The evolution and effect of the velocity model on various aspects of surface seismic data analysis

Well log data, which are measured in depth, provide detailed information only in the vicinity of the boreholes, while surface seismic data, which are measured in units of time, can reveal important structural and stratigraphic changes beyond well locations. Integrating seismic with well data provides a much more extensive understanding of the subsurface and is key for modelling and developing a subsurface reservoir. Therefore, developing a good velocity model that converts seismic data from the time domain to the depth domain provides an effective means to integrate data from the two domains, depth and time. This study examines the value of additional well and seismic data acquired over the course of four years, and the respective merits of updated velocity models. The Illinois Basin - Decatur Project (IBDP), is a demonstration project funded by the US Department of Energy, and involves injection of CO₂ into a highly porous, deep sandstone reservoir several kilometres below the surface. Four velocity models have been created to support a carbon dioxide (CO₂) geological storage project located at the Archer Daniels Midland Company (ADM) in Decatur, Illinois, USA. Predicted formation depths from the different velocity models for subsequently drilled wells will be compared. As the CO₂ storage project has matured and new data have become available, velocity models have been developed representing the years: 2010, 2011, 2012, and 2013. The models enabled depth predictions of geologic strata in subsequently drilled wells in late 2010 and in 2012. The 2012 model integrated the measurements from three wells and two seismic surveys to update the reservoir model. In 2013, a thorough study comparing well and seismic data identified weaknesses in some measurements, for example, the 3D surface seismic data was negatively influenced by inter-bed multiple reflection events and indicated the need for seismic reprocessing of the 3D data. The evolution of the velocity models has been driven by the progressive availability of new data. Because uncertainty in velocity models occur away from wells used for calibration, velocity model updates with newly acquired data will continue to be beneficial given the demands for accurate estimation of structure and properties in the spaces between wells.