Objective and Methods  During volume fracturing or multi-round hydraulic fracturing of deep coal reservoirs, pressure fluctuations induced by the propagation of fracture tips and fluid accumulation within fractures cause cyclic perturbations to surrounding rocks. To clarify both the mechanical responses of coals and the mechanisms behind fracture evolution within coal reservoirs under such cyclic perturbations, this study conducted triaxial cyclic loading-unloading tests on high-rank coals under varying confining pressures, stress amplitudes, and loading modes. In combination with field engineering data, this study proposed strategies for optimizing deep coal reservoir stimulation.

Results The stress-strain curves of coals under cyclic loading and unloading exhibit pronounced hysteresis loops. Residual stress primarily accumulates during initial loading-unloading cycles, with the stress magnitude controlled by confining pressure, loading mode, and the damage/failure mode. The cyclic process promotes the compaction of coal matrix, as well as the closure of pores and fractures. Consequently, the load-bearing structure of coal reservoirs evolves from a heterogeneous framework providing weak support into a homogeneous skeleton with a high bearing capacity. Accordingly, the elastic modulus generally increases first and then stabilizes with increasing number of cycles. Under the condition of low confining pressure in shallow parts, weak structural constraint and significantly concentrated residual stress are collectively prone to collectively induce the formation of interconnected brittle fractures and the efficient release of residual stress. In contrast, in an environment with high confining pressure in deep parts, strong structural constraint and evaluated load transfer efficiency jointly lead to relatively dispersed energy distribution. In this case, fractures primarily suffer localized, discontinuous damage or progressively propagate along primary fractures, leading to a low release efficiency. During multi-round fracturing, continuous fluid injection causes cyclic perturbations to the vicinity of main fractures. As a result, the accumulation of residual stress elevates the stress threshold for initial cracking. In field fracturing operations, such elevation is manifested by the phenomenon where the fracturing pressure is the lowest in the first round and then significantly increases and tends to stabilize in subsequent rounds. Only when the net pressure within fractures exceeds the strengthened stress barrier can flank fractures be activated. Otherwise, fracture network complexity and effective stimulated rock volume (SRV) increase to a limited extent with increasing number of cycles. Furthermore, repeated re-opening of main fractures results in a decrease in the overall energy utilization efficiency. To achieve efficient stimulation of deep coal reservoirs, the key is to optimize the energy input approach and improve energy utilization efficiency rather than increasing the fracturing scale blindly. It is recommended to adopt the loading strategies of combining gradually increasing injection rates with staged pump shutdown or implementing low-intensity, intermittent fluid injection. These strategies promote residual stress release and stepwise fracture activation, thereby facilitating effective fracture propagation and increasing the formation efficiency of complex fracture networks. The results of this study provide a theoretical basis and experimental support for optimizing the hydraulic fracturing design for deep coal reservoirs.