Optimal control strategies with nonlinear optimization for an Electric Submersible Pump lifted oil field

Abstract

In an Electric Submersible Pump (ESP) lifted oil eld, the ESP of each oil well should be operated inside its operating window. The total power consumed by the ESPs in the oil eld should be minimized. The speed of the ESPs and the production choke valve opening should be optimally chosen for maximizing the total oil produced from the oil eld. At the same time, the capacity of the separator should not be exceeded. In this paper, nonlinear steady state optimization based on Sequential Quadratic Programming (SQP) is developed. Two optimal control structures are proposed in this paper. In the rst case, the optimal pump speed is controlled by a PI controller by varying the electrical excitation signal to the motors. The optimal uid ow rate through each oil well is controlled by another PI controller by varying the production choke valve opening. The paper shows that the production choke valve for each oil well has to be always 100% open to maintain the optimal uid ow rate. In the second case, the production choke valves are considered to be always 100% open as hard constraints. The optimal uid ow rate through each oil well is controlled by a PI controller by varying the pump speed. It is shown that when the optimal uid ow rate is tracked by the controller, the speed of each of the pumps is equal to the optimal pump speed calculated by the optimizer. This basically means that we can achieve the optimization objective with the same optimal results as in the rst case by using only a single PI controller. The limitations of these two optimal control structures for very low values and for very high values of the separator capacity are discussed. For the feasible range of separator capacities, the optimal locus of the uid ow rate and the pump speed are shown in this paper.