Auk Field Development: A Case History Illustrating the Need for a Flexible Plan

Publisher Website
Google Scholar
Abstract
The Auk field produces from thin, but highly permeable, Zechstein dolomites. The reserves of the field were defined poorly after the initial four wells were drilled, primarily because of the lack of seismic control. This paper contrasts the initial development plan with the historical sequence and shows how unexpected reservoir response was monitored and interpreted and how it influenced development drilling. Introduction: The Auk field, discovered in 1971, is not typical of the North Sea. This small carbonate oil reservoir already has reached an advanced stage of development. All except one of the 10 slots on the drilling/production platform have been drilled - some deviated at angles up to 65°. The cumulative production is already more than one-half the estimated ultimate recovery. The field is unusual in that the very thin Zechstein carbonate producing horizon could not be mapped directly by seismic data, so that both the structural attitude and areal spread of the accumulation initially were defined very poorly. Potential difficulties during development of the Auk field were apparent at an early stage, and we realized that a flexible approach would be needed. A small integrated team of specialists was responsible for planning the development strategy. This team included a seismologist, geologist, reservoir engineer, production engineer, and an economist. Planning decisions thus have benefited from multidisciplinary analysis, and conclusions and recommendations were those of the whole team. This approach proved both necessary and effective. The main aim of this study is to illustrate how the continuous flow of information necessitated radical revisions of our initial concepts of the Auk field and how development plans were modified continually. Because of space limitations, it is impossible to detail how the views of the various disciplines interacted when formulating each proposal. However, the overriding factors are pointed out, and the reasoning behind changes of policy are indicated. Discovery and Appraisal: Shell/Esso's Field A in the U.K. North Sea (Auk) was discovered when Well 30/16-1, drilled in about 280 ft (85 m) of water, encountered oil-bearing, thin, vugular Zechstein dolomites unconformably overlain by Cretaceous chalk.1 The well tested 5,900 B/D (938 m3/d) of 37°API (0.84 g/cm3) oil with an extremely high productivity index. The underlying Rotliegend formation, which had formed the main objective of the well on the pronounced Auk horst structure, was found to be water-bearing. Because the thin Zechstein formation could not be distinguished on seismic sections, the extent of the accumulation was in doubt. The high productivity of the Zechstein formation also could have been a local phenomenon because tight Zechstein was present in nearby exploration wells.

This publication has 0 references indexed in Scilit: