Industry operators using downhole water separation technology in bottom water drive reservoirs with water coning problem have employed two major approaches. One approach uses downhole hydrocyclone and pumps to separate water from oil in one co-mingled production stream. This approach involves elaborate completion design and high cost equipment. The second approach, termed "gravity segregation" uses dual completion technology with zonal isolation packer to separately produce the water and the oil and so counter the cone development at the wellbore. Both methods, currently, do not completely eliminate the problem of contaminated water production but reduces it to, perhaps, some manageable level. The growing emphasis on environmental-friendly oil production operation and increasing cost of water handling requires the production of oil-free disposable water in a simple completion design.

This paper presents case studies of the pre-installation design of gravity segregation method for three fields (West Africa, Gulf Canada and Louisiana) with strong water coning problems. The study confirms that old oil fields that have suffered severe water coning exhibit a transitional saturation profile and dispersed oil-water contact. In addition, the imbibition/drainage process of water cone development and reversal induced by gravity segregation creates relative permeability hysteresis effect. The effects should be included in the pre-installation modeling.

The results using numerical simulator, indicate that the combined effects of capillary transition pressures and relative permeability hysteresis are responsible for the production of contaminated water experienced with application of the gravity segregation approach in old oil fields. It also shows how to add an envelope to the well's inflow performance window to accommodate the transition zone. Inside the envelope production of oil-free water from the bottom completion is possible with production of minimal water cut oil at the top. This uncontaminated water could be disposed while operators maximize the use of their pipeline and water separation facilities as well as improve oil recovery.

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