The downhole pressure environment is one of the most important factors affecting the rate of penetration. It is believed that impermeable rocks experience high differential pressures due to 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 Energy (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 data acquisition system. In the experiments, a 13-mm 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, visual inspections of cuttings, etc), a new theory is 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 (WOB, type of drilling fluid and its properties, etc) and the rates of penetration.
Low rates of penetration and reduced drilling efficiencies have always been a challenge to the drilling industry. To overcome this challenge a fundamental understanding of the mechanics of the rock-cutting process is necessary. However due to the complicated nature of the cutter-rock interaction at downhole conditions, the understanding of the process has been very limited.
Until the early fifties, the industry was unaware of the effect of pressure on the rock and the drilling process. Kühne (1952) suggested that downhole pressure strengthens the rock and a Mohr-Coulomb criterion may be used for taking the effect of strengthening into account. Cunningham and Eenink (1958) used the Terzaghi's effective stress law and showed in their experiments that it is actually the differential pressure, the difference between the bottomhole mud column pressure (borehole pressure), and the pressure inside the pore spaces of the rock (pore pressure) that strengthens the rock matrix and makes it more difficult to drill.