Open PhD position! Scaling Friction from Single Grain to Aggregate Grain (Fault-Gouge) Contacts
Along with natural gas extraction, the subsurface of the earth is getting more and more important in the current period of energy and CO2 storage as well as the realization of renewable geothermal energy sources. Energy coming from sun and wind is a renewable but less stable source of energy, meaning that there are energy buffering needs. For example, salt caverns and natural gas reservoirs are currently considered for different storage purposes and could function as energy buffers for energy coming from renewable sources. Both the process of subsurface energy storage and extraction interfere with the load carrying capacity of the overbearing reservoir (fault) rocks. This has resulted in unwanted events such as induced seismicity, subsidence, and leakage. Specifically in the Groningen gas reservoir, in the Netherlands, the ‘induced’ seismicity is attributed to fault reactivation caused by gas extraction.
Earthquakes reflect the phenomenon of sudden slip when the frictional strength of faults in the Earth’s crust is overcome. Therefore, earthquakes being a stick-slip phenomenon, frictional instabilities play a crucial role in earthquakes, determining their size and peak slip velocity. Geo-mechanical models of earthquake slip typically use rate-and-state friction law, but are currently mainly phenomenological, lacking a solid physical foundation in many cases. The physics underlying the frictional phenomena in faults are highly relevant at the microscale. Some work has been done in developing microphysical models , but this topic is basically unexplored. Main reasons are the limited accessibility of the subsurface as well as the wide variations in length and time scales in studying earthquakes through experiments and on-field studies. Within the field of tribology, theories and experimental techniques have been developed for modelling contact and frictional phenomena that can be used to develop better microphysical friction models that can be used in larger geo-mechanical models. A deeper understanding of microphysical is fundamental for a better understanding of the dynamics of the subsurface of the earth. This is not only highly relevant for earthquakes, but also for other current developments in energy transition and sustainability. All in all, there is a clear scientific need to model frictional phenomena in geomechanics. This scientific niche of ‘geotribology’, at the boundary of geology and tribology, is almost unexplored and provides a great opportunity for research in this societal relevant topic.
Figure 1. a) Ongoing µFault project at the University of Twente merges geophysics and tribology fields (PhD project vacancy) b) to study friction in reservoir faults.
The recently approved µFault project  which aims for ‘scaling friction from micro-contacts to faults at the reservoir scale’ is one of the first marked attempt in the Netherlands to merge fields of tribology and geomechanics through a dedicated project in ‘geo-tribology’ (see figure 1). One of the main objectives of the µFault project is scaling of friction from single contact to aggregate grain (fault-gouge) contacts through a dedicated PhD project (vacancy) . Fractures and discontinuities in rocks, i.e. faults in natural gas reservoirs are typically filled with crushed rocks grains called gouges. Understanding the friction between the gouge-grains is key to modelling fault slip. Frictional phenomena in individual gouge-grain contact can be experimentally simulated at the microscale as the relevant scale where physical frictional interactions take place. Using results from single grain experiments, a physical basis will be formulated for friction phenomena in microphysical and numerical granular models for fault-gouges (see figure 2). This project seeks to understand the physical processes controlling this sudden failure through numerical modelling and lab experiments and applying the results to acquire large-scale models for induced seismicity caused by natural gas extraction.
Figure 2. Scaling of contacts from natural fault (m-scale) contact up to single grain micro-scale contact, critical to understanding of the underlying frictional (tribological) phenomena .
All in all, increasing activity will take place in the subsurface of the earth, leading to questions of stability of the earth subsurface. For this, there is a clear and urgent need to better understand frictional phenomena and develop models, as there is very limited knowledge available at this moment. The proposed research is therefore considered to be highly relevant and timely regarding the transition into cleaner, cheaper, and more efficient energy, adding precision to earthquake prediction models and a deeper understanding of the earth subsurface in general. Gaining deeper knowledge of frictional mechanisms in faults at different length scales will help improve forecasting of earthquakes and other subsurface phenomena, caused by growing human intervention.
 J Chen & CJ Spiers (2016), Rate and state frictional and healing behaviour of carbonate fault gouge explained using microphysical model. J. Geophysical Research: Solid Earth, 121, 8642-8665.