Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis
Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis
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根据钻孔破坏分析，巴塞尔（瑞士）下方地下室 5 公里深度的应力方向
DOI：
10.1007/s00015-009-1335-z
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发表时间：
2009
影响因子：
3.1
通讯作者：
K. Evans
中科院分区：
文献类型：
--
作者：
B. Valley;K. Evans
根据从两个1公里远的近垂直钻孔中获得的超声波电视图像获得的井筒破坏观测结果，获得了瑞士城市巴塞尔地下5公里深处最大水平主应力SHmax方向的垂直剖面图：一个2755米的勘探井（OT 2）从2550米到2753米，穿过2649米处的花岗岩基底-沉积物界面;以及一个5 km深的钻孔（BS 1），完全在2569 m至4992 m的花岗岩内成像。与应力相关的井眼破坏形式为崩落或钻井引起的张力裂缝（DITF），其发生在以崩落为主的整个测井深度范围内。在花岗岩内，DITF是间歇性的，和突破或多或少连续存在于所有，但最上面的100米，他们是稀疏的。DITF的平均SHmax方向为151 ± 13°，而断裂产生143 ± 14°，发生频率加权的组合值为N144°E ± 14°。在数百米深度间隔内平均的平均SHmax方向没有明显的深度依赖性。花岗岩的平均SHmax方向与巴塞尔以北和以南区域内10-15 km深度之间发生的地震的震源机制解的总体反演结果一致，也与水库内发生的事件的T轴一致（Deichmann和Ernst，本卷）。OT 2中沉积物-基底界面上方和下方识别的DITF和崩落表明，Rotliegendes砂岩内的SHmax方向在其与基底的界面附近变为N115°E ± 12°。20-30°变化的起源及其横向范围尚不确定。测井曲线在界面上方延伸不超过80 m，因此数据不能确定是否有应力方向的进一步变化发生在碳酸盐岩处。在距离巴塞尔50 km范围内进行的近地表测量表明，平均方位为南北向，尽管变化很大，蒸发岩层内和上方850 m深处的水力压裂方位以及距离巴塞尔50 km范围内的活动盐底辟也是如此。因此，现有证据支持这样的观点：蒸发岩上方的SHmax方向平均而言更偏向南北方向，因此与根据BS 1/OS 2井眼破坏数据和地震数据推断的基底NW-SE方向不同。应力方向随深度的变化可能对EGS储层的开发产生重大的实际影响，并有助于强调从目标岩体内获得估计的重要性。
A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city of Basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (BS1) imaged entirely within the granite from 2569 m to 4992 m. Stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DITFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DITFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. The mean SHmax orientations from DITFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean SHmax orientation averaged over several hundred meters depth intervals is evident. This mean SHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10–15 km within regions immediately to the north and south of Basel, and with the T-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DITFs and breakouts identified in OT2 above and below the sediment-basement interface suggest that a change in SHmax orientation to N115°E ± 12° within the Rotliegendes sandstone occurs near its interface with the basement. The origin of the 20–30° change is uncertain, as is its lateral extent. The logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of Basel suggest a mean orientation of N–S, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of Basel. Thus, the available evidence supports the notion that the orientation of SHmax above the evaporites is on average more N–S oriented and thus differs from the NW–SE inferred for the basement from the BS1/OS2 wellbore failure data and the earthquake data. Changes in stress orientation with depth can have significant practical consequences for the development of an EGS reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.