Microseismic monitoring at a shallow CO2 injection site, the CaMI Field Research Station in Alberta, Canada

Abstract:

The Containment and Monitoring Institute (CaMI) Field Research Station (FRS), based in Newell County in southern Alberta, was established in 2017 by the CMC Research Institutes Inc. in collaboration with the University of Calgary. The FRS serves as an experimental laboratory for the development and assessment of robust seismic monitoring approaches for the early detection of CO₂ leakages and for the accurate tracking of subsurface fluid migration. The injection target is a thin water filled sandstone formation located at 300 m depth, 75 m below the aquifer. Bottom well pressures were kept constant since January 2020 and appear to have triggered the onset of microseismic activity in the same month. In this presentation, we will characterize the microseismic activity which was continuously recorded since February 2019 on 3 components by a 2D surface array of 10 Hz geophones and a downhole array in an observation well 20 m away from the injector. Different detection methods were explored to capture the small micro-earthquakes. The time dependence of noise levels, interference from other fieldwork activities, animal disturbances and generally weak signal amplitudes all posed some challenges for establishing a robust detection workflow on this continuous dataset. Detection methods employed include recursive STA-LTA (e.g. Trnkoczy, 2012) and earthquake similarity search approaches such as Fingerprint And Similarity Thresholding (FAST) (Yoon et al., 2015), matched-filter detection (e.g. Chamberlain et al., 2017) and the Matrix Profile approach, a fast time series mining analysis technique deployed on GPU clusters (Zimmerman et al., 2019) that we have adapted to our dataset. Pros and cons of each approaches are explained in this presentation. Between January and May, nearly 20k events were detected by this ensemble of methods. Events are dominated by high S-wave energy and are largely located in the shallower 150 m, above the reservoir. During periods of high microseismic activity, we also observed tremor-like long period, long duration signals with dominant frequency in the 10-15 Hz range. These signals last from a few minutes to almost half an hour. Polarization analysis using the downhole array shows high signal rectilinearity, suggesting a superposition of body waves.