X‑ray computed tomography‑based experimental investigation of CO₂ miscible zone formation mechanisms and degree of miscibility

Abstract

Carbon dioxide capture, utilization, and storage–enhanced oil recovery (CCUS–EOR) depends on the progressive development of CO₂–oil miscibility, yet the pressure-controlled transition from interfacial mass transfer to porous-media miscible-zone propagation remains difficult to quantify. In this study, X-ray computed tomography (CT) was combined with a time-series dual-energy CT material decomposition fusion algorithm (TDEMDF) to visualize CO₂–oil contact, quantify density redistribution, and reconstruct saturation during direct-contact and core-flooding experiments. The interfacial-tension-extrapolated minimum miscibility pressure (MMP) was 14.21 MPa; however, CT observations showed that MMP marks a mechanism transition rather than complete density equilibration. A miscible zone appeared at 9 MPa and evolved through three stages: CO₂ dissolution dominated at 9–14 MPa, oil-component extraction dominated from the MMP to the density-equilibrium threshold pressure (P DE ) of 18.62 MPa, and miscible-zone expansion dominated above P DE . Although complete CT-density parity between the CO₂-rich and oil-rich regions would require an extrapolated pressure of approximately 36.03 MPa, density-difference reduction became much less pressure-sensitive above 18.62 MPa. A density-difference-normalized apparent miscibility index coupled with Tsallis entropy revealed stepwise miscibility enhancement. Core-flooding results further showed that pore structure restricts CO₂–oil contact, lowering the peak Tsallis entropy from 2.92 in direct-contact experiments to 2.64 in porous media. Compared with the conventional method, TDEMDF reduced MAE from 16.41 to 6.23, improving saturation-estimation performance by 50.2%. These results demonstrate that CO₂–oil miscibility continues to evolve beyond the MMP and provide a CT-based framework for pressure optimization, miscible-zone monitoring, and saturation reconstruction in CCUS–EOR.