Molecular dynamics simulation has emerged as a powerful tool to shed light on the fundamental mechanisms that govern fluid behavior across multiple phases, such as gas, liquid and solid, at the molecular-scale. In shale reservoirs, understanding nanoscale phenomena such as microscopic friction, oil-gas interactions during CO2 huff-n-puff processes, and the influence of co-solvents, is crucial for enhanced oil recovery and informing strategies for CO2 geological sequestration in shale. This paper explores the unique role of molecular dynamics simulations in revealing the microscopic mechanisms of fluid transport and enhanced recovery during CO2 injection, with particular attention to the effects of hydration film, CO2 affected areas and co-solvents. The regulatory mechanism of hydration films on the friction behavior of montmorillonite provides new insights into interfacial mechanics, with implications for the mobility of confined fluids. In tight reservoir systems, the microscopic oil recovery mechanisms of CO2 under varying sweep conditions water film thicknesses highlight the complexity of fluid displacement at the nanoscale. Furthermore, the co-injection of CO2 with selected co-solvents is shown to enhance both oil mobilization and carbon storage efficiency within shale nanopores, offering a promising pathway to improve recovery outcomes under diverse reservoir conditions. By providing a molecular-level understanding of these critical processes, this work lays the groundwork for bridging atomistic insights with field-scale applications for unconventional resources development. Document Type: Perspective Cited as: Bao, J., Du, J., Zhan, S., Wang, L., Luo, Y. A molecular perspective on the microscopic mechanisms of CO2 injection and water films in fluid transport and enhanced oil recovery. Advances in Geo-Energy Research, 2025, 16(3): 288-292. https://doi.org/10.46690/ager.2025.06.08

The underlying mechanism of shale gas migration behavior is of great importance to understanding the flow behavior and the prediction of shale gas flux. The slippage of the methane molecules on the surface is generally emphasized in nanopores in most predicted methods currently. In this work, we use molecular dynamic (MD) simulations to study the methane flow behavior in organic (graphene) and inorganic (quartz) nanopores with various pore size. It is observed that the slippage is obvious only on the graphene nanopores and disappeared on the quartz surface. Compared with the traditional Navier-Stokes equation combined with the no-slip boundary, the enhancement of the gas flux is nonnegligible in the graphene nanopores and could be neglected in the quartz nanopores. In addition, the flux contribution ratios of the adsorption layer, Knudsen layer, and the bulk gas are analyzed. In quartz nanopores, the contributions of the adsorption layer and the Knudsen layer are slight when the pore size is larger than 10 nm. It is also noted that even if the Knudsen number is the same, the flow mode may be various with the effect of the pore surface type. Our work should give molecular insights into gas migration mechanisms in organic and inorganic nanopores and provide important reference to the prediction of the gas flow in various types of shale nanopores.

Clarifying the mechanisms of confined fluid flow in nanoscale space and calculating the permeability and other microscopic parameters are crucial to enhance the recovery of shale oil, which have attracted great attention in recent years. We review the micro flow mechanisms and numerical methods of shale oil, and discuss the effects of the solid-liquid molecular interaction force which leads to boundary slip and viscosity/density heterogeneity on the shale oil flow behaviors. Firstly, we introduce the four analytical models of confined fluid flow in nanopores which are the region-separation model, the effective viscosity model, the apparent viscosity model and the apparent slip length model. Then, we summarize the simulation methods and the applications in the study of fluid flow in nanoporous media Among them, the direct simulation methods are the molecular dynamics simulation (MDS) method and the pore-scale lattice Boltzmann simulation method (LBM), and the indirect simulation methods are the pore network model (PNM) and the coupled direct-indirect simulation method . We discuss the advantages and disadvantages of each simulation method. Finally, on the basis of summarizing the advantages and disadvantages of research results, the future research directions are put forward.