Borehole Seismic Monitoring of Injected CO2 at the Frio Site
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The recently completed CO2 injection in the brine aquifer of the Frio Formation in southeast Texas provided an opportunity to test borehole seismic monitoring techniques. Designed tests included time-lapse VSP and crosswell surveys which investigated the detectability of CO2 with surface-to-borehole and borehole-to-borehole measurement. The VSP method uses surface seismic sources in conjunction with borehole sensors to measure the seismic properties (such as velocity and reflection strength) in the vicinity of the borehole. By moving the source location, seismic properties can be mapped spatially around the sensor well. A large change (about 70%) in VSP reflection amplitude from the Frio zone was observed. Because of the relatively small amount of CO2 injected (about 1600 tons), and the thin injection interval (about 6 m thick at 1500 m depth), CO2 detectability by the VSP method was not an assumed certainty. The initial result is therefore quite promising for use of the VSP method. The crosswell method measures wave propagation between wells and can tomographically image the interwell volume. The crosswell survey was conducted using the injection well (for sensors) and a nearby monitoring well (for the source) which is about 30 m offset. Crosswell source locations were centered on the injection interval. The crosswell sensors were also centered on the injection interval, which is the 6-7 m thick, upper C sand in the Frio formation which is at a depth of about 1500 m. Initial analysis of the crosswell data shows good quality P- and S-wave direct arrivals. Time-lapse tomographic imaging maps the changes in velocity (up to 1 km/s) due to the CO2 plume.