Nanoscale CO2-oil miscibility and minimum miscibility pressure in tight reservoirs: nanofluidic slim-tube visualization coupled with thermodynamic modeling

Fluid miscible behavior in nanoporous media governs various critical processes, including carbon capture, utilization, and storage (CCUS), enhanced hydrocarbon recovery, and subsurface pollution remediation. At nanoscale, confinement effects cause fluid miscibility to deviate significantly from bulk behavior, yet the mechanisms governing multicomponent fluids’ miscibility remain poorly understood due to experimental and theoretical limitations. Here, we employed a nanofluidic slim-tube method to directly visualize CO 2 displacement of multicomponent hydrocarbons in nanoporous media for determining minimum miscibility pressure (MMP). We resolved the coupled thermodynamic-kinetic processes underlying nanoscale fluid displacement and, for the first time, identified the three-stage immiscible displacement and the five-stage miscible displacement at nanoscale. Notably, during immiscible displacement, a fast-propagating CO 2 diffusion front precedes the displacement front, inducing a diffusion-dominant stage that contributes ∼33% of the total hydrocarbon recovery. During the miscible displacement, two distinct diffusion fronts propagate in a piston-like manner, while local CO 2 dissolution contributed ∼55% of the recovery—highlighting a dominant yet previously overlooked mechanism. In parallel, we develop a theoretical model that incorporates molecular confinement, critical point depression, and capillarity, enabling MMP prediction across varying pore sizes and geometries (defined by aspect ratio). The model aligns well with nanofluidic observations (RMSRE = 3.75%), and reveals that reductions in pore size and aspect ratio lead to greater MMP deviation from bulk values. This work establishes the mechanistic framework of understanding multicomponent fluid miscibility under nano-confinement, and informs the design of CO 2 injection strategies in tight formations.