  Underground gas storage is a versatile tool for managing energy resources and
addressing pressing environmental concerns. While natural gas is stored in
geological formations since the beginning of the 20th century, hydrogen has
recently been considered as a potential candidate toward a more flexible and
sustainable energy infrastructure. Furthermore, these formations can also be
used to sequester environmentally harmful gases such as CO2 securely. When such
operations are implemented in faulted basins, however, safety concerns may
arise due to the possible reactivation of pre-existing faults, which could
result in (micro)-seismicity events. In the Netherlands, it has been recently
noted that fault reactivation can occur "unexpectedly" during the life of an
underground gas storage (UGS) site, even when stress conditions are not
expected to cause a failure. The present two-part work aims to develop a
modeling framework to investigate the physical mechanisms causing such
occurrences and define a safe operational bandwidth for pore pressure variation
for UGS operations in the faulted reservoirs of the Rotliegend formation, the
Netherlands. In this paper, we investigate in detail the mechanisms and crucial
factors that result in fault reactivation at various stages of a UGS project.
The mathematical and numerical model described in Part 1 is used, also
accounting for the effect of geochemical dissolution on reservoir and caprock
weakening. The study investigates the risks of fault activation caused by the
storage of different fluids for various purposes, such as long-term CO2
sequestration, CH4 and N2 injection and extraction cycles, and N2 permanent
storage. The results show how geomechanical properties and reservoir operating
conditions may increase the risk of fault reactivation at various UGS stages.
Finally, operational guidelines for improving secure storage operations are
presented.
