Abstract

In 2004, injection of carbon dioxide (CO2) to be stored at depth began at the In Salah Carbon Capture and Storage (CCS) site and a pilot microseismic monitoring array was installed in 2009. The In Salah project presents an unusual dataset since it is the first major non-Enhanced Oil Recovery (EOR) CCS project to be monitored for microseismicity. This paper outlines an extensive seismological study using a range of techniques, relying mainly on data from a single three-component geophone. Important information is derived from the data, such as event locations, event magnitudes and fracture characteristics, that could be used in real-time to regulate the geomechanical response of a site to CO2 injection. The event rate closely follows the CO2 injection rate, with a total of 9506 seismic events detected. The locations for a carefully selected subset of events are estimated to occur at or below the injection interval, thereby ruling out fault or fracture activation caused by CO2 migration at shallow depths. A very small number of events (11) with less well-constrained locations may have occurred above the injection interval. However, there is no microseismic evidence that these events are correlated with CO2 injection and we suggest they are caused by stress transfer rather than CO2 migration into the caprock. The observed maximum moment magnitude, Mw=1.7, is consistent with estimated fracture dimensions at the injection depth. Fracture orientation estimated using shear-wave splitting analysis is approximately NW-SE, in agreement with fracture orientations inferred from logging data. During periods of high injection rates the degree of anisotropy increases slightly and then falls back to original values when injection rates fall. This implies the CO2 is opening pre-existing fractures which then close as pressure decreases.This an important proof-of-concept study that proves the value of microseismic monitoring of CCS projects, even with a limited array. We thus recommend that microseismic monitoring arrays are installed prior to CO2 injection at future CCS sites to enhance our understanding by making baseline and comparative studies possible. This would also provide real-time monitoring of the geomechanical response to injection, allowing operators to modify injection parameters and to help ensure the safe operation of a project.

Highlights

  • Carbon Capture and Storage (CCS) projects, where CO2 is injected into the ground to be stored at depth, is one technology with the potential to reduce anthropogenic CO2 emissions to mitigate global warming

  • The project was the first non-Enhanced Oil Recovery (EOR) >1 million tonnes (Mt) storage project to be monitored by a microseismic array and the results presented above prove the usefulness of the data in understanding the geomechanical response of the site to CO2 injection

  • We suggest that the repeating events in the clusters occur along the NW-SE oriented pre-existing fracture zone with events occurring on neighbouring fractures with very similar orientations, the similarity in waveforms and tsp times within the clusters

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Summary

Introduction

Carbon Capture and Storage (CCS) projects, where CO2 is injected into the ground to be stored at depth, is one technology with the potential to reduce anthropogenic CO2 emissions to mitigate global warming. At Weyburn CO2 has been injected for enhanced oil recovery (EOR) since 2000, with some increase in pore pressure (15–20 MPa) and a small number of induced seismic events, ∼100 between 2003 and 2010 (Verdon et al, 2011). To date the Sleipner field in Norway is the site of the world’s largest non-EOR CCS project where around 14 million tonnes (Mt) of CO2 have been sequestered since 1996 and where pressures are reported to have remained close to pre-injection levels (Chadwick et al, 2012). This makes significant geomechanical deformation and seismic activity at the site unlikely. The techniques applied here are important and relevant to controlling site response in other industries that require fluid injection (e.g., wastewater injection; hydraulic fracturing and enhanced geothermal stimulation)

The In Salah CO2 storage site
Microseismic instrumentation and data
Data analysis
Event rate
Event clustering
Event locations
Event magnitudes
Fracture strike and density using shear-wave splitting
Importance of experimental set-up
Combining microseismic and geomechanical studies
Findings
Comparison with other CCS sites
Conclusion
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