Hydraulic fracturing is used to create pathways for fluid migration and to stimulate production. Usually, water is the injected fluid, although alternative fluids such as carbon dioxide ( C O 2 ) have been used recently. The amount of fracturing that C O 2 can induce is also of interest for the security of carbon capture and storage. Hydraulic fracturing is usually monitored using passive seismic arrays, detecting microseismic events generated by the fracturing. It is of interest to compare the amount of seismicity that C O 2 injection can generate in comparison with water. With this in mind, we have analyzed a passive seismic data set monitoring the injection of water and supercritical C O 2 under very similar conditions, allowing us to make a direct comparison be-tween the fluids. We examined event locations and event magnitudes, and we used shear-wave splitting to image the fractures that are generated. For both fluids, the event locations map the formation of fractures moving away from the injection well with normals parallel to the minimum principal stress. The events during water injection are limited to the injection depth, while during C O 2 injection, activity migrates above the injection depth. Event magnitudes are similar in both cases, and larger event magnitudes appear to correlate with higher injection pressures. Shear-wave splitting suggests that water injection generates more fractures, though the data quality is not good enough to make a robust conclusion about this. The comparability between water and C O 2 injection means that lessons can be learned from the abundant experience of conventional water injection.

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