Bit - Tooth Penetration Under Simulated Borehole Conditions
Authors: Maurer William C. 
Published: December 2007

Abstract
A study of bit-tooth penetration, or crater formation, under simulated borehole conditions has been made. Pressure conditions existing when drilling with air, water and mud have been simulated for depths of 0 to 20,000 ft.
These crater tests showed that a threshold bit-tooth force must be exceeded before a crater is formed. This threshold force increased with both tooth dullness and differential pressure between the borehole and formation fluids.
At low differential pressures, the craters formed in a brittle manner and the cuttings were easily removed. At high differential pressures, the cuttings were firmly held in the craters and the craters were formed by a pseudoplastic mechanism.
With constant force of 6,500 lb applied to the bit teeth, an increase in differential pressure (simulated mud drilling) from 0 to 5,000 psi reduced the crater volumes by 90 per cent. A comparable increase in hydrostatic fluid pressure (simulated water drilling) produced only a 50 per cent decrease in volume while changes in overburden pressure (simulated air drilling) had no detectable effect on crater volume.
Crater tests in unconsolidated. sand subjected to differential pressure showed that high friction was present in the sand at high pressures. Similar friction between the cuttings in craters produces the transition from brittle to pseudoplastic craters.

INTRODUCTION
The number of wells drilled below 15,000 ft increased from 5 in 1950 to 308 in 1964.1 Associated with these deep wells are low drilling rates and high costs. High bottom-hole pressures produce low drilling rates by increasing rock strength and by creating bottom-hole cleaning problems. This paper describes an experimental investigation of crater formation under bottom-hole conditions simulating air, water and mud drilling.
Although numerous investigators have studied bit-tooth penetration (cratering) at atmospheric pressure conditions, only limited work has been done on cratering in rocks subjected to pressures existing in oil wells. Payne and Chippendale2 have studied cratering in rocks subjected to hydrostatic pressure using spherical penetrators. Garner et al.3 conducted crater tests in dry limestone by varying overburden pressure and borehole fluid pressure independently and using atmospheric formation-fluid pressure. Gnirk and Cheathem4,5 have studied crater formation in several dry rocks subjected to equal overburden and borehole pressure and atmospheric formation pressure. Podio and Gray6 studied the effect of pore fluid viscosity on crater formation using atmospheric borehole and formation-fluid pressure and varying overburden pressure. Although these studies have provided useful information On crater formation under pressure, they were limited in that the three bottom-hole pressures could not be varied independently and, therefore, that many drilling conditions could not be simulated.
The pressure chamber used in this study allowed visual observation of the cratering mechanism and independent control of the borehole, formation and confining pressures. By using different fluids in the chamber, pressure conditions existing in air, water and mud drilling to depths of 20,000 ft were simulated. The mechanisms involved in cratering at these different pressure conditions were studied for teeth of varying dullness and at different loading rates. High-speed movies (8,000 frames/sec) and closed-circuit television were used to visually study the crater mechanism under pressure.

EXPERIMENTAL PROCEDURE

PRESSURE CHAMBER
The pressure chamber in Fig. 1 was used to simulate bottom-hole pressure conditions. This chamber has been pressure-tested to 22,500 psi and is normally operated at pressures up to 15,000 psi. The chamber contains four lucite windows7 used for illuminating and observing the crater mechanism under pressure. A closed-circuit television and a Fastax camera (8,000 frames/sec) have been used in these studies.
Cylindrical rock specimens (8-in. diameter×6-in. long) were subjected to three independently controlled pressures simulating overburden, borehole fluid and formation-fluid pressures. Overburden pressure, which corresponds to the stress induced by the overlying earth mass, was applied by exerting fluid pressure against a rubber sleeve surrounding the rock. Borehole pressure, which is the pressure exerted by the column of mud in the wellbore, was simulated by applying pressure to the fluid overlying the rock in the chamber. Formation pressure was simulated by applying pressure to the water saturating the rock. The borehole and formation pressures were equal except when mud was used in the chamber, in which case the differential pressure between these fluids acted across the mud filter cake.

PRESSURE CHAMBER
The pressure chamber in Fig. 1 was used to simulate bottom-hole pressure conditions. This chamber has been pressure-tested to 22,500 psi and is normally operated at pressures up to 15,000 psi. The chamber contains four lucite windows7 used for illuminating and observing the crater mechanism under pressure. A closed-circuit television and a Fastax camera (8,000 frames/sec) have been used in these studies.
Cylindrical rock specimens (8-in. diameter×6-in. long) were subjected to three independently controlled pressures simulating overburden, borehole fluid and formation-fluid pressures. Overburden pressure, which corresponds to the stress induced by the overlying earth mass, was applied by exerting fluid pressure against a rubber sleeve surrounding the rock. Borehole pressure, which is the pressure exerted by the column of mud in the wellbore, was simulated by applying pressure to the fluid overlying the rock in the chamber. Formation pressure was simulated by applying pressure to the water saturating the rock. The borehole and formation pressures were equal except when mud was used in the chamber, in which case the differential pressure between these fluids acted across the mud filter cake.