Designing robust hole-opening BHA to drill two consecutive sections with interbedded formation in a deep HPHT appraisal well is exceptionally challenging. The two hole-opening sections, 17in × 20in and 14-3/4in × 17in need to retain strict verticality requirement and the final hole diameter sizes must be guaranteed to allow for smooth casing running operations. Conventional method, consisting of pilot hole drilling and a dedicated hole-opening run, is time consuming. Hence, the Operator decided to proceed with single run hole-opening while drilling approach for significant time saving benefit.
Utilizing finite element analysis modeling approach, tandem reamer BHA were designed and proposed for both hole-opening sections. The BHA consists of a PDC bit, a hydraulic-actuated reamer near the bit and a mechanical-actuated reamer above MWD tools. This BHA design concept can deliver both hole-opening and rat-hole cleaning operations in a single run. BHA optimization modeling was also done to select robust and appropriate cutting structures that can yield the most stable drilling dynamics with suitable drilling parameters.
Applying the FEA modeling recommendations in the 17in × 20in hole-opening section, the operation went smoothly as planned for the first 1184m until a vibration level increase was observed, believed to be due to an unexpected sudden increase in the hardness of the interbedded formation. Realizing the potential of similar risk happening for the next hole interval, a conventional two-runs approach was implemented in the 14-3/4in × 17in section. A pilot hole was first drilled with a steerable motor BHA for trajectory adjustment and then followed by a dedicated under-reaming only (URO) operation with a bullnose BHA. The 937m under-reaming interval was executed successfully without any downhole issues, despite lower ROP than initially expected.