Water coning is a serious problem in maturing reservoirs with bottom water drive where depleted oil strata overlays sizable water zone. The resulting high values of water cut and minimal oil rate lead to early shut downs of the wells without sufficient recovery of hydrocarbons in place. Over many years, reservoir engineers have looked for some effective ways to control water coning, but only a few successful oil field examples have been reported. Usually, in the presence of bottom water wells are completed in the upper section of the pay zone and produce oil below its critical rate in order to prevent or delay water coning. Typically, however, the critical rate is too low to be economical so that high water cut becomes inevitable.
Downhole water sink (DWS) technology --- a relatively new and effective method to control water coning --- has been increasingly considered by engineers worldwide. Theoretical calculations and field practice have shown that DWS can improve oil production rate, reduce water cut, and increase oil productivity index of a well. However, the technique requires draining and lifting of large amounts of water from the aquifer, which increases the cost of production. In order to solve the DWS water lifting problem, a new water coning control method is developed --- downhole water loop (DWL) installation. It has the same advantages as the DWS technique but it does not need to lift water to the surface.
This paper addresses feasibility of DWL installation and analytically examines the mechanism of producing water-free oil from the DWL from the bottom water drive reservoirs. The DWL feasibility is tested with a simple analytical model derived in this study. Also, a good match is obtained when comparing the model --calculated data to real production data.
Oil reservoirs with bottom water drive have high oil recovery due to supplemental energy from the aquifer. When the oil production rate exceeds critical rate, water is also produced due to the upwards invasion of the oil-water contact known as water coning. After the water enters the well completion (breakthrough) it may finally cause a part of oil bypassed. In order to control the coning problem, many authors had investigated the mechanism and developed methods to calculate critical rate, predict breakthrough time and reduce water cut (Muskat and Wyckoff, 1935; Elkins, 1958; Fortunati, 1962; Karp et al., 1962; Smith and Pirson, 1963; Chierici et al., 1964; Romero-Juarez, 1964; Kuo et al, 1983; Wojtanowicz, 1995; Gunning et al, 1999; Ould-amer and Chikh, 2004; Utama, 2008 and so on).
It has been established that each reservoir-well system would give a maximum water-free oil production rate (critical rate) and for higher rates there would be a time period (breakthrough time) needed for the bottom water to reach the well's oil completion. After the breakthrough, the water phase would progressively dominate the total production.
