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ABSTRACT Significant quantities of hydrocarbon reserves are contained in North Sea high temperature, high pressure reservoirs. Development of these reserves will require the drilling of deviated wells. This paper outlines the planning. drilling and testing of the ftrst deviated HTHP well in the UK North Sea. INTRODUCTION Within the oil industry there is a general term "high temperature, high pressure (HTHP) drilling". This term has slightly varying definitions in different areas. In this paper the definition used for HTHP drilling is that of the UK Health and Safety Executive (HSE) which has the statutory responsibility for safety in the UK. This definition is contained in Continental Shelf Operations Notice (CSON) 591 and is that the undisturbed bottom hole temperature must exceed 300°F (149°C) and have a pore pressure gradient in excess of O.8psi/ft or require the use of well control equipment in excess of l0,000psi working pressure. These wells may be characterised by the following:A rapidly rising pore pressure profile.Convergence of pore and fracture pressures.High gas levels from the source rocks.Loss gain phenomena.Potentially long sections leading to high over balances.Elevated temperatures. The high temperature requires mud systems, downhole equipment and tools designed to work at elevated temperature. The convergence of pore and fracture pressures (see figure 1) leads to problems due to the narrow band of mud weight between inducing losses and inducing a kick. This is the aspect of these wells which most probably leads to most trouble. The high mud weights required for well control leads to a situation, common in the North Sea, where due to the large difference between formation fluid and mud pressure gradients (see figure 2) mud overbalance becomes so high at the bottom of long permeable hole sections that differential sticking becomes likely. These problems are magnified when drilling small diameter directional holes. The most important single factor is mud, it's weight and system design. BACKGROUND Well 22/22b-2 was a farm in exploration well. The objective was to test a Jurassic fault block by drilling to 16,000ft. The well was planned to penetrate the Jurassic in an up-dip location on the upthrown side of the large fault therefore the well had to be deviated (see figure 3). WELL DESIGN Casing The casing design was conventional by North Sea standards (see figure 4 and table 1); Krus and Prieur2 have described the factors in HTHP well design in U.K. waters. Despite it's conventional appearance there were two major points which had to be addressed. The first was the placing of the 13-3/8" casing shoe. The question here was whether to place the shoe above or below the Palaeocene sands. There are two schools of though on this. One is that the shoe should be placed above the Palaeocene with the top of the deep casing cement below the bottom of the Palaeocene. |
