Zeitschrift für Geologie und Geophysik
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A Depth Conversion Methodology by Optimally Integrating Seismic Velocity with Well Velocity for Effectively Resolving the Depth Uncertainties

Zubair Z

The accuracy of velocity model plays a critical role in optimal time-to-depth conversion. Velocity models attained by traditional approaches are far from the reality as the velocity variation in the subsurface is much more complex than we perceive. Therefore, it is vital to accurately resolve the geological structures, even minor errors of 2 to 3 meters are significant for horizontal well placement near the base of reservoir. In order to have better understanding of geological structure to enhance recovery, we need to have an accurate velocity model to enhance depth imaging. Traditional layer cake depth conversion method needs a series of layers for building up velocity model. The layer surfaces tied to well tops, have good tie in depth domain, since velocity is calculated as an average of layer. However, intra-zone reflections due to changes in velocity are not well characterized by overall average zone values. This may lead to depth mis-ties or uncertainties within intra-zone layers. In developed fields with many wells, we can use existing well data to enhance the velocity estimation which therefore improves quality of depth imaging. By incorporating and adjusting seismic velocity according to well velocity we can resolve to some extent the lateral and vertical velocity variations in the inter-well space and the intra-zone velocity changes and manages to get an improved tie for intra- zone reflections and depth conversion away from well control.