Critical Displacement Ratio and Its Effect on Wellbore Measurement of Residual Oil Saturation

Abstract

Introduction Residual oil saturation is one of the most basic concepts in reservoir engineering; knowledge of its value is virtually indispensable in determining the economic attractiveness of a waterflood project or a tertiary recovery operation. Its measurement, which is frequently made in situ, depends on the condition of true residual oil saturation around the wellbore. This condition is usually created by injecting a fluid to reduce the existing oil saturation to a residual value. Measurement techniques include various log-inject-log schemes, minitests, the tracer method, and even the conventional method of movable oil determination from logs, which depend on the true residual oil saturation created by mud filtrate in the flushed zone. All these methods sample the relatively small part of the formation that is subjected to the largest pressure gradients because of the logarithmic pressure distribution around the wellbore. In this region close to the well, the large pressure drop (or high velocity of the displacing phase) may cause the parameter Delta p/L sigma to exceed the "critical" value and mobilize some of the residual oil in this region. The result may be a conservative value of measured residual oil saturation and an optimistic value of movable oil saturation. Others have commented on this danger and have recommended that injection rates be kept low enough to minimize the possible displacement of residual oil before the logging test possible displacement of residual oil before the logging test is run. An extreme case of stripping of the residual oil may occur in coring operations when very large pressure gradients exist near the core bit. pressure gradients exist near the core bit. While it is undesirable to mobilize the residual oil before its in-situ measurement, there are cases in which the reduction of oil saturation immediately adjacent to the wellbore is highly desirable. The relative permeability to water increases with even a small reduction of permeability to water increases with even a small reduction of residual oil saturation; since most of the pressure drop in an injection well takes place within the first few feet from the wellbore, the well injection pressure can be reduced significantly by a deliberate displacement or reduction of the residual oil in that area of the wellbore. A preinjection well treatment is then necessary and the displacement ratio, Delta p/L sigma, during the well treatment must exceed the critical value. This paper describes a method for determining the injection rates that will not disturb the residual oil saturation for any specified distance from the injection well. The method also will be useful for determining the combinations of interfacial tension and injection rates that can be used to reduce the residual oil saturation at various distances from the injection well. Displacement Ratio in Linear and Radial Systems The fact that some residual oil can be displaced from most porous materials, if the ratio of viscous to capillary forces is large enough, has been reported by a number of researchers. Some authors reported their results in terms of a dimensionless group such as v mu/sigma, where v and mu are linear velocity and viscosity, respectively, of the water phase and sigma is the interfacial tension between the oil and water. For convenience, we prefer to use the ratio Delta p/L sigma, where Delta p is the prefer to use the ratio Delta p/L sigma, where Delta p is the pressure drop (psi) across the distance L (ft) and sigma is pressure drop (psi) across the distance L (ft) and sigma is the interfacial tension (dynes/cm) between oil and water. JPT P. 865