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Abstract The Kuparuk Coiled Tubing Drilling (CTD) program began with the development of dynamically overbalanced CTD techniques1 in 1998. Although many of the first wells were technical successes, the economic and productive results were less than expected.
Starting with a step change in 2002, improvements in the economic results of the CTD program have materialized over the past 3 years fueling excitement about CTD in the Kuparuk River Unit (KRU). Results from the 11 well 2005 CTD program suggests that the successful application of CTD technology will continue in the Greater Kuparuk Area (Fig. 1).
Technological advances, changes in target focus, improved well designs, improved CT drilling techniques, and persistence all paid off. The 2004 & 2005 CTD programs delivered wells yielding rates beyond expectations and, in the case of 2005, were achieved "at or below" the authorized expenditure budget. In addition to the overall CTD program success story, what the authors believe as 3 new world records were set on one particular well: Deepest 3" CTD sidetrack - 18,350' MD; deepest CTD openhole sidetrack - 17,935' MD; and deepest CTD liner run - 18,320' MD. All drilling objectives were achieved and the well completed under the AFE (Authorization for Expenditure).
Introduction CTD has been considered for some years as a potential tool to tap unswept reserves in the faulted and compartmentalized formation of the Kuparuk River field. Waterflooding has left numerous small targets requiring a cost effective solution to access. The past success of CTD in the neighboring North Slope Prudhoe Bay field, due to the low cost of thru-tubing intervention and strong horizontal productivity in lower quality pay sands, suggested this technology could be transferred into Kuparuk success as well.
Due to the significant faulting present in this waterflooded reservoir, trapped pressure between adjacent fault blocks is commonplace. It is also the location of trapped reserves requiring access by drilling while operating in overpressured or high-differential pressure regimes. CT's fully contained pressure control equipment, ability to operate with surface pressure, and options for a closed circulation system provides a unique advantage over standard rotary drilling.
The majority of KRU (Kuparuk River Unit) completions consist of 3–1/2″ and 2–7/8″ tubing with a smaller subset of 4–1/2″. In 1998, CTD Bottom Hole Assemblies (BHA's) were downsized from equipment to drill through the 4–1/2″ production tubing at Prudhoe Bay to work through 3–1/2″ tubing in Kuparuk. 2–3/8″ nominal BHA's were developed with a 2.74″ nominal bi-center bit to provide a 3″ drilled openhole. The first generation of 2–3/8″ MWD tools2 -deployed with 2″ OD by 0.156″ wall coil tubing - were pulsed systems with a gamma ray tool for logging and a hydraulic orienter to direct the bent motor for directional control.
One disadvantage of working through 3–1/2″ tubing is the high annular friction incurred in the 2″ coiled tubing by ∼3″ ID production tubing and openhole annuli. In Kuparuk, this is a critical concern when drilling in a high pressure area if typical overbalance mud weights are utilized. The sum of the high annular friction and mud weight increases the bottom hole pressure, or ECD (equivalent circulating density), and can result in fracturing the rock. To maintain the ECD below fracture gradient, the dynamically overbalanced CTD (DOB CTD1) method was developed. DOB-CTD uses the combination of underbalanced drilling fluid hydrostatic pressure, annular friction pressure, and surface choke pressure to maintain a moderate and consistent overbalance on the formation. This method is similar to techniques now widely referred to as managed pressure drilling.
With these small tools available, CTD programs consisting of 2–5 wells per year were initiated from 1998 to 2001. Even though some of these sidetracks were technical successes, they lack the competitive economic and production benefits sought after. The CTD program of 2000–2001 was expanded to include sidetracks above the Kuparuk formation in Kalubik shale and to extend several rotary horizontal wells. Due to numerous problems, predominately shale instability, only 1 out of the 7 candidates was successfully completed as planned and only 1 was considered an economic success.
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