Critical aspects of the well design, notably, the casing design are reviewed in relation to the drilling of the well. The role of pore pressure prediction and the relevance of kick tolerance is discussed. Problems associated with drilling situations where pore pressure, mud weight and fracture pressure converge are discussed. It is concluded that mud weight control is crucial and that different mud weights may be required for drilling and tripping.
The design criteria that led to an integrated 20,000 psi pressure control system for a well in the Central Graben area of the North Sea are reviewed. The problems of designing and incorporating a dual control 15,000/20,000 psi system are discussed in relation to jack-up rig design.
The impact of downhole problems is discussed and it is shown that successful remedial work is possible with careful planning and patience.
The importance of human factors in the successful management of a project of this type is also reviewed and it is concluded that offshore drilling in the U.K. to HPHT objectives where 15,000 psi BOP equipment ratings are exceeded is possible, with a small management team coordinating the skills of manufacturers, suppliers and contractors.
Exploration drilling and evaluation in the Central Graben has proved difficult. This has not been due to the absolute reservoir parameters but because of the pore pressure profile of these wells. The rapid rise in over pressure occurs over a very short vertical interval, which cannot be picked geologically. It varies from well to well and can occur in the Lower Cretaceous, Kimrneridge or the top of the reservoir sands. The pore pressure profile has the potential to rise from 13.5 ppg to 18.0 ppg EMW over an interval of less than 100 ft while the fracture gradient of the permeable formations is less than 18.5 ppg. Thus, the successful drilling of these wells is focused on pore pressure prediction to pick the intermediate casing shoe and maintenance of the required bottom hole pressure by mud weight control. The well design model, has developed since 1984 ref [ 1] as is shown in Fig 1.
The deepest prospects being explored in the Central Graben in the late 1980s were nearing the limits of 15,000 psi equipment on current design criteria. The maximum anticipated pressure to be encountered in a prospect can be evaluated in several ways. Perhaps the simplest is to assume that the depth at which the top of the potential reservoir sand is encountered has the highest pore pressure gradient known to occur in that area, typically 0.9 psi/ft. Therefore, the reservoir pressure is given simply by:
The maximum allowable surface pressure at the blowout preventer is given by:
Max. Allowable Surface Press.. BOP Press. Rating/Safety Factor
The need for a safety factor is questionable, however a value of 1.1 was suggested by the legislative authorities in the U.K. to allow for temperature effects, shock loading and the difference between design and operating pressure. Therefore 15,000 psi BOP stacks are effectively downrated for use in the North Sea as follows.