Molecular Insights into the Salinity Effects on Movability of Oil–Brine in Shale Nanopore-Throat Systems

Abstract

Low-salinity flooding has been well recognized as a promising strategy to increase shale oil recovery, but the underlying mechanism remains unclarified, especially for complex nanopore networks filled with oil–brine fluids. In this study, the pressure-driven flow of an oil–brine fluid with varying salinities in shale nanopore-throat channels was first investigated based on molecular dynamics simulations. The critical pressure driving oil to intrude into a nanothroat filled with brine of varying salinities was determined. Simulation results indicate that the salinity of brine exhibits great effects on the movability of oil, and low salinity favors the increase of oil movability. Further analysis of the interactions between fluid and pore walls as well as the displacement pressures reveals dual effects of brine salinity on oil transportation in a nanopore-throat. On the one hand, hydrated cations anchoring onto throat walls enlarge the effective flow width in the throat before the hydration complexes reach the maximum. On the other hand, the interfacial tension between oil and brine increases with the brine salinity, which increases the capillary resistance and leads to a higher displacement pressure. These findings highlight the effects of brine salinity on oil movability in a nanopore-throat, which will promote the understanding of oil accumulation and dissipation in petroleum systems, as well as help to develop enhanced oil recovery.

Occurrence and Flow Behavior for Oil Transport in Mixed Wetting Nanoscale Shale Bedding Fractures

“…Shale oil reservoirs are often characterized by the development of nanopores, complex mineral composition, and diverse fluid occurrence states. − The flow behavior and physical properties of fluids in nanoconfined spaces are completely different from those under bulk conditions. This is mainly reflected in the fact that (1) the interaction between the fluid and the solid wall becomes non-negligible, which causes changes in the physical properties of the fluid in nanoconfined spaces; − (2) the slip of the fluid on the surface of solid wall during the flow process in nanoconfined spaces becomes non-negligible; , (3) the flow in nanoconfined spaces is more easily affected by the wettability of the pore wall than in bulk conditions. , Considering these changes comprehensively, conventional theories cannot be directly applied to describe flow behavior in nanoconfined spaces. Therefore, it is crucial to accurately characterize the occurrence states and flow behavior in nanoconfined spaces in shale reservoirs.…”