3D geomechanical modeling for CO2 geological storage in faulted formations. A case study in an offshore northern Adriatic reservoir, Italy

One of the six CO2 carbon capture and storage (CCS) demonstration projects recently selected within the European Energy Programme for Recovery (EEPR) is located in Italy. In the framework of the feasibility study, the selection of a geological formation suitable to store the required 1 Mt/yr of CO2 over 10 years and the safety of the CO2 disposal are two major issues. In the present modeling study, we investigate the role played by geomechanics in assessing the maximum CO2 amount that can be sequestered into a 2000 m deep multi-compartment reservoir seated in the off-shore northern Adriatic sedimentary basin. We use a three-dimensional finite element-interface element geomechanical model to simulate the possible mechanical failure in both the injected formation and caprock, the fault reactivation, and the ground surface displacement. The faulted geological structure is reproduced based on detailed seismic surveys, with petrophysical/geomechanical properties based on the several well-logs available from several oil/gas explorations in the area. The pore pressure distribution due to two injection wells is provided by a fluid-dynamic simulator and a sensitivity analysis is carried out to investigate the role of the major uncertainties in the geomechanical setting. The modeling results suggest that a safe and permanent containment may be secured over a few years only. Afterwards, mechanical failure by shear stress is likely to be experienced by a significant portion of reservoir's injected compartments. Shear failure and fault reactivation can occur much before attaining the hydraulic fracturing pressure, hence represent two major issues in assessing the maximum allowable CO2 injection overpressure.