Bubble-Point Pressure Deviations and Metastable Nucleation in Nanoconfined CO2-Shale Oil Systems: Effects of Temperature, Composition, Gas-Oil Ratio, and Pore-Scale Coupling.

Abstract

Bubble-point behavior of shale oil under nanoconfinement is critical for understanding phase evolution during CO₂-enhanced shale-oil recovery. In this study, visual micro/nanofluidic PVT experiments were conducted to systematically investigate bubble-point pressure deviations of pentane-CO₂, hexadecane-CO₂, pentane/decane/hexadecane-CO₂, and shale oil-CO₂ systems. Measurements were performed at gas-oil ratios of 130 and 260 and temperatures of 65-125 °C using a 10 nm single-scale pore model and a coupled 10 μm-10 nm pore-network model. Experimental results were compared with theoretical predictions from a confinement-modified Peng-Robinson equation of state incorporating capillary pressure, surface adsorption, and critical-property shifts. Furthermore, a metastable nucleation limit was introduced to describe the experimentally observed phase-transition pressure below the equilibrium bubble-point boundary. Pronounced downward deviations toward the metastable region were observed in isolated 10 nm pores. For the pentane-CO₂ system at 65 °C and GOR = 130, the experimentally observed phase-transition pressure was approximately 3.5 MPa, compared with an equilibrium prediction of 4.3 MPa, corresponding to a relative deviation of 18.35%. The deviation decreased as temperature approached the critical region and was reduced at higher GOR. For example, at GOR = 260, the deviation of the hexadecane-CO₂ system decreased from 40.63% at 65 °C to 10.01% at 125 °C. Fluid composition also strongly affected confinement sensitivity, with heavier hydrocarbon systems exhibiting stronger shifts toward metastable behavior. In contrast, bubble-point pressures in the coupled 10 μm-10 nm network remained close to the equilibrium predictions of the micrometer-scale pores and deviated significantly from the metastable limit. For the shale oil-CO₂ system at 65 °C and GOR = 130, the relative deviation decreased from 29.77% in the isolated 10 nm pore model to only 2.43% in the coupled 10 μm-10 nm network. These results demonstrate that nanoconfined bubble-point behavior is governed by the coupled effects of thermodynamic conditions, fluid composition, metastability, and pore-scale connectivity.