Measurement of Some Mechanical Properties of Rocks And Their Relationship to Rock Drillability
Authors: Gstalder S. , Raynal J. 
Published: December 2007

Consideration was given to simple tests which could be performed on rocks to give a measure of rock drillability. Various methods of breaking rocks were considered and the hardness test developed by Schreiner was selected for this study. The test involves application of an increasing load on the rock face through a flat-faced cylindrical punch until rupture occurs.

Test results show that hardness is a good measure of the breaking strength of rocks. Useful relationships are shown to exist between hardness and other physical quantities such as specific disintegration, Young's modulus and sonic velocity. Specific disintegration (volume of rock broken per unit of work input) provides a possible means of comparing the effectiveness of other methods of rock breakage.

The relationship between hardness and sonic velocity may be very significant for it may be possible to deduce rock drillability from sonic log data, provided that a mineralogical factor is taken into account. To test further the results of the work, laboratory drilling tests were performed which closely simulated downhole pressure conditions. Correlations of drilling results with rock hardness measurements were quite good. It was concluded that rock hardness, as measured in the laboratory or as deduced from sonic logs, could be used in relations for predicting rock drilling performance.

INTRODUCTION

Drillability of rocks cannot be defined in an absolute manner by a single quantity or measured by a single test. Resistance of rock to drilling depends to a large extent upon the means used for rock destruction. This complicates drillability classification of rocks for there may be as many classifications as there are methods of rock breakage without known functional relations between them.

The objective of this work was to develop, in terms of rock destruction behavior, means of selecting the best drilling method for a given rock or the conditions for optimum output from a given drilling method.

ANALYSIS OF ROCK DESTRUCTION TESTS

Gauthier and Baron1 have reviewed the many test methods which have been developed for determining the resistance of rocks to breakage. These include drilling tests, punch tests, scratch tests, resistance to shock, wear tests, etc.

A standard drilling test might be to determine the time required to drill a standard depth (1/16 in.) in a rock, using a bi-cone microbit (1 1/4-in. diameter) when a standard load (417 lb) is applied at a standard speed of bit rotation (110 rpm). From the standpoint of similarity, the results of microbit drilling tests would seem to be most representative of the several methods suggested above. However, evaluation of drilling performance by this method is only qualitative and cannot be applied directly to actual drilling conditions.

Punch tests basically measure rock hardness. Punches of various shapes and sizes, including small cones for Vickers' and Knoop's tests, the wedge for Epstein's test and the right circular cylinder for Schreiner's test, have been used. Results of these tests lead to a quantitative hardness classification which would seem to be related to the resistance of rock to bit-tooth penetration. Scratch tests are related to the resistance of rocks to abrasion such as would be encountered in drilling with diamonds. Either the width of the scratch obtained at a fixed load or the load required to give a scratch of a fixed width may be used to evaluate surface hardness of the rock. This should be related to the resistance of the rock to wear.

The above tests, and probably many others, give relative measurements of the breakage or destruction characteristics of rocks. There is no direct relation between results of these tests and the drill ability of rocks. Also, the tests each measure different quantities which have different dimensions and they do not permit direct comparison of numerical values.

As one step in rectifying the above difficulties, it is proposed that the results of all test methods be expressed in terms of volume of rock destroyed per unit of work required; this is referred to as "specific disintegration". Test results may then be reported in a form which shows both a classification of rocks in regard to each method and a comparison of methods with regard to each rock.