The Scratch Test: a High Resolution Log of Rock Strength with Applications to Geomechanics and Petrophysics

In this paper, we present the principle of the scratch test, a novel state of the art laboratory test carried out on core samples. The test provides a unique high precision continuous profile of the rock strength along the core sample. The standalone portable and semi-automated testing equipment, dubbed "the Wombat", allows testing up to 3 meters of core sample per hour. This simple and robust test requires minimal sample preparation (carried out with the machine), is applicable over a wide range of sedimentary rock with strength ranging from 0.5 to 200 MPa, and is only partially destructive with the extent of damage limited to a groove of 10 mm wide and few millimetres depth along the core sample. The results are not affected by the sample size, and yield a continuous profile of the uni-axial compressive strength of the rock with a precision of less than 1 MPa. Besides direct applications to geo-mechanics, this high resolution log provides a unique measure of the scales, dispersion and distribution of heterogeneities along the core sample offering new perspectives in petro-physics with application such as strategy for core plugs selection, correlations with measurements on plugs but also well logs.