Single-Blow Bit-Tooth Impact Tests on Saturated Rocks Under Confining Pressure: II. Elevated Pore Pressure
Authors: Yang J.H. , Gray K.E. 
Published: December 2007

Abstract
Results of single-blow bit-tooth impact tests on saturated rocks under elevated confining pressures and zero pore pressure were reported in a previous publication. This paper presents an extension of the earlier work to include a study of crater formation during tooth impact on both gas- and liquid-saturated Berea and Bandera sandstones at elevated confining and pore pressures. The basic data obtained were force-time, displacement-time, velocity-time and force-displacement curves during crater formation. Crater volume was also measured and the mode of crater formation determined. Bit tooth geometry, depth of penetration and velocity of impact were held constant. Results indicate that, with pore fluid present in the rock, failure trends from brittle to ductile as pore pressure is increased at constant confining pressure (pore pressure and borehole pressure were equals For a given rock type, the mode of crater formation was dependent not only upon the nominal effective stress, but also upon the fluid which saturated the rock pore space. When confining pressure and pore pressure were equal (zero nominal effective stress), bit-tooth impact resulted in brittle failure for nitrogen-saturated Berea, and brittle to transitional failure for nitrogen-saturated Bandera; when saturated with liquid both rocks failed in a ductile manner at zero nominal effective stress.

Introduction
Dynamic wedge penetration tests have been conducted by investigators in several fields, but the failure mechanism of rock under dynamic stresses is not understood completely. The complex action of drilling bits, even considering the action of a single tooth, may be considered as a combination of drag bit and rolling cutter action. Thus, as a first step in understanding rock breakage in oil well drilling, single chisel impact and rock planing are of fundamental importance. For example, Gray and Crisp studied drag bit cutting action at brittle stress states. Simon and Hartman studied the reaction of rocks to vertical impact by means of drop tests. The depth of penetration, crater volume and force-vs-time curves during crater formation were observed. The significance of indexing single-bit impacts has been noted. Garner et al, reported impact tests on impermeable Leuders limestone at atmospheric and elevated confining pressures. In all cases the tests were accomplished on dry rock and pore pressure was considered to be zero. The importance of both confining pressure and pore pressure on the failure characteristics of rock was described. It was found that the yield strength and ductility of porous rock depend on the state of stress under which the sample is tested. The importance of pore pressure on drilling rate in microbit experiments was noted by Cunningham and Eenink, Robinson also pointed out that in drilling the most important parameter in rock failure is the effective stress, where effective stress is defined as confining pressure Pc minus pore pressure Pp. The effect of pore pressure and confining pressure on rock strength was also noted by Serdengecti and Boozer in strain rate tests, and by Gardner, Wyllie and Droschack in elastic wave studies. Until recently all reported wedge impact studies under simulated wellbore stress states have been conducted on dry rock. Maurer reported impact tests on samples saturated with deaerated water. Borehole and formation fluid pressures were equal in these tests except when mud was used in the borehole. With mud in the borehole and a high borehole-to-formation fluid pressure differential, Maurer observed "pseudoplastic" crater formation. Podio and Gray reported impact tests on Berea and Bandera sandstone saturated with pore fluids having wide ranges in viscosities. In Podio and Gray's tests, confining pressure was elevated, but pore pressure and borehole pressure were held fixed at atmospheric pressure.