Characterization of Microscopic Residual Oil Distribution during Waterflooding Using In Situ Computed Tomography

In this study, a comprehensive and pragmatic experimental framework has been developed to quantitatively characterize the occurrence and categorization of microscopic residual oil distribution via in situ computed tomography (CT) techniques. More specifically, waterflooding displacement experiments were conducted and monitored through online CT scanning. In addition to incorporating the oil–water distribution within pores and throats, a three-dimensional (3D) digital core was subsequently reconstructed through threshold segmentation, facilitating the identification and reclassification of microscopic residual oil distribution based on its formation mechanisms and occurrence state. Finally, 3D digital core modeling was performed to determine the microscopic residual oil saturation at various occurrence states, and the microscopic residual oil saturation in pores and throats with different radii was determined by the use of the pore space intersection algorithm. For the microscopic residual oil after being displaced during waterflooding, continuous oil distribution of its original network has gradually evolved into a discontinuous one with multiple states and small discrete volumes. According to its formation mechanisms, microscopic residual oil can be divided into three types: Nondisplaced residual oil, throat-plugging residual oil, and adsorbed residual oil. Nondisplaced residual oil includes cluster-shaped residual oil and throat-shaped residual oil with a saturation of 16.76 and 2.70%, respectively. Throat-plugging residual oil includes droplet-shaped residual oil with a saturation of 9.23%, while adsorbed residual oil includes corner-shaped residual oil and film-shaped residual oil with an absolute content of 2.00 and 1.45%, respectively. When waterflooding reaches the high water-cut period, the microscopic residual oil primarily accumulates in the intermediate and large pores controlled by small throats.