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X-ray micro-tomography combined with a high-pressure high-temperature flow apparatus and advanced image analysis techniques were used to image and study fluid distribution, wetting states and oil recovery during low salinity waterflooding (LSW) in a complex carbonate rock at subsurface conditions. The sample, aged with crude oil, was flooded with low salinity brine with a series of increasing flow rates, eventually recovering 85% of the oil initially in place in the resolved porosity. The pore and throat occupancy analysis revealed a change in fluid distribution in the pore space for different injection rates. Low salinity brine initially invaded large pores, consistent with displacement in an oil-wet rock. However, as more brine was injected, a redistribution of fluids was observed; smaller pores and throats were invaded by brine and the displaced oil moved into larger pore elements. Furthermore, in situ contact angles and curvatures of oil–brine interfaces were measured to characterize wettability changes within the pore space and calculate capillary pressure. Contact angles, mean curvatures and capillary pressures all showed a shift from weakly oil-wet towards a mixed-wet state as more pore volumes of low salinity brine were injected into the sample. Overall, this study establishes a methodology to characterize and quantify wettability changes at the pore scale which appears to be the dominant mechanism for oil recovery by LSW.

This work aims at studying the origin of spontaneous emulsification occurring at the oil/water interface. This phenomenon was observed for the five crude oils tested as well as at the interface of an asphaltene toluene mixture and water. The kinetics of appearance of water micro-droplets was slowed down for increasing salt concentrations and the micro-droplet formation ceases when the chemical potential of water they contain is equal to the one of the water in the bulk solution. Nucleation events occur at the oil-water interface and at the solid surface/liquid interface: some water microdroplets are stuck together close to the oil/water interface, others grow in oil and sediment or nucleate at the oil/solid surface. This suggests the following mechanism: water molecules diffuse from the water reservoir into the oil phase, and then create droplets. These droplets are simultaneously fed by hydrosoluble "osmogeneous" species increasing the osmotic pressure, inducing an osmotic pumping of water molecules into micro-droplets WATER OIL

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