North Sea High Pressure Drilling And Testing Techniques
Authors: Floyd Keith , Myers L. Roger 
Published: December 2007

Over the last two decades the oil industry has been faced with numerous drilling and production problems in the hostile environment of the North Sea. The progress in rig and platform design has kept pace with these environmental problems and currently the industry is producing hydrocarbons such that the United Kingdom is approaching self sufficiency in the supply of oil and gas. To maintain this impetus the hostile environment is now changing to greater water depths and deeper unknown areas beneath the sea-bed and therefore during the last two to three years the search for hydrocarbons has resulted in the requirement to drill deeper wells. This increase in depth, from a range of 3000 m to 3500 m to a range of 4500 m to 5500 m, has in general terms resulted in higher abnormal reservoir pressures and therefore demanded of the industry an equipment capability to handle these higher pressures in both the drilling and testing of a well.

This paper discusses one approach to the problem of increase in BOP rating from 10,000 psi to 15,000 psi working pressure with the additional auxiliary equipment required, namely kill and choke lines and choke manifold. The problem of basic casing design and selection is also discussed together with the approach to storage (deckload) and running and cementing a deep 9 5/8" casing string. The paper also takes a close look at the philosophy behind the design and preparation of a downhole and surface equipment system capable of handling a well test up to a working pressure of 15,000 psi shut in.

The results from recent wells are included to indicate that a safe and successful operation can in fact be obtained and although the wells have not been tested to date, the ground work is now prepared for application to the first field operation.

The paper concludes by stating that while the problem of drilling and testing such deep high pressure wells are currently being solved, should such wells be successful, the problem of producing commercial hydrocarbons from such wells have yet to be answered.

Introduction

During the last two to three years, exploration requirements in the North Sea have demanded the drilling and testing of deeper wells. Previously a number of deep wells had been attempted, however due to the high abnormal reservoir pressures encountered it was considered that the equipment available at the time was not capable of safely containing these high reservoir pressures during both drilling and testing phases of an exploration well. The industry was therefore faced with a problem of the application of proven technology to enable the drilling, testing and possible future development of such wells in the North Sea.

Fundamentally the problem is one of depth and the resultant high reservoir pressure. Consider two normal pressure reservoirs at depths of 10,000 ft. and 17,000 ft., then the pressures encountered will be 4330 psi and 7360 psi respectively for a hydrostatic gradient, however, in general terms, as depths increase it has been found that pressure gradients increase. For gradients of 0.65 psi/ft and 0.85 psi/ft the resultant pressures can be safely controlled at depths of 10,000 ft. with 10,000 psi rated equipment, however at depths of 17,000 ft. higher rated equipment is required. (Refer Table 1).