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 Frigg Sand oil reservoir in the Nini field, offshore Denmark. The mechanical stability of the Frigg Sand reservoir, shale caprock and key intersecting faults was investigated using a field-scale coupled finite element model built for the target storage reservoir, taking into account the forecasted CO2 injection plan. The analysis shows that the calculated threshold pressure to initiate tensile failure in the caprock is considerably above the planned CO2 injection 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 for reactivation in the reservoir with high injection pressures. Overall, the most likely mode of failure is possible tensile failure in the caprock and this defines the safe operational limit for the Frigg Sand reservoir. Thorough analyses using a field-scale finite element simulation coupled with the reservoir flow dynamic simulations helped to capture the risk associated with the CO2 injection operation and to optimize the injection plan. 1. INTRODUCTION Project Greensand is a geological CO2 storage project located in offshore Denmark. The storage layer is the depleted Frigg Sand reservoir approximately 1800 m below the seabed in the Nini field. The Frigg Sand reservoir has a potential storage of 0.5 million tonnes of CO2 per year by 2026 with an expected storage capacity of 5.0 million tonnes. Modeling and monitoring the subsurface behavior throughout the CO2 injection is key to a permanent, efficient and secure storage. Subsequently, as part of the injection design, a comprehensive geomechanical analysis was carried out to investigate the impact of the CO2 pressure on the caprock and fault stabilities near the injection zone and estimate the upper pressure limit of the CO2 injection.