Partly gravity dominated water floods take place in several North Sea oil fields.A characteristic feature of some of these water floods is a significant reduction of the oil saturation behind the water front versus time.Field data that support this statement is data from observation wells where saturation logs are run on approximately a yearly basis [1].This reduction of the oil saturation behind the water front is explained phenomenological by interpreting the rock as being mixed wet for the oil-water-rock system involving oil flow through films [2].There is a fundamental question to this recovery/displacement process concerning the relative permeability characteristics,residual oil saturation and how representative data from standard SCAL experiments are.For strongly wetting systems it is possible to scale the viscous and gravity forces for prediction of residual oil saturation [3].However,for mixed wet systems it is difficult to perform properly scaled SCAL experiments reproducing the characteristics of a gravity dominated displacement over a vertical distance of several tens of meters or more.The main difficulty is maintaining the wetting properties while reducing the capillary forces in a small scale model.A closer inspection of the relevant scaling groups governing the dynamic process on the pore scale and on the macroscopic scale,shows that a properly scaled experiment to model the gravity dominated water flood process requires a long vertical flow rig,which is flooded at a relatively low rate to ensure a gravity stable water flood.The length requirement depends on the capillary forces and the vertical extent of the capillary transition zone.One of the main phenomena,which is difficult to scale,is the flow of oil through continuous films over long distances driven by gravity and capillary forces.In this study the wetting behaviour was prepared by using fresh core,reservoir fluid and aged material.
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