Conventional Capillary Number theory predicts that residual oil will not be mobilized until a critical capillary number (2E-05) is exceeded. This theory was tested to determine residual oil saturation mobilization after the forced water imbibition. In the literature, high residual oil saturation was firstly established from water flooding at a certain pressure. Then gradually increasing pressure was applied for water injection until residual oil production was observed. At this research, firstly, it is confirmed that this critical capillary number (2E-5) is applicable when the initial residual oil saturation was estimated from a spontaneous water imbibition tests. Then the same type of experiments by building up the initial residual gas saturation from spontaneous oil or water imbibition tests were applied to the same samples. The entire procedures, which include spontaneous imbibition tests followed by a forced imbibition tests, were monitored by an online NMR system. Through analyses of experimental results, the critical capillary number for mobilizing residual gas from water imbibition and oil imbibition tests were estimated. It was found that the critical capillary number for gas- liquid system is very different from that of an oil-water system and the same rock.
Berea sandstone plugs were used in all the experiments. The reason for using Berea sandstone plugs is due to their relative homogeneous pore structure. Additional Western Canada sandstone plugs were used for testing gas-water systems to confirm the results obtained from the Berea Sandstone plugs. Understanding the different mechanisms of producing discontinuous residual oil or residual gas is important today. Hopefully this research could show some new insight for recovering additional gas from gas reservoirs with active aquifers.
Increasing the capillary number has long been investigatedas a strategy for improving oil recovery. Many methodologies either tested in the laboratory or applied in the field evolved around increasing the capillary number. Capillary number is defined as the ratio of viscous forces to capillary forces. Evaluation of the capillary number can be used to describe the relative importance of viscous forces to capillary forces during immiscible displacements. There are various forms of the capillary number. The most common versions of capillary number are those of Shaffman and Taylor1:
Equation(1) (Available in full paper)
Equation(2) (Available in full paper)
When the non- wetting phase is trapped in the porous media, the pressure gradient required to move it through a capillary tube is much higher than what would be predicted by the pipe flow equation, due to the pressure discontinuity at the wetting/non-wetting interface. Because of the contact angle hysteresis, this discontinuity is not of the same magnitude on the both sides of the discontinuous non-wetting phase. For example, in a water-wet medium, on oil droplet represents the discontinuous non-wetting phase. As the oil droplet is pushed through a pore throat, its downstream end gets squeezed into a much narrower segment, making its radius of curvature much smaller than the upstream part. When analyzing the possibility of mobilizing such residual phase droplets, the interfacial curvatures play a most important role in predicting mobilization
