Assessing CO₂ Leakage Risks and Fault Stability: A Coupled Flow and Geomechanics Simulation with Uncertainty Analysis of Fault Properties

This study investigates coupled flow and geomechanics simulations of carbon dioxide (CO₂) injection within a hypothetical saline aquifer. We intentionally set a scenario to enable fluid migration and fault reactivation in the synthetic model of the SEG Advanced Modeling Corporation (SEAM) CO₂ project. The numerical model consists of a main turbidite aquifer, along with several geological layers including a capping shale layer, middle, as well as upper aquifers. In addition, two faults, western and eastern, cut across the section striking North-South. We assume the western fault is a sealing fault while the eastern side has either no fault or is an open fault with a high-permeability damage zone. Our modeling approach employs fine grids near the faults to explicitly represent the fault's 6-m thin damaged zone. We also use a Mohr-Coulomb failure envelope for nonlinear plastic analysis into a two-way coupling scheme. We undertook a stochastic analysis to quantify the impact of fault properties on their post-failure behavior. Using the stress path along the fault plane over time, we show that the characteristics of faults can significantly affect areas of reactivation. Assuming uniform fault properties, an increase in pore pressure can cause shear failure at the upper or lower parts of the fault where it intersects with the target aquifer, because of a significant increase in shear stress. Sensitivity analysis of various parameters, suggests a time evolution of the fault response, with an early emphasis on parameters like cohesion and friction angle affecting reactivation timing. Over time, the permeability of the damage zone gains prominence, influencing the extent of fault reactivation due to its effect on the pressure propagation of the damage zone. Coupled flow and geomechanics simulation can provide useful insights into establishing robust measurement and monitoring plans to mitigate fault reactivation and fluid migration through faults. While the choice of the Mohr-Coulomb model was consistent with our scenario-based approach to "magnify" the effects of fluid migration for this study, we note that in real field situations, other fit-for-purpose plasticity models with different fault permeability evolution may be more appropriate.