Damage Evolution Behavior of Coal Reservoirs under Cyclic Disturbance and Optimization Suggestions for Deep Reservoir Stimulation

【Objective and Methods】During volumetric stimulation or multi-stage hydraulic fracturing of coal reservoirs, pressure fluctuations induced by fracture-tip propagation and fluid-pressure build-up within fractures can impose cyclic perturbations on the surrounding rock. To clarify the mechanical response and fracture-evolution mechanisms of coal under such cyclic perturbations, triaxial cyclic loading–unloading experiments were conducted on high-rank coal under varying confining pressures, stress amplitudes, and loading modes. Field engineering data were further integrated to propose optimization strategies for deep coal-reservoir stimulation.【Results】(1) The cyclic stress–strain curves exhibit pronounced hysteresis loops. Residual stress accumulates mainly during the initial cycles, and its magnitude is controlled by confining pressure, loading mode, and the damage/failure pattern. (2) Cyclic loading promotes matrix compaction and closure of pores and fractures, driving the load-bearing structure to evolve from a heterogeneous weak-support framework to a relatively homogeneous, high-capacity skeleton. Accordingly, the elastic modulus generally increases first and then stabilizes with increasing cycle number. (3) Under shallow, low-confining-pressure conditions, structural constraint is weak and stress concentration is prominent, favoring through-going brittle fractures and efficient release of residual stress. Under deep, high-confining-pressure conditions, structural constraint and load-transfer efficiency are enhanced, the energy distribution becomes relatively dispersed, fractures are dominated by localized discontinuous damage or progressive growth along pre-existing fractures, and the release efficiency of residual strain energy is low. (4) During multi-stage fracturing, continuous fluid injection induces cyclic perturbations around the main fracture, causing residual-stress accumulation and elevating the initiation threshold. Field operations typically show the lowest breakdown pressure in the first stage, followed by a marked increase and subsequent stabilization in later stages. Only when the net pressure within the fracture exceeds the strengthened stress barrier can wing fractures be activated; otherwise, the incremental increases in fracture-network complexity and effective stimulated volume with additional stages remain limited, and repeated re-opening of the main fracture reduces overall energy-utilization efficiency. (5) Efficient stimulation of deep coal seams depends on optimizing the energy-input strategy and improving energy-utilization efficiency rather than blindly enlarging the stimulation scale. A loading strategy combining stepwise-increasing injection rate with staged shut-in or low-intensity intermittent injection is recommended to promote residual-stress release and progressive fracture activation, thereby enhancing effective fracture growth and complex fracture-network construction. These findings provide experimental evidence and mechanistic support for optimizing the design of deep-coal hydraulic-fracturing programs.