An Analytical Model to Predict Segregated Flow in the Downhole Water Sink Completion and Anisotropic Reservoir
Authors: Utama Fia Adhi 
Published: June 2014

Abstract
Water coning, a phenomenon of water cone shape which moves upward to the perforation set, is one of the serious problems that occur in many oil fields. The water coning generally is caused by large drawdown in the oil zone where the reservoirs have active bottom water aquifer.
A conventional method which normally has been used to prevent or delay the water coning is completing the upper section of the pay zone and producing oil below its critical rate. However, producing oil below its critical rate is proven not economical. Fortunately, alternative methods that have been investigated by many authors, so called Downhole Water Sink (DWS) Completion, is proposed to solve these problems. The DWS primary mechanism is done by giving an adversary pressure drawdown in the oil zone and producing the water from the water zone. Moreover, the pressure drawdown created in the water zone would allow a higher oil production
The success key of the DWS is a segregated flow which only oil produced at the top perforation and only water produced from bottom perforation, separated flow between oil and water. In the end, a stable Water Oil Contact can be maintained through the production rate ratio between oil and water.
This paper analytically examines critical flow rate that will provide segregated flow under Downhole Water Sink completion and in anisotropic reservoir. The author's equation results included the calculation have been compared to numerical simulations by plotting it into Inflow Performance Window (IPW) curve 4 and comparing to the field data 9.

INTRODUCTION
Many authors, Muskat and Wyckoff (1935), Meyer and Garder (1954), Schols (1972), Wheatly (1985), Hoyland, and others, had developed methods to prevent water coning in a vertical well. They have generated analytically critical rate methods to anticipate water coning. However, their methods have not been economical to oil production due to small flow rate. Thus, a new method has been developed to get a better economical production rate, so called Downhole Water Sink (DWS) method. The method is using dual completion to perforate oil zone and water zone to create a balance in pressure drawdown to stabilize Water Oil Contact. Schematic comparison between the conventional completion (single completion) and the DWS completion is shown in Fig. 1 which illustrate one of the DWS completion type that can accommodate the separations flow between oil and water, producing oil at top perforation through annulus and producing water from bottom perforation through the tubing.
Wojtanowicz et. al.8 proposed and showed this method can increase ultimate oil recovery, prevent the water coning, and increase the oil production rate without water breakthrough. Recently, many authors have studied optimization of the DWS performance. Fisher, Letkeman, Tetreau, and Prima 5 have used numerical simulation to observe performance of the Downhole Water Sink increasing the oil recovery and reducing effect of the water coning.
Marhaendrajana and Alliyah 3, and Astutik 4 have used numerical simulation to provide a single Inflow Performance Window (IPW) curve as a guideline to justify oil and water rate combination, which should cause a segregated flow, water coning, reverse coning, or unstable contact into a plot.