Stress Levels Control Microstructural Damage in Mudstone Caprocks

Researchers have identified the maximum cycling stress as the primary factor controlling microstructural damage in mudstone caprocks. This finding, derived from a numerical study published in Geomechanics and Geophysics for Geo-Energy and Geo-Resources, shifts the focus from macroscopic sealing failures to the underlying microstructural mechanisms that determine caprock integrity. The study, led by Xueying Lyu and colleagues, utilized a Particle Flow Code (PFC) numerical model calibrated by rock mechanics experiments to investigate how stress levels and loading-unloading cycles affect the internal structure of mudstone. By taking the PG2 underground gas storage (UGS) in the Bohai Bay Basin as a case study, the team provided a quantitative basis for analyzing caprock stability and designing operational parameters for gas storage projects.

The motivation for this research stems from a gap in previous studies, which largely focused on macroscopic sealing failure without systematically quantifying microstructural damage. The authors noted that the specific mechanisms governing this damage, particularly under varying stress levels and loading-unloading cycles, remained unclear. To address this, the study developed a numerical model that simulates the microstructural evolution in mudstone caprocks.

The results of the numerical simulations indicate that the maximum cycling stress exerts primary control over microstructural damage, rather than the minimum cyclic stress or the number of loading-unloading cycles. This conclusion is supported by three key indicators: particle displacement velocity, contact force chain strength, and the number of microcracks. An increase in the maximum cyclic stress was found to significantly contribute to the propagation of microcracks and internal damage. While a decrease in the minimum cyclic stress and a higher number of loading-unloading cycles also contribute to damage, the maximum stress level emerged as the dominant factor.

The study further proposes a quantitative characterization model for microstructural damage, based on the relationship between crack density and strain. This model provides a predictive tool for assessing the service life of mudstone caprocks under specific operational conditions. According to the model’s predictions, 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 model predicts that mudstone caprocks can withstand a maximum cyclic stress ratio of 0.96 when the number of loading-unloading cycles reaches 60. This finding suggests that there is a critical threshold beyond which the caprock’s sealing capacity is compromised. The study emphasizes that once the cycle count exceeds this critical threshold, the accumulated microstructural damage can lead to the failure of the mudstone caprock’s sealing function.

The findings of this study have important implications for the design and operation of underground gas storage facilities. By identifying the key factors controlling microstructural damage, the research provides a novel, quantitative basis for caprock stability analysis.

Subject of Research: Microstructural damage mechanisms and quantitative characterization in mudstone caprocks for underground gas storage

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

Article References: Lyu, X., Yun, L., Su, Y.-N., Zhang, J., & Xu, J.-G. (2026). The key factors controlling the microstructural damage and its quantitative characterization in mudstone caprocks. Geomechanics and Geophysics for Geo-Energy and Geo-Resources. https://doi.org/10.1007/s40948-026-01241-4

Image Credits: AI Generated

DOI: 10.1007/s40948-026-01241-4

Keywords: mudstone caprocks, underground gas storage, microstructural damage, Particle Flow Code, cyclic stress, caprock stability, geomechanics, Bohai Bay Basin, crack density, loading-unloading cycles, factors, controlling