Fast X-ray micro-CT study of the impact of brine salinity on the pore-scale fluid distribution during waterflooding

Many studies indicate that the recovery of crude oil by waterflooding can be improved by lowering the salinity of injected water. This so-called low-salinity effect (LSE) is often associated with the change of the wetting state of rock towards more water-wet. However, it is not very well understood how wettability alteration at the pore level could lead to an increase in production at the Darcy scale. Therefore, this study aims at direct observation of the wettability-change-driven fluid reconfiguration related to a lower-salinity (LS) flood at the pore-network scale, using synchrotron beamline-based fast X-ray computed tomography. Cylindrical outcrop rock samples were initialized by first saturating them with high-salinity (HS) brine, then displacing the HS brine with crude oil down to residual water saturation. After this initialization the rock samples were aged to establish wettability states assumed to be close to mixed-wet conditions. During the flooding experiments, the pore-scale fluid distribution was imaged at a voxel resolution of 3 μrn and (under flowing conditions) a time resolution of 10 s for a full 3D image. The micro-CT flow experiments were conducted on both sandstone and carbonate rocks, all in tertiary mode and at identical field relevant flow rates. The real-time imaging shows the presence of an oil/water structure in addition to the oil and water phases and a saturation change during the HS waterflood which approaches a stable equilibrium at its end. During flow of both HS and LS brine we observe (re-)connection and disconnection of the oil phase which are characteristics of ganglion dynamics. In addition, we observe relatively slow pore-filling events that we believe to be characteristic of the mixed-wet state of the sample. Preliminary analysis indicates that upon lowering of injection brine salinity individual pores change fluid occupancy, however further research is required to draw definitive conclusions.