Abstract

A unique hydraulic turbine driven submersible pump, (HSP), was successfully trialled in a multiphase oil production well on Texaco's North Sea Captain development. The express purpose of the trial was to prove the pump's field performance on high GOR flows as a precursor to its wider deployment in the second phase of development of the Captain field, Area"B".

The paper briefly describes the pump concept and the application specifics. Also presented are the operational results from both the field trial and early experience from Area B, indicating the performance of the pumpset and well system. The paper describes the ways in which the HSP installation and well completion has been designed to accommodate a very wide range of operating conditions, and how full use is made of downhole monitoring to optimise management of the start-up and operation of the wells under these variable conditions. The new design of HSP has proved to be capable of extended operation on both single and multiphase heavy oil, and has generally operated successfully under difficult transient downhole pumping conditions.

Introduction

The continuing drive to maximise recovery rates and quantities from oil assets is increasing the requirement for the development of reliable artificial lift systems which are flexible enough to operate in free gas cap areas, at greater drawdowns and on well streams where the bottom hole pressures are closer to the bubble point. Such applications are becoming increasingly common as we attempt to tie in previously considered problematic assets to existing infrastructure for low cost incremental production, and where there is a desire to operate with lower bottom hole pressures tosqueeze more barrels from mature wells.

One of the main technical challenges associated with this trend is the development of pumping systems that are capable of operating on varying well streams containing significant quantities of free gas, either as a homogenous mixture or as slugging flow. Conventional downhole centrifugal pump designs are limited in the amount of free gas void fraction, (GVF), that they can handle to around 40% unless fitted with in-line separation equipment. This performance also falls off dramatically at off-design conditions, thus limiting the effective operating range. A further drawback in the use of mechanical rotary separators at pump inlet is that they are an additional complexity that can compromise the system reliability through imposition of higher shaft loadings. However, with significant reserves capable of being produced at gas void fractions above this level, there remains a clear need for the development of enhanced pump designs capable of continuous, reliable pumping at elevated gas levels.

Texaco and Weir have therefore collaborated in the development of an enhanced version of Weir's HSP, which promised the ability to pump fluids containing from 0 to in excess of 60% free gas at pump suction.

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