Microstructural evolution and damage development of interlayers in CAES salt caverns: A low-field NMR investigation

The interlayer within compressed air energy storage (CAES) salt caverns represents a critical geological weak zone, whose microstructural evolution is pivotal to the long-term stability and airtightness of the cavern. In this study, uniaxial fatigue tests were conducted on the interlayer samples, while low-field nuclear magnetic resonance (NMR) technology was applied to non-destructively monitor microstructural variations. The results indicated that increasing maximum upper limit stresses promoted the evolution of the pore structure, with a threshold effect observed at a maximum upper limit stress ratio of 0.40. Below this threshold, pore structure complexity increased due to compaction, accompanied by simultaneous reductions in porosity and permeability. Above this threshold, pore expansion dominated the microstructural evolution, characterized by enlarged pore diameters, reduced structural complexity, and enhanced seepage capacity. With increasing loading cycles, pore structure complexity exhibited a two-stage evolution: an initial increase followed by a subsequent decrease. Porosity and permeability displayed a non-linear trend characterized by an initial slow decline, a rapid decline, and a subsequent slow increase. Notably, when the number of cycles reached 600, the sensitivity of pore structure evolution to further cycling declined markedly. Considering the varying contributions of pore size heterogeneity to damage progression, a pore-size-weighted damage model was proposed. Under the applied uniaxial cyclic loading conditions, the pore–size–weighted damage variable exhibits a two-stage evolutionary pattern and progressively approaches a quasi-stable state. These observations represent laboratory-scale behavior and offer preliminary insights into the long-term mechanical response of interlayers under cyclic loading.