Microseismic monitoring using a fibre-optic Distributed Acoustic Sensor (DAS) array

Abstract

We present a case study demonstrating the use of an “L”-shaped downhole fibre-optic array to monitor microseismicity. We use a relatively simple method to detect events from continuous waveform data, and develop a workflow for manual event location. Locations are defined with a cylindrical coordinate system, with the horizontal axis of the DAS cable being the axis of symmetry. Events are located using three manual “picks”, constraining (1) the zero-offset “broadside” channel to the event (2) the P-S wave arrival time difference at the broadside channel, and (3) the angle, ? of the event from the array. Because the one-component DAS array is unable to record P-wave energy on the broadside channel, the P-wave pick is made indirectly by ensuring that the modeled P- and S-wave moveout curves match the observed data. The ? angle requires that signal is observed on the vertical part of the array, in our case this is possible because an engineered fiber, rather than standard telecommunications fiber, provided a significant reduction in the noise level. Because only three picks need to be made, our manual approach is significantly more efficient than equivalent manual processing of downhole geophone data, where picks for P- and S-waves must be made for each receiver. We find that the located events define a tight cluster around the injection interval, indicating that this approach provides relatively precise and accurate event locations. A surface microseismic array was also used at this site, which detected significantly fewer events, the locations of which had significantly greater scatter than the DAS array locations. We conclude by examining some other aspects of the DAS microseismic data, including the presence of multiple events within very short time windows, and the presence of converted phases that appear to represent scattering of energy from the hydraulic fractures themselves.

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

“…VSP methods are now applied in other environments, such as at glaciers (Booth et al., 2020). Recently, DAS has been applied to passive seismic investigations including: the study of tectonic earthquakes (Ajo‐Franklin et al., 2019; Dou et al., 2017; Jousset et al., 2018; Lindsey et al., 2019; Sladen et al., 2019; Wang et al., 2018); ambient noise studies (Ajo‐Franklin et al., 2019; Dou et al., 2017; Martin et al., 2018; Spica et al., 2020; Zeng et al., 2017); and microseismicity in a variety of settings including hydraulic fracture reservoir stimulation (Baird et al., 2020; Karrenbach et al., 2019; Stork et al., 2020; Verdon et al., 2020), geothermal seismicity (Li & Zhan, 2018), and alpine glacier icequakes (Walter et al., 2020).…”

Seismic Applications of Downhole DAS

“…For certain events, it is possible to detect arrivals in the deviated and vertical parts of the well, in which case many previously degenerate planes can be resolved. Furthermore, [ 108 ] uses deviated well recordings to approximately estimate event location without the need for individual channel picking. Instead, they measure several geometrical quantities, evident in the DAS records, and locate events assuming a constant background model.…”

Application of machine learning to microseismic event detection in distributed acoustic sensing data

“…DAS data provides only a single-component recording (the fiber is sensitive to changes in strain along the fiber, but insensitive to changes broadside to the cable) whereas standard geophones and seismometers can provide three-component recordings. Nevertheless, processing techniques are advancing to determine microseismic event locations (Webster et al, 2016;Verdon et al, 2020) and source mechanisms (Cole et al, 2018;Baird et al, 2020) from DAS data. The technology is also being explored for microseismic monitoring in other industrial settings, for example at geothermal sites (Mondanos and Coleman, 2019), geological CO 2 storage sites and for volcano monitoring.…”