Velocity, attenuation, and microseismic uncertainty analysis of the Niobrara and Montney reservoirs

Abstract

Time-lapse reservoir characterization with surface seismic provides greater spatial information about reservoir physical properties, and delineates reservoir scale changes. Identification of reservoir deformation due to hydraulic fracturing and production improve reservoir models by mapping non-stimulated and non-producing zones. Monitoring these time-variant changes improves the prediction capability of reservoir models, which in turn should lead to improved well and stage placement. In Wattenberg Field, the Reservoir Characterization Project (RCP) at the Colorado School of Mines (CSM) and Anadarko Petroleum Corporation (APC) collected time-lapse, multicomponent seismic data in order to characterize the reservoir fracture changes caused by hydraulic fracturing and production in the Niobrara Formation and Codell Sandstone member of the Carlile Formation. Three seismic surveys help understand the dynamic reservoir changes caused by hydraulic fracturing and production of eleven horizontal wells within a one-square mile section (Wishbone Section). A baseline survey was recorded immediately after the wells were drilled, another survey after stimulation, and a third survey after two years of production. A robust layer stripping method is used to quantify 4D velocity and attenuation from pre-stack seismic data. Processing of the data before attenuation analysis includes noise reduction, regularization of amplitudes, and statics. Data show that time-lapse, pre-stack velocity and attenuation estimates are sensitive to hydraulic stimulation and production. Time-lapse velocity and attenuation results are integrated with image logs, surface microseismic, tracer data, and production information to analyze how faults, joint sets, and well spacing, affect stimulation, early term production, and late term production of the eleven horizontal wells in the Wishbone Section. Data demonstrate that faults in the reservoir limit lateral stimulation and allow hydraulic fracture fluids to move to other reservoir facies vertically within the Wishbone Section. Attenuation and velocity changes are observed in the western portion of the survey. Higher producing wells are also located the western portion of the study area. Borehole microseismic is a common tool used to evaluate hydraulic stimulation. A challenge in microseismic monitoring is quantification of survey acquisition and processing error, and how these errors jointly affect estimated locations. Quantifying error and uncertainty has multiple benefits, such as more accurate and precise estimation of locations, anisotropy, moment tensor inversion, and, potentially, allowing for detection of 4D reservoir changes. Processing steps are applied to a downhole microseismic dataset from Pouce Coupe, Alberta, Canada. A probabilistic location approach is implemented to identify the optimal bottom well location based upon known source locations. Probability density functions (PDF) are utilized to quantify uncertainty and propagate it through processing, including in source location inversion to describe the 3D event location likelihood. Event locations are calculated and an amplitude stacking approach is used to reduce the error associated with first break picking and the minimization with modeled travel-times. Changes in the early processing steps have allowed for understanding of location uncertainty and improved the mapping of the microseismic events. The overall research illustrates that reservoir heterogeneity significantly affects hydraulic stimulation and production. Integration of multi-disciplinary data is vital for reservoir characterization in shale reservoirs. Additionally, this work shows how the assessment of data uncertainty is necessary for future development of reservoir characterization tools in anisotropic reservoirs. The developed workflows provide new approaches for appraising shale reservoirs, and quantifying uncertainty associated with geophysical data.