Abstract
Fluid injection is known to induce seismic events if the injection causes fracturing of the surrounding rock or if resulting pressure changes reactivate pre-existing faults and fractures. Carbon dioxide (CO2) storage projects where CO2 is injected into deep geological formations for permanent containment are one sector where induced seismicity has been observed. The Aquistore storage project in Saskatchewan, Canada began CO2 injection into the basal Cambrian sandstone at ∼3.2 km deep in April 2015 and the site has been extensively monitored for seismicity. Passive seismic monitoring instrumentation includes a small network of broadband seismometers, a continuously recording array of near-surface geophones and temporary deployments of downhole geophones at depths from 2950 m to 3010 m in an observation well. To date no injection-related induced seismicity has been observed. The seismic arrays are functioning as expected and local mine blasts, orientation shots and perforation shots have been detected using standard detection algorithms. Data stacking algorithms have also been tested on short-periods of data. Using synthetic data added to noise models, the estimated minimum detectable event local magnitude is −0.8 for the broadband stations and between −1.6 and −0.6 for the near-surface geophones. Thus far, small volumes of CO2 have been injected at Aquistore (∼140 kt) and injection has generally occurred below the fracture pressure. As a result, predicted pore pressure changes are small and periods without injection have allowed relaxation of the pressure plume. Geomechanical modelling suggests insignificant effective stress changes at an identified fault near the Aquistore injection well. It is therefore not surprising that no induced seismicity has been detected. With further injection, continued seismic monitoring is essential to provide warning of any fault reactivation and thus any potential increase in seismic risk or CO2 leakage risk.
Highlights
Geological storage of carbon dioxide (CO2) is one method proposed to reduce anthropogenic emission of the greenhouse gas to mitigate against climate change
Since there have been no seismic events detected that have been attributed to CO2 injection at Aquistore, we look at the performance of each seismic monitoring array separately to highlight the detection capabilities and potential limitations of each type of deployment
An analysis of passive seismic data collected at the Aquistore site between April 2015 and March 2018 has not identified any induced seismicity associated with CO2 injection
Summary
Geological storage of carbon dioxide (CO2) is one method proposed to reduce anthropogenic emission of the greenhouse gas to mitigate against climate change. Deep fluid injection has the potential to induce seismicity. To detect induced seismicity the Aquistore site is equipped with passive seismic monitoring networks including an intermittently deployed array of five downhole geophones (Fig. 1; Nixon et al, 2017) in a deep observation well 150 m to the north-northeast of the injection well; a continuously recording near-surface array of up to 65 geophones and an array of 3–5 broadband seismometers. Verifying an aseismic response to injection at a site is difficult to achieve because there may always be small magnitude seismic events resulting from fault slip or brittle failure that remain undetected due to low signal-to-noise (SNR) ratios. This study presents a thorough investigation to detect induced brittle failure and fault slip seismic events in the Aquistore passive surface seismic data. We attempt to interpret the paucity of induced seismic events within the context of current geomechanical understanding at the site
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