Downhole Water Sink (DWS) technology controls water coning in dual-completed wells by concurrently producing water from the bottom completion - below the oil-water contact, and oil from another completion at the top of the oil sand. It has been shown that DWS improves well productivity, increases oil recovery, and could produce oil-free water for direct injection or overboard dumping offshore. To date, DWS has been applied in natural flow wells or wells where a downhole pump can be easily installed. However, in many areas, such as Louisiana Gulf Coast and Venezuela - where water coning is commonplace, the only production method is gas lift.
This paper presents a feasibility study and a design method for dual gas lifting in DWS wells. The design employs a two-tier nodal analysis for several combinations of two tubing strings - one for oil and second one for water - installed in a production casing. First, the nodal analysis is performed separately for the water and oil legs in order to define their operational range. Then, the two solutions are combined and coupled with the Inflow Performance Window (IPW) describing the operational domain (top Inflow rate vs. bottom inflow rate) of the well.
Using the new method, a simulation study was conducted using data from actual wells. The results indicate that the gas lift would limit performance of DWS wells. Moreover, the design can be optimized for maximum production of oil with controlled water withdrawal from the bottom completion. Limitations such as limited gas injection rate, or injection gas pressure are discussed. This paper presents an example of theoretical performance of a gas lift DWS well.
Gas lift is a flexible method - it is possible to use it in wells of few barrels per day, or wells of thousand barrels per day. There are several methods of producing through gas lift, but continuous gas lift is most common. (Presently, there are at least 1500 wells in Venezuela producing by intermittent gas lift.) Also, single well completions are the most used gas lift installations around the world.
For any type of gas lift method, there is a maximum production rate, which is function of depth, reservoir, completion, and surface conditions. This maximum production requires a specific optimized rate of gas injection. Any additional gas injection rate would result in an oil production rate smaller than the maximum rate.
To optimize gas lift for a given well, selection of tubing, size limitations and maximum available gas flow rate, it is necessary to find the best depth of injection point and the gas injection pressure.
Although dual gas lift has been theoretically developed, it has not been much practiced since single well completions are the most used around the world. In fact, only few field cases of dual gas lift have been reported in the literature.1–8 Some of them indicate that accurate reservoir and well information becomes a key factor for successful installation.