Abstract

Sub-vertical natural fracturing is critical for ensuring economic gas production from tight sand reservoirs in many provinces. Over the last seven years, the US Department of Energy (DoE) has sponsored a series of programmes in the continental United States to develop cost-effective technologies for investigating naturally fractured reservoirs. The results from a study of a Laramide-age faulted anticline reservoir in the Lower Fort Union Formation of the Wind River Basin, Wyoming are presented. At the field site a multi-azimuth, multi-offset 3D compressional wave seismic survey was acquired and processed using azimuth-dependent processing followed by multi-azimuth attribute analysis. Geophysical attributes such as velocity, frequency, reflectivity and amplitude variation with offset, together with geological attributes such as depth below closure and distance from faults, were correlated to estimated ultimate recovery of gas for established wells within the field. The correlations were made using linear statistical methods, non-linear rank methods and neural networks to combine attribute results. The geophysical attributes and other geological information were combined into prospectivity maps for areas of highest fracture density. Based on the geophysical and geological results an 80% success rate was achieved at prospecting well locations.

A major aspect of the DoE programmes has been the investigation of costeffective protocols for fracture detection. From this study the following procedure is recommended for cost-effective mapping of fractures and associated gas: (1) perform field and remote-sensing reconnaissance of structural trends; (2) acquire 3D P-wave surveys with offsets equal to or greater than target depth in all azimuths; (3) process in at least two dominant structural azimuths; (4) combine best correlated geophysical and geological attributes to fractures for future fracture zone mapping. The implications of the results from this survey have important consequences for the design and implementation of multi-azimuth, multi-offset data acquisition on land and for marine surveys where multi-component ocean bottom cable surveying is used.

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