Vertical seismic profiling in deviated wells: a study of the signal processing considerations when determining travel times and velocity anisotropy estimates from a vertical incidence and walkaway survey conducted in a deviated production well

McCallum, David Skye (2008) Vertical seismic profiling in deviated wells: a study of the signal processing considerations when determining travel times and velocity anisotropy estimates from a vertical incidence and walkaway survey conducted in a deviated production well. Masters thesis, Memorial University of Newfoundland.

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Abstract

Travel times are essential for processing and velocity determination from VSP surveys. Even though many wells drilled today are deviated from vertical, there is very limited geophysical literature on the effects of well deviation on travel time measurements and processing issues when a VSP is conducted in a deviated well. An investigation of the processing considerations associated with acquiring precise travel times from a vertical incidence and walkaway survey conducted in a deviated well was carried out. Compensation for well deviation by rotating the vertical component in-line with the downgoing P-wave particle motion for each source-receiver pair of the vertical incident survey had a negligible effect on acquired travel times. Rotation of the vertical component in-line with the downgoing P-wave particle motion for each source-receiver pair of the walkaway survey proved to follow conventional practice and the deviation of the well was not an issue. -- The issue of well deviation when acquiring travel times from VSP surveys conducted in deviated wells is a concern when characterizing a reservoir by determining aspects like anisotropic characteristics of a specific rock layer. Using the travel times acquired from both the vertical incidence and walkaway surveys, it is demonstrated, by producing a percent velocity anisotropy estimate of 17.1% for a marine shale, that the travel time-inversion method is well suited for a deviated well setting. This estimate is appropriate when compared to published values and to an independent estimate of 17.9% obtained by modifying the phase-slowness method using the same assumptions that govern the travel time-inversion method. Modifying the phase-slowness method also made it operationally less intense. In general, with the application of reasonable assumptions, velocity anisotropy measurements can be obtained within a deviated well without rigorous computational adjustments.

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