Transient and steady-state flow mechanisms of immiscible supercritical CO2–oil two-phase flow in a single rough rock fracture

Abstract To clarify the immiscible two-phase flow mechanisms of supercritical CO 2 (scCO 2 ) and oil in a single rough rock fracture, this study developed temperature- and pressure-dependent models for the density and viscosity of scCO 2 and incorporated these models into numerical simulations of transient and steady-state immiscible scCO 2 –oil flow in such fractures. The effects of fracture aperture, fractal dimension, contact angle, and scCO 2 injection pressure were systematically investigated. During the transient stage, the migration behavior of the scCO 2 –oil interface and the dynamic evolution of scCO 2 saturation were revealed, along with their dominant controlling factors. An effective flow channel ratio was proposed to quantify the flow capacity of a single rough fracture, and the influence of fracture geometric features on this parameter was clarified. During the steady-state displacement stage, the root-mean-square (RMS) value of the first-order derivative of the fracture profile was introduced to quantify fracture roughness. The response of the scCO 2 volumetric flow rate to the pressure gradient—under varying fracture apertures and surface roughness conditions—was analyzed, and the critical pressure gradient marking the transition from linear to nonlinear flow was determined. Furthermore, a calculation method for the non-Darcy inertial coefficient in the Forchheimer equation was developed, and a critical Reynolds number criterion for the onset of nonlinear flow of scCO 2 in a single rough fracture was proposed. The research findings deepen our understanding of scCO 2 –oil two-phase flow in rough fractures and provide theoretical support for the design and optimization of CO 2 -fracturing-based oil displacement technology.