Understanding PDC Cutting Structure Behaviors While Drilling Through High Pressured Non-Homogenous Rock Types

Abstract As the drilling industry continues to push for longer unconventional intervals requiring polycrystalline diamond compact (PDC) drill bits to drill shoe to shoe at rig limits, the complexity of drilling challenges and encountered trade-offs become performance limiters. Whether drilling for hydrocarbons or geothermal energy sources, one such tradeoff is the vast contrasting formation types or lithologies experienced in a drilling interval. This presents the potential problem of designing cutting structures that provide competitive performance in a softer, easier to drill formation, while maintaining durability in a tougher, more complex interbedded section, as these require contrasting design philosophies. While one approach to this issue is to improve PDC cutter technology or introduce various shaped PDC cutters, this still leaves room for cutting structure optimization. This paper details the full-scale testing and optimization of varying PDC cutting structures designed to overcome differing drilling challenges that originate from drilling multiple formation types. These include tangential overloading of cutters from lateral and torsional vibrations, drilling through transitioning formations, and the cutting evacuation issues of drilling soft shale at high rate of penetration (ROP). A series of drilling lab tests were devised to collect comparative performance data on these cutting structures. This included drilling, for example, layered carbonate, shale, granite, and anhydrite cores under pressure to simulate downhole conditions. During this testing, the PDC drill bits were fitted with high frequency vibration monitoring sensors which were later aligned with the high frequency rig performance data. These tests resulted in the comprehensive comparative analysis of cutting structures behaviors through all high frequency lab drilling data and allowed for the categorization of each cutting structure for each drilling challenge or trade off. By categorizing the cutting structure selection criteria, it was possible to optimize for further field testing initially in North America land where one operator faces challenging interbedded abrasive formations oscillating from below 10kpsi to up to 35kpsi unconfined compressive strength (UCS), making the ROP fluctuate below 20ft/hr and up to 70ft/hr in the softer rock and then expanding the knowledge to Oman. Oman (KZN) is always challenging drilling through multiple lithologies including sandstones, shales, and carbonates. Long intervals require smooth transmission from one formation to another to prevent cutter wear and overload which leads to excessive vibrations levels that can result in reduced run length or low ROP. In some instances, these cutting structure improvements allowed the operator to apply 17% higher weight on bit, and drill 20% further at a higher rate of penetration. By conducting comprehensive comparative testing of drilling performance in multiple rock types in a controlled, full-scale, and high-pressure laboratory environment, it was possible to create a selection criterion for various PDC cutting structure types. By utilizing a design experiment, the laboratory and vibration data removed the variation one might expect to see from field testing and provided clean, conclusive results. This data was also used in conjunction with a three-dimensional tetrahedral drilling simulation software to augment selection and minimize risk to the operator.