S012-0018
Seismic monitoring with downhole DAS – examples from the FORGE Enhanced Geothermal System project

Tuesday, 8 December 2020
Poster
Ariel Lellouch1, Ryan Schultz2, Nathaniel J Lindsey3, Biondo Leonardo Biondi2 and William L Ellsworth4, (1)Stanford Earth Sciences, Stanford, CA, United States, (2)Stanford University, Geophysics, Stanford, CA, United States, (3)Stanford University, Dept. of Geophysics, Stanford, CA, United States, (4)Stanford University, Department of Geophysics, Stanford, CA, United States
Abstract:
Seismic monitoring is a crucial element of Enhanced Geothermal Systems (EGS) projects. First, there is an induced seismicity hazard following large-volume fluid injection. Two major EGS projects, in Basel (Switzerland) and Pohang (South Korea), have been halted due to large induced earthquakes. Therefore, high-sensitivity long-term seismic monitoring is both a regulatory need and a mitigation tool. In addition, as fractures open during the stimulation of the reservoir, microseismic events are generated. The analysis of these events is the primary proxy in understanding the developing fracture network. If no connectivity is established between the injection and production wells, or if the path short-circuits via a few high-permeability fractures, the energy density per volume of fluid transmission becomes uneconomical.

Downhole DAS deployments can be beneficial for both induced seismicity and microseismic monitoring. Using the FORGE EGS experiment, we show how an engineered fiber DAS array, deployed vertically to a depth of approximately 1 km, can be used for monitoring. About ten days of continuous data were acquired, during which the reservoir was stimulated, and natural earthquakes occurred in the region. Using various array processing techniques, we utilize DAS records for earthquake and microseismic event detection, location, and magnitude estimation.

We compare DAS results for earthquake detection with that of the regional array maintained and analyzed by the University of Utah Seismograph Stations. During the experiment, 82 events were detected by DAS, compared to 4 by the regional surface array. DAS-derived locations, which do not include event azimuth, appear to coincide with seismically active areas. The magnitude completeness of DAS is approximately an order of magnitude better than for the surface array. However, for microseismic monitoring, downhole geophones outperform DAS, yielding magnitude completeness better by M 0.3. Set aside the lack of azimuthal information in DAS, locations are very close, and so are the moment magnitude estimations. We conclude that thanks to its operational benefits, particularly the potential for long-term deployment of a fiber close to the high-temperature reservoir, DAS is a promising alternative for EGS monitoring.

<< Previous Abstract | Next Abstract >>