Pore-scale mechanisms during low salinity waterflooding: Oil mobilization by diffusion and osmosis

Pore-level oil mobilization is studied during low salinity waterflooding by microscopic visualization of water diffusion and osmosis in sandstone silicon-wafer micromodels. The two-dimensional water-wet micromodels apply a controlled, state-of-the-art experimental approach, with a high accuracy pore network, sharp edges and surface roughness to observe transport and displacement processes during low salinity waterflooding. Residual and capillary trapped oil is mobilized when a salinity contrast is established between high-saline connate brine in matrix and low salinity water flowing in an adjacent fracture. The driving force is the difference in chemical potential between the aqueous phases. The focus of this work is on water transport by diffusion and osmosis, mechanisms that are both present in low salinity waterflooding, but less reported in literature. The micromodel system makes it possible to distinguish diffusive and osmotic effects from other well-known mechanisms such as wettability change and fines migration. Transport of water occurs by diffusion through film-flow resulting in film-expansion along water-wet grains. In presence of an osmotic gradient the oil-phase act as a semi-permeable membrane allowing transport of low salinity water into high-saline water-in-oil emulsions.