The pore structure of caprock plays an important role in underground gas storage security, as it significantly influences the sealing capacity of caprock. However, the pore structure evolution of caprock with the cyclic stress perturbations triggered by the cyclic gas injection or extraction remains unclear. In this study, the pore structure changes of mudstone caprock under cyclic loading and unloading were obtained by the nuclear magnetic resonance (NMR) tests system, then the influence of the changes on the breakthrough pressure of caprock was discussed. The results indicated that the pore structure changes are depending on the stress loading-unloading path and stress level. In the first cyclic, at the loading stage, with the increase of confining stress, the NMR T₂ spectrum curve moved to the left, the NMR signal amplitude of the first peak increased, while the amplitude of the second peak decreased gradually. This indicated that the larger pores of mudstone are compressed and transformed into smaller pores, then the number of macropores decreased and the number of micro- and mesopores increased. For a certain loading-unloading cycle, the porosity curve of mudstone in the loading process is not coincide with that in the unloading process, the porosity curve in the loading process was located below that in the unloading process, which indicated that the pore structure change is stress path dependent. With the increase of cycle numbers, the total porosity shown an increasing trend, indicating that the damage of mudstone occurred under the cyclic stress load-unload effects. With the increase of porosity, the breakthrough pressure of mudstone decreased with the increase of the cyclic numbers, which may increase the gas leakage risk. The results can provide significant implication for the underground gas storage security evaluation.

盖层孔隙结构对地下储气库安全起着重要作用，它显着影响盖层的密封能力。然而，循环注气或抽采引发的循环应力扰动下盖层孔隙结构演化仍不清楚。本研究利用核磁共振(NMR)测试系统获取循环加卸载下泥岩盖层的孔隙结构变化，探讨这些变化对盖层突破压力的影响。结果表明，孔隙结构的变化取决于应力加载-卸载路径和应力水平。在第一个循环加载阶段，随着围压的增加，NMR T ₂谱曲线向左移动，第一个峰的NMR信号幅度增加，而第二个峰的幅度逐渐减小。这表明泥岩中较大的孔隙被压缩并转化为较小的孔隙，大孔隙数量减少，微孔隙和中孔隙数量增加。对于一定的加卸载循环，泥岩在加载过程中的孔隙度曲线与卸载过程中的孔隙度曲线并不重合，加载过程中的孔隙度曲线位于卸载过程中的下方，表明孔隙结构发生了变化是应力路径依赖的。随着循环次数的增加，总孔隙度呈增加趋势，说明泥岩在循环应力加卸载作用下发生了破坏。随着孔隙率的增加，泥岩突破压力随循环次数的增加而降低，可能增加漏气风险。研究结果可为地下储气库安全评价提供重要启示。