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Distributed Optical Sensing at Aquistore and Brady Hot Springs

February 15, 2018 @ 2:00 pm - 3:00 pm

About the Speaker: Douglas Miller

Douglas Miller holds a Ph.D. in Mathematics (UC Berkeley, 1976) and is retired from a career at Schlumberger’s research labs in Ridgefield CT, Cambridge UK, and Cambridge MA. He is presently a Research Affiliate at MIT’s Earth Resources Laboratory, Principal Scientist at Miller Applied Science LLC, and Scientific Advisor for Silixa LTD. He has published work on a broad range of topics in applied mathematics and geophysics, focusing on the relation between measurements, mathematical models, and data analysis. A compendium of his work can be found at www.mit.edu/~demiller/.


About the Seminar:

Distributed optical sensing (DOS) is a rapidly maturing commercial technology to continuously monitor temperature (DTS) and dynamic strain (DAS) as a function of time and location within optical fibers that are deployed in geophysical environments of interest.

LBL scientists have helped to organize and conduct extensive field surveys to calibrate and qualify the use of DOS in monitoring applications for carbon capture and storage (CCS) at the Aquistore site near Estevan Saskatchewan, and for enhanced geothermal systems (EGS) at Brady Hot Springs near Reno Nevada. Interrogators made by Silixa Ltd (ULTIMA-STM DTS and iDASTM) were used at both sites. Douglas Miller has been an active participant in ongoing analysis of Silixa data acquired at each site.

The talk will quickly review DAS calibration results from Aquistore and then discuss the extensive integrated survey that was performed at Brady, 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 horizontal cable (DASH and DTSH) for 15 days and the vertical 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 shows 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. 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 deeper process. 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.

Host: Tom Daley


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