Continuous crosswell seismic monitoring of a small-scale CO2 injection was accomplished with the development of a novel tubing-deployed piezoelectric borehole source. This piezotube source was deployed on the CO2 injection tubing, near the top of the saline aquifer reservoir at 1657-m depth, and allowed acquisition of crosswell recordings at 15-minute intervals during the multiday injection. The change in traveltime recorded at various depths in a nearby observation well allowed hour-by-hour monitoring of the growing CO2 plume via the induced seismic velocity change. Traveltime changes of 0.2to1.0ms (up to 8%) were observed, with no change seen at control sensors placed above the reservoir. The traveltime measurements indicate that the CO2 plume reached the top of the reservoir sand before reaching the observation well, where regular fluid sampling was occuring during the injection, thus providing information about the in situ buoyancy of CO2.

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