Geomechanical Analysis of Caprock Integrity and Fault Stability for Greensand CO2 Storage Project Feasibility

Abstract A comprehensive geomechanical assessment is carried out to investigate the upper pressure limit of CO2 injection for the depleted oil Frigg Sand reservoir in Nini field, offshore Denmark. The mechanical stability of the Frigg Sand reservoir, shale caprock and key intersecting faults were investigated under the forecasted CO2 injection plan using a field-scale coupled finite element model built for the target storage reservoir. The field-scale geomechanical model is built by preparing a 3D grid which covers the entire Greensand field structure including the basement and overburden up to the seabed. The geomechanical properties and parameters are constrained using the well-centric data and propagated in 3D grid using geostatistical approach combined with surface seismic data. A finite element model is prepared and coupled with the reservoir flow dynamic model for the interested zone of the studied area. The results of the finite element model are used to investigate the integrity of the caprock and stability of the faults over the life of the field. The results of the finite element simulation shows that the calculated minimum principal stress does not change its state to tension in any part of the field. However, a reservoir pressure above the minimum principal stress magnitude in base of the caprock may increases the risk of failure in the caprock. Hence, considering the uncertainty for the lower bound of the minimum principal stress, the risk of failure in the caprock is low until the Frigg Sand reservoir pressure exceed the estimated threshold pressure. The results also show that there is no risk of shear failure in the caprock for the entire studied area. The fault stability analysis shows that no risk of fault reactivation exists in the overburden. However, some faults may reach the critical stress state in the reservoir with high injection pressures. A reactivation in the reservoir layer may extend to the overburden and increase the risk of caprock integrity failure. Overall, the most critical mode of failure is the possible tensile failure which defines the safe operational limit for the Frigg Sand reservoir. The thorough analysis using field scale finite element simulation coupled to the reservoir flow dynamic helped to capture the risk associated to the CO2 injection operation and optimize the injection plan.