Fluid injection-induced seismicity considering secondary damage and heterogeneity in the surrounding rock

Rock is a heterogeneous material with primary damage and defects, which can greatly affect the mechanical properties of the rock and the slip on a fault. Additionally, slip on a fault can generate secondary damage in the surrounding rock. Therefore, this paper focuses on investigating injection-induced seismicity considering the heterogeneity and secondary damage in surrounding rocks with the FEM-based numerical code COMSOL. First, a heterogeneous model was established using the digital image processing technique, and the elemental microscale parameters were determined through comparison with testing results from a homogeneous model. Subsequently, using the defined damage variable and the rate and state friction law, numerical modeling was performed with the established homogeneous and heterogeneous models while considering the heterogeneity, fluid pressure, and generated secondary damage. The results showed that fluid pressure and heterogeneity can significantly influence injection-induced earthquakes. With increasing fluid pressure, the initial time for shear stress drop decreases, both the stress drop and the area of secondary damage increase, and the probability of unstable slip increases in a homogeneous rock matrix. Compared with the homogeneous numerical model, the heterogeneous model has a reduced time, a higher stress drop, a higher probability of seismicity, and a larger area of secondary damage. In addition, secondary damage is generated at two ends of the fault. The area of the secondary damage zone increases when unstable slip is induced, and the rate increases with slip velocity. The findings in this paper could facilitate better understanding of the mechanisms of fluid injection-induced seismicity and hence may be helpful for predicting, evaluating, and controlling induced seismicity.