Crude oil/brine interactions and spontaneous formation of microdispersions in low salinity water injection

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.