While planning a wildcat exploration well, an operator faced formation depth uncertainty as a result of poor seismic imaging as well as limited offset pore pressure and fracture pressure data. A steep pressure ramp was anticipated at some depth below surface casing, and casing design included a 16-in liner to secure this interval. To properly evaluate and understand the lithology and pressure while drilling would require high-quality LWD data, but it was also essential that the liner and casing could be fully run to the bottom of the hole. Underreaming near the bit while drilling would compromise the LWD data, but underreaming above the LWD tools would not enable the casing to be fully run to bottom. Two hole-enlargement-while-drilling (HEWD) BHAs with the option for rathole elimination (RHE) were proposed and enabled these objectives to be efficiently achieved without compromise. A combined HEWD and RHE BHA is not new in the industry, but it is challenging in large hole sizes. With multiple cutting structures in the BHA, drillstring dynamics become complex. The resultant vibration and shock can lead to diminished drilling performance, or even failure of BHA components.

A prejob finite-element analysis (FEA) modeling simulation was used to investigate the dynamic drilling behavior of the initially-planned BHAs. The simulation results from initial BHA design predicted high vibrations. A BHA optimization analysis was then conducted to determine the most favorable BHA configuration for effective LWD tool placement and minimized vibration issues, taking into account the BHA geometry and stabilization points, cutting structures, and formation type. The simulation results from the final BHA exhibited a significant reduction in shock and vibration levels. Suitable drilling parameters were identified, and hydraulics simulations were performed to ensure that both the HEWD and RHE underreamers could be reliably actuated.

The operator implemented the recommended BHAs, cutting structures, and parameter roadmaps on both the 17-in × 20-in and 14¾-in × 17½-in sections, and the solution successfully drilled 501-m and 605-m intervals, respectively. Each interval was drilled and enlarged in a single trip. Both HEWD BHAs exhibited low levels of vibration during the original underreaming, enabling quality LWD data to be obtained. After maximizing section depth, the HEWD underreamer was deactivated and the RHE underreamer activated, enabling the rathole to be underreamed and subsequently the liner and casing strings run fully to the bottom of the hole.

Advanced dynamic drillstring modeling can simulate downhole drilling conditions, enabling improved prejob planning and thus more efficient drilling operations. Proper design of HEWD and RHE BHAs can help the operator efficiently maximize the quality of LWD data while drilling.

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