Stress Orientation To 5 Km Depth In The Basement Below Basel (Switzerland) From Borehole Failure Analysis

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Journal Article: Stress Orientation To 5 Km Depth In The Basement Below Basel (Switzerland) From Borehole Failure Analysis

A vertical profile of maximum horizontal principal stress, SHmax, oriental ion to 5 km depth Was obtained beneath the Swiss city of Basel front observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 in exploration well (OT2) imaged from) 2550 in to 2753 in across the granitic basement-sediment interface in 2649 m: and a 5 km deep borehole (BSI) imaged entirely within the granite front 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 log's with breakouts predominant. Within file 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 degrees whereas breakouts yield 143 +/- 14 degrees. tire combined value Weighted for frequency Of occurrence being N144 degrees E +/- 14 degrees. 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 tire 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 (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 degrees E +/- 12 degrees within the Rotliegendes sandstone occurs near its interface with the basement. The origin of tire 20-30 degrees change is uncertain as is its lateral extent. The logs do not emend higher than 80 in 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 kin 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 all active salt diapir, also within 50 kin of Basel. Thus. the available evidence supports the notion that tire orientation of SHmax above tire evaporites is oil 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 front within the target rock mass.

B. Valley and K. F. Evans

Published Journal 
Swiss Journal of Geosciences, 2009

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B. Valley,K. F. Evans. 2009. Stress Orientation To 5 Km Depth In The Basement Below Basel (Switzerland) From Borehole Failure Analysis. Swiss Journal of Geosciences. (!) .