Oil Recovery Improvement from Low Salinity Waterflooding in a Clay-free Silica Core

Laboratory and field trials have demonstrated that oil recovery may be improved by injecting low salinity brine as a simple and cost-effective EOR method to sweep oil, provide pressure support and produce trapped oil. A single physiochemical theory for this improved recovery mechanism has remained elusive due to the complex nature of rock-fluid and fluid-fluid interactions. Wettability alteration has been identified as a consequence of low salinity water injection, which is supported by underlying theory governing surface forces. This paper delineates the potential mechanisms involved in this process and demonstrates that low salinity water has a positive response in a homogeneous borosilicate core that is absent of clay. Utilising this core material demonstrates great synergy between previously reported 2D micromodel observations with a 3D pore network at reservoir conditions. Various injection scenarios are presented. Injecting low salinity brine into the aged core first, gave a recovery of 60% of the initial oil in place (IOIP) at the end of the first injection phase. Repeating the experiment afresh with high salinity brine gave 50% IOIP whereas high salinity injection into an un-aged core gave 34% IOIP at the end of the first stage of injection. Ultimate recovery was highest by cyclic injection of low salinity brine followed by high salinity brine. Chemical changes in the produced oil were measured with infrared and supported with visual observations of recovered oil and brine that indicated stable micro-emulsions during low salinity brine injection. This highlights the importance of fluid-fluid interactions as an area of required investigation.