Fundamental investigation of underlying mechanisms behind improved oil recovery by low salinity water injection in carbonate rocks

The underlying mechanisms of additional oil recovery by low salinity water injection (LSWI) have remained poorly understood. The main objective of this investigation is to examine the relative dominancy of different processes taking place during LSWI in carbonate rocks. An integrated approach was employed to study the role of fluid/fluid (micro-dispersion formation) and rock/fluid (geochemical processes) interactions that can impact oil recovery. Firstly, a number of different crude oils were analysed, screened and ranked, based on their propensity to form micro-dispersions. From this bank of samples, two crude oil samples were selected with similar physical properties but high and low propensities for forming micro-dispersion. In addition, a third sample was prepared from the positive crude oil (the high micro-dispersion sample) by removing oil compounds associated with micro-dispersion formation. Three coreflood experiments were designed to assess the role of crude oil/brine interactions in oil recovery by LSWI in tertiary scenarios. The outcome of the coreflood experiments using the same rock and brine compositions should demonstrate the dominant mechanism in improving oil recovery. When low salinity water (below 10,000 ppm) was injected in tertiary mode, the crude oil with a positive tendency to form micro-dispersions demonstrated a significant additional oil recovery of 6.3%. In contrast, no additional oil recovery was observed from LSWI with the crude oil sample that had reacted poorly to low salinity water (i.e. not formed micro-dispersion) nor with the treated sample. Using a novel method, the mineralogy of the rock was identified, which indicated the presence of anhydrite minerals. A series of spontaneous imbibition tests were also performed, which indicated similar mixed-wet conditions for the systems, which would make the systems comparable. The results of this fundamental work have demonstrated that despite the occurrence of geochemical processes, micro-dispersion formation appears to be the more dominant mechanism behind the IOR obtained by LSWI in carbonate rocks.