Abstract
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.
