Summary: The underlying mechanism of oil recovery by low-salinity-water injection (LSWI) is still unknown. It would, therefore, be difficult to predict the performance of reservoirs under LSWI. A number of mechanisms have been proposed in the literature, but these are controversial and have largely ignored crucial fluid/fluid interactions. Our direct-flow-visualization investigations (Emadi and Sohrabi 2013) have revealed that a physical phenomenon takes place when certain crude oils are contacted by low-salinity water, leading to a spontaneous formation of micelles that can be seen in the form of microdispersions in the oil phase. In this paper, we present the results of a comprehensive study that includes experiments at different scales designed to systematically investigate the role of the observed crude-oil/brine interaction and micelle formation in the process of oil recovery by LSWI. The experiments include direct-flow (micromodel) visualization, crude-oil characterization, coreflooding, and spontaneous-imbibition experiments. We establish a clear link between the formation of these micelles, the natural surface-active components of crude oil, and the improvement in oil recovery because of LSWI. We present the results of a series of spontaneous- and forced-imbibition experiments carefully designed with reservoir cores to investigate the role of the microdispersions in wettability alteration and oil recovery. To further assess the significance of this mechanism, in a separate exercise, we eliminate the effect of clay by performing an LSWI experiment in a clay-free core. Absence of clay minerals is expected to significantly reduce the influence of the previously proposed mechanisms for oil recovery by LSWI. Nevertheless, we observe significant additional oil recovery compared with high-salinity-water injection (HSWI) in the clay-free porous medium. The additional oil recovery is attributed to the formation of micelles stemming from the crude-oil/brine-interaction mechanism described in this work and our previous related publications. Compositional analyses of the oil produced during this coreflood experiment indicate that the natural surface-active compounds of the crude oil had been desorbed from the rock surfaces during the LSWI period of the experiment when the additional oil was produced. The results of this study present new insights into the fundamental mechanisms involved in oil recovery by LSWI and new criteria for evaluating the potential of LSWI for application in oil reservoirs. The fluid/fluid interactions revealed in this research can be applied to oil recovery from both sandstone and carbonate oil reservoirs because they are mainly derived from fluid/fluid interactions that control wettability alteration in both sandstone and carbonate rocks.

Low salinity water injection (LSW) has recently attracted much attention as a technique for improving oil recovery. Several mechanisms have been put forward for oil recovery by LSW, however, their contribution to the observed additional oil recovery has not been well established and currently a consistent mechanism that can adequately explain both successful and unsuccessful results of LSW is lacking. Most of the proposed mechanisms attribute the low salinity effects to wettability alteration towards more water-wet conditions. This has resulted in many studies of LSW being mainly focused on the role of rock and its constituents whereas the role of fluid/fluid interactions has been largely ignored. We have recently reported the results of a series of direct flow visualization of LSW, which have revealed spontaneous formation of micro-dispersions of water in oil when a crude oil comes in contact with low salinity brines (Emadi and Sohrabi 2013). In the current work, we further investigated interactions between crude oil and brine by performing a series of fluid characterization tests. Five different crude oil samples were used to individually bring them in contact with brines with various salinity levels to identify experimentally subtle changes in the oil composition. The process of contacting the crude oils and brines was designed and performed very carefully to avoid physical agitation between the two phases in order to replicate reservoir flow conditions. Then, samples of the oil were taken for qualitative and quantitative analysis including ESEM, FTIR, and Karl Fischer Titration. The results demonstrate spontaneous formation of water-in-oil dispersions when salinity of the brine is reduced to below 2000 ppm. This phenomenon is associated with detectable changes in oil composition, which occurred consistently in the tested crude oils. Our quantitative analysis shows that the concentration of micro-dispersions varies depending on the SARA content of the oil and there is a brine salinity threshold at which microdispersion concentration in the oil changes sharply. These new findings coupled with our micromodel visualizations (Emadi & Sohrabi 2013) reveal some new aspects of LSW and may lead to the development of relatively simple screening tests to identify suitability of a crude oil for LSW.