Characterizing imbibition and void structure evolution in damaged rock salt under humidity cycling by low-field NMR

Abstract

The excavation damaged zone (EDZ) of salt cavities for compressed air energy storage (CAES) undergoes daily humidity cycling. This study investigated moisture imbibition and its impact on void structures in differently damaged rock salt in a cyclic humidity environment using periodic low-field nuclear magnetic resonance (NMR) tests. NMR tests were also conducted on centrifuged and saturated specimens to assist with the analysis. Fluid-accessible voids in rock salts encompass mesovoids (2–50 nm) and macrovoids (>50 nm). In the wet stage of the first cycle, imbibition in the mesovoids lagged behind that in the macrovoids, whereas dehydration in the mesovoids was more pronounced and rapid in the subsequent dry stage than in the macrovoids. Throughout the long-term treatment, imbibition persisted, and the macrovoid volume in all three specimens notably decreased, whereas those of the mesovoids slightly increased. Collectively, these effects significantly reduced the fluid-accessible porosity. In our interpretation, the greater viscous force in the mesovoids impeded their imbibition and brine supplementation, resulting in a difference in the tests of the first cycle. The continuous imbibition of brine activated two self-healing mechanisms in the macrovoids: diffusion mass transfer and recrystallization. These mechanisms transformed consecutive macrocracks into isolated voids filled with recrystallized microstructures, effectively transforming macrovoids into mesovoids. In summary, this study characterized the complex imbibition and void structure evolution affected by self-healing mechanisms of rock salt under humidity cycling, offering new insights for evaluating the long-term stability and integrity of CAES salt cavities.