  Underground gas storage (UGS) is a worldwide well-established technology that
is becoming even more important to cope with seasonal peaks of gas consumption
due to the growing uncertainties of the energy market. Safety issues concerning
the reactivation of pre-existing faults might arise if the target reservoir is
located in a faulted basin, where human activities can trigger
(micro-)seismicity events. In the Netherlands, it has been observed that fault
activation can occur somehow "unexpectedly" after the primary production (PP),
i.e., during cushion gas injection (CGI) and UGS cycles, when the stress regime
should be in the unloading/reloading path. To understand the physical
mechanisms responsible for such occurrences, a 3D mathematical model coupling
frictional contact mechanics in faulted porous rocks with fluid flow is
developed, implemented and tested. The final aim of this two-part work is to
define a safe operational bandwidth for the pore pressure range for UGS
activities in the faulted reservoirs of the Rotliegend formation. Part I of
this work concerns the development of the mathematical and numerical model of
frictional contact mechanics and flow in faulted porous rocks. A mixed
discretization of the governing PDEs under frictional contact constraints along
the faults is used. A slip-weakening constitutive law governing the fault
macroscopic behavior is also presented. The model is tested in the setting of
an ideal reservoir located in the Rotliegend formation. The analyses point out
how fault reactivation during PP can lead to a stress redistribution, giving
rise to a new equilibrium configuration. When the fault is reloaded in the
opposite direction during the CGI and/or UGS stages, further activation events
can occur even if the stress range does not exceed either the undisturbed
initial value or the maximum strength ever experienced by the formation.
