Microscopic occurrence state of remaining oil in low permeability reservoirs using CT technology

Accurate characterization of the microscopic occurrence states of remaining oil is essential for improving oil recovery in low-permeability reservoirs during the high water-cut stage. To advance the characterization of remaining oil from static description to dynamic quantitative analysis and to clarify the evolution of remaining oil within rock pore spaces during water flooding, a multi-parameter coupled dynamic characterization method is developed. Microscopic displacement experiments are conducted using computed tomography (CT) scanning technology. By comprehensively analyzing parameters including the total number of remaining oil clusters, average cluster volume, contact area ratio, shape factor, and Euler number, the coupled evolution of remaining oil quantity, spatial distribution, and morphological characteristics throughout the entire displacement process is quantitatively characterized. The results show that during water flooding, the total number of remaining oil clusters continuously increases while the average cluster volume decreases, resulting in an increasingly dispersed distribution; and the greater the core permeability, the more pronounced these trends become. As displacement proceeds, the oil phase is progressively detached from pore walls, leading to a continuous reduction in the oil-rock contact area ratio. Although the cores are water-wet, strong reservoir heterogeneity causes portions of residual oil remain in contact with the pore walls during the late development stage, suggesting the further recovery can be achieved by reducing interfacial tension in subsequent development stages. In addition, well-connected cluster-type remaining oil is mobilized and gradually transforms into other occurrence types during water flooding. At the residual oil stage, cluster-type remaining oil dominates, forming a complex occurrence system in which cluster residual oil coexists with other morphologies. This observation indicates that the enhanced oil recovery strategies should shift from a single displacement mechanism to a comprehensive mobilization mechanism targeting multiple residual oil morphologies. This study provides critical insights linking microscopic mechanisms to macroscopic development strategies and offer an important scientific basis for the efficient development of the G76 fault-block reservoir in the JD Oilfield during the high water-cut stage.