Zbinden, Dominik (2020) Coupled multi-phase hydromechanical modeling of induced seismicity associated with geo-energy applications. PhD thesis, ETH Zurich.
Full text not available from this repository.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 fluids 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 fluid 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 fluids 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 flow 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 fluids 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 influenced by multi- phase fluid interactions during the gas kick, which precludes a simple interpretation of the occurrences. We first 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 fluid 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 significantly to enhance the induced seismicity.Although the interaction between gas flow and seismicity is certainly specific 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 fluid flow can have a strong influence on induced earthquakes during both fluid injection and production operations. Therefore, in future geo-energy projects, the potential influence of gas should be adequately taken into account.
| Item Type: | Thesis (PhD) |
|---|---|
| Subjects: | Methodology > Other-additional study Region > Switzerland > St. Gallen Inducing technology > Geothermal energy production |
| Project: | S4CE > ST GALLEN: geothermal project |