On the role of thermal stress and fluid pressure in triggering seismic and aseismic faulting at the Brawley Geothermal Field, California.
On the role of thermal stress and fluid pressure in triggering seismic and aseismic faulting at the Brawley Geothermal Field, California.
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DOI：
10.1016/j.geothermics.2021.102238
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发表时间：
2021-09-04
期刊：
影响因子：
3.9
通讯作者：
Avouac, Jean-Philippe
中科院分区：
文献类型：
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作者：
Im, Kyungjae;Avouac, Jean-Philippe
地表变形和地震群在空间和时间上与南加州布劳利地热田的运行相关。地震活动在 2012 年达到顶峰，即地热活动爆发约 2 年后，发生了 M5.4 地震。这些地震发生的深度超过 5 公里，远大于地热井约 1 公里的范围，引发了人们对触发机制的疑问。地表变形表明，与地热储层相交的正断层上的地震滑动先于该群，并可能引发该群。在这项研究中，我们利用地质力学模型来研究地震和地震滑动序列是如何展开的。该模型考虑了地热作业引起的热应力和孔隙弹性应力变化，并考虑了储层和周围介质的非弹性变形和断层。该模拟成功地再现了操作员报告的流量和井口压力以及测量的地表沉降。通过改变模型参数，我们表明地表沉降是由于热收缩和正断层造成的。故障重新激活是由压力变化和热松开驱动的。压力驱动的再激活快速且影响更大的区域，而温度驱动的再激活则更加渐进且更集中于注入井附近。在我们的模拟中，地热作用驱动的抗震正断层会通过储层体积内的屈服和断层导致弹性应力释放，相反，会导致储层下方的应力积聚，2012 年群就在那里形成。这种压力转移为 2012 年布劳利群提供了合理的解释。我们的研究表明，原则上，地热作业如何通过地热场内构造应力的抗震释放来有助于减轻地震灾害，但也指出了减轻周围地区应力转移造成的危险的困难。
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 me-dium. 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.