Well A, drilled by KN, an oil & gas operator in Indonesia, is a deepwater vertical exploration well located offshore Bali. The high cost of a deepwater operation demanded all drilling components to be as efficient as possible. One of the optimizations performed was a single-trip hole-enlargement-while-drilling (HEWD) and rathole elimination (RHE) operation, in which drilling the pilot hole, opening the hole, and eliminating the rathole were all executed in one run. The key component of the single-trip HEWD and RHE was the on-demand hydraulically actuated reamer, an underreamer with advanced technology by which activation based on flow index is used instead of a conventional ball-drop system; this enables the tool to be positioned below measurement- and logging-while-drilling (MLWD) tools, closer to the bit. This advanced technology, however, required extensive preplanning activities, especially for bottomhole assembly (BHA) vibration and hydraulics modeling. Multiple BHA alternatives were evaluated using simulation based on finite element analysis (FEA) in terms of stability, and the most stable BHA was selected. In regard to hydraulics, sufficient pressure drop below the reamer and rotary steerable system (RSS) tool needed to be achieved to ensure the full cutter-block opening capability, as well as the RSS functionality. The integrated BHA design and modeling successfully delivered an excellent single-trip HEWD and RHE operation in two different hole sections. The 12 1/4-in × 14 1/2-in BHA used an RSS to drill a 1,400-ft interval in HEWD mode and eliminated 157 ft of rathole using the on-demand reamer. The 10 5/8-in × 12 1/4-in BHA drilled 1,418 ft with HEWD and eliminated 182 ft of rathole. A savings of 57 operating hours (equal to USD 1,425,000) was realized as a result of eliminating a dedicated RHE run. The 11 3/4-in liner and 9 5/8-in casing were smoothly run to bottom, indicating a good hole quality produced by a stable HEWD and RHE system.
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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