Laboratory characterization of induced seismicity

Subsurface fluid injection causes an increase in seismic activity near injection sites. To ensure public safety and improve public acceptance of subsurface fluid injection, it is important to understand the underlying mechanisms giving rise to these behaviors and characterize the factors affecting induced seismicity. The work reported here aims to better understand induced seismicity in faulted/fractured subsurface rock formations. A series of experiments are conducted in core flooding and triaxial configurations on test samples with a preexisting fracture/fault to study frictional slipping at small and large scales respectively. The geometries of both surfaces on either side of the fault are characterized using x-ray CT. The extent of fault gouging in a specimen after slipping is quantified by defining a cumulative gouging parameter using x-ray CT measurements before and after the experiment. Acoustic emission (AE) events emanating from the specimens in both configurations are monitored. The results demonstrate that (1) the orientation of fault relative to the major principal stress direction affects the slip characteristics, (2) frictional slipping at a fault can occur at different length scales, (3) with an increase in the scale of slipping, the spectral ratio of low-frequency bins increases while high-frequency bins reduces, and (4) locations on the fault surface that exhibit a sudden change in surface normal are most susceptible to gouging/damage during frictional slipping.