Cyclic mechanical response and damage evolution of shale under different fluid saturation conditions

In oil and gas development engineering, shale may serve either as a reservoir subjected to repeated hydraulic fracturing or as a caprock subjected to long-term gas injection and production in underground gas storage. In both cases, it is exposed to the coupled effects of cyclic loading and fluid invasion. Therefore, clarifying the deformation behavior, damage accumulation, and instability mechanisms of shale under such coupled conditions is of great significance for understanding its mechanical response and evaluating its engineering stability. In this study, Fuling shale was selected as the research object. Uniaxial monotonic loading tests and graded cyclic loading tests were conducted under three conditions, namely dry, oil-saturated, and water-saturated states. In addition, cyclic loading tests within a high-stress range were carried out under the water-saturated condition. Through these tests, the evolution laws of shale strength, deformation, residual strain, and energy dissipation under different fluid conditions were systematically analyzed, and the effects of fluid state on the cyclic mechanical behavior of shale were further compared. The results show that dry shale exhibits the highest overall strength, whereas water saturation significantly weakens both the strength and stiffness of shale and markedly increases the proportion of energy dissipation during deformation and failure. Oil saturation has only a limited influence on the elastic modulus, but it enhances the residual deformation and energy dissipation during cyclic loading. Under all three fluid conditions, cyclic instability occurs when the stress level is still lower than the peak stress under monotonic loading, indicating that shale may become unstable before reaching its monotonic peak strength under repeated loading. Meanwhile, the corresponding peak total strain at instability is close to that at monotonic failure. Before the onset of instability, the growth rate of residual strain shows a pronounced increasing trend, reflecting the accelerated accumulation of irreversible deformation. Under all three conditions, the evolution of cyclic residual strain exhibits clear stage characteristics: the deformation in the first cycle is relatively large, then decreases and tends to stabilize, and finally increases rapidly again as failure approaches, showing an obvious stage-dependent pattern. These findings indicate that, compared with traditional evaluation methods based only on peak stress or uniaxial compressive strength, the criteria based on total strain, the growth rate of residual strain, and the energy dissipation ratio are more suitable for evaluating wellbore stability during shale oil and gas development and during long-term gas injection and production in underground gas storage. Therefore, these parameters can provide more appropriate indicators for stability assessment under coupled cyclic loading and fluid invasion conditions.