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.