The Simulation of Percussion Drilling in the Laboratory By Indexed-Blow Studies
Authors: Hartman Howard L. 
Published: December 2007

HARTMAN, HOWARD L., PENNSYLVANIA STATE U., UNIVERSITY PARK, PA. MEMBER AIME

Abstract
The drop tester has proved an invaluable tool for the investigation of percussion drilling in the laboratory in "slow motion". It has allowed the process of rock penetration by impact to be studied a single blow at a time. In the present work, the relation of cratering to indexing has been determined under conditions simulating those in the bottom of the hole at atmospheric pressure.Indexed blows form craters which are influenced by others adjacent to them. Two unique effects are involved:the provision of additional free faces in proximity to the point of impact andthe creation of subsurface damage by the previous blow(s).
Both have a pronounced influence on the volume of rock removed per blow, which governs the rate of penetration in actual drilling. The emphasis in this study was placed on determining effects of index distance and impact energy on crater volume when blows were struck on a previously "drilled" or damaged surface, a situation most representative of down-hole conditions. The results indicated thatthe optimum index distance on a damaged surface is greater than on a fresh, undamaged rock surface;the volume of rock broken at optimum index distance is about the same for both surface conditions with die-shaped chisels but tends to be greater on a cratered surface with wedges;the optimum index distance and maximum crater volume are proportional to the energy level;at a given energy level, the volume of rock broken by dies generally exceeds that by wedges; andmaximum crater volume varies, approximately, inversely with width of die but is nearly independent of included angle of wedge.

Introduction
This is the third of a series of papers reporting on one phase of a continuing program of drilling research being conducted in the Rock Mechanics Laboratory at The Pennsylvania State U. The phase under consideration is crater geometry - blow energy relations in percussion drilling. While the previous papers have discussed mainly craters produced by single blows, indexed-blow studies are the subject of this paper.The basic mechanism of drilling under study here - that of penetration of a chisel-shaped bit subjected to impact - is the one employed in probably 90 per cent of all rock drilling in mining, percussion drilling. It also forms the fundamental action in nearly all oil well drilling with the so-called rotary (roller-bit) method. Here, the action is more complex, however, and may in actuality comprise rotary-percussion drilling. Certainly, the down-hole air and mud hammer drills, which superimpose percussion on rotation, belong in this latter category. In any event, impact blows constitute one of the principal mechanisms in the vast majority of drilling, mining or petroleum, and it is essential in drilling research to study percussion in all its basic aspects.Percussion drilling can be simulated in the laboratory by means of an impact drop tester. Striking one blow at a time with chisel-shaped bits, the drop tester permits study of most of the parameters in the drilling process except blow frequency and the interrelationship of certain variables. Single-blow craters, the simplest to produce, allow the determination of basic relationships, such as volume vs blow energy; while indexed craters most closely resemble the action in real drilling and provide data for the establishment of optimum operating criteria.Indexed-blow craters are those which are influenced by others in proximity. The simplest case to visualize, and the one studied in detail here, is that in which a given crater is formed adjacent and parallel to a previous crater, such that the second crater breaks into the first. A more complex situation usually exists downhole, where indexing is achieved by rotation of a radial-winged or rolling-cutter bit. But, admittedly, the phenomenon of indexing is more likely to be understood eventually if it is first studied in its simplest form, that related to parallel craters.Limited research has been conducted on indexed blows. The most complete study was that by Simon, who proposed universal indexing curves to relate the important variables.
SPEJ
P. 214^