We performed a series of hydraulic stimulations at 1.1 km depth in the Bedretto underground laboratory, Switzerland, as part of an overall research strategy attempting to understand induced seismicity on different scales. Using an ultra-high frequency seismic network we detect seismic events as small as M_w < −4, revealing intricate details of a complex fracture network extending over 100 m from the injection sites. Here, we outline the experimental approach and present seismic catalogs as well as a comparative analysis of event number per injection, magnitudes, b-values, seismogenic index and reactivation pressures. In our first-order seismicity analysis, we could make the following observations: The rock volume impacted by the stimulations in different intervals differs significantly with a lateral extent from a few meters to more than 150 m. In most intervals multiple fractures were reactivated. The seismicity typically propagates upwards toward shallower depth on parallel oriented planes that are consistent with the stress field and seem to a large extent associated with preexisting open fractures. This experiment confirms the diversity in seismic behavior independent from the injection protocol. The overall seismicity patterns demonstrate that multi-stage stimulations using zonal isolation allow developing an extended fracture network in a 3D rock volume, which is necessary for enhanced geothermal systems. Our stimulations covering two orders of magnitude in terms of injected volume will give insights into upscaling of induced seismicity from underground laboratory scale to field scale.

中文翻译：

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瑞士 Bedretto 地下实验室百米级压裂系统对水力增产的地震响应


我们在瑞士 Bedretto 地下实验室 1.1 公里深处进行了一系列水力刺激，作为试图了解不同尺度的诱发地震的整体研究策略的一部分。使用超高频地震网络，我们探测到小至 M_w < -4 的地震事件，揭示了从注入地点延伸超过 100 m 的复杂裂缝网络的复杂细节。在这里，我们概述了实验方法并介绍了地震目录以及每次注射的事件数量、震级、b 值、地震指数和再激活压力的比较分析。在我们的一阶地震活动分析中，我们可以进行以下观察：受不同间隔刺激影响的岩石体积差异很大，横向范围从几米到 150 多米不等。在大多数间隔中，多发性骨折被重新激活。地震活动通常在平行方向平面上向上传播到较浅的深度，这些平面与应力场一致，并且似乎在很大程度上与先前存在的开放裂缝有关。该实验证实了独立于注入方案的地震行为的多样性。整体地震模式表明，使用纬向隔离的多阶段增产允许在 3D 岩石体积中开发扩展的裂缝网络，这对于增强地热系统是必要的。我们的刺激在注入量方面涵盖两个数量级，将深入了解诱发地震活动从地下实验室规模放大到现场规模。