About geomechanical safety for UGS activitiesin faulted reservoirs
P. Teatini, C. Zoccarato, M. Ferronato, A. Franceschini, M. Frigo, C. Janna
Dept. of Civil, Environmental and Architectural Engineering, University of Padova, Padova, Italy
G. Isotton
M3E Srl, Padova, Italy
ABSTRACT
A critical issue concerning geomechanical safety for UGS (underground gas storage) in
compartmentalized reservoirs is fault reactivation. Indeed, the displacement (land subsidence,
land upheaval) and the stress fields caused by the seasonal injection and production of CH4
into and from deep reservoirs is peculiar. The need of improving our understanding of
compartmentalized reservoir behavior and to define safe bounds for the pressure fluctuation in
order to prevent undesired land movements and induced seismicity is becoming even more important.
This also in view of the expected energy transition when large amount of green energy will
potentially be stored and recovered through UGS of compressed air or hydrogen. In this
framework, an in-depth modelling investigation has been carried out for the typical UGS
geological setting and operations in The Netherlands. The specific goals of the study
are the following: (i) explaining the possible mechanisms responsible for seismic events
unexpectedly recorded during UGS phases; (ii) understanding which are the critical factors
(e.g. the geological configuration, the geomechanical properties, and the reservoir operations)
that increase theprobability of fault reactivation during the various UGS stages; and (iii)
advancing possible guidelines for safe UGS operations. This contribution summarizes the
main outcomes obtained by the modelling simulations: the combinations of factors causing
fault reactivation during primary production (PP) are also more prone to generate fault
failure during cushion gas injection (CG) and UGS. In fact, fault activation during PP
leads to a stress redistribution and a new (deformed) equilibrated configuration that is newly
loaded, in the opposite direction, when the pressure variation changes the sign because of CG
and/or UGS. Finally, the various combinations have been ranked to highlight the conditions where
the fault system is most likely reactivated during CG and UGS operations: the initial
stress regime of the system, the geomechanical properties of the fault, and dislocation of
the reservoir compartments are the major influencing drivers to fault instability.
