Experimental evidence of chemical osmosis-driven improved oil recovery in low-salinity water flooding: Generation of osmotic pressure via oil-saturated sandstone

Chemical osmosis caused by semipermeability and salinity gradient of reservoir rock has been considered to improve oil recovery obtained by low-salinity water flooding. However, the generation of osmotic pressure by rocks containing crude oil has not been reported. In this study, an oil-saturated sandstone generated osmotic pressures of up to 37 kPa at a salinity difference of 0.6-0.1 M NaCl and expelled crude oil from the surface facing low-salinity water (LSW). The results demonstrate that chemical osmosis can drive directional oil migration toward LSW on the scale of rock. The tested sandstone exhibited a clay content of 17 wt%; however, the oil-free core did not generate osmotic pressure. In contrast, the oil-saturated core generated osmotic pressures at the high-salinity water (HSW) facing its surface five days after being exposed to the salinity difference. The delayed pressure generation can be attributed both to the water intrusion into potential membranes (i.e., clay minerals, crude oil, and the interface between oil and minerals) and to the evolution of salinity gradient in the core. In the last experiment, on the same core using a salinity difference of 0.9-0.1 M NaCl, osmotic pressure developed immediately because both the active membranes and saline water already existed across the core. These results suggest that the potential semipermeability of reservoir rocks containing crude oil and saline water can promptly cause chemical osmosis upon exposure to the change in salinity difference. Although the cumulative flux driven by the osmotic pressure likely exceeded the volume of injected crude oil, chemical osmosis continued generating osmotic pressure until the end of the experiments. These contradictory results suggest that the chemical osmosisdriven directional oil migration toward the LSW occurs preferentially through conductive large-sized pores, and the residual oil in small-sized pores and clay minerals exert long-lasting semipermeability to cause chemical osmosis further toward the HSW.