Downhole and Subsea Completion Design for a High-Pressure North Sea Gas Condensate Field

Abstract

This paper details the overall system design for a 15,000 psi subsea, gas condensate well in the North Sea. Currently, significant industry interest exists in the technology required to complete wells of this type, which would enable substantial hydrocarbon reserves to be developed. Subsea wells such as this have not yet been completed. INTRODUCTION A significant number of high pressure discoveries with surface pressures in excess of 10,000 psi have been made in the Central Graben of the North Sea which await attractive project economics to allow their development. A reduction in cost and risk associated with such a development may provide the incentive required. The technology needed to develop these discoveries economically is likely to include subsea production systems as these are now a proven means of providing a low cost development. Subsea technology has not previously been applied to these high pressure wells and the downhole technology has only previously been applied from platform or land based developments. This paper presents the issues concerned with the completion of such a well and where possible makes recommendations for a special design. In areas where it has not been possible to specify a design comments are made regarding the issues to be considered or resolved and suggestions are made for additional industry development. The work is based upon in-house designs and external studies performed for Marathon Oil UK in order to allow the future development of it?s high pressure prospects and discoveries. This design isbased upon the known and anticipated data from one such North Sea reservoir, Figure 1 shows the formation pressure of this reservoir in relation to existing fields in the North Sea and elsewhere and to other as yet uncompleted prospects. Table 1 lists the operating conditions. The pressures and temperaturesencountered in this design are not as severe as some other discoveries and do not meet some definitions of'hostile? (reference 1) or high pressure / high temperature (HP/HT) (reference 2). However, both these definitions are somewhat arbitrary, and since 15,000 psi drilling and production equipment isrequired for the field, it is a significant departure from previous subsea completion experience. The issues raised in this paper are relevant to the completion of wells with greater pressures and temperatures than those in this well and as such this design can be seen as a 'stepping stone? to overcoming the problems likely to be encountered in these wells. The wide ranging nature of the subject prevents an in depth analysis of each issue but is intended to stimulate and encourage further work in septic areas. Certain aspects and recommendations are likely to be controversial but it is hoped that bypublishing the work and sharing ideas, the development of 15,000 psi subsea wells may become one step closer.