Solutions to Problems Drilling a High-Pressure, High-Temperature Well
- S.D. Cassidy (Texaco North Sea U.K. Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1995
- Document Type
- Journal Paper
- 238 - 241
- 1995. Society of Petroleum Engineers
- 5.1.6 Near-Well and Vertical Seismic Profiles, 5.2.1 Phase Behavior and PVT Measurements, 1.11 Drilling Fluids and Materials, 2.4.3 Sand/Solids Control, 1.7 Pressure Management, 1.10 Drilling Equipment, 1.6 Drilling Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.2.3 Materials and Corrosion, 3 Production and Well Operations, 1.12.1 Measurement While Drilling, 1.6.10 Running and Setting Casing, 5.1.7 Seismic Processing and Interpretation, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.14.1 Casing Design, 4.2 Pipelines, Flowlines and Risers
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This paper is a case history of a deep, high-temperature, high-pressure (HTHP) well, drilled in the U.K. sector of the North Sea. The topics covered include the use of a vertical seismic profile (VSP) ahead-of-the-bit prediction combined with measurement-while-drilling (MWD) information to assist in picking a critical casing point. This technique permits penetration of the high-pressure transition zone without reaching a previously identified kick zone. Also discussed is an assessment of the suitability of different casing elevators to run a heavy casing string. New laboratory data indicate that some elevators will initiate localized yielding of the casing material at tensile loads much lower than those predicted by theory. Field data are presented that demonstrate the limitation on oil-based mud (OBM) circulating rates caused by flowline temperatures approaching the base oil flashpoint.
Although this material relates to a HTHP well, much of the information may be applied to improve the safety and efficiency of drilling operations in other deep, overpressured, or close-to-balance drilling environments.
In 1991, Texaco North Sea U.K. Co. drilled a 16,600-ft well in Block 23/26 in the Central graben of the North Sea. Situated in 300 ft of water, this subsea well was drilled by the semisubmersible rig Henry Goodrich. Although Well 23/26b-15 (Fig. 1) was the fourth well drilled by Texaco in the block, several new studies and practices were performed during the planning and execution of this difficult well.
Well 23/26b-15 was planned primarily to appraise Jurassic sands below 15,700 ft. The Jurassic formations were expected to be overpressured to ±17.6 lbm/gal mud weight equivalent (MWE) at a temperature of 350°F. A short (±150-ft) lower-Cretaceous interval was predicted to overlay the Jurassic. In previous wells, where this lower-Cretaceous zone was absent, 18-lbm/gal gas-condensate kicks had been taken in the upper-Cretaceous limestone only 80 ft above the Jurassic. We believed that the formation pressures within the upper Cretaceous changed from±9.5 lbm/gal to±18 lbm/gal MWE over an interval of 300 to 1,000 ft. Two previous casing shoes set only 300 ft above the known kick zone had failed to provide sufficient seat integrity to penetrate the kick zone safely. As a result of this previous experience, locating the last intermediate casing shoe deep enough to provide adequate seat integrity, but shallow enough to avoid encountering the kick zone, was of critical importance to the success of the well.
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