The development of an oilfield in the North Sea is characterized by costly appraisal drilling and very long lead times necessary to design and build platforms. This paper describes the evolution of development plans for the North Sea Brent field, a field that is composed of two oil plans for the North Sea Brent field, a field that is composed of two oil reservoirs, both of which are overlain by condensate-rich gas caps. Introduction The development of a North Sea oil field such as the Brent field, which consists of two reservoirs covering an area about 17 x 5 km in Block 211/29, is characterized by costly appraisal drilling and very long lead times necessary to design and build platforms. As a result, a large amount of capital must be committed and spent before a clear understanding of the reservoir has emerged. The present program includes pressure maintenance largely by injection of treated sea water and provision for a gas sales scheme with British Gas Corp. provision for a gas sales scheme with British Gas Corp. This means that full secondary recovery projects will have been committed before the reservoir has been produced. The potential reservoir performance for various producing mechanisms was predicted using numerical and physical models that were constructed on the basis of a seismic structure map and control from only the first two wells. Most early decisions were based on the results of this work. This paper discusses the early work and describes how development plans have evolved as additional data have been obtained from new wells and as more detailed studies have been completed. Fig. 1 shows the location of the Brent field. It is situated about 150 km from the Shetland Islands in an area of the North Sea with a water depth of 470 ft and where wind speeds up to 125 miles/hour and waves of 100 ft have been observed. Early exploration drilling in this area appeared promising. The regional seismic picture showed some very large structures with estimates of picture showed some very large structures with estimates of potential reserves ranging up to 2 billion bbl. The Brent potential reserves ranging up to 2 billion bbl. The Brent field discovery well was drilled in the summer of 1971. Because of the severe weather conditions in the Brent field area, semisubmersible drilling vessels available at that time could only operate from May through August and appraisal drilling was delayed until the following summer. Since that time a large number of exploration wells have been drilled in the northern North Sea and the great potential of this environmentally hostile area has been confirmed by the discovery of other major gas and oil fields. Significant technological advances in rig and platform construction, offshore loading systems, subsea well completions, and deep-water pipeline laying are being made to develop these fields. Historical Development A feasibility study for the development of North Sea oil fields was carried out at the beginning of 1971. The aim of the study was to evaluate the economics of developing North Sea oil fields for a wide range of environmental and reservoir conditions. One of the potential fields studied was an anticlinal seismic event in Block 211/29, and development of both an undersaturated reservoir and a reservoir having a gas cap were considered. In Aug. 1971 oil was discovered in Well 211/29-1, the most northerly well to be drilled in the North Sea at that time. Log evaluation indicated a 180-ft gross hydrocarbon-bearing column within an 800-ft-thick Middle Jurassic sand interval. JPT P. 1190

A study to determine the most sui?able pressure evaluation techniques for Leman Gas Field is described. The field is of such a size that pressure behaviour in the reservoir is still transient, so only transient evaluation techniques have been~onsidered.Three main considerations influenced the final choice of procedures and the methods of evaluating them. Firstly, the type of field operation allows little time for detailed wel testing using bottom hole pressure recorders. Secondly, the data obtained from the evaluation of well tests is required as input for a 2-D areal, numerical model of the whole field. Thirdly, the field geology is complex and affects the pressure behaviour of most wells even during short well tests. As a direct consequence of the latter factor, existing evaluation techniques have been modified to make them more suitable for Leman Field conditions. These three factors and the evaluation of the procedures finally adopted are discussed.The procedures described in this paper fall into two separate categories: firstly, References and illustrations at end of paper. methods for measuring the decline in reservoir pressure; secondly, methods for analysing bottom hole pressures, recorded during multi-rate flow tests and build-p surveys. The former methods are based on measurement of tubing head pressure, supplemented by a meanE of estimating reservoir pressure from an extrapolation of short pressure build-ups. The latter analyses have been based on methods already accepted in the industry but modified to suit the complex Leman Field reservoir geology.

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