Improving Plugs Selection for Single-Stage Triaxial Tests with Micro-Rebound Hardness
Authors: Gonzalez Hector , Algarni Salem , Alqroos Ahmed 
Published: August 2024

ABSTRACT:
The primary objective of this study is to characterize the full description mechanical properties of rocks. The study aims to achieve this characterization using single stage triaxial (SST) tests instead of multistage triaxial (MST) test by grouping non-consecutives plugs from the same rock mechanical type. A novel approach to address the challenge of obtaining enough samples from the same depth or target layer involves grouping samples based on their hardness using the Micro-rebound technique. This method offers a potential solution to the issue of limited samples and could help improve the reliability of failure envelope parameters, especially for cohesion and friction angle in the case of the Mohr-Coulomb failure envelope. The SST test results on the selected plugs using the hardness criteria shows very good calibration of the Mohr-Coulomb failure envelope and the possibility to study the post-failure behavior of the rock as function of the confining pressure.

1. INTRODUCTION
A comprehensive laboratory rock mechanics characterization applicable to the majority of geomechanics applications on the oil & gas industry should include the determination of the static & dynamic elastic properties at different confining effective pressures, at least, up to the mean effective in-situ stress and the full description of the stress-strain behavior including the post-failure description or the residual strength.
The primary objective of this study is to characterize the mechanical properties of rocks, specifically focusing on static elastic properties in relation to effective confining pressure, and key strength parameters such as unconfined compressive strength (UCS), residual strength, cohesion, and friction angle. The study aims to achieve this characterization using single stage triaxial (SST) tests instead of multistage triaxial (MST) test by grouping plugs from the same rock mechanical type. The study employs two variations of triaxial tests, namely the MST and SST tests and micro-rebound measurements. The key difference between these methods is that MST requires only one sample, while SST necessitates at least three samples for a comprehensive mechanical characterization. The MST test is favored when there are limited homogeneous samples within the target formation or layer. The SST test is considered a more reliable procedure for a comprehensive mechanical characterization of rock behavior. The micro-rebound test was used to measure the hardness of 12 plugs as a classification mechanical parameter for grouping purpose. MTS tests have high probability getting the rock failure before reaching the last stage or high probability of having premature stopping point when the test is manual of fully controlled by an operator. Also, the cumulative damage on each stage may influence the elastic properties, like having Poisson ratio equal or higher than 0.4 after the first stage could drive the evaluation of non-representative properties. Additionally, estimating the failure envelope from the yield envelope introduces uncertainty due to the wide range of mechanical behaviors exhibited by different rock types, and it is not possible to study the residual stress as a function of effective confining pressure. By grouping samples using the Hardness criteria and performing SST tests results indicate that the Mohr circles from each SST test are matching very well a Mohr-Coulomb failure envelope model and the full stress-strain curve is available to study the residual strength, however, some degree of variability was observed on the static elastic Young's modulus. One novel approach to address the challenge of obtaining enough samples from the same depth or target layer involves grouping samples based on their hardness using the micro-rebound technique. This technique assigns a mechanical parameter for grouping sets of three or four samples for the SST test at different confining pressures. This method offers a potential solution to the issue of limited samples and could help improve the reliability of failure envelope parameters, especially for cohesion and friction angle in the case of the Mohr-Coulomb failure envelope.