Time-lapse seismic imaging of enhanced coalbed methane production: a numerical modelling study

Sarah E. Richardson and Don C. Lawton

ABSTRACT

Coalbed methane (CBM) production relies on dewatering coal seams to allow gas flow, and the injection of carbon dioxide to maintain effective reservoir pressure. Injection of carbon dioxide into CBM strata also serves as an effective method of subsurface greenhouse gas sequestration. Both dewatering and gas injection alter the acoustic impedance and hence reflectivity of coal strata. Physical tests and numerical modelling suggest that time-lapse seismic imaging may be used effectively to image the changes in a coalbed methane reservoir that result from CBM production. Seismic responses of coal seams change due to differences in the acoustic impedance of the coal, and decreased velocities within coal zones results in delayed reflections from deeper horizons. Modelling provides "proof of concept", and provides parameters to be considered in survey design prior to a field trial involving the Ardley coal zone in Alberta.

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