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As long as underreamers have existed in the oil field, operators have wanted a clearer understanding of how these tools are operating in real time. A new integrated underreamer that provides real-time communication via the same mud pulse telemetry system as the familiar MWD/LWD/RSS (Measurement While Drilling/Logging While Drilling/Rotary Steerable System) tools and allows the placement of the underreamer closer to the bit (to aid vibration management and minimize rat hole length), has been used. This advanced underreamer system reveals the current condition of the tool and the position of the blades (implied hole caliper) to the operator, additionally, it uses internal hydraulic oil pumped-pressure activation (and deactivation) of the blades. This integrated underreamer was used to simultaneously drill and underream an 8½-in. × 9⅞-in. pilot hole section interval and then side-track to drill an 8½-in. × 9⅞-in. mainbore lateral section to the top reservoir horizon. A case study in the UK North Sea Harding field is included as part of this paper describing the critical challenges of simultaneously drilling and underreaming a development well, including the equivalent circulating density (ECD) management within narrow pressure margins. Underreaming an 8½-in. × 9⅞-in. wellbore all the way to TD was viewed as the most acceptable method to manage ECD and be able to get the production liner to bottom. Other ECD control methods used on this well, and detailed in this paper, include a combination of adjustments of rate of penetration (ROP) and rotations per minute (RPM), drilling fluid rheology management and pumping of sweeps at different stages of drilling to aid hole cleaning. The PDC (polycrystalline diamond compact) bit design was matched to that of the reamer with a proprietary technology to optimize the drilling assembly and minimize stick-slip or coupled lateral vibrations. The reaming system was used to drill and underream 6,645 ft. in two runs totalling 288 hours on-bottom drilling, with minimal vibrations. The well was drilled with an inclined trajectory of up to 90° with as much as 5°/100 ft. dog leg severity. The reamer appeared to provide an in-gauge borehole allowing for successful running, rotation and cementing of approximately 4,300 ft. of 7-5/8″ liner without any issues, demonstrating superior borehole quality. This paper outlines the problems anticipated in this challenging well, the thinking behind the unusual reamer selection, the method for proper synchronization with the pilot bit, and the keys to overall success.

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The new hybrid drilling technology which combines the two primary rock failure modes of crushing and shearing was effectively deployed in the challenging drilling environment on the west coast of Ireland. It improved drilling efficiency by reducing the mechanical specific energy (MSE) and mitigating stick-slip and excessive torsional oscillations. The 12¼-in. section was drilled through a relatively homogenous cretaceous limestone sequence with a small percentage of chert bands. The limestone became progressively harder with depth but remained drillable using the PDC bit. The initial bit, an eight-bladed PDC design, was considered the best choice to drill through the limestone sequence. The initial rate of penetration (ROP) through the top of the limestone was 20 to 30m/hr. After 30 to 40m of drilling, the ROP decreased rapidly to 0 to 2 m/hr over a 10m interval. The bit was pulled. When on surface, the dull condition of the bit confirmed the presence of interbedded chert. When the hybrid drill bit was run into the hole it drilled an average of 5 to 7m/hr with an instantaneous ROP of 22 to 24m/hr. Isolated chert bands occasionally reduced the ROP to a 3 to 4m/hr range. However, the bit returned to the average ROP upon exiting the chert. The operator, therefore, selected a second identical bit to complete the section. Both hybrid bits endured similar wear on the PDC and TCI cutting elements without any evidence of bearing or seal damage. The hybrid cutting action generated smoother rotation and lower vibrations throughout the drilling system which improved ROP and extended bit life. The bit thus provided a viable solution to drilling firm limestone with interbedded chert stringers.