Frictional properties of simulated fault gouges from the Groningen gas field and implications for induced seismicity.

Since the 1990’s, Europe’s largest gas field, the Groningen field, N.E. Netherlands, has exhibited numerous small earthquakes, reaching magnitude 3.6. These are caused by gas production from the porous Slochteren sandstone, located at 3 km depth. Reduction in gas pressure leads to compaction of the sandstone and to stress build-up on ancient, long-inactive faults that cut the rock layers. When fault strength is overcome, sliding may occur either slowly or in a sudden burst, leading to earthquakes and ground shaking. To evaluate under what conditions, and in which faulted rock layers, earthquakes can be generated, it is crucial to understand the fault strength and sliding properties. In this thesis, I report experiments in which fault motion is simulated, under Groningen conditions, using crushed rock materials mimicking those found in faults that cut the main layers present in the Groningen field. Slow sliding and sudden earthquake slip, simulating magnitude 3 to 4 events, are explored – for the first time. A key finding is that the frictional strength of faults varies strongly between layers. Faults containing material derived from the chalky rock layer above the reservoir can weaken rapidly when motion starts. These are therefore especially earthquake prone, as is the sandstone to some extent. Clay-rich layers, below and at the reservoir top, are less so. The results provide new insight into how fault strength and stability vary in the Groningen field and are key for improving assessments of earthquake activity and risk during the final years of field operation, and beyond.