Hindcasting injection-induced aseismic slip and microseismicity at the Cooper Basin Enhanced Geothermal Systems Project

Taiyi A. Wang, & Eric M. Dunham

Published September 11, 2022, SCEC Contribution #11948, 2022 SCEC Annual Meeting Poster #130 (PDF)

There is a growing recognition that subsurface fluid injection can produce not only earthquakes, but also aseismic slip on faults. A major challenge in understanding interactions between injection-related aseismic and seismic slip on faults is identifying aseismic slip on the field scale, given that most monitored fields are only equipped with seismic arrays. We present a modeling workflow for evaluating the possibility of aseismic slip, given observational constraints on the spatial-temporal distribution of microseismicity, injection rate, and wellhead pressure. Our numerical model simultaneously simulates discrete off-fault microseismic events and aseismic slip on a 3D main fault during fluid injection.

The model consists of four components: the well, fluid flow and pressure diffusion in the fault damage zone, aseismic slip on the main fault in the center of the damage zone, and microseismicity on secondary faults outside of the damage zone. Wellhead pressure is computed from bottom hole pressure by correcting for the hydrostatic pressure difference and pressure loss from turbulent pipe friction. Fluid flow in the fracture zone is represented by radial Darcy flow outward from the injector. The main fault is modeled with velocity-strengthening rate-and-state friction. Off-fault seismic patches are represented with the spring-slider model with velocity-weakening rate-and-state friction.

We apply the workflow to the 2012 Enhanced Geothermal System injection episode at Cooper Basin, Australia, which aimed to stimulate a water-saturated granitic reservoir containing a highly permeable fault zone. We find that aseismic slip likely contributed to half of the total moment release. In addition, fault weakening from pore pressure changes, not elastic stress transfer from aseismic slip, triggers the majority of observed microseismic events, given the inferred stress state. To our knowledge, this is the first time injection-induced aseismic slip in a granitic reservoir has been inferred, suggesting that aseismic slip could be widespread across a range of lithologies.

Key Words
Aseismic slip, induced seismicity, poroelasticity, rate and state

Citation
Wang, T. A., & Dunham, E. M. (2022, 09). Hindcasting injection-induced aseismic slip and microseismicity at the Cooper Basin Enhanced Geothermal Systems Project. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)