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The objective of this study is to monitor time-variant changes in the velocity of seismic waves associated with dewatering during methane production at the Big George Coal, Powder River Basin (PRB), Wyoming. The common practice for primary Coal Bed Methane (CBM) recovery at the PRB is to depressurize the coal, usually by pumping water from CBM production wells installed in coal beds for several months. At the initial stage of dewatering, gas production is negligible, but increases and stabilizes as dewatering progresses over an appreciable period. Below (figure 1) is a schematic summary of the results of crosswell seismic imaging surveys, processing and interpretation performed to monitor changes in P-wave velocity associated with the dewatering process. This field test showed that the velocity changes caused by gas in coal are detectable by the seismic method. Also, observed changes are consistent with theoretically derived coal physics models and observations from production data (Akintunde et al. 2004). Data acquisition for this study was sponsored by the Western Resources Project.
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In seismology, variations in the subsurface of the earth are often present on many scales: from scales much larger than the typical seismic wavelength down to scales that are much smaller (Figure 1). A new variable grid-spacing and time-step finite-difference method is developed for efficient seismic modeling in multi-scale heterogeneous media. This method accommodates multi-scale features by allowing fine grid-spacing and time-steps for zones with small-scale features, and coarse grid-spacing and time-steps for zones with large-scale structures. Furthermore, a parallel version of the algorithm has been developed for efficient computation of large problems.
One application of the developed technique is to model cross-well field data with inclusion of tube waves and tube-wave-related arrivals caused by the presence of the two perforated cased boreholes. The good agreement between the synthetic (Figure 2) and the field (Figure 3) observations shows the validity of the method.
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