Coiled Tubing in High-Pressure/High-Temperature Wells-Technical Enhancements in the North Sea

Abstract

Abstract This paper presents an overview of coiled-tubing (CT) innovations and perforating-tool enhancements that have been developed in recent years to meet the challenges of coiled-tubing perforating in high-pressure, high-temperature (HPHT) well environments in the North Sea, specifically on TotalFinaElf's Elgin and Franklin fields. Key areas covered include: the manufacturing and inspection of coiled tubing; new pressure-control equipment for pressures up to 15,000 psi and temperatures up to 390°F; installation, operation, and maintenance of surface equipment; downhole tools; perforating tools; elastomers; well fluids; contingencies; best practices; and future solutions. Introduction TotalFinaElf's Elgin and Franklin Fields are located in the central North Sea Graben Area, east of Aberdeen, Scotland. The two fields consist of 14 wells, in water 295 ft (95 m) deep, with the combined capabilities of 770 MM BOE reserves and a design rate of 170,000 bbl/day (14.6 MM m3) condensate. The reservoir conditions of these wells are rated at 16,000-psi (1,100-bar) pressure, 392°F (200°C) temperature, with formation depths of 18,000 ft (5,500 m). The primary components of the completions consist of 4 1/16-in. Christmas trees, 5-in. 25% chrome tubing, safety valves with communication nipples, permanent production packers, and isolation packers specific to the Franklin Field (two reservoirs). The Elgin field required single-run selective perforating of up to 200-m intervals with ± 2-m depth accuracy. The Franklin field required selective perforating of up to 334-m intervals with ± 2-m depth accuracy for the two reservoirs. The second perforating operation was run in a live well. The depth accuracy was needed to avoid perforating high-permeability zones that would cause early sand production. A number of perforating methods were considered, and finally, perforating through coiled tubing with an accurate depth-control system was chosen for the following reasons:Only one or two runs would be required for the entire interval. In HPHT wells, minimal intervention is advantageous.The method would allow at or underbalanced perforation.Accurate depth control could be maintained.There would be no requirement to drop the guns (a sump was unavailable due to drilling constraints). However, the team also recognized that this method would require the development of some critical equipment and a careful selection of tools to achieve a safe and accurate execution. Project Initiation On commencement of the project, a dedicated team was assigned to the operation from both TotalFinaElf and Halliburton. The primary requirements identified for a coiled-tubing system working in an HPHT environment were:A CT string capable of running and pulling 330+ m of guns.All critical parts of the system rated for worst-case conditions for the surface pressure-control equipment.An accurate depth-control system that would not require feeding electric line through the coil.A simple deployment system and gun connectors.Perforating guns and firing systems qualified for 392°F (200°C).