3D seismic data were acquired at a site where enhanced coalbed methane production and carbon sequestration were being tested. The data were acquired nine months after 180 tons of gaseous CO2 were injected at a depth of 0.4 km into the Ardley coals of Alberta, Canada. Poststack inversion of the data showed a low acoustic-impedance anomaly updip and along strike of the preferential fluid pathway in the target coal zone. The size and location of the anomaly concur with the expected imprint of the injected CO2, and it has been interpreted as the flood. The magnitude of the apparent reduction in acoustic impedance cannot be explained by the Gassmann fluid substitution model, suggesting that the elastic moduli of the coal rock frame has been reduced by exposure to the CO2. Other authors have shown that such reduction in the elastic moduli of coal could be achieved by changes in the surface energy of the pore wall or by organic dissolution of the coal frame.

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