The presence of faults in the subsurface, crossing geothermal targeted aquifers introduces concerns related to induced seismicity. Improving the understanding of geomechanical processes due to geothermal operations is therefore important. Previous research has indicated that thermally induced stresses can have a significant effect on fault stability, especially in the long term. In depleting gas fields, the effect of fault offset has been studied and found to promote the onset of fault reactivation. However, little research has been conducted on this effect in geothermal reservoirs. This thesis will investigate the effect of juxtaposition of sandstone and claystone across faults on thermo-elastic stress changes and induced fault slip in geothermal projects targeting porous sandstone reservoirs.
This research question will be answered by constructing three different model geometries in a semi- analytical 2D plane-strain model assessing thermo-elastic stress changes and fault slip. The geology and properties of the Pijnacker Geothermie reservoir, located in the West Netherlands Basin and targeting the Delft Sandstone Member as its reservoir rock are used for the input of the model. The three geometries include a model scenario in which sandstone is fully juxtaposed against sandstone, a scenario in which sandstone is fully juxtaposed against claystone and lastly, a scenario with half-a-reservoir-thickness fault offset, juxtaposing sandstone against both sandstone and claystone. These geometries were chosen to represent a fault that experiences cooling on both sides, on a single side and on both sides, but with an offset. A comparison was made between the three model scenarios using base cases and evaluating the differences in stress changes and fault slip. A sensitivity analysis was performed in order to determine the key parameters influencing the results the most. Then, three metrics were extracted from the base cases and plotted against the varying key parameters. These metrics are: the temperature change needed to reactivate the fault, the maximum slip and the slip patch length. Lastly, heterogeneous friction was used in the model in order to assess its effect and make a step towards a slightly more realistic model. The results showed that the double sided cooling case with fault offset was the most destabilizing configuration, needing the least amount of cooling before reaching the onset of fault reactivation and also consistently experiencing the largest maximum slip. The single sided cooling model scenario is the most stable, however its slip patch length is the most sensitive to variations in key parameters, generating the largest slip length for destabilizing values of the key parameters. The inclusion of heterogeneous friction had a destabilizing effect on all scenarios. As the degree to which sandstone is juxtaposed against sandstone affects the amount of thermo-elastic stress changes and slip a fault experiences, it should be considered to include fault offset when assessing fault slip in geothermal projects. Not including fault offset will likely underestimate the thermo-elastic stress changes and subsequent induced fault slip. Panther can capture the effect of thermo-elastic stress changes in a first-order assessment on a simple reservoir geometry containing the essential parameters while avoiding extra model complexity. Further improvements to the model can be made to capture the effect of certain aspects that have been proven to affect thermo-elastic stress changes. These include property contrasts between the reservoir and surrounding rock as well as the addition of clay-rich layers in the sandstone formation and vice versa. Further geological work could be performed to decrease the uncertainty around the location of
the fault and its dip angle.

This study investigates the impact of the viscous properties of the Zechstein formation on the state of stress and (a)seismic slip in depleting gas reservoirs. I have put the focus on the Rotliegend and Zechstein-2 Carbonate gas fields in the Northeast Netherlands. Six geomechanical models (four Rotliegend and two Carbonate) different were created in Plaxis, representing a fault in various gas fields. For the Carbonate models and two Rotliegend models, the halite is juxtaposed to the reservoir, while for the other two Rotliegend models, the halite is not juxtaposed to the reservoir. The models were simulated with both elastic and viscous properties for the Zechstein halite. Deformations and stresses were calculated around and on the fault using the finite element method.

The viscous property led to a significant increase in strain deformation in the halite and the reservoir juxtaposed to the halite, resulting in a greater compaction in the reservoir next to the halite. The change in strain in other layers was limited with the viscous property.
With depletion the stress ratio decreased in the reservoir and juxtaposed layers, increasing the risk of fault reactivation. In the layers above and below the reservoir, the stress ratio increased, moving away from criticality. While the pattern of stress change with depletion was similar for both elastic and viscous models, the magnitude differed.
For the viscous model, the stress ratio in the layers next to the halite are larger with respect to the elastic model. The high horizontal stresses of the halite significantly decreased the horizontal stresses and, consequently, the stress ratio in the Basal Zechstein below the Halite. Deeper in the reservoir, the decrease in vertical stress between the elastic and viscous was larger than the decrease in horizontal stress, resulting in a slightly larger stress ratio for the viscous model, moving the fault away from criticality. In some models, a slip patch was identified for the elastic model but not for the viscous model. Therefore, it can be concluded that a larger pore pressure depletion is needed for fault reactivation.
The difference in stress and slip between the elastic and viscous model were more pronounced when the halite was juxtaposed to the reservoir. Smaller stress changes and differences between the elastic and viscous were observed for the Carbonate reservoir compared to the Rotliegend reservoirs, due to the greater resistance to deformation. This results in a lower risk of fault reactivation with reservoir depletion. Fault reactivation is also not identified for the Carbonate models.

Using a field classification, I have assessed the likelihood that a gas field caused a seismic event. I have put the focus on all of the minor gas fields located in the Rotliegend sandstone formation in the Netherlands. These gas fields were divided into five geologically distinctly different regions: the North Holland Platform (NHP), the Groningen Platform (GRO), the Lauwerszee Trough High Pressures (LTHP), the Frisian Platform (FP) and the Lauwerszee Trough (LT). I have investigated which reservoir factors are most influential in deciding whether or not seismicity occurs. The stress ratio has proven to be the most critical parameter. Then, for the stress ratio, I have computed the stress ratio range for each region at which failure will occur. Above the stress ratio, seismic activity will occur, and below the stress ratio, no seismic activity will occur. To answer the research question "Can the reservoir properties of small gas fields in the Netherlands predict the occurrence of seismicity?", I have taken the following steps.
First, I have examined the probability of all small Dutch gas fields being responsible for a seismic event. Based on the distance between the gas field and the nearest event, as well as the presence of other fields in the surrounding area, I have derived a classification for the likelihood that the field was associated with induced seismicity. Second, I have run a sensitivity analysis to identify which parameter was most significant. I have accomplished this by implementing a semi-analytical model that computed and depicted depletion-induced stresses and fault slip along an inclined fault. The model calculated the depletion pressure at which seismic slip starts to occur, here called the onset pressure, based on reservoir data and fault characteristics. The reservoir data contains the compaction coefficient, critical slip weakening distance, dip angle, dynamic friction coefficient, initial reservoir pressure, Poisson's ratio, porosity, reservoir depth, reservoir thickness, shear modulus, static friction coefficient and stress ratio while the fault characteristics included the absolute fault offset and whether the fault is bounding or not.
Afterwards, I have plotted the onset pressure versus the relative fault offset and assessed the sensitivity of the onset pressure whilst changing input parameter settings, namely the critical slip weakening distance, dip angle, dynamic friction coefficient, initial reservoir pressure, Poisson's ratio, porosity, reservoir thickness, shear modulus, static friction and stress ratio. The most critical parameter has turned out to be the stress ratio. I have examined the stress ratio ranges for each field in order to assess whether this parameter could predict the occurrence of seismicity for an entire region. Some fields that do not suit the optimal regional stress ratio have been considered anomalies and investigated further. The main explanations for the anomalies have been geological complexity, assumed synthetic fault offsets, over- or underestimated offsets, unregistered seismic events and substantial overpressure.
The answer to the research question is, yes, it is possible to predict whether seismicity will occur in a small Dutch gas field located in the Rotliegend formation based on reservoir data and fault characteristics. A regional stress ratio has been determined for each of the five regions. The regional stress ratio is 0.51 for NHP, 0.58 for Groningen, 0.69 for LTHP, 0.46 and 0.47 for FP and 0.50 and 0.56 for LT. Salt layers have most likely contributed to higher regional stress ratios for GRO, LT and LTHP.