Comparative Study of Phase Behavior and CO2 Miscibility Characteristics of Live-Oil Samples from Different Members of the Yanchang Formation, Ordos Basin

Carbon dioxide-enhanced oil recovery (CO2-EOR) in ultra-low-permeability reservoirs requires a reliable understanding of CO2–oil phase behavior and minimum miscibility pressure (MMP), particularly where multiple stratigraphic members are co-developed. In this study, recovered live-oil samples from the Chang 4 + 5-producing Well 272 and the Chang 6-producing Well 651 of the Yanchang Formation in the Ordos Basin were compared using compositional analysis, conventional PVT tests, CO2-swelling experiments, slim-tube displacement tests, and Peng–Robinson equation-of-state (PR-EOS) characterization. The PR-EOS model used pseudo-component characterization, grouped binary interaction parameters and CO2-swelling saturation-pressure constraints to support the interpretation of EOS-predicted phase behavior and the evaluation of internal consistency. The two recovered samples have similar measured bubble-point pressures and dead-oil densities, whereas the Well 651 sample has a lower live-oil viscosity and a slightly higher C2–C12 hydrocarbon fraction. Slim-tube tests gave approximate MMPs of 17.8 MPa for the Well 272 sample at 59.94 °C and 16.1 MPa for the Well 651 sample at 69.30 °C. Because the samples were tested at their respective reservoir temperatures, temperature and composition varied simultaneously and their independent contributions cannot be separated using the present dataset. Accordingly, the comparison is limited to the two sampled well-fluid systems under the documented sampling and test conditions; the results should not be generalized to interval-wide Chang 4 + 5 or Chang 6 fluid properties without additional representative sampling.