Experimental Study of MSE of a Single PDC Cutter Interacting With Rock Under Simulated Pressurized Conditions

The downhole pressure environment is one of the most important factors affecting the rate of penetration (ROP). It is believed that impermeable rocks experience high differential pressures because of shear dilatancy and become stronger and, thus, more difficult to drill. However, recently performed atmospheric and pressurized single-cutter experiments show that contrary to this belief, even at low pressures (100-200 psig) and even with permeable rocks, significant increase in mechanical specific enemy (MSE) is observed compared to atmospheric tests. The experiments were carried out in a single-cutter high-pressure testing facility refurbished with high-precision sensors and a data acquisition system. In the experiments, a 13-mm polycrystalline-diamond-compact (PDC) cutter was used to cut Carthage marble and Indiana limestone samples with depths of cut ranging from 0.025 to 0.050 in. More than 70 high-precision tests were performed on these two rock types under confining pressures ranging from 0 to 1000 psig. The confining fluids were either water or mineral oil. Unexpectedly, analysis of the MSE consistently showed that increases in the confining pressure as small as 150 psig can increase the MSE of the cutting process significantly and reduce the cutting efficiency by half. These reductions in the cutting efficiency that were even more dramatic in the permeable and saturated Indiana limestone could not be explained by the strengthening of the rock under confining pressures. Upon analysis of the results of experiments (cutting forces, volume of cuts, and visual inspections of cuttings), a new theory was proposed to explain this unexpected behavior. This new theory, based on the frictional forces and the cutting mechanism under pressure, gives useful insights into the physics of cutter-rock interaction. Such insights are invaluable to the improvements of drilling practices selection [weight on bit (WOB), type of drilling fluid, and its properties] and the ROPs.