Chemical Osmosis-Driven Thermodynamically Coupled Processes: Mechanistic Insights into Oil Recovery from Core-Scale Experiments

Abstract

Abstract Chemical osmosis-driven oil migration is a viable mechanism for improved oil recovery (IOR) obtained by low-salinity water flooding (LSFW) in various reservoirs, including shale, sandstone, and carbonate rocks. Chemical osmosis generates a driving force on crude oil trapped alongside connate high-salinity water (HSW) in hydraulically stagnant zones that are not directly accessible by injected low-salinity water (LSW). However, the extent to which chemical osmosis contributes to oil recovery remains unclear. This uncertainty arises partly due to the limited experimental evidence directly demonstrating chemical osmosis-driven oil migration in actual rocks and mainly because the underlying processes have not been comprehensively clarified. This study re-examines the thermodynamically coupled processes involved in chemical osmosis-driven oil migration at the pore scale. Building on the underlying mechanisms, previous studies indicating osmotic effects were reviewed to gain mechanistic insights. These studies specify the necessary factors enabling chemical osmosis-driven oil recovery in LSWF: leaky semipermeability, permeability gap, and hydraulic dead-end boundary. With these factors, chemical osmosis via pores with membrane effects generates effective osmotic pressure and volumetric increase in HSW, driving oil migration through pores with less or no membrane effects from hydraulically stagnant to conductive zones. These thermodynamically coupled processes continue in a dynamic equilibrium state until the salinity difference eventually vanishes between the hydraulically stagnant and conductive zones. Therefore, chemical osmosis-driven oil recovery lasts long and progresses into a hydraulically stagnant zone at the scales of pores, pore networks, and rocks. "Effective" osmotic pressure acts on HSW as a driving force and breakthrough pressure to counteract the viscous and capillary forces working on the oil to migrate. However, the magnitude of the effective osmotic pressure is subtle or non-detectable in rocks with large pores, causing chemical osmosis-driven oil migration to resemble static oil displacement, seemingly caused by the volume increase of the HSW and/or oil phase, as observed in microfluidic pore models.