Morphology Classification and Quantitative Analysis of Residual Oil during CO 2 Miscible Displacement Based on Digital Core

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CO 2 enhanced oil recovery technology, as a critical component in the field of carbon capture, utilization, and storage, enables simultaneous geological sequestration and efficient utilization of CO 2 . Current numerical simulations of CO 2 miscible displacement predominantly concentrate on investigating the seepage processes while lacking a comprehensive quantitative analysis of the residual oil distribution. Addressing this research gap, this study employs digital core technology to reconstruct the actual three-dimensional rock pore structures and establishes a mathematical model coupling fluid flow with interphase mass transfer. The process of CO 2 miscible displacement is simulated to elucidate its seepage mechanisms in the complex pore structures, classify the occurrence morphologies of the residual oil, and conduct quantitative analysis. Further investigations examine the impacts of CO 2 injection velocity, fluid viscosity ratio, and rock wettability on the displacement dynamics and the reduction of the residual oil. Simulation results reveal that the displacement process can be divided into three stages. During the early stage, a rapid CO 2 breakthrough into the crude oil is observed. During the intermediate stage, CO 2 plumes are formed, establishing the dominant flow pathways that sweep into the surrounding pores from the main flow zone; meanwhile, the displacement efficiency decreases. During the late stage, the morphological types of residual oil within the pore region tend toward stability. After the displacement, the residual oil can be divided into seven categories according to the different occurrence forms, with the descending remaining quantities as follows: lump-shaped (32.1%), blind-end-shaped (21.8%), sheet-shaped (19.7%), strip-shaped (15.5%), columnar-shaped (8.2%), annular-shaped (2.3%), and corner-shaped (0.4%) residual oils. Among these morphologies, the lump-shaped, blind-end-shaped, sheet-shaped, and strip-shaped residual oils account for nearly 90%, making them the primary mobilizable types. Increasing the injection velocity accelerates the displacement process, with the reduction in lump-shaped, sheet-shaped, and strip-shaped residual oils being the most substantial, accounting for 34.7%, 29.0%, and 16.5% of the total reduction, respectively. On the other hand, reducing the viscosity ratio enhances the piston-like displacement efficiency, significantly decreasing the sheet-shaped and strip-shaped residual oils adhering to the pore walls and the difficult-to-mobilize corner-shaped residual oil, with relative reduction rates of 67.8%, 54.2%, and 73.7%, respectively. When the rock wettability transitions from the oil-wet to the nonoil-wet conditions, the CO 2 seepage resistance decreases, and the displacement efficiency for the sheet-shaped and strip-shaped residual oils adhering to the pore surfaces is significantly improved, with reductions accounting for 43.2% and 28.8% of the total reduction, respectively, while the blind-end-shaped residual oil is also notably mobilized, contributing 11.5% to the total reduction. This study achieves the classification and quantitative characterization of the occurrence morphologies for different types of residual oil during CO 2 miscible displacement in the three-dimensional complex pore structures and quantitatively reveals the mobilization potential of each type of residual oil, thereby providing a theoretical basis and quantitative support for optimizing targeted injection strategies and enhancing oil recovery.

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