High-Resolution 3D VSP Processing - An Example From the Middle East

Abstract

Abstract The 3D VSP method is being increasingly employed as a tool to produce high-resolution images for detailed reservoir characterization and to address reservoir challenges. These challenges include thin layer reservoirs, thief zones and stratigraphic features that affect recovery. The main challenges in processing VSP datasets are twofold: First to ensure that the high frequency and better vector fidelity is being used and carried through to the final image. This requires special care and appropriately adapted processing techniques to the smaller scale and high frequency contained in the VSP data. Second, is dealing with the unique geometry of a 3D VSP, which has laterally varying fold coverage and aperture that has to be accounted for in order to minimize any footprint on the final image. In this project VSP processing advances have been made using data from the largest 3D VSP recorded to date, which was acquired in an Abu Dhabi oil field. Different types of static corrections were tested and optimized to recover the high frequencies required for optimum event delineation. A combination of static corrections that takes full advantage of the 3D VSP geometry and includes surface seismic data results that helped achieve optimal coherency of events. A careful analysis of the irregular fold geometry resulted in good target imaging using a detailed illumination analysis. Such an analysis aids in the correct treatment of the high resolution events and helps to interpret their character along the area illuminated. This analysis provides critical information about the velocity model and the corresponding kinematics. The ability of VSP's to recover high frequencies is demonstrated in this processing flow, by showing the difference in resolution between new high resolution surface seismic and the final 3D VSP image. Introduction The availability of 3D VSP data has resulted in more detailed characterizations of the reservoir because of the high resolution given by the VSP data compared to surface seismic techniques. Its usage includes detailed stratigraphic analysis of thin and often deep targets that the surface seismic cannot adequately image. In addition the VSP technology has been used in areas within complex near surface environments or areas where there is limited surface access. The use of receivers within the well has led to seismic images in the vicinity of the well that have high resolution and high signal to noise ratio. More importantly receivers in the borehole environment have led to high frequency data because of shorter travel paths. In the VSP case less energy is attenuated as it only travels once through the near surface weathering layer or complex overburdens. The high frequency recorded by the borehole array (Figure 1) consequently results in smaller Fresnel zones at the target in the vicinity of the well, therefore enhancing its lateral characterization.