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.