An experimental investigation was carried out on mobilization and very slow flow of oil slugs in a capillary tube. The pressure drop of the slug flow was measured at every stage of mobilizing and moving the oil slugs as a function of capillary number in the range of 4 × 10−7–6 × 10−6. The pressure drop across the oil slug experienced three stages: build-up, hold-up, and steady stages. During the build-up stage, the convex rear end of the slug was becoming concave into the oil slug and the convex front end of the slug moved ahead to form a new portion of the slug. At the hold-up stage, both the concave rear end and the front end continued to advance, and the initial contact line of the oil slug with the tube wall through a very thin water film was being shortened. At this stage, the pressure drop reached a maximum value and remained nearly constant. At the steady stage, after the oil slug was completely mobilized out of the original contact region, the differential pressure had a step-drop first, and then the oil slug flowed at a lower differential pressure depending on the flow rate. Numerous slug flow tests of this study showed that the hold-up pressure drop was always higher than the steady stage pressure drop. Results also showed that the measured extra pressure drop was significantly high compared to the pressure drop calculated from Poiseuille equation, which is still commonly used in network modeling of multiphase flow in porous media.
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