Acoustic impedance inversion and CO₂ flood detection at the Alder Flats ECBM project

Michael Jason McCrank, Donald C. Lawton

The 3D post-stack vertical component data from the Alder Flats site were inverted to estimate the acoustic impedance of the Upper and Lower Ardley coal zones. The theory of two methods of inverting the data, a model-based algorithm and a constrained sparse spike algorithm, is reviewed. The inversion with each of the algorithms shows a similar result. While the band-limited nature of the seismic data and the resulting inversion does not resolve each sub-zone of the Ardley Coals, the parameter estimation appears to be accurate and unbiased. Both inversions show a low acoustic impedance anomaly approximate areal extent of 1300 - 1800 m2 to the northeast of a well that was used to inject 180 tonnes of CO₂ into one of the sub-zone coals of the Ardley Coal zone prior to acquisition of the seismic survey. Gassmann modelling shows that a simple fluid substitution of formation water for gaseous CO₂ could explain the degree of anomaly seen, however, other interpretations are possible.