Dynamic Fluid Flow and Geomechanical Coupling to Assess the CO2 Storage Integrity in Faulted Structures

Abstract

The SiteChar research on the Southern Adriatic Sea site focused on the investigation of the geomechanical and hydrodynamic behaviour of the storage complex in the case of CO2 injection in a reservoir consisting of fractured carbonate formations. Special attention was paid to the effects that natural faults and fractures might have on CO2 migration, and the effects that injection might have on the stability of faults. This assessment was originally performed via a hydro-geomechanical one-way coupling which relies on an adequate representation of faults in the model, allowing one to simulate fluid flow along the fault plane and inside faults as well as evolution of the stress state due to CO2 injection. The geological model was populated with petrophysical and geomechanical parameters derived either from laboratory measurements performed on samples from a reservoir analogue, or published literature. Since only sparse data were available, various scenarios were simulated to take into account the uncertainties in the fluid flow and geomechanical properties of the model: the different state of faults (i.e., open or closed) and various in situ stress state, commonly named geostatic stresses as the earth’s crust deformation is assumed to be slow regarding the short-term study. Various fluid flow parameters were also considered, although only one set of petrophysical data corresponding to the most realistic ones is considered here. Faults modeled as volumetric elements behave as flow pathways for fluids when they are conductive. The injected CO2 migrates inside and through the Rovesti fault, which is located near the injection well. The fluid flow also induces overpressure in the faults. The overpressure in the Rovesti fault reaches 2.2 MPa while it reaches 4.4 MPa at the bottom hole of the injector. Extending to about 30 km, the pore pressure field reaches the Gondola fault located at 15 km from the injection zone but the overpressure does not exceed 0.1 MPa at such a distance from the injection well. Using this overpressure as loading in the geomechanical model allows one to compute the effective stress variation in the whole geological model. The total effective stress is then computed by adding an estimation of the regional stress. Post-processing is performed to derive the likely damage of the faults according to the Mohr-Coulomb criterion. The results are illustrated on the Rovesti fault, which is located near the injection well and consequently the most likely to be reactivated. On the basis of available data, for all the modeled scenarios (various initial stress regimes, closed or open fault), no fault damage is observed, as the stress state stays below the Mohr-Coulomb criteria.