Effect of force levels and indenter types on rock mechanical parameters of tight formation from instrumented indentation tests

Rock mechanical properties such as deformation modulus and hardness are important for geological and engineering applications. Instrumented indentation testing provides a practical approach for characterizing these properties using small rock specimens while simultaneously measuring multiple mechanical parameters. Although different indenter geometries and force levels have been widely used in rock testing, their combined influence on indentation-derived mechanical properties remains insufficiently understood. This study performed instrumented indentation tests on tight sedimentary rocks from the Doig Formation of the Montney Shale Play, Canada, using Brinell and Vickers indenters over a wide macro-indentation force range. Contact stiffness, indentation modulus, and three hardness measures were evaluated and compared. Results from 160 indentation datasets show that contact stiffness increases with indentation force for both indenters. Brinell indentation modulus and hardness increase with force and then approach stable values, whereas Vickers indentation modulus and hardness decrease with force before reaching a plateau. Vickers indentation consistently produces higher indentation modulus and hardness values and greater variability than Brinell indentation. Mean indentation stress analysis indicates that differences in contact-area evolution between the two indenter geometries lead to distinct stress trajectories with increasing force. Grain-scale sampling-volume analysis shows that increasing force substantially increases the number of grains contributing to the measured response. Discrete-element and finite-element simulations further demonstrate that the Vickers indenter generates more concentrated and deeper stress-transfer paths and stress fields than the Brinell indenter. The results indicate that indenter geometry, contact volume, and stress-transfer characteristics collectively influence indentation-derived mechanical properties in tight sedimentary rocks and should be considered when selecting testing parameters for rock characterization.