An Experimental Study of Single Bit-Tooth Penetration Into Dry Rock at Confining Pressures 0 to 5,000 psi
Authors: Gnirk P.F. , Cheatham J.B. 
Published: December 2007

Abstract
Single bit-tooth penetration experiments under static load were conducted on six rocks at confining pressures of 0 to 5,000 psi using sharp wedge-shaped teeth with included angles ranging from 30 to 120°. In general, the force-displacement curves for all rocks exhibit an increasingly nonlinear and discontinuous behavior with decreasing confining pressure. The confining pressure at which a rock exhibits a macroscopic transition from predominantly ductile to predominantly brittle behavior during penetration varies from about 500 to 1,000 psi for the limestones to greater than 5,000 psi for dolomite. The correlation between calculated values of force per unit penetration based on plasticity theory and experimental values is quite encouraging, even at confining pressures as low as 1,000 psi. A qualitative correlation between volume of fragmented rock per unit energy input for a single bit-tooth and drilling rate for microbits appears to exist over a confining pressure range of 0 to 5,000 psi.

INTRODUCTION
Laboratory experiments utilizing a small-scale drilling apparatus have demonstrated that penetration rates are reduced considerably as a result of increasing the confining pressure ham atmospheric to a few thousand psi.1–3 This undesirable situation can, in general, be attributed to a combination of decreased efficiency of chip removal at the bottom of the borehole, increased rock-failure strength, and a possible change in the mechanism of chip generation and rock fragmentation with increasing confining pressure. To more fully understand the principles underlying the last circumstance, it is the purpose of this investigation to experimentally study the mechanism of single bit-tooth penetration into dry rock at low confining pressures and, in particular, to establish the confining pressure at which the penetration mechanism may undergo a brittle to ductile transition for various rock types commonly encountered in drilling. Confining pressure as used here refers to the differential pressure between the borehole fluid pressure and the formation-pore fluid pressure.

EXPERIMENTAL PROCEDURE
Using an experimental apparatus previously described,4 a single, sharp wedge-shaped tool was forced under a "statically" applied load into an effectively semi-infinite dry rock sample subjected to a prescribed confining pressure. To prevent the invasion of the confining-pressure fluid into the pores of the rock sample during penetration, the exposed surface of the rock was jacketed with a layer of silicon putty.* Electrical instrumentation incorporated into the apparatus yielded a graphical plot of force on the tool as a function of penetration or displacement of the tool into the rock during an experiment.
During the course of the experimentation the following conditions were maintained constant:pore pressure - atmospheric (i.e., the rock was dry);temperature - 75F;rate of loading - essentially static (approximately 0.002 in./sec);bit tooth - a sharp wedge-shaped tool loaded normal to the rock surface;rock surface smooth and flat;drilling fluid - hydraulic oil; andmaximum depth of penetration - approximately 0.1 in.
In addition, each experiment was performed on a different rock sample so the rock surface is free of a layer of cuttings and of any previous indentation craters. The influence of the corners of a borehole was neglected, since each rock sample was cemented into a section of aluminum tubing to simulate a semi-infinite body.