Influence of Viscosity on ESP Performance
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 110661, "On the Influence of Viscosity on ESP Performance," by Gilmar Amaral and Valdir Estevam, Petroleo Brasileiro S.A, and Fernando A. Franca, SPE, State University of Campinas, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11–14 November. The paper has not been peer reviewed.

Experimental data and a preliminary analysis of the operation of centrifugal pumps with viscous liquids are presented. Two centrifugal pumps, a conventional radial and a semiaxial electrical submersible pump (ESP), were instrumented and tested with water and clear glycerin. The glycerin viscosity was varied from 67 to 1,020 cp by changing the temperature, encompassing the viscosity range of light-to-heavy oils. The main purpose of these tests, besides measuring the influence of viscosity on the pump overall performance, was to supply detailed information on the energy-transfer processes taking place in the pumps' internal components.

Introduction

ESPs have been used increasingly as an artificial-lift method to produce medium-to-heavy oils in deep offshore fields. The pump may be inside the well or, depending on technical requirements, above the seabed to reduce intervention costs, as is the case in Jubarte field in Campos basin, Brazil. When the oil viscosity is too high to allow efficient pump operation, the oil viscosity can be reduced by injecting light oil (usually referred to as a downgrading technique) or solvents upstream of the pump suction or by raising the mixture temperature. When installed on the seabed, series- or parallel-arranged pump systems can be used to reduce the power required by each pump. Thus, new operation strategies and different equipment allocation and arrangements have been used to exploit heavy oils in these harsh environments.

In Jubarte field, an ESP was installed in the production riser, just above the tree, to lift the raw mixture to the platform. When the ESP is on the seabed, the fluid temperature may be lower than the reservoir temperature. The heat generated in the ESP, which reduces the fluid viscosity and increases pump performance, is partially lost to the surrounding cold water. As the pumped mixture cools and attains a higher viscosity, pump performance is affected. Additionally, a pump at seabed level operates at a reduced inlet pressure compared with an in-well pump and may increase the free-gas content, additionally reducing pump performance. Therefore, fluid viscosity is a key technical issue when selecting an ESP to lift medium-to-heavy oil in offshore fields. The performance of pumps operating with liquids of various viscosities is obtained by either empirical or deterministic methods. Empirical methods usually try to adjust the pump performance starting with the published pump-characteristic curves for water. Nondimensional parameters and correcting coefficients for hydrostatic head and efficiency for a given flow rate and fluid viscosity are used to estimate the actual characteristics on the basis of those for water. However, the database is limited in terms of pump characteristics and operational conditions.