Experimental Study on CO2–Crude Oil Phase Behavior, Miscibility and Threshold Pressure Gradient for CCUS in Ultra-Low Permeability Reservoir

The Chang 8 reservoir in the Baima Middle Block of the Ordos Basin is an ultra-low-permeability reservoir that has faced formation-pressure depletion, a rising water cut, and poor remaining-oil recovery during long-term water-injection development. CO2 flooding in CCUS can simultaneously enhance oil recovery and achieve carbon sequestration; however, comprehensive and integrated research on the CO2–crude oil phase behavior, miscibility characteristics, and nonlinear flow in porous media for this block is still lacking. To address this gap, this study used crude oil and natural cores from the block and natural cores to conduct a series of experiments at 70 °C, including PVT high-pressure physical-property tests, CO2 constant-mass-expansion tests, slim-tube minimum-miscibility-pressure (MMP) tests, pendant-drop oil–gas interfacial-tension measurements, and CO2-flooding threshold-pressure-gradient measurements in cores of different permeability. The results showed that as the CO2 mole fraction increased from 0% to 61.78%, the bubble-point pressure of the crude oil rose from 17.424 to 22.575 MPa, the viscosity decreased from 9.026 to 4.906 mPa·s, and the volumetric expansion coefficient increased from 1.0000 to 1.1454. As the water cut increased from 0% to 80%, the apparent MMP of CO2 increased from 15.83 to 17.69 MPa. The oil–gas interfacial tension gradually decreased with increasing system pressure, and droplet instability and atomization occurred under near-miscible high-pressure conditions. The CO2-flooding threshold pressure gradient decreased with core permeability following a power-law relationship, and a knee-like trend appeared near 0.5 mD within the limited permeability dataset of this study; this trend cannot be generalized without further experimental evidence. This study comprehensively reveals the coupled mechanisms of phase behavior, miscibility, and nonlinear flow during CO2 flooding in the target block, providing experimental and theoretical support for optimizing the injection pressure and reservoir-stimulation schemes of CCUS-EOR in ultra-low-permeability reservoirs.