T his second part of an article about a large 3D VSP survey in Abu Dhabi describes the interpretation effort which quantifies the value that a 3D VSP seismic image can bring when supplementing even a 640-fold, highresolution surface seismic volume.It is understood that for recovery to be optimized and bypassed resources to be minimized, especially in later stages of field production, more accurate models of a reservoir's architecture and characteristics are needed. This first 3D VSP survey in Abu Dhabi characterized details of the reservoir that could not be derived from surface data or well-log data alone. The higher-quality, higher-resolution images made it possible to map detailed stratigraphy and important but previously unknown faults. The improved structural map and updated geologic model were verified by wells drilled inside the 3D VSP image areas. The effect of receiver array length and source effort on VSP qualityTo better understand the value in acquiring 3D VSP data with long borehole receiver arrays, processing tests using a conventional 12-level receiver configuration were conducted by using a subset of the 126-level VSP data. Using the same statics, velocity model, and other relevant processing parameters developed for the 126-level array data, a 3D VSP image was produced with data from only 12 geophone levels. Figure 1 compares VSP common depth point (VCDP) gathers at different offsets between the 12-and 126-level data. At an offset of 400 m, both the 12-and 126-level gathers show well organized energy from primary events. Good VSP images up to 400 m away from the well should be possible with both data sets. At the longer offset of 700 m, there does not appear to be any indication of primary events on the 12-level gather. However, due to higher fold, primary events are clearly visible on the 126-level gather. This result suggests the reason; in 12-level walkaway VSPs, it is difficult to image distances greater than 500 m from the wellbore, even though offsets up to 4 km
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.
Abu Dhabi Company for Onshore Oil Operations (ADCO) undertook a two-well 3D VSP pilot project in 2007. Because it was acquired concurrently with a high-resolution wide-azimuth surface seismic survey, it was at the time the largest 3D VSP ever recorded. The project consisted of four main parts: acquisition, processing, interpretation, and quantifying value. In part 1 of this paper the acquisition and processing of the 3D VSP is described with an emphasis on the lessons learned. Significant advances in processing are described that demonstrate how larger 3D VSP images with better amplitudes and structural preservation can be produced. In part 2, the results of the 3D VSP interpretation and economic evaluation effort are described and illustrate different ways that a VSP image can help characterize a hydrocarbon reservoir.
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