The key factors controlling the microstructural damage and its quantitative characterization in mudstone caprocks

Abstract Previous studies on mudstone caprock damage in underground gas storage (UGS) have largely focused on the macroscopic sealing failure, while the microstructural damage mechanism and the key controlling factors remain unclear. In particular, the microstructural damage has not been quantified systematically considering the stress levels and loading–unloading cycles. Therefore, this study developed a Particle Flow Code (PFC) numerical model calibrated by rock mechanics experiments to investigate the microstructural evolution in mudstone caprocks, taking the PG2 UGS in the Bohai Bay Basin as a case study. The results show that the maximum cycling stress, rather than the minimum cyclic stress or the loading-unloading cycles, exerts primary control over the microstructural damage in mudstone caprocks. Which can be reflected from the particle displacement velocity, the contact force chain strength and the microcrack number. An increase in maximum cyclic stress, a decrease in minimum cyclic stress, and a higher number of loading–unloading cycles will contribute to the propagation of microcracks and internal damage, ultimately leading to the failure of mudstone caprock sealing once the cycle count exceeds a critical threshold. Furthermore, a quantitative characterization model for microstructural damage was proposed, based on the relationship between crack density and strain. The model predicts that the maximum service life of mudstone caprocks is 160 cycles when the maximum cyclic stress ratio is 0.8 and the minimum cyclic stress ratio is 0.3. Additionally, the mudstone caprocks can withstand a maximum cyclic stress ratio of 0.96 when the loading–unloading cycles reaches 60. These findings provide a novel, quantitative basis for caprock stability analysis and operational parameter design in UGS.