Gullfaks 'A' Subsea Wells: Four Years' Operational Experience

Abstract

ABSTRACT This paper presents operational experience gained during four years operations of subsea wells tied up to the Gullfaks A (G17A) platform in the North Sea. The paper includes experience gained during production, hook-up of a new subsea well to the production platform, damage of control umbilical and flowline, logging and other field operations. The paper also describes precautions taken for safe operations. Experience which can be applied to future developments are highlighted. 'Ihc conclusion is that the Gullfaks subsea system is a simple, versatile concept with potential also in deeper waters. INTRODUCTION The satellite well development at Gullfaks A was Statoil's first subsea development. In this project, five producers and one water injector have been individually tied in to the Gullfaks A platform,(fig. I). These wells served three main purposes: They allowed accelerated production, they were needed to produce the oil outside the normal range of the platform wells and they provided early reservoir information. Our experience has shown that they served their purpose well and still do. The early production period gave some very valuable information about the complex Gullfaks reservoirs as well as earlier income from oil production. This information later contributed significantly to our justification of increased offtake rates, beyond those originally planned. When Statoil started the planning of subsea wells for Gullfaks in 1982, we had seen both successes and failures of subsea wells in the North Sea. Being our first subsea well experience, we were therefore well aware that, basically, we should go for a safe and simple concept. It was also recognized that Statoil would need subsea production system in the future. More Gullfaks subsea wells would be needed, some in deeper water, and application on other deep water field developments were foreseen. An important aspect of the Gullfaks subsea system, was to gain experience with diverless installations and maintenance systems. The water depth was well within diving capability should diving assistance be called for. All six wells are installed in approximately 140 meters water depth. During the first two years, one of the subsea wells sustained production of approximately 5000Sm3 (31500 bbl) oil per day. Due to water breakthrough and some sand problems the average production rate by January 1991 is approximately 1000Sm3 (6300 bbl) oil per day per well. Since installation the wells have produced with a high degree of availability. Certain problems have occurred and will be mentioned in this paper. INVESTMENT The investment in the subsea wells for the A-platform totals nearly 1.8 billion NOK (USD 300M) which includes engineering, integration testing, drilling & completion, flowliness & umbilicals and platform related equipment. The cost of future wells can be substantially reduced by use of the same concept and the same installation tools. The average cost excluding drilling and completion per well of 210 mill NOK (1989) (USD 35 M) was considered acceptable, but of course is too high related to todays need for more cost effective subsea development.