Geochemical controls on wettability alteration at pore-scale during low salinity water flooding in sandstone using X-ray micro computed tomography

Abstract

We performed a pore-scale multiphase flow experiment on a sandstone core sample. We imaged the core sample at initial oil saturation, residual oil saturations after high salinity and low salinity water flooding. Moreover, we examined fluid occupancy maps and water cluster size distribution at pore-scale before and after low salinity waterflooding. Furthermore, we performed a geochemical study to relate physicochemical process at sub-pore scale to account for in-situ wettability alteration at pore-scale.Micro-CT imaging shows that low salinity waterfloodng yielded 5% of residual oil saturation reduction after high salinity water flooding. Fluid occupancy maps within pore network show water film propagation at pore surface during low salinity water flooding, suggesting the oil film detachment from pore surfaces due to in-situ wettability alteration. Micro-CT imaging analysis also shows that the large size water cluster (greater than 107 µm3) occupies 87.7% of water volume after high salinity water flooding, whereas the same size water cluster occupies 89.6% pore volume after low salinity water flooding, implying that water clusters coalesce into each other to transport in pore network during low salinity water flooding in line with fluid occupancy maps. Geochemical modelling predicts a pH increase (from 7 to 8.9) during low salinity water flooding largely due to ankerite and albite dissolution. This study sheds light on the significance of geochemical controls over wettability alteration at pore-scale through water film propagation.