Prediction of Rock Parameters from Micro-Indentation Measurements: The Effect of Sample Size

Abstract

Abstract Micro-indentation measurements have been performed in order to investigate the possibility of extracting rock mechanical properties from small rock samples. The tests were performed with a 1 mm flat indenter. Two parameters were determined when analyzing the indentation measurements: The Indentation Modulus (IM) and the Critical Transition Force (CTF). IM is the slope of the force-displacement curve, corrected for deformations in the load frame. CTF is defined as the force level where the material deforms without significant change in the applied force. The rock samples were casted in a mounting material (Demotec 30), in order to stabilize the sample during testing, and simplify surface preparation. The upper and lower surfaces of the samples were made flat and plane parallel by grinding. 15 materials were tested, including sandstones, limestones and shale/clay materials with different strengths and stiffness. The micro-indentation measurements showed that both IM and CTF were significantly affected when the experiments were performed on small samples with volumes ranging from 0.04 Cm3 to 0.7 cm3. Correlations combining the Uniaxial Compressive Strength (UCS), Young's Modulus (E) and porosity () with IM and CTF have been made. For measurements performed on small rock samples (embedded in Demotec 30) the following correlation with Uniaxial Compressive Strength was found: UCS = 0.149 CTFR2=0.90 The correlation is only valid for small samples embedded in a mounting material with the same indentation properties as Demotec 30. It is not recommended to use the micro- indentation measurements to predict porosity and Young's Modulus. P. 487