Experimental analysis of the damping characteristics of longmaxi shale under graded cyclic loading with different confining pressures

In the process of shale gas field development, the surrounding rock has been in a three-way high-pressure stress state for a long period of time and has undergone complex cyclic loading during hydraulic fracturing and high-pressure fracturing fluid return, and its mechanical response and mechanism of action have important impacts on the stability of the reservoir. To reveal the damping characteristics of Longmaxi shale under different peripheral pressures, graded cyclic loading tests were carried out under 0 MPa, 10 MPa and 30 MPa peripheral pressures to systematically analyze the stress‒strain hysteresis characteristics of the shale and the evolution of the damping ratio. The results show that the damping ratio of the Longmaxi shale shows a staged evolution with increasing stress level, which is characterized by "decreasing, stabilizing, and then slightly increasing". During cyclic loading, the axial and radial damping ratios both decreased and stabilized with increasing number of cycles. The radial damping ratio was always greater than the axial damping ratio under the same circumferential pressure conditions, and the difference was more significant under high circumferential pressure conditions. Shale exhibits typical characteristics of hysteresis and damping response evolution during dynamic loading, reflecting a progressive phase shift wherein strain increasingly lags behind stress. This phenomenon primarily arises from the accumulation of plastic deformation, the continuous propagation of microfractures, and the sequential activation of multistage energy dissipation mechanisms. The damping ratio follows a staged evolution pattern characterized by an initial decline, a period of stability, and a final surge. This trend indicates a transition in the material's response mechanism-from being dominated by structural compaction to being governed by microscopic damage processes.