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

Despite the growing popularity of renewable energy and alternative materials for petroleum products, petroleum exploration and production has continued to increase with increased demand for energy and growth of population. While petroleum development has significantly advanced, the field still faces various challenges. One of the main problems is the limitations of the conventional oil recovery mechanisms, especially those obtained via low-salinity water (LSW) flooding. Since these mechanisms mostly focus on (or target at or address)residual oil in hydraulically conductive zones, knowledge of chemical osmosis that could recover the oil left behind in hydraulically stagnant zones like low-permeability layers in sedimentary formations is extremely important.

By principle, chemical osmosis is the migration of water molecules driven by the differences in water chemical potential across semipermeable membranes. Thus, salinity differences and the presence of semipermeable membranes are the two primary conditions required for chemical osmosis in reservoir formation to occur. Previous results revealed that chemical osmosis induced by salinity gradient and semi permeability of the reservoir rock could improve oil recovery obtained via low-salinity water flooding. Unfortunately, the generation of osmotic pressure via crude oil-containing rocks is underexplored. In addition, although oils can be moved from high-salinity water (HSW) to LSW via chemical osmosis, its impact on large-scale directional oil migration and removal of crude oil from hydraulically stagnant zones is still poorly understood.

Herein, Dr. Mikio Takeda and Dr. Mitsuo Manaka from National Institute of Advanced Industrial Science and Technology and Mr. Daisuke Ito from Japan Petroleum Exploration Co., Ltd. proposed a chemical osmosis experiment to validate the generation of osmotic pressure via oil-saturated sandstone with relatively low permeability. While the proposed experiment is akin to conventional core-flooding experiment, it relied on salinity gradient across the rock core instead of pressure gradient. Its configuration was designed to simulate hydraulically stagnant zones not directly accessible to the injected low-salinity water in the reservoir formation. The experiments were performed on oil-free and oil-saturated sandstone cores, with the former utilized as a control experiment. Their work is currently published in the research journal, Journal of Petroleum Science and Engineering.

The research team showed that chemical osmosis generates an effective chemical osmotic pressure that can drive directional oil migration towards LSW on the scale of rock. The expulsion of crude oil was more significant on the core surface facing the LSW than on the surface facing the HSW. The oil-saturated sandstone exhibited osmotic pressures of up to 37 kPa at a salinity difference of 0.6 – 0.1 M NaCl and clay content of 17 wt.%, while the oil-free core failed to generate osmotic pressure. The observed delays in pressure generation were attributed to two main reasons: potential water intrusion into the membranes and salinity gradient evolution in the core. Furthermore, while the osmotic pressure-driven cumulative flux exceeded the injected crude oil volume, the total semi-permeability was still responsible for the continued generation of osmotic pressure.

In summary, the authors presented the first experimental evidence demonstrating the improvement of oil recovery in LSW flooding via osmotic pressure generated through chemical osmosis. The findings suggest that chemical osmosis driven oil migration progresses into the hydraulically stagnant zones in reservoirs with HSW, with water molecules invading small-sized pores towards HSW (chemical osmosis) and with crude oil flowing out of large-sized pores towards LSW (Osmotic pressure driven flow). The long-lasting semi-permeability in the hydraulically stagnant zones would be exerted by the residual oil in small-sized pores and electrical double layers surrounding the clay minerals as demonstrated by the core-scale experiment of this study.. In a statement to Advances in Engineering, First and corresponding author Dr. Mikio Takeda stated that their findings would contribute to the development of the petroleum production field in terms of oil recovery.