On the role of thermal stress and fluid pressure in triggering seismic and aseismic faulting at the Brawley Geothermal Field, California.

Abstract

Surface deformation and earthquake swarms are correlated in space and time with operations at the Brawley geothermal field in southern California. The seismicity culminated in 2012, about 2 years after the onset of geothermal activity, with a M5.4 earthquake. These earthquakes occurred at a >5km depth, much larger than the ~1km reach of the geothermal wells, raising questions about the triggering mechanism. Surface deformation shows that aseismic slip on a normal fault intersecting the geothermal reservoir preceded the swarm and possibly triggered it. In this study, we resort to geomechanical modeling to investigate how the sequence of aseismic and seismic slip unfolded. The modeling accounts for thermo-and poro-elastic stress changes induced by the geothermal operations and allows for inelastic deformation and faulting of the reservoir and surrounding medium. The simulation successfully reproduces the flow rates and well-head pressures reported by the operator as well as the measured surface subsidence. By varying the model parameters, we show that the surface subsidence is due to thermal contraction and normal faulting. The fault reactivation is driven by pressure changes and thermal unclamping. The pressure-driven reactivation is rapid and influences a larger area, while the temperature-driven reactivation is more gradual and more localized near the injection wells. In our simulation, aseismic normal faulting driven by the geothermal operation leads to elastic stress release via yielding and faulting within the reservoir volume and, conversely, to stress build-up beneath the reservoir, where the 2012 swarm developed. Such a stress transfer provides a plausible explanation for the 2012 Brawley swarm. Our study shows how a geothermal operation can, in principle, contribute to seismic hazard mitigation through the aseismic release of tectonic stresses within a geothermal field but points to the difficulty of mitigating the hazard posed by stress transfers in the surrounding area.

Detection of Aseismic Slip and Poroelastic Reservoir Deformation at the North Brawley Geothermal Field From 2009 to 2019

“…This is consistent with the overall drop in microseismicity rates observed at the geothermal field (Figure 4). This finding is also consistent with the interpretation that the 2012 swarm and its associated aseismic slip released strain energy that accumulated tectonically (Im & Avouac, 2021), resulting in minimal moment release in the years after 2012.…”

“…During 2009–2012, before the Brawley swarm, the mostly subsidence‐dominated deformation can be reproduced with a growing aseismic slip feature on a normal fault beneath the geothermal field (S. Wei, Avouac, et al., 2015; Im & Avouac, 2021; Figure 4). Deformation during the Brawley swarm interval (T3) is consistent with a combination of normal and strike‐slip faulting, similar to other studies of coseismic deformation during the swarm (Figure 7).…”

“…Additionally, in the 2005 earthquake swarm near the Salton Sea Geothermal Field, geodetic evidence suggested aseismic slip accounted for up to 80% of the moment released during the swarm (Lohman & McGuire, 2007). Finally, geomechanical modeling of the North Brawley Geothermal Field (NBGF) reservoir indicates that an aseismic slip transient may have contributed to triggering the 2012 Brawley swarm (Im & Avouac, 2021).…”

Probabilistic Evaluation of Susceptibility to Fluid Injection-Induced Seismicity Based on Statistics of Fracture Criticality

“…Here, the gradient of critical fluid pressure change Δp c /h is utilised as the fracture slip susceptibility metric. However, it is acknowledged by the authors that in many subsurface fluid injection applications, particularly in geothermal systems where cold fluids are injected to reservoirs, a number of seismicity triggering mechanisms are at play, including the elevated fluid pressure, poroelastic stress, thermal effects, interactions between seismicity, and the Kaiser effect (Izadi and Elsworth 2015;Ghassemi and Tao 2016;Brown and Ge 2018;Garcia-Aristizabal 2018;Im and Avouac 2021;Cao et al 2022). The overpressure-driven seismicity has a larger areal extent of influence and occurs rapidly after injection, whilst the temperature-driven seismicity is more constrained around injection wells and may be dominant in the long term (Izadi and Elsworth 2015;Ghassemi and Tao 2016).…”

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

“…In the Sichuan Basin, China, prevalent well casing deformation is attributed to aseismic fault slip associated with hydraulic fracturing treatments, which has significantly hindered shale gas production 8 . Geothermal power extraction at the Brawley Geothermal Field, California, has been linked with earthquake swarms preceded by aseismic slip in sedimentary rocks 5,15,16 . These observations indicate that injection-induced aseismic fault slip may be a prevalent, yet potentially under-detected phenomenon.…”