Perched water occurs when oil invades a reservoir and eventually traps pools of water at the bottom of the reservoir. The pools of perched water are separated laterally. When the reservoir is unsegmented and in equilibrium, with both water and oil being continuous phases, the saturation curves have to be identical in the upper part of the reservoir. However, the water pressure is increased inside the trap due to the push of the invading oil. As long as the reservoir rock is water-wet, water will try to escape the trap through the continuous water film on the rock surface. The escaping water will have to overcome the capillary pressure building up from the new shallow free water level inside the perched water pool. The perched water pool will become stable at an elevated pressure compared to the water in the main body of the reservoir.

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