3-D AVO ANALYSIS AND MODELING APPLIED TO FRACTURE DETECTION IN COALBED METHANE RESERVOIRS

Over the years, amplitude variation with-offset (AVO) analysis has bee n used successfully to predict reservoir properties and fluid contents , in some cases allowing the spatial location of gas-water and gas-oil contacts. In this paper, we show that a 3-D AVO technique also can be used to characterize fractured reservoirs, allowing spatial location of crack density variations. The Cedar Hill Field in the San Juan Basi n, New Mexico, produces methane from the fractured coalbeds of the Fru itland Formation. The presence of fracturing is critical to methane pr oduction because of the absence of matrix permeability in the coals. T o help characterize this coalbed reservoir, a 3-D, multicomponent seis mic survey was acquired in this field. In this study, prestack P-wave amplitude data from the multicomponent data set are used to delineate zones of large Poisson's ratio contrasts (or high crack densities) in the coalbed methane reservoir, while source-receiver azimuth sorting i s used to detect preferential directions of azimuthal anisotropy cause d by the fracturing system of coal. Two modeling techniques (using ray tracing and reflectivity methods) predict the effects of fractured co al-seam zones on angle-dependent P-wave reflectivity. Synthetic common -midpoint (CMP) gathers are generated for a horizontally layered earth model that uses elastic parameters derived from sonic and density log measurements. Fracture density variations in coalbeds are simulated b y anisotropic modeling. The large acoustic impedance contrasts associa ted with the sandstone-coal interfaces dominate the P-wave reflectivit y response. They far outweigh the effects of contrasts in anisotropic parameters for the computed models. Seismic AVO analysis of nine macro bins obtained from the 3-D volume confirms model predictions. Areas wi th large AVO intercepts indicate low-velocity coals, possibly related to zones of stress relief. Areas with large AVO gradients identify coa l zones of large Poisson's ratio contrasts and therefore high fracture densities in the coalbed methane reservoir. The 3-D AVO product and P oisson's variation maps combine these responses, producing a picture o f the reservoir that includes its degree of fracturing and its possibl e stress condition. Source-receiver azimuth sorting is used to detect preferential directions of azimuthal anisotropy caused by the fracturi ng system of coal.