Geomechanical simulation of underground gas storage in a faulted reservoir: a case study in northern Italy

Underground gas storage (UGS) is a very common practice currently adopted to meet the growing energy demand, especially in North America and Europe. The gas is stored in reservoirs through a seasonal cycle of injection and removal. Factors such as the reservoir depth, the pressure change, and the geomechanical properties of the soil control the expansion and contraction of the reservoir and, consequently, the movements of the land surface. Furthermore, faults may exist and, as a result of pressure variations, they might be reactivated, raising several safety and economical issues due to the induced seismicity phenomena. For these and many other reasons, a geomechanical simulation of the soil response during UGS activities is of paramount importance. In this work, the simulation of a case study in northern Italy is performed applying a one-way coupled modeling approach with the Finite Element method that is used to solve the partial differential equations of the flow and the equilibrium. The geomechanical simulator is calibrated using land movements provided by an interferometric synthetic aperture radar (InSAR) technique. The aim of the study is the analysis of the geomechanical response of the system mainly during the storage cycles, with a particular attention to the possible fault reactivation. The model outcomes show that the displacements associated to the UGS cycles do not exceed 4 mm, both in the vertical and horizontal directions, and are concentrated above the center of the reservoir. Moreover, the maximum displacements will remain limited to 5 mm also in scenarios of future development, when the pressure change during the injection-production phases will be larger than those experienced during the last years. The stress field change caused by the UGS activity does not propagate more than 20 m above the top of the unit used for UGS activities, and also the horizontal propagation within the aquifer hydraulically connected to the reservoir is limited. The safety factor for the regional faults remains equal to 0.1, in a range [0,1] where 0 is the safest condition, leading to the conclusion that UGS activities in this reservoir cannot be responsible of induced seismicity.