4D processing and time-lapse azimuthal amplitude analysis using legacy surveys for Niobrara reservoir characterization, Wattenberg field, Colorado

Abstract

Understanding changes in reservoir properties with time due to hydraulic fracture stimulation and production is important. Time-lapse (4D) seismic data analysis enables investigation of stimulated and produced reservoir volumes and helps forecast reservoir performance. Time-lapse seismic has proven beneficial for monitoring conventional high porosity reservoirs but its application to unconventional reservoirs is still in its infancy. Monitoring unconventional reservoir properties relies on the investigation of the natural fracture network affected by stimulation and production. This study investigates the potential of legacy seismic surveys for time-lapse azimuthal anisotropy analysis in Wattenberg Field, Denver Basin, Colorado. A workflow for co-processing and methods of analyzing reservoir changes using legacy surveys is developed. The workflow and methods can be used in Wattenberg and potentially in other resource plays. The application of legacy 4D surveys to analyze unconventional reservoirs requires careful applications of cross-equalization for preservation of time-lapse amplitude changes. Despite the differences in acquisition geometries and parameters this study resulted in normalized root mean square (NRMS) differences of .28 or 28% in the overburden interval above the Niobrara. A widely accepted threshold of .30 or 30% is considered as a measurable index of excellent repeatability. Azimuthal amplitude variation analysis is used to detect the orientation of the isotropy plane. The azimuth of the isotropy plane indicates the dominant orientation of the fractures affecting seismic anisotropy. Fracture orientation does not always coincide with the maximum horizontal stress. A reduction in amplitude variation with azimuth is observed near faults and is postulated to be a result of multiple fracture orientations. This study provides new insight by illuminating fracture orientations that were not captured by microseismic data or image logs in horizontal wells. Time-lapse analysis in azimuthal amplitude variation shows anisotropy increases in the reservoir interval. The increase in anisotropy from baseline to monitor is interpreted as being caused by gas coming out of solution in the more fractured parts of the reservoir. Investigating changes in the azimuth and magnitude of anisotropy is beneficial for monitoring unconventional reservoirs.