Cross‐well seismic monitoring of Coal Bed Methane (CBM) production: A case study from the Powder River Basin of Wyoming

Abstract

The primary mechanism for Coal Bed Methane (CBM) recovery in a typical biogenic, low-rank, shallow and subbituminous coal like that of the Powder River Basin (PRB) of Wyoming is by dewatering or depressurization of the coal beds. In this case history, cross-well seismic experiments were designed to image changes in seismic signatures emanating from the dewatering process. Quantitative estimation of these changes is vital to the technical viability of the CBM reservoir process optimization and monitoring. The pre-and post-CBM production cross-well seismic surveys were executed in July, 2002 and August, 2003 respectively at the PRB's Big George coal beds, Wyoming.Following 2D tomography of the acquired timelapse data sets, the resulting tomograms manifest rocks with different seismic velocities and petrophysical properties. Consequently, we identified 4 discrete subsurface layers: shaly-sand, sandy-shale, the low-velocity coal-bearing formation and sandstone. This result is consistent with the qualitative interpretation of the gamma ray logs in the 2 observation wells that were used for the surveys. Moreover, comparison between the two travel times-inverted tomograms yields P-wave velocity changes of 5% to 7% due to gas production. These observed changes are corroborated by our theoretically derived velocity models, underscoring the effectiveness and applicability of cross-well seismic experiments in imaging the CBM production process.