Mechanisms of Improved Oil Recovery from Sandstone by Low Salinity Flooding

Oil recovery mechanisms in four Berea sandstones were probed by flow experiments in mini-plugs and analysis of registered images of the resulting distributions of mineral components and oil deposits in pores in the before and after states. After single-phase flow of brine, from high to low salinity, x-ray -CT was able to resolve some slight differences in mineral distribution in pores, while SEM imaging of cut faces more clearly showed the migration of clay particles and other fines. Two-phase experiments involved drainage of high salinity brine by crude oil, followed by aging and flooding by high or low salinity brine. Using the SEM technique, fines migration was observed to be greater than for the single-phase experiments, even for the traditional case of high salinity connate and injected brine. Low salinity flooding gave even greater movement of fines, although in all systems the changes were presumed to be insufficient to drastically alter overall rock permeability. SEM images typically show migrated fines intermingled with the oil deposits. The overall amount of oil deposits from spectroscopy also increases from high to low salinity flooding. Results are interpreted in terms of the moving oil-brine meniscus stripping loosely bound clays and other fines, which can lower its interfacial tension and increase its stability and ability to sustain flow paths for oil recovery.