of the imposed salinity gradient. Given that osmosis could be the underlying mechanism for low salinity EOR, two important changes in research strategy are proposed: Firstly, the use of spontaneous imbibition tests as evidence for wettability alteration in low salinity water should be critically reinvestigated. This is because observed production could have stemmed from “osmotic expansion” of the connate water rather than wettability change. Secondly, research focus should be shifted from sandstone reservoirs to fractured oil-wet carbonates, as osmosis is expected to yield larger responses for the latter type.