Multiphase nanoconfined fluid flow mechanisms in nanopores, insights derived from molecular dynamics

Multiphase flow in nanoporous media is ubiquitous in various fields, including geophysics, physical chemistry, and bioengineering. Tight sandstone reservoirs have relatively well-developed nanoscale pores. Understanding the mechanism of multiphase flow in nanoporous media is crucial for enhanced oil recovery. In this study, the pore distribution characteristics of tight sandstone cores were visually observed and analyzed through Wood's metal injection experiments and scanning electron microscopy. Subsequently, molecular dynamics simulations were used to establish a multiple-pore model with/without bound water. The mainstream channel (MC) was selected as the research pore, and multiple groups of molecular dynamics models with different oil-gas-water ratios were established to simulate multiphase flow. The results showed that the micro-nano-scale pore throats of the tight sandstone were well developed, necessitating a study of fluid flow characteristics within the nanopores. In the process of CO2 injection production, the bound water could occupy dead-end pores (DEP) and small channels (SC), which was beneficial to the oil recovery. When there was no bound water, the MC mainly relied on the CO2 displacement for oil recovery, while the DEP of the MC, the DEP of the SC, and the SC mainly relied on oil expansion and CO2 extraction. In the MC nano-confined fluid flow process, the larger the proportion of CO2 was, the lower the viscosity of the oil was, and the more easily it could flow. However, when water was present, three flow modes formed: water drop oil-water flow (WDOWF), water film oil-water flow (WFOWF), and oil drop oil-water flow (ODOWF), among which ODOWF had the greatest influence on the flow of bound fluid in the nanopores. The injection of CO2 could improve the flow in the WDOWF and WFOWF modes and had no positive effect on the ODOWF.