Abstract

This paper presents experimental data and a preliminary analysis on the operation of centrifugal pumps with viscous liquids. Two centrifugal pumps, a conventional radial (Ns = 1000) and a semi-axial electrical submersible pump - ESP (Ns= 3850) were instrumented and tested with water - 1 cP, and clear glycerin. The glycerin viscosity was varied from 67 cP to 1020 cP, through changes in temperature, encompassing the viscosity range of light to heavy oils. The main purpose of these tests, besides measuring the influence of viscosity upon the pump overall performance, was to supply detailed information on the energy transfer processes taking place in the pump internal components. To accomplish with that the pressure distribution along the flow path, through impellers and diffusers, from the pump inlet eye to the discharge section, was measured. Thus, besides measuring the flow rate, the total pressure difference, the speed, the power and the mean operation temperature, for a working liquid with various viscosities in a full range of operational conditions, the data showed the pressure evolution inside the pump. Later analyses revealed that there was a strong relationship between the flow hydrodynamics in successive pump devices in terms of head gain and viscous dissipation. This relationship was set by the pump operational operational conditions, flow rate and speed, and the fluid viscosity. In other words, how a diffuser performed depended on Reynolds number and the upstream flow through the impeller. Thus, any generalizing model dealing with the influence of fluid viscosity on the pump performance should account for these phenomena. For this reason, it is highly improbable that "black-box" approaches that neither consider the flow hydrodynamics of the successive connecting devices nor assess the influence of the Reynolds number on them could give proper viscosity correction factors for different pumps.

Introduction

Electrical Submersible Pumps - ESPs - have been increasingly used as an artificial lift method for producing medium-to-heavy oils in deep offshore fields. The pump may be located inside the well or, depending on technical requirements, even above the seabed in order to diminish intervention costs. This is just the case in Jubarte, a pioneering heavy oil field in Campos Basin, Brazil. Economic and technical analyses showed that using ESPs was the sole option to produce this low gas-to-oil ratio heavy-oil field. When the oil viscosity is too high to allow for the pump efficient operation, injecting light oil (usually referred to as a downgrading technique) or solvents before the pump suction or acting on the mixture temperature to reduce the oil viscosity are some of the strategies to be considered. When installed on the seabed, series-parallel arranged pump systems might be employed, in order to reduce the power of every single pump. Thus, new operation strategies and different equipment allocation and arrangements have been used to exploit heavy oils in these harsh environment.

In Jubarte field an ESP was installed in the production riser, just above the Christmas tree, to lift the mixture to the platform without any previous gas-liquid separation. In this scenario accurately sizing the ESP is, for a number of reasons, very important. When the ESP is on the seabed, the fluids may be at a temperature somewhat bellow the reservoir temperature. Moreover, the heat generated by the viscous dissipation inside the ESP, which contributes to reduce the fluid viscosity and increases its performance when in-well systems are considered, is partially lost to the surrounding cold water. Thus, in both cases the pumped mixture attains a higher viscosity, which imparts the pump performance. Additionally, when at the seabed level, the reduced pressure at the pump inlet compared to an in-well pump, may increase the free gas content - GOR, additionally depleting the pump performance. As seen, the fluid viscosity is a key technical issue when using ESPs to lift medium to heavy oils in offshore fields and as such deserves proper attention when selecting the equipment.

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