Influences of Test Parameters on the Meso-Mechanical Characteristics of Coal Rock in the Longtan Formation, Sichuan Basin

As a typical heterogeneous, porous, organic–inorganic composite rock, the microscopic mechanical properties of coal rock significantly impact wellbore stability during drilling in deep fractured formations. Traditional macro-mechanical experiments struggle to reveal the mechanical behavior of its microscopic fabric. This study investigates the local indentation response of Longtan Formation coal rock from the Sichuan Basin under different peak loads, dwell times, and loading/unloading durations. Elastic modulus and hardness are evaluated by contact analysis, together with an apparent indentation fracture index derived from an empirical energy partition model. The plotted responses exhibit protocol-dependent variation and local scatter, without establishing a common optimum for all measured quantities. The observations are discussed in relation to contact-area uncertainty, microstructural sampling, and time-dependent penetration during unloading. These explanations remain conditional because phase-correlated damage observations and independent fracture validation are not reported. The study clarifies the need to distinguish local measurement sensitivity from intrinsic rock properties and provides a basis for designing subsequent correlative and multiscale experiments. Application to wellbore-scale behavior requires independent validation and explicit consideration of rock mass discontinuities and in situ conditions.