Deciphering fluid extraction-induced earthquake nucleation in Groningen under rate-strengthening friction

Induced seismicity triggered by fluid injection or extraction has been studied extensively in recent years. However, models relying on a Mohr-Coulomb yield criterion for interseismic loading or using a linear slip-weakening friction law for dynamic earthquake rupture cannot quantify well how much aseismic slip accumulates prior to nucleation or how to explain nucleation. Instead, a rate-and-state friction law is extensively utilized in earthquake cycle models to resolve and understand earthquake nucleation. Moreover, laboratory experiments indicate that the relevant lithologies in the Groningen subsurface are velocity-strengthening under in-situ temperature, pressure and fluid chemistry conditions [1]. This property should in theory lead to a lower chance of earthquake nucleation, which makes it difficult to explain the occurrence of earthquakes in Groningen. We study how to explain earthquake nucleation under velocity-strengthening friction and how much aseismic slip can be expected. In this study, we model the normal-fault setup of the Groningen field under reservoir depletion with rate-and-state friction. Initial conditions are chosen to mimic healing over millions of years prior to gas production. We implement fault loading due to fluid pressure reduction and validate our loading stresses with analytical predictions in Jansen et al. [2]. We provide constraints on how much aseismic slip to expect during nucleation and evaluate its relevance to induced seismicity in Groningen. We systematically investigate scenarios with various fluid extraction rates and different rate-and-state friction properties (including rate-strengthening, rate-weakening and a mixture of both) of surrounding lithologies using constraints from laboratory observations. In this way we explore the rate of stress change needed for nucleation under rate-strengthening friction. Currently, we produced an event with slip rate below seismic rate. If seismic rates cannot be reached, we will add a second state variable describing cohesion weakening with time to assess how it affects earthquake nucleation. The impact of frictional property and stress rate to aseismic slip build-up and earthquake nucleation is compared to what is caused by varied reservoir off-set distance and fault dipping angle. Sequences of earthquakes and aseismic slips are studied to understand the long-term effect of fluid extraction, with the influence of the planned gas production termination taken into account. We find that during continuous fluid depletion earthquakes reoccur at increasing recurrence interval. Large dipping angle and relatively low Poisson ratio are necessary to achieve this if reservoir offset is zero. Slip or strain nucleation and distribution patterns produced by our models provide hints that can guide seismologists to identify aseismic slip from natural observations, which can in turn, support this study and constrain simulated fault properties. Ultimately, this will help to better understand the nucleation of induced seismicity with similar lithologies that are present across northwestern Europe and lead to a better understanding of the relevance of aseismic slip.

 

References

[1] Hunfeld, L. B., Niemeijer, A. R., & Spiers, C. J. (2017).  Journal of Geophysical Research: Solid Earth, 122(11), 8969-8989.

[2] Jansen, J. D., Singhal, P., & Vossepoel, F. C. (2019).  Journal of Geophysical Research: Solid Earth, 124, 7193– 7212.