Coupled Large Scale Hydromechanical Modelling for Caprock Failure Risk Assessment of CO2 Storage in Deep Saline Aquifers

This work presents a numerical strategy of large scale hydromechanical simulations to assess the risk of damage in caprock formations during a CO2 injection process. The proposed methodology is based on the development of a sequential coupling between a multiphase fluid flow (TOUGH2) and a hydromechanical calculation code (Code_Aster) that enables us to perform coupled hydromechanical simulation at a regional scale. The likelihood of different caprock damage mechanisms can then be evaluated based on the results of the coupled simulations. A scenario based approach is proposed to take into account the effect of the uncertainty of model parameters on damage likelihood. The developed methodology is applied for the caprock failure analysis of deep aquifer of the Dogger formation in the context of the Paris basin multilayered geological system as a demonstration example. The simulation is carried out at a regional scale (100 km) considering an industrial mass injection rate of CO2 of 10 Mt/y. The assessment of the stress state after 10 years of injection is conducted through the developed sequential coupling. Two failure mechanisms have been taken into account, namely the tensile fracturing and the shear slip reactivation of pre-existing fractures. To deal with the large uncertainties due to sparse data on the layer formations, a scenariobased strategy is undertaken. It consists in defining a first reference modelling scenario considering the mean values of the hydromechanical properties for each layer. A sensitivity analysis is then carried out and shows the importance of both the initial stress state and the reservoir hydraulic properties on the caprock failure tendency. On this basis, a second scenario denoted “critical” is defined so that the most influential model parameters are taken in their worst configuration. None of these failure criteria is activated for the considered conditions. At a phenomenological level, this study points out three key aspects for risk management. The maximum overpressure is reached rapidly after a couple of years, the lateral extension of the “overpressurized” zone induced by the injection is very large (> 50 km) and the most critical zone is the injection near zone (distance < 100 m) at the interface between the caprock and the reservoir layer.