Outline History of Underreamer Blocks and Problem Description Conical Diamond Elements in Underreamer Blocks Complementary Design Features 4D Simulation Software Introduction 4D Simulation and Design Approach Vibration and Drilling Efficiency Lab Testing 18.125-in × 22-in HEWD Job and 4D Simulation Study 12.25-in × 14.75-in URO Job 8.5-in × 9.0-in HEWD Job Conclusion
As part of an exploration campaign in the Sub Andean region in Bolivia, Repsol faced a unique challenge to reduce well construction costs while drilling through harsh and abrasive Caipipendi Block formations. The Caipipendi Block poses several drilling challenges that often lead to premature bit and underreamer cutting structure damage. Two main points were targeted as fundamental to accomplish this objective: reduce the number of runs to drill the Carboniferous section and improve borehole enlargement efficiency. A multidisciplinary group comprising a service company and Repsol team members conducted a detailed and thorough investigation of previous failure modes to identify an integrated approach to improve the performance. Along with offset history, the team was able to identify lithology characteristics to accurately diagnose the underlying root causes. To tackle the hard and abrasive Upper Carboniferous section, a customized polycrystalline diamond compact (PDC) drill bit was equipped with 3-D cutter (3DC) technology and powered by a combination of an enhanced power section and a robust rotary steerable system (RSS) tool with a new specific steering pad feature. A new reamer concept was also applied in this well to mitigate the anticipated drilling shock and vibration associated with underreaming through the challenging strata—without compromising drilling efficiency. To achieve the desired performance goals, 3DC elements were applied to the drill bit, along with a robust RSS designed with a pioneering steering pad developed to sustain performance in high levels of formation abrasiveness. To increase borehole enlargement efficiency in the Devonian section, a new cutter block was designed. An advanced 4D transient drillstring dynamics modeling package was used to analyze failure modes, design the reamer and drill bit cutting structures, predict the drilling dynamics of drill bit and reamer, and recommend drilling parameters for the run. The objective was successfully achieved where Repsol was able to set a new benchmark for the block, drilling the programmed depth of the well in 350 days, 17% less than AFE curve and 25% less compared with the best result in the block even with a significant change in lithology sequence from the program. The innovative 3DC technology boosted overall drilling performance. The new PowerDrive* rotary steerable system pads enabled the customized drill bit to drill farther and avoid unexpected bottomhole assembly (BHA) trips. The use of a new cutter block equipped with 3DC technology enhanced stability and reduced lateral displacement and vibration.
This case study highlights the challenge of identifying the root cause of bit failures and provides an iterative design solution for maximizing drilling performance on a deepwater Gulf of Mexico (GoM) field development project. The well design and project economics required drilling through two distinct lithologies: salt followed by subsalt sediments consisting of plastic shale, abrasive sands, and marl. The operator wanted to drill through these in the same interval, in one run, while simultaneously achieving high rates of penetration (ROP) and maintaining durability for the entire interval. Offset wells from previous operations in the area provided an initial starting point for the design, despite being quite different from the new well plan. A new drill bit and reamer combination were developed and built for the first well. In addition, the bottomhole assembly (BHA) design was optimized and detailed drilling parameter roadmaps were provided for the field crew. When the first run resulted in a premature bit failure, an in-depth post-run analysis was completed. A rapid response root-cause analysis was conducted, which resulted in a new bit design and expedited manufacturing to deliver the new bit within the tight timeline between consecutive wells. Historical data from the Exploration &Appraisal (E&A) wells in the field indicated coring as the prevalent form of bit failure in the subsalt interval and loosely correlated with drilling of a middle Miocene sandstone. However, the wide range of bit vendors, bit sizes, and dull conditions in the data created uncertainty on root cause of failure. Although a good ROP was achieved in the salt section on the first field development well, the bit suffered premature wear in the subsalt interval, resulting in two additional trips to drill the interval. Each worn bit was thoroughly analyzed in conjunction with the drilling parameters and mud logging data from each bit run. Against the previous assumption of coring being a result of drilling abrasive sandstones, it was now determined that the failure mechanism was most likely a result of drilling shales with high bottomhole pressure causing plastic behavior, regrinding of cuttings at the bit face, and insufficient cuttings evacuation. An entirely new bit design was generated to prevent this failure mode and manufactured in time for the next well. The new bit design combined with revised drilling parameters strategy resulted in the most successful run ever in the field: 10,854 ft drilled (7,000 ft of reaming) in one run at high ROP in the salt followed by good ROP and excellent durability in subsalt formations.
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