Downhole Distributed Temperature Sensing in Fractured Rock
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|Created:||Dec 29, 2017 at 10:49 p.m.|
|Last updated:|| Apr 09, 2018 at 8:48 p.m.
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Newmont’s Leeville Mine is located on the western side of the Tuscarora Range in northeastern Nevada. A 587 meter geotechnical pilot borehole in silty carbonate was drilled during 2012 prior to excavating a 10 m diameter vent shaft using ground freezing technology. Fiber optic cable was installed in the borehole and backfilled with cement-bentonite grout. In 2013 a distributed temperature interrogator was installed, which measured temperature along the fiber optic cable at 0.25 m and 0.02 °C resolution. After initial data were taken, a lower-resolution interrogator was installed for longer-term data collection, which measures temperature along the fiber optic cable at 1 m and 0.2 °C resolution. The intent was to utilize temperature for aquifer characterization. Daily measurements were programmed and run from December 21, 2013 through June 20, 2015 for a total of 546 daily temperature traces comprising 321,048 data points. Descriptive data analysis is presented, indicating a saturated zone 122 m below ground surface where a rapid temperature decrease of near 2 °C was seen. Similar downhole temperature changes are observed at 367 and 528 m coinciding with fracture flow zones where lateral water flow is suspected. Distinct slope profiles of the local geothermal gradient are evident between the fracture flow zones, indicating separate hydrostratigraphic units. The close-by ground freezing provided a thermal tracer of water movement. Temperature generally decreased over time because of the freeze process. Greater than 7 °C difference was observed in zones interpreted as lower permeability where conductive heat transfer was likely dominant. Advective flow at the 366 and 526 m depths showed substantially less temperature variability. It is at these depths where water flows through fractures at a greater velocity than the bulk rock and where the water temperature is indicative of the greater aquifer system. The smallest temperature change was less than 2 °C at 579 meters depth, which is interpreted to be the upper zone of the semi-saturated Roberts Mountain Thrust Fault.
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