Downhole water sink (DWS) technology is an alternative to conventional limited-entry completions and downhole water separation in controlling water production in wells with bottom water drive. DWS wells comprise two completions: the bottom completion produces water and keeps the top completion open to oil inflow. The system performance depends on the top and bottom rates to maximize oil productivity and produce oil-free water from the bottom completion.
Conventional nodal analysis methods cannot provide a solution for DWS wells because of the complex fluid dynamics depending on the top and bottom rates where critical rate for water coning changes with water drainage rate. Analytical modeling to date estimates DWS well's inflow conditions (water coning, reverse/oil coning, and segregated inflow) from a characteristic plot of critical rates for oil and water, but falls short in accurately predicting pressure interference for water coning, because the model couldn't account for distributed saturation around the well and resulting multiphase flow effects. A reservoir simulator is therefore used to model twophase flow to the dual completions using algorithms to automatically create, queue, and analyze simulation runs and to build accompanying tubing performance models.
In this paper, a nodal analysis approach for dual completed wells is proposed to evaluate natural flow rate. The approach identifies the operational range of top and bottom rates with water coning at the top completion and oil-free water production at the bottom completion limited to the maximum rate of water disposal by injection. Also, inflow and tubing performance relationships accounts for the bottom rate dependent critical rate and water cut response. Since the operational range changes in time, time-dependent analysis is required to evaluate the maximum oil and clean water production rate for a given increment of cumulative production. As the process of water-free oil recovery in short time cannot be practically accomplished via conventional wells, DWS wells are shown to be effective in improving recovery for a given production increment.
Water cones often develop in response to oil production in partially completed wells. Water movement toward the oil zone completions changes the saturation profile near the wellbore. The vertical movement of water is time and rate dependent. The partial completion prevents water breakthrough if the oil rate is under the critical rate. However, the critical rate is too low to be economically viable for most wells, and therefore these wells produced above critical rate where the well experiences a multiphase flow both in inflow and in tubing.
With the onset of water cone, the area where oil flows into the wellbore decreases while there is also a water saturation distribution in water cone (Figure -1). The presence of cone decreases the oil productivity via blocking oil flow path, and degrading the total mobility due to relative permeability effects (Figure-1.) So, a representative well production analysis must consider water coning. Production systems (Nodal) analysis seeks the highest oil or gas production rate.
