Abstract

Providing a stable production stream with minimal GLR fluctuations to surface is a critical factor when designing gas-lift systems for subsea applications. As well intervention methods are very costly, the sub-surface gaslift equipment must be designed for the life of the well and the unloading and operating procedures must be tailored to minimize erosion of the gaslift valves and orifice.

For the Angola Kuito Development, dynamic modelling was used to determine the optimum orifice size for long term operability over a given range of injection rates, water cuts, and productivity indexes. The dynamic model also aided in the development of start-up procedures that minimized erosional effects on the downhole orifice and helped to answer questions regarding cyclic production pressures at the subsea wellhead. Through a variety of charts and graphs, this paper will describe the methodology used for this detailed design process. Results of the study led to the purchase of different downhole and FPSO (floating production storage and offloading vessel) components for the gas injection system than originally planned and greatly modified the well completion and start-up procedures.

Using dynamic modeling for gas lift design is relatively new to the oil and gas industry. Dynamic modelling is a powerful tool, that when used properly, leads to better gas lift designs and increases confidence levels in the reliability and performance of the system.

The field was placed on production late 1999 and the gas lift system is expected to start-up February 2000.

Introduction

The Kuito field, discovered in April 1997, lies offshore Cabinda province in Angola's deepwater Block 14 (Fig.1). The initial phase of development was a 12-slot subsea production manifold and a remote gas injection well with the crude oil being produced to a floating production storage and offloading vessel (FPSO) for processing. (Fig. 2)

Gas lift was the artificial lift method selected due a solution GOR of 200 scf/bbl and a "zero flare" production philosophy. In late 1998, Chevron Petroleum Technology Company was commissoned by Cabinda Gulf Oil Company to design the downhole gas lift system. Table 1 summarizes the parameters given.

Subsea Gaslift Design Philosophy

Gaslift designs for subsea wells have several requirements that are not normally encountered in traditional gaslift designs. First, because the cost of intervention in a subsea well is considerably higher than for a traditional completion, the subsurface gaslift equipment must be designed with special attention to reliability and longevity. Secondly, the sizing of the port in the operating valve must anticipate production conditions for the life of the well.

Normally, failed equipment or a change in production conditions would be dealt with by using light intervention methods (i.e. wireline or coiled tubing) to place new equipment into the sidepocket mandrels. The Kuito wells are equipped with sidepocket mandrels that would allow replacement but the cost of doing so is such that the special attention is paid to eliminating possibilities of failure, and designing the operating valve for a wide range of production conditions.

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