The value of the mechanical specific energy (MSE) concept to analyze drilling efficiency and bit performance is well established. Recent operator-driven research has concentrated on predicting and maximizing rate of penetration (ROP). The specific energy ROP model has been successfully implemented in numerous high-cost/high-profile environments. To further advance the mechanical specific energy (MSE) concept, an engineering team is developing a methodology to optimize polycrystalline diamond compact (PDC) bit design for the entire hole section based on modeled MSE and unconfined compressive strength (UCS) values.

Traditionally, PDC design engineers adjust cutter density/size, back rake, blade count and nozzle placement to optimize the bit for a specific application. The goal is to maximize ROP and total footage capabilities by minimizing damaging vibrations (axial, lateral, torsional) in a predetermined series of formations. Generally, the selected design has the least potential for vibration across all lithology types. PDC bits can generally drill a homogeneous formation without issues, but when transitioning zones or penetrating interbedded/unexpected formations vibrations can lead to performance degradation and cutter damage.

The industry requires a method to unify bit efficiency throughout the entire section on a meter-by-meter basis. To calculate a bit efficiency factor (Em), engineers are utilizing a sophisticated, integrated, dynamic engineering modeling system. They start with a meter-by-meter correlation between actual borehole lithology and a digitized rock library (UCS). MSE is then calculated using the modeled drilling parameters, torque, and ROP. The resulting two curves are then standardized by calculating the mean value of MSE/UCS fraction to derive a hard value bit efficiency factor. By overlapping the two curves, design and field engineers can identify lithologies that have the optimum correlation then compute the mean efficiency factor for the interval and use it as baseline for the entire hole section.

In the test case study a 38m thick shale section offered the best match, yielding a median efficiency factor. Using the fixed median efficiency factor, engineers designed and optimized a PDC bit for the entire 16-in. hole section instead of each specific formation. The resulting ROP was significantly improved, as is documented in a case study that will be presented.

You can access this article if you purchase or spend a download.