This experimental study identifies osmosis as an oil-mobilization mechanism during low-salinity waterflooding using capillary tubes and micromodels with different wetting properties. Oil-wet capillary tube tests verify that oil acts as a semipermeable membrane for water transport when separating brines of different salinity. Osmotic oil mobilization was investigated using 2D silicon-wafer micromodels for direct optical visualization of fluid interactions. Osmosis was observed at both strongly water-wet and oil-wet conditions influenced by the pore-level distribution of oil and water. At strongly water-wet conditions osmosis displaced oil by expanding water-in-oil emulsions. At oil-wet conditions low-salinity water was transported by osmosis to otherwise inaccessible regions of high-saline brine. Osmotic oil mobilization is a fluid- fluid interaction and will occur in both sandstone and carbonate reservoirs. The experimental results support this and provide evidence of osmotic water transport and oil mobilization at various wettability conditions.
Low-salinity waterflooding (LSW) denotes injection of diluted brine concentrations in either secondary or tertiary recovery mode to increase oil recovery. Whereas conventional waterflooding uses formation brine or seawater predominantly to maintain reservoir pressure, LSW also improves microscopic sweep (Morrow and Buckley, 2011). LSW has been reported to enhance oil production in a series of coreflood experiments (Tang and Morrow, 1997) and it has been tested in various field operations (e.g., Ligthelm et al., 2009). There is a consensus that an optimal wetting preference exists at which LSW is favorable for oil recovery and it is generally expected to shift reservoir wettability towards more water-wet conditions.
Initial research on LSW emphasized on fluid-rock interactions and the mechanisms that alter wettability of a rock surface. Coreflood results in sandstone provided a variety of theories aiming at defining the low-salinity effect (LSE), as summarized by Sheng (2014). One of the more accepted hypotheses is the concept of multicomponent ion exchange (MIE), where inorganic Ca2+ and Mg2+ cationic exchange, between the negatively charged sandstone and the injected low-salinity water, replace organic polar components at the rock surface to shift wettability towards water-wet (Lager et al., 2008). The influence of cationic exchange was partially confirmed by others, who also reasoned that a major contribution was reduction of ionic brine strength (Ligthelm et al., 2009). Reduced ionic strength lowers the cationic screening potential that result in electrostatic repulsion and expansion of the electrical diffuse double layers surrounding clay and oil particles. Oil is desorbed from the surface and the initial wetting state is altered (Ligthelm et al., 2009).
