Successful Production Allocation through ESP Performance in Peregrino Field
Authors: Brazil Helge Olsen-Statoil , Sheth Ketan , Pessoa Rui 
Published: April 2012

The Peregrino field is located in Campos Basin approximately 85 km offshore from Cabo Frio in Brazil. The field started production in April 2011 by two drilling capable wellhead platforms tied back to a central FPSO. A total of 30 producer wells, (either horizontal or multilateral) and 7 water injector wells are planned for this phase in water depths that range from 350 to 400 ft depth. Producer wells will be either horizontal or multilateral.

The oil in Peregrino is heavy with a gravity of 14° API, reservoir pressure averaging 3350 psi. Bubble point (700 psi) however it is not enough to bring fluids to surface due the vertical depths averaging 7500 ft. Because of this, it was decided that the use of artificial lift by Electric Submersible Pumps (ESPs) was the best alternative based on production expectations, flexibility and for production allocation purposes.

Flow allocation using ESP operation parameters utilizes the known models of the ESP system, the well and fluids parameters. This technique has been used with success, in wells with very high water cuts but it's a novelty and a challenge with heavy oil, where the pump performance is strongly affected by the fluid viscosity in the absence of free gas.

Flow allocation devices like test separators are bulky and expensive due to the large capacities required to operate heavy oil and one of the most critical factors in offshore operations is physical space. In addition to the separator vessel, the instrumentation and heaters required for control and efficient operation are high space consumers. In addition, a test separator only provides spot tests of the flow from a well.

More compact multiphase flow meters are also available for flow allocation at the well head, but their accuracy is highly dependent on correct fluid characterization. Specifically, oil viscosity and phase fractions are important parameters. The slow/transient gas flashing out of heavy oils affects the representativeness of necessary pressure-volume-temperature characterization.

Downhole Venturi meters were also considered for Peregrino. Though these are technically favored by the lack of free gas, they require a significant initial investment to cover topside equipment and a downhole unit per well. Limited data available regarding their uncertainty with viscous oil was the main driver to discourage their use during the production sanctioning phase. The changing oil/water conditions in Peregrino are a challenge from a viscosity modeling point of view. In addition, the Venturi meter has no redundancy in case of sensor/system failure.

Estimating the flow rate based on the ESP operation parameters can be made both based on the measured head and alternatively the brake horsepower (BHP), thus providing a robust redundancy. Based on the above considerations, the decision was made to perform individual well flow allocation based on ESP performance.

To achieve maximum accuracy of this approach, calibration of the three submersible pump types selected for Peregrino field was carried out. The tests were performed in a high-viscosity flow loop in Claremore, Oklahoma, USA, and covered the expected field operating range of viscosity, pump speed, head, flow rate and BHP. This paper outlines the results of the calibration work and presents the principles of the ESP flow allocation method. It also presents the field experience via a comparison of the ESP allocation method against the FPSO's fiscal meter, where a deviation less than the 5% required by the Brazilian regulatory agency ANP was observed.