Collective properties of injection‐induced earthquake sequences: 1. Model description and directivity bias
Collective properties of injection‐induced earthquake sequences: 1. Model description and directivity bias
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DOI：
10.1002/2015jb012550
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发表时间：
2016-05
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影响因子：
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通讯作者：
D. Dempsey;J. Suckale
中科院分区：
文献类型：
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作者：
D. Dempsey;J. Suckale
在油气、地热和碳封存作业中，诱发地震活动越来越受到关注，近年来引发了几起50亿桶的地震事件。模拟在了解地震活动的原因和控制地震灾害方面起着重要作用。在这里，我们研究了诱发地震序列的集体性质及其物理基础。在两部分系列的第一篇论文中，我们重点关注指向性比，它量化了断层破裂是由一个（单边）还是两个（双边）传播前沿主导。在第二篇论文中，我们重点研究了诱发地震活动的时空和震级频率分布。我们开发了一个模型，该模型将1 - D断层破裂的断裂力学描述与分形应力非均质性和注入井周围不断变化的孔隙压力分布相结合，从而引发地震。断层破裂的程度是根据以震源为中心的扩展裂缝的两个尖端的运动方程来计算的。在构造荷载条件下，我们的模型显示出单边破裂和震源位置的正态分布，这两个特征与地震观测相一致。另一方面，当注入直接发生在断层上时，诱发事件的目录显示出倾向于向注入井传播的破裂。这种偏向是由于高压羽流中存在相对有利的破裂条件。指向性偏倚的强度取决于许多因素，包括压力积聚的类型、断层离破裂的距离和事件的震级。对于触发地震的断层注入，模拟的指向性偏差很小，对于实际探测来说可能太弱。对于两种假设的注入情况，我们估计了检测指向性偏差所需的地震观测次数。
Induced seismicity is of increasing concern for oil and gas, geothermal, and carbon sequestration operations, with several M > 5 events triggered in recent years. Modeling plays an important role in understanding the causes of this seismicity and in constraining seismic hazard. Here we study the collective properties of induced earthquake sequences and the physics underpinning them. In this first paper of a two‐part series, we focus on the directivity ratio, which quantifies whether fault rupture is dominated by one (unilateral) or two (bilateral) propagating fronts. In a second paper, we focus on the spatiotemporal and magnitude‐frequency distributions of induced seismicity. We develop a model that couples a fracture mechanics description of 1‐D fault rupture with fractal stress heterogeneity and the evolving pore pressure distribution around an injection well that triggers earthquakes. The extent of fault rupture is calculated from the equations of motion for two tips of an expanding crack centered at the earthquake hypocenter. Under tectonic loading conditions, our model exhibits a preference for unilateral rupture and a normal distribution of hypocenter locations, two features that are consistent with seismological observations. On the other hand, catalogs of induced events when injection occurs directly onto a fault exhibit a bias toward ruptures that propagate toward the injection well. This bias is due to relatively favorable conditions for rupture that exist within the high‐pressure plume. The strength of the directivity bias depends on a number of factors including the style of pressure buildup, the proximity of the fault to failure and event magnitude. For injection off a fault that triggers earthquakes, the modeled directivity bias is small and may be too weak for practical detection. For two hypothetical injection scenarios, we estimate the number of earthquake observations required to detect directivity bias.