Modeling Approach Mitigates Premature PDC Failure in Transitional Drilling

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 220789, “Modeling PDC Cutter Loads When Drilling Interbedded Formations at Constant Penetration per Revolution vs. Weight on Bit” by Paul E. Pastusek, SPE, Pastusek and Associates; Mason M. Cherry, SPE, SLB; and Gregory S. Payette, SPE, ExxonMobil, et al. The paper has not been peer reviewed. _ The industry is placing increased importance on drilling harder, faster, and longer, giving rise to the challenge of premature bit failure caused by drilling through transitions of varying rock strength. To address this issue, new strategies have emerged, such as using automatic drillers operating with a constant rate of penetration (ROP). In this study, a method was developed to analyze the effects of drilling through transitions on bit-cutting structures and construct an ideal drilling strategy to mitigate the forces overloading cutters using a detailed drilling model. Testing Methodology A physics-based drilling-simulation environment was used to study the effects of drilling through formation transitions on polycrystalline diamond compact (PDC) cutting structures. The environment can be used to model only a drill bit or a drill bit attached to a drilling assembly. The environment allows a given PDC drill-bit design to be run through various rock formations or lithologies and can generate a set of outputs that characterize bit performance. First, rock lithologies and a drilling path must be set along with corresponding depths. Prescribing rock lithology as a function of depth allows for various transitions to take place in a simulation. Once the drilling path and lithologies are set, an existing bit design can be analyzed or a new design created. After setting a cutting structure layout, the drilling parameters may be configured, such as the rotational speed of the bit and a target value for ROP or weight on bit (WOB). Simulated Case Studies For the current work, a seven-bladed 17.5-in. PDC drill bit was selected for analysis, with target parameters chosen based on applicable field data. In this work, two bit designs have been considered that only differ in that one includes depth-of-cut (DOC)-limiting features in the cone (Fig. 1). Simulations that target a constant WOB were run at 60,000 lbf. For simulations that target a constant ROP, 50 ft/hr was used. A simulated soft salt with unconfined compressive strength (UCS) of 2,000 psi and a simulated hard limestone with UCS of 23,000 psi were used for the respective soft and hard formations. All simulations were performed with the same soft-to-hard and hard-to-soft transitions, using a simple horizontal bedding plane perpendicular to the wellbore trajectory and bit axis. Outputs from simulations include the at-bit ROP, WOB, and torque as a function of time and depth. They also include the resultant normal forces on each cutter at every simulated timestep.