@unpublished{epos2289,
           title = {Coupled multi-phase hydromechanical modeling of induced seismicity associated with geo-energy applications},
          school = {ETH Zurich},
          author = {Dominik Zbinden},
            year = {2020},
             url = {https://episodesplatform.eu/eprints/2289/},
        abstract = {Industrial activities to extract deep underground energy resources can induce earthquakes. Such induced seismicity can jeopardize the public acceptance of associated
industrial projects and, with increasing size, cause damage to infrastructure and pose a
risk to the population. It is therefore necessary to develop methods to better control
and mitigate the seismicity. In order to achieve this, the physical mechanisms leading to
induced earthquakes need to be studied more accurately. For instance, it is known that
the injection or withdrawal of ?uids causes changes in stress and pore pressure in the
subsurface, which can lead to the reactivation of faults. However, various processes are
still poorly understood, especially when multiple ?uid phases are present. Since most
of the processes are hidden underground, physics-based numerical models can help to
improve the understanding of the interactions between rock and ?uids underground.
In this thesis, we use numerical models to simulate the physical processes involved in
different geo-energy projects. Firstly, we model gas extraction from a compartmentalized
reservoir intersected by a fault to more accurately understand the processes leading to
production-induced seismicity. The results show that gas production causes poroelastic
stress changes and gas ?ow promoting fault reactivation. Simulations aimed at avoiding
induced earthquakes indicate that shut-in of the production well and gas reinjection
into the depleted reservoir compartment can stabilize the fault, while injection into
the adjacent compartment may not be successful in preventing the fault from being
reactivated.Secondly, we numerically model the induced seismicity in the deep geothermal
project in St. Gallen, Switzerland, conducted in 2013 . After a stimulation phase, a gas
kick forced the operators to inject ?uids to combat the kick, followed by hundreds
of minor seismic events and a ML 3 . 5 earthquake. The induced seismicity occurred
several hundreds of meters away from the well and may have been in?uenced by multi-
phase ?uid interactions during the gas kick, which precludes a simple interpretation
of the occurrences. We ?rst attempt to determine the predominant mechanism that
led to fault reactivation: poroelastic or direct pressure effects. The results show that
changes in Coulomb stress can be three orders of magnitude greater in the case of a
permeable hydraulic connection, i.e., when the injected ?uid directly pressurizes the
fault. In combination with multiple independent observations, we conclude that the
direct pressure effect was the predominant mechanism in the St. Gallen reservoir. We
then model the gas kick, the subsequent injection and the induced seismicity. Assuming
the gas to be initially sealed by the fault and released after fault reactivation, our model
is successful in reproducing the spatio-temporal evolution of the induced seismicity.
The model suggests that the gas could have contributed signi?cantly to enhance the
induced seismicity.Although the interaction between gas ?ow and seismicity is certainly speci?c in
St. Gallen, the release of overpressurized gas has been suggested as a cause of after-
shocks in naturally occurring earthquake sequences. Therefore, thirdly, we simulate
the effect of single- and multi-phase overpressurized in-situ conditions on the timing
and magnitude of induced earthquakes. On the one hand, the results show that such
conditions can lead to earlier fault reactivation. On the other hand, while the size of
the induced earthquakes increases when overpressurized gas prestresses the fault, an
initially overpressurized reservoir does not result in a larger event.
The overall results of the thesis show that multi-phase ?uid ?ow can have a strong
in?uence on induced earthquakes during both ?uid injection and production operations.
Therefore, in future geo-energy projects, the potential in?uence of gas should be
adequately taken into account.}
}