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Geomechanical stress conditions to induce half-moon events during hydraulic fracturing
Geophysics ( IF 3.3 ) Pub Date : 2021-07-13 , DOI: 10.1190/geo2019-0681.1
Nepomuk Boitz 1 , Serge A. Shapiro 1
Affiliation  

Half-moon events are a special type of microseismic source mechanism that is found at various hydraulic-fracturing sites, but hardly observed in natural seismicity. This event type can be explained either by a vertical slip on a nearly vertical fault plane or by a horizontal slip on a nearly horizontal fault plane. For this, special stress conditions are required, for instance, nearly equal horizontal and vertical compressional stresses and significant shear stresses. Such conditions are created during hydraulic stimulation as shown by our numerical simulations. By applying fracture pressure to the surface of the hydraulic fracture, the stress field in the vicinity of the hydraulic fracture can locally rotate and horizontal or vertical faults become optimally oriented. We found that such rotations can occur in locations where the elastic properties of rocks change (i.e., the fracture crosses a layer interface) or at the tips of the hydraulic fracture. Depending on the stress regime, our model explains half-moon events generated by slip on nearly horizontal fault planes in strike-slip environments and by slip on nearly vertical fault planes in normal faulting tectonics. Moreover, our models explain several common characteristics observed in multiple case studies. This includes the observation of the high portion of half-moon events and opposed shear senses in different depths and on opposite sides of the fracture.

中文翻译:

水力压裂过程中诱发半月事件的地质力学应力条件

半月事件是一种特殊类型的微震震源机制,在各种水力压裂现场都有发现,但在自然地震活动中几乎没有观察到。这种事件类型可以用近垂直断层面上的垂直滑动或近水平断层面上的水平滑动来解释。为此,需要特殊的应力条件,例如几乎相等的水平和垂直压缩应力以及显着的剪切应力。如我们的数值模拟所示,此类条件是在水力增产过程中产生的。通过向水力压裂表面施加压裂压力,水力压裂附近的应力场可以局部旋转,水平或垂直断层得到最佳定向。我们发现这种旋转可能发生在岩石弹性特性发生变化的位置(即裂缝穿过层界面)或水力裂缝的尖端。根据应力状态,我们的模型解释了由走滑环境中近水平断层面上的滑动和正常断层构造中近垂直断层面上的滑动所产生的半月事件。此外,我们的模型解释了在多个案例研究中观察到的几个共同特征。这包括在不同深度和裂缝的相对两侧观察半月事件的高部分和相反的剪切感。我们的模型解释了由走滑环境中近乎水平断层面上的滑动和正常断层构造中近乎垂直断层面上的滑动所产生的半月事件。此外,我们的模型解释了在多个案例研究中观察到的几个共同特征。这包括在不同深度和裂缝的相对两侧观察半月事件的高部分和相反的剪切感。我们的模型解释了走滑环境中近水平断层面上的滑动和正常断层构造中近垂直断层面上的滑动所产生的半月事件。此外,我们的模型解释了在多个案例研究中观察到的几个共同特征。这包括在不同深度和裂缝的相对两侧观察半月事件的高部分和相反的剪切感。
更新日期:2021-07-14
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