Dynamic Mechanical Behaviour and Energy Evolution of Coal Measure Sandstone and Implications for Coalbed Methane Development

ABSTRACT Triaxial loading and unloading tests on sandstone samples were conducted under various stress paths to investigate the mechanical properties and energy evolution during rock deformation. The results show a positive correlation between the peak strength and peak strain of sandstone and confining pressure. Both the elastic modulus and Poisson's ratio also increase with confining pressure. Notably, the peak strength and peak strain are minimised under conditions of constant axial stress (CAS) while unloading confining pressure (UCP). For different stress paths, sandstone primarily stores elastic energy with minimal energy dissipation during the initial loading stage, while the proportion of dissipated energy increases with rising confining pressure. During the weakening stage, the threshold value gradually decreases to below 80%, leading to a gradual transformation from rock brittleness to ductility. Under conventional triaxial compression (CTC), rock failure predominantly occurs through shear failure. In the involving increasing axial stress (IAS) while UCP test, a complex tensile‐shear mixed failure mode is observed, whereas no main shear plane forms under CAS‐UCP. Sandstone predominantly undergoes tensile failure under cyclic loading and unloading (CLU). Additionally, an increased loading rate inhibits the initiation and propagation of internal cracks in sandstone at the early stage but promotes damage during the damage development and accelerated development stages. Therefore, hydraulic fracturing design must concurrently account for both tensile and shear fracture mechanisms, with precision optimisation of operational parameters to maximise fracture network complexity.