The withdrawal of fluid from only one layer of a vertically stratified immiscible fluid system is often referred to as selective withdrawal. For a given withdrawal location, the ability to predict the maximum flow rate at which fluid from one layer can be withdrawn before an adjacent layer of fluid is also entrained is critical for many applications. The Strategic Petroleum Reserve (SPR) offers one such example where oil is stored above an underlying brine layer in large underground caverns. When oil is added to a cavern, a corresponding volume of brine must be withdrawn through a hanging string that extends through the oil into the brine layer. If the depth of the string below the oil-brine interface is insufficient for a given flow rate, oil may be inadvertently withdrawn along with the expected brine — potentially introducing oil into the brine handling system and leading to costly cleanup to prevent environmental contamination. Laboratory experiments with two immiscible liquids (silicone oil and water or brine) have been conducted to investigate this behavior for typical SPR cavern conditions. In these experiments the higher density fluid is withdrawn through a tube below the liquid-liquid interface. As the withdrawal point is raised closer to the interface for a given flow rate, or the flow rate is increased for a given submergence, the overlying lower density layer begins to entrain along with the higher density liquid. The critical withdrawal depth at which transition to light layer entrainment occurs is measured for a given flow rate of the lower liquid, and the process is repeated for different flow rates. Most prior literature concerning the transition from selective withdrawal has examined removal of the lower density fluid and the transition to entraining the higher density fluid, whereas this work focuses on the inverse. Experiments were performed for a range of different light layer silicone oils and heavy layer water or brine, covering a range of density and viscosity ratios. Three separate withdrawal tubes of differing diameter were placed in two orientations to establish the depth at which selective withdrawal began as a function of fluid properties. The data show that, at the highest flow rates, the transition to light layer entrainment can occur when the withdrawal point is up to two diameters below the liquid-liquid interface, depending on the lower fluid density. Particle Image Velocimetry (PIV) was performed to map the instantaneous velocity field. The strength of the velocity vectors increased dramatically near the withdrawal tube opening showing the region in which inertial forces dominated the flow pattern.

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