MULTICOMPONENT 3-D CHARACTERIZATION OF A COALBED METHANE RESERVOIR

Methane is produced from fractured coalbed reservoirs at Cedar Hill Fi eld in the San Juan Basin. Fracturing and local stress are critical to production because of the absence of matrix permeability in the coals . Knowledge of the direction of open fractures, the degree of fracturi ng, reservoir pressure, and compartmentalization is required to unders tand the flow of fluids through the reservoir. A multicomponent 3-D se ismic survey was acquired to aid in coalbed methane reservoir characte rization. Coalbed reservoir heterogeneities, including isolated pressu re cells, zones of increased fracture density, and variable fracture d irections, have been interpreted through the analysis of the multicomp onent data and integration with petrophysical and reservoir engineerin g studies. Strike-slip faults, which compartmentalize the reservoir, h ave been identified by structural interpretation of the 3-D P-wave sei smic data. These faults form boundaries for pressure cells that have b een identified by P-wave reflection amplitude anomalies. The analysis of polarizations, traveltimes, and reflection amplitudes from the shea r-wave seismic data has allowed the identification of zones of variabl e fracture direction and fracture density. There is good agreement bet ween stresses inferred from the structural interpretation and those in dicated by the shear-wave polarizations. Reflection amplitudes have be en calibrated to seismic velocities and reservoir pressures through th e use of petrophysical data taken from core samples. New methods have been developed for the statistical analysis of prestack shear-wave pol arizations, poststack polarizations, and the accurate determination of travel-time anisotropy. The prestack polarization analysis method all ows for rapid and efficient determination of a dominant polarization d irection. Shear-wave anisotropy has been quantified over the reservoir zone using both traveltime and thin-bed reflection response with exce llent agreement between the two methods. Crack densities computed from the anisotropy show two regions of high crack density, one coinciding with a sealed overpressured cell and the other in the region of the H amilton #3 well. This indicates the potential for monitoring productio n of coalbed methane reservoirs using multicomponent seismology.