Energy Evolution of Coal During Uniaxial Compression under Different Bedding Intersection Angles and its Controlling Effect on the Failure Process

Energy evolution is of great significance for exposing the failure mechanism of coal reservoirs and evaluating the fracturing effect. This study conducted uniaxial compression experiments with bedding intersection angle (θ) as a variable, calculated the energy and analyzed their controlling effects on coal deformation, failure and brittleness. The results show that, with increase in θ, the failure mode of coal changes from tensile fracture to shear failure, and the strength, elastic energy (U^e) and total energy (U) of coal all show a trend of first decreasing and then increasing. Based on the comprehensive criteria of the elastic energy ratio (U^e/U) and stress–strain curve, a new division method of the compression stage is proposed, and the determination of the critical point between stages can avoid the influence of human subjective factors. The transformation of U^e and external loads jointly provides the energy source for the release of dissipated energy (U^d). U^e is the main energy type in the pre-peak stage, and a significant amount of energy is converted into U^d in the failure stage. A new brittleness index was constructed by utilizing the relationship between U^e and U^d. It was found that brittleness is negatively correlated with θ and positively correlated with U^e/U at the peak point. A model of the mechanical strength of coal with structured planes has been constructed and validated by experimental data, which can be used to analyze the effect of structural planes on coal strength. When the hydraulic fracturing direction is parallel or at a low angle to the bedding plane, it is favorable for the formation of extensive fractures.