Relationships between fault characteristics and seismic responses in a large lab-scale tri-axial injection test conducted on a faulted Castlegate Sandstone

To investigate mechanisms causing microseismicity (M-w < 2) at a CO2 injection site, a large-scale triaxial block experiment was carried out on a faulted (saw-cut) cubic-meter of Castlegate Sandstone. The experiment consisted of injection tests at varying differential stresses, while monitoring and recording pore pressure and acoustic emissions (AEs). During the experiment, similar to 33,000 AEs and-14 mm of horizontal displacement/slip, like a strike-slip fault movement, occurred. To un-derstand the AE responses and ascertain fault characteristics near the located AEs, we modeled the topography of the fault surface, fault aperture, and fault-gouge thickness using pre-and post-experiment laser scans of the fault surface on each half of the block. Additionally, we characterized surface roughness parallel and perpendicular to slip. Models show crushing and flattening of the fault surfaces can be linked to the spatiotemporal distribution of AEs within 50 mm of the fault surface. Approximately 65% of AEs were in areas with small aperture (<= 300 mu m); thicker fault gouge was observed in adjacent areas with wider aperture and shows a two-fold reduction in grain size relative to unaltered Castlegate Sandstone. This work provides a conceptual understanding on fault surface evolution, which can be applied towards modeling of seismic slip.

Automated summary

In this study, a large-scale triaxial block experiment was conducted on Castlegate Sandstone to investigate the mechanisms causing microseismicity at a CO2 injection site. The experiment revealed that the crushing and flattening of fault surfaces were correlated with the spatiotemporal distribution of microseismic events near the fault. The presence of thicker fault gouge and smaller grain size in areas with wider aperture suggests a link between fault characteristics and microseismic activity.