(155f) CO2-Tunned Pressure-Driven Oil?Water Two-Phase Flow in Mesopores: A Molecular Dynamics Study

In recent years, CO₂ injection is not only one of the potential means to alleviate carbon emissions through geological CO₂ sequestration (GCS), but also a successful enhanced oil recovery (EOR) technology which has been widely applied in North American reservoirs. On one hand, it is well-known that CO₂ facilitate volume expansion and viscosity reduction of oil. On the other hand, oil−water interfaces play an important role in two-phase flow behavior. Specially, there is liquid-liquid slippage at the interface between two immiscible flowing liquids (Physical review letters 2006, 96 (4), 044505), which exists in oil-water interfacial region (Fuel 2020, 262, 116560). Additionally, the presence of soluble gas replenishes the liquid–liquid interface, increasing the viscous coupling and decreasing slip length between the liquids (Journal of Fluid Mechanics 2022, 938, A35).

Although the extensive investigations on oil-water flow in nanochannels are conducted, the effect of CO₂ on oil−water flow behaviors, especially in the interface region, remains largely unknown. To investigate pressure-driven oil-water flow with CO₂, molecular dynamic (MD) simulations are conducted. More specifically, we study the effect of varying CO₂ concentrations on liquid-liquid slippage at the interface, as well as viscosity reduction in the bulk oil−CO₂ miscible region. In this work, octane is used to represent oil phase. The pore size is kept at ~5.5 nm as a representative α-quartz (a common reservoir mineral) mesopore, with temperature and pressure of 323 K and 20 MPa, respectively (a typical reservoir condition). The external pressure gradient is set as ~6 MPa/nm in the flow direction.

Water film forms on the pore surface due to strong surface hydrophilicity, while octane molecules tend to occupy the middle of pore. The accumulation of CO₂ occurs in the octane-water interfacial region, with a peak density value that is 2-3 times greater than that observed in the bulk region. As CO₂ content increases, water distributions remain unchanged, while octane molecules gradually deplete in both interfacial and bulk regions. As a result, the bulk oil viscosity gradually decreases. There exists a critical CO₂ fraction below which the liquid-liquid slippage length remains constant, beyond which it gradually decreases with CO₂ content. Herein, CO₂ acts as an adhesive between water and octane, since it can not only form hydrogen bonding with water, but also have strong van der Waals interactions with octane. As the number of CO₂ is sufficient in the interfacial region, the viscous octane−water coupling is enhanced, leading to weak liquid-liquid slippage. Overall, the presence of CO₂ exhibits dual impacts in oil flux, while its impact on water flow is negligible. On one hand, oil mobility is enhanced through bulk viscosity reduction. On the other hand, the liquid-liquid slippage is weakened with a high concentration of CO₂, leading to a decrease in oil flux.

Collectively, by using molecular simulations to explicitly consider molecular characteristics and intermolecular interactions which are largely ignored in conventional continuum flow model (such as Hagen-Poiseuille equation), we observe the unique features of CO₂ tuning on oil-water two-phase flow behaviors in nanoporous media. The fundamental understanding from this work provides important insights into the optimization and rational design of EOR and GCS processes.