This paper presents, for the first time, a theoretical model for the bottomhole gas separation efficiency in Electrical Submersible Pump Installations. The model is based only on fundamental physical principles. New experimental data, collected in a field scale apparatus, and covering a wide range of liquid flow rates, GLRs, pressures and rotational speeds, are also presented. Predictions of the model are verified against the experimental data and limited published field data.
It was detected that, when rotary separators are used, two possible operating regions exist on a map of separation efficiency versus liquid flow rate and pressure. In one region the separator is quite effective and in the other the separator is not effective at all. The transition from the high efficiency zone to the low efficiency zone, in terms of liquid flow rate, is sharp. This behavior has never been reported in the literature before.
The model is simple enough that a small subroutine can be easily written, from the equations presented in the paper, to be included in design and troubleshooting programs for ESP installations. Overall agreement of the model's predictions with the experimental and field data was good in both high and low efficiency zones.
When designing an ESP installation, usually the goal is to attain the maximum flow rate from the well without generating conditions that would cause premature failure of the downhole equipment. One condition to be monitored is the amount of free gas at the pump depth. Because of the reservoir productivity characteristics, any additional amount of oil can only be obtained by decreasing the bottomhole flowing pressure. With lower bottomhole flowing pressures, higher percentages of free gas are present at the pump depth. Although some bottomhole separation can almost always be achieved, part of the free gas may eventually reach the pump. The submersible centrifugal pump, as for most pumps, cannot handle a large amount of free gas without having its efficiency highly deteriorated. Furthermore, with a large amount of free gas, the pump starts surging and protective electric devices for the downhole motor are triggered off. Frequent restarts of the system eventually damage the motor. The whole installation ends up having a short running time. An optimum design obtains a good compromise between well productivity and equipment running time. One has to select a bottomhole flowing pressure low enough to still allow a sufficiently high flow rate, but high enough not to cause problems for the submersible centrifugal pump.
Basic information needed to achieve this optimum ESP installation design includes, first, how much free gas the pump can handle and, second, the bottomhole gas separation efficiency (i.e., what fraction of the produced free gas is vented through the annulus and does not have to be handled by the pump).