DAS and DTS at Brady Hot Springs: Observations about Coupling and Coupled Interpretations

TitleDAS and DTS at Brady Hot Springs: Observations about Coupling and Coupled Interpretations
Publication TypeConference Proceedings
Year of Conference2018
AuthorsMiller, D, Coleman, T, Zeng, X, Patterson, J, Reinisch, E, Wang, H, Fratta, D, Trainor-Guitton, C, Thurber, C, Robertson, M, Feigl, K, Team, TPoroTomo
Conference Name43rd Stanford Workshop on Geothermal Reservoir Engineering
Abstract

In March 2016 an extensive integrated survey was performed at the geothermal field at Brady Hot Springs, Nevada, where highly permeable conduits along faults appear to channel fluids from shallow aquifers to the deep geothermal reservoir tapped by the production wells. The data set included two fiber-optic cable Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) systems. One cable was arranged horizontally in a zigzag trench 8700 m in length. The second cable was deployed into the accessible 363m portion of a vertical well near another monitoring well. Both cables contained both single-mode and multi-mode fibers with optical U-bends at the ends. Silixa DTS and DAS interrogators were operated to continuously monitor the trenched cable (DASH and DTSH) for 15 days and the borehole cable (DASV and DTSV) for 8 days. In addition to providing active and passive seismic waveform data, the DAS was processed to extract fiber slow strain at a rate comparable to the DTS (2 samples/min). Combined analysis of both DAS and DTS in both horizontal and vertical deployments show details, including puzzling observations, where the combined datasets help to identify and interpret anomalous coupling of the fiber measurements to environmental signal. Patterns in DTSH response as a function of both time and position document thermal response to daily temperature cycles and to changes in injection and production pressure. A magnitude 4.3 regional earthquake from a source in Hawthorne NV, 100 km south of Brady, was clearly detectable by both DASH and DASV. Comparison with Nodal geophones confirmed and cross-calibrated the instrument response of each system. Local earthquakes detectable by the DASH installation include all of those catalogued by the local LBL Brady seismic array plus several additional events of likely interest. Slow strain measured by the DASV is highly correlated to temperature change measured by DTSV. Synchronous patterns in DASV and DTSV document repetitive cycles of thermal exchange both at the expected fluid level in the well and at the level of the slotted liner. DASV documents resonant acoustic behavior associated with the process. Events in DASV data suggest that thermal reaction to borehole rewarming periodically breaks the frictional coupling between cable and borehole wall causing slippage. Patterns in the DASV and DTSV data suggest that the upper section of casing is backed by a fluid annulus that is hydraulically connected to the main bore. Low-frequency (6.4 Hz) resonant pressure transients detected by the DASV at the slotted liner correlate to quasi-periodic (semihourly) thermal events at the same location detected by the DTSV array. Both the earthquake arrival and VSP waveforms extracted from the DASV active-source recordings show a vertical compressional propagation velocity close to 2 km/sec.

URLhttps://pangea.stanford.edu/ERE/db/GeoConf/papers/SGW/2018/Miller.pdf

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