Drilling a large directional hole section through the Middle East's hard carbonate formations using a polycrystalline diamond compact (PDC) bit and a rotary steerable system (RSS) bottomhole assembly (BHA) equipped with an expandable underreaming-while-drilling (UWD) tool presents a number of challenges. An operator was experiencing high-lateral vibration, stick/slip, and whirl-induced drilling dysfunctions, causing premature downhole tool fatigue failures. To reduce reaming runs and bit trips, a finite element analysis (FEA) -based dynamic simulation system was used to quantitatively analyze the complex interaction of various manufacturers’ downhole tools to optimize the BHA configuration and drilling parameters. Formation-related challenges were present in the carbonate formations, typically limestone and dolomite, known to be hard and interceded, along with the chances of well kick or complete losses, in addition to the shale formations, which are reactive as well as tectonically unstable. The shale formations tend to fall out if incorrect mud weight is applied. Also, the abrasive sandstone formations, comprised of interbedded shales, are reactive and mechanically unstable.

The goal of the drilling operation was to effectively match the PDC and reamer cutting structures to minimize vibration. A modeling tool was used to define an optimized set of operating parameters for efficiently drilling the upper limestone, multiple limestone and dolomite transitions, and the difficult lower dolomite formation. The modeling enabled the drilling team to configure a BHA, which would efficiently deliver directional requirements in the curve and tangent as well as to mitigate potential push-pull issues in the critical transition zones. The BHA, run as modeled with the optimized operating parameters, produced good quality results. The six-blade PDC- RSS combination and underreamer BHA successfully drilled and opened the 12.25-in. × 13.5-in. wellbore with minimal lateral vibration and stick/slip. The BHA drilled 2,082 ft of wellbore, including the challenging hard and interbedded formations, which consisted of 6,000 to 25,000-psi limestone and shale with moderate- to high-impact indices at a 15 to 17-ft/hr average rate of penetration (ROP) and within authority for expenditure (AFE). The BHA delivered a high-quality, in-gauge borehole and efficiently delivered all directional objectives, building hole angle from vertical to 17.8° as required. The run was the first successful directional large-hole hard-rock UWD operation with a PDC-RSS BHA in Kuwait.

The successful operation is the result of a proactive planned initiative to mitigate BHA shock loading, which included real-time monitoring using a predictive compressive-strength analysis system. The success also increased the confidence in the FEA-based modeling system's ability to accurately identify the root cause of damaging vibrations in the planning stage while underreaming the long sections of hard carbonates in the Middle East.

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