Effects of fault-zone architecture on earthquake magnitude and gas leakage related to CO2 injection in a multi-layered sedimentary system

The presence of fluid within a fault zone can cause overpressure and trigger earthquakes. In this work, we study the influence of fault-zone architecture on pore pressure distribution and on the resulting fault reactivation caused by CO2 injection. In particular, we investigate the effect of the variation and distribution of lithological and rock physical properties within a fault zone embedded in a multi-layer sedimentary system. Through numerical analysis, we compare several models where the complexity of the fault-zone architecture and different layers (such as caprock and injection reservoir) are incrementally included. Results show how the presence of hydraulic and mechanical heterogeneity along the fault influences the pressure diffusion, as well as the effective normal and shear stress evolution. Hydromechanical heterogeneities (i) strengthen the fault zone resulting in earthquakes of small magnitude, and (ii) impede fluid migration upward along the fault. We also study the effects of the caprock and aquifer thickness on the resulting induced seismicity and CO2 leakage, both in heterogeneous and homogeneous fault zones. Results show that a thin caprock or aquifer allows smaller events, but a much higher percentage of leakage through the caprock and into the upper aquifer. The amount of leakage reduces drastically in the case of a multi-caprock, multi-aquifer system.