Energy evolution and distribution patterns of sandstone and its microscopic mechanism under multistage cyclic loading

Abstract

Sandstone, as a fundamental engineering material in depleted oil and gas reservoir gas storage systems, is susceptible to damage and failure under periodic stress disturbances. In this study, multi-level multi-cyclic loading tests were carried out on sandstone samples over the confining pressures range of 5–40 MPa, accompanied by real-time acoustic emission (AE) monitoring and periodic nuclear magnetic resonance (NMR) measurements. This study investigats the effects of confining pressure, stress level, and the number of cycles on energy evolution and energy distribution in rock, revealing the micromechanisms of energy evolution during cyclic loading. The results indicate that during the first cyclic loading, the input energy is primarily converted into dissipated energy through the compression of small pores and some medium pores. In subsequent loading cycles, the input energy is primarily converted into dissipated energy through the initiation and propagation of internal microcracks. Under high confining pressure, the rock transitions from brittle to ductile behavior, enabling it to withstand greater deformation. Additionally, at high confining pressure, rocks accumulate more strain energy, while energy dissipation is higher compared to lower confining pressures. Throughout the cyclic loading, dissipated energy consistently accounts for less than 30 % of the total input energy across all stress levels. The linear energy storage coefficient remains independence from stress level and cycle number, but exhibits an inverse relationship with confining pressure. There is an obvious linear relationship between rock dissipation energy and AE energy. Higher AE energy indicate that the rock dissipates more strain energy.