Evolution of High-Pressure Gas-Injection Centrifugal Compressors

Abstract

This paper discusses the design features and operational case histories of high-pressure, centrifugal gas-injection compressors being operated in the Ekofisk North Sea project. Results indicate that centrifugal compressors will satisfy any known gas injection requirements. Introduction Energy conservation requires that natural gas (which is entrained in high-pressure crude oil) be recovered through injection. Injection not only retains the gas for future marketing but also maintains the field pressure at peak production. Crude oil production, which at the peak production. Crude oil production, which at the Ekofisk field North Sea platform (Fig. 1) has exceeded 300,000 B/D, requires injection of large amounts of gas into the geological reservoir. The conservation-minded Norwegian government allows flaring of only a limited amount of gas each day. Unless the remainder of the gas (in excess of 480 MMscf/D) can be injected, oil production must be curtailed. production must be curtailed. To meet gas injection requirements, it is necessary to use high-flow, high-pressure compressors. Previously, most companies used positive-displacement reciprocating compressors for injection. Since platform space was at a premium, it was decided to use centrifugal compressors for gas injection. This resulted in a compact compression package that was smaller and lighter than comparable reciprocating equipment by a factor of 5 to 10. Subsequent operational history at Ekofisk established the centrifugal compressor as the most practical means for gas injection. The natural gas used for injection is recovered from crude oil in three separation stages. The high-pressure crude is flashed progressively to 1,000, 260, and 20 psia. At the low pressure level, the gas may be flared to atmosphere or compressed to 265 psia by an Elliott Co. Model 46M centrifugal compressor. From this pressure level, the gas is compressed to 1,000 psia by a Model 38MB centrifugal compressor and then transported by pipeline to the injection platform, where it is compressed pipeline to the injection platform, where it is compressed by two identical trains of high-pressure centrifugal compressors, each driven by an ISO (International Standards Organization) rated 25,469-hp gas turbine. The gas is compressed to the injection-pressure level in two compression stages. The first stage of compression, performed with a Model 25MBH centrifugal compressor, performed with a Model 25MBH centrifugal compressor, receives the gas at 975 psia and increases the pressure to 3,500 psia. The gas is then intercooled and sent to a second stage of compression, performed with a Model 25MBHH centrifugal compressor, which increases the pressure to the required injection-pressure level. pressure to the required injection-pressure level. This pressure, which is a function of gas flow and the number of injection wells, can vary from 6,000 to 8,700 psia. The design and development of this centrifugal compressor were major state-of-the-art breakthroughs; the problems confronted and their solutions have greatly problems confronted and their solutions have greatly advanced engineering design technology. As a result of problems encountered at Ekofisk, the design of the problems encountered at Ekofisk, the design of the Model 25MBHH centrifugal compressor evolved in three separate phases. Phase I was the original compressor design, Phase II was an interim design, and Phase IV was the final design. (Phase III was discarded in favor of Phase IV.) Phase IV.) Model 25MBHH Centrifugal Compressor JPT