The Thunder Horse challenge: Optimize perforating performance in an HP/HT deepwater well
The well’s initially proposed completion design was to set a 7-in. 38 lbm/ft casing with a drift diameter of 5.875 in. (14.9 cm) across the reservoir. Under these conditions, the selected perforation gun system for this job was a 3 3⁄8-in. (8.6-cm) HSD gun loaded with deep penetrating charges at 99° phase and six shots per foot (spf) shot using 3 3⁄8-in. perforating hardware. This gun system was projected by perforation simulation software to deliver a mechanical skin factor less than one. The 3 3⁄8-in. gun system has a published gun swell (OD) of 3.66 in. (9.3 cm); this gives approximately 2 in. (5.1 cm) of casing clearance. When the drilling approached total depth and the LWD data were available, engineers could figure roughly how long the perforation interval would be. Consequently, the required 3 3⁄8-in. guns with the necessary hardware and backup were ordered. But, because of the drilling complications explained earlier, those guns could not be used inside the 4 ½-in. liner.
The challenge
The pay sand was originally drilled with an 8 ½-in. bit and a final average caliper of 9 in. (22.9 cm). Because of drilling problems, the open hole in the pay zone was exposed to mud for an extended period. Finally, it was cased with 4 ½-in. 17 lbm/ft casing with a drift diameter of 3.7 in. (34.8 cm). This mandated that a smaller OD gun system be used, even though the cement sheath around the casing increased to over 2 in. The small liner generated a lot of debate amongst the various design disciplines. The team’s greatest challenge was to deliver a good completion, minimize operational risk, and maximize well performance, given the wellbore changes. This forced a re-evaluation of the perforating system while the completion operations were being started.
Gun systems evaluated
Perforation modeling analysis was performed on the characteristics of the five HSD gun systems with various explosive loads, including 3 3⁄8-in., 2 ½-in., and 2 7⁄8-in. The characteristics included charge explosive load, phasing, formation penetration, and enhanced hole diameter. The first gun size, 3 3⁄8-in., is the original gun system planned for the 7-in. liner. The remaining gun systems are options which include deep penetrating charges and new systems with enhanced deep penetrating charges.
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The 2 7⁄8-in. (7.3-cm) HSD gun system with the enhanced deep penetrating charge (99°, 6 shots per foot [spf]) run with 1 11/16-in. (4.3-cm) perforating hardware was qualified for 4 ½-in. (11.4-cm) casing as per Thunder Horses best practices.
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The perforation performance modeling results on the same gun systems clearly indicated that the 2 7⁄8-in. gun system with the new enhanced deep penetrating charge could deliver skin factor and productivity index values closest to those anticipated with the original 3 3⁄8-in. gun system with the deep penetrating charge.
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Volume 68 Issue 9
September 2008
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