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

Abstract

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 distributionof microseismicity, injection rate, and wellhead pressure. Our numerical model simultaneously simulates discrete off-fault microseismic events and aseismic slip on a main fault during fluid injection. 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, induces the majority ofobserved 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.

Control of Seismicity Migration in Earthquake Swarms by Injected Fluid Volume and Aseismic Crack Propagation

“…Wang and Dunham (2022) successfully reconstructed the seismicity observed during fluid injection in the Cooper Basin, Australia, using a numerical model that assumes a velocity‐strengthening fault, explained by the potential presence of phyllosilicates in the fault zone. For the Soultz 1993 sequence, the presence of repeating events, asperity interactions, slip rate decay suggest that the observed seismicity is due to a combined effect of velocity‐weakening on seismic asperities embedded in a velocity‐strengthening matrix (Figure 3, Bourouis & Bernard, 2007).…”

“…Similar observations have been made in association with anthropogenic injections at depth; for example, a swarm in the Brawley, California geothermal field in 2012 was preceded and likely triggered by an aseismic slip transient (Wei et al., 2015), while hydraulic fracturing injections in northwestern Canada led to two major slow slip episodes not accompanied by significant seismic events (Eyre et al., 2022). Numerical modeling also showed that fluid injection can trigger aseismic transients, which in turn can trigger seismicity (Eyre et al., 2019; Wang & Dunham, 2022; Wynants‐Morel et al., 2020).…”