Design, Drilling, and Testing of a Deviated HTHP Exploration Well in the North Sea
- K.P. Seymour (Consultant) | Robert MacAndrew (Consultant)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1994
- Document Type
- Journal Paper
- 244 - 248
- 1994. Society of Petroleum Engineers
- 1.6.1 Drilling Operation Management, 4.1.6 Compressors, Engines and Turbines, 1.14.1 Casing Design, 1.10 Drilling Equipment, 1.1.3 Trajectory design, 5.1.1 Exploration, Development, Structural Geology, 1.11 Drilling Fluids and Materials, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.5 Drill Bits, 1.6.6 Directional Drilling, 1.6 Drilling Operations, 5.1.2 Faults and Fracture Characterisation, 1.3.2 Subsea Wellheads, 1.6.2 Technical Limit Drilling, 2.4.3 Sand/Solids Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 5.8.8 Gas-condensate reservoirs, 2.2.2 Perforating, 1.14 Casing and Cementing, 1.7.5 Well Control, 1.7.7 Cuttings Transport, 1.1 Well Planning
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Significant quantities of hydrocarbon reserves are contained in North Sea high-temperature, high-pressure (HTHP) reservoirs. Development of these reserves will require deviated wells. This paper outlines the planning, drilling, and testing of the first deviated HTHP well in the U.K. sector of the North Sea.
Within the oil industry, the general term "HTHP drilling" has slightly varying definitions in different areas. In this paper, we use the definition of the U.K. Health & Safety Executive, which has the statutory responsibility for safety in the U.K. This definition, contained in Continental Shelf Operations Notice 59, states an HTHP well must have an undisturbed bottomhole temperature >300 F and that the pore-pressure gradient must exceed 0.8 psi/ft or require the use of well-control equipment at >10,000-psi working pressure.
These wells may be characterized by (1) a rapidly rising pore-pressure profile, (2) convergence of pore and fracture pressures, (3) high gas levels from source rocks, (4) loss/gain phenomena, (5) potentially long sections that lead to high overbalances, and (6) elevated temperatures.
The high temperature requires mud systems, downhole equipment, and tools designed to work at elevated temperatures. The convergence of pore and fracture pressures (Fig. 1) leads to problems owing to the narrow band of mud weight between inducing losses and inducing a kick. This aspect of these wells probably causes the most trouble. The high mud weights required for well control leads to a situation where, owing to the large difference between formation-fluid and mud pressure gradients (Fig. 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 in controlling these problems is the mud system design.
Well Background and Design
Well 22/22b-2 was a farm-in exploration well. The objective was to test a Jurassic fault block by drilling to 16,000 ft. The well was planned to penetrate the Jurassic in an updip location on the upthrown side of the large fault; therefore, the well had to be deviated (Fig. 3).
Casing. The casing design was conventional by North Sea standards (Fig. 4 and Table 1). Krus and Prieur have described the factors in HTHP well design in U.K. waters.
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