Modeling PDC Cutter Loads when Drilling Interbedded Formations at Constant Penetration Per Revolution vs. Weight on Bit
Authors: Pastusek Paul E. , Cherry Mason M. , Payette Gregory S. , Bijai Rahul R. , Gjertsen Ole J. , Durairajan Bala 
Published: September 2024

Abstract
The demands of the oil and gas industry are placing an increased importance on drilling harder, faster, and longer, giving rise to the challenge of premature bit failure due to drilling through transitions of varying rock strength. To address this, new strategies have become more prevalent, 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.
A digital environment was created to model transitional drilling with the capability to predict downhole conditions when provided with various complex inputs. A polycrystalline diamond compact (PDC) bit design, including cutting structure and other features was first loaded into the model. The desired drilling parameters were provided, and the lithologies of the rock transitions throughout the interval were set. Drilling was then simulated within this digital environment, producing the bit performance data. The data included several high-level performance indicators, such as weight on bit (WOB), ROP, and torque, as well as detailed transient information on individual cutter loading and stress levels. Additionally, the simulation can be viewed as an animation to allow intuitive visualization of the effects of varying transitions on bit performance.
By analyzing the outputs from the simulated transitional drilling, conclusions were drawn relating certain drilling parameters and rock strengths to areas of cutting structure damage. When drilling with a constant WOB, it was observed that transitioning from soft to hard formations causes a large increase in instantaneous forces on the cutters in the nose of the bit. In contrast, when transitioning from hard to soft formations, the forces are increased on the cutters in the cone and shoulder. These observations correlate with drilling data and field dull observations. It was found that when drilling with a constant downhole ROP, the cutting structure does not experience transient spikes in cutter loads. The loads approach a new steady state distribution based on the compressive strength of the newly entered formation with little to no overshoot. However, when drillstring compliance was accounted for it was found that drilling at a constant surface ROP can still lead to transient spikes in cutter loads, especially when transitioning from hard to soft formations. Transient load overshoot can be minimized through the use of depth of cut (DOC) limiting features when drilling from hard to soft formations.
Based on these observations, the ideal strategy for drilling interbedded transitions is to use a customized cutting structure including DOC features and drill at constant, controlled ROP. These findings, along with the virtual environment and digital simulation capabilities, have the potential to save operators time and resources, and decrease the overall environmental impact of future oil and gas exploration.