Leaky compressional mode is a dispersive fluid mode that propagates at formation compressional slowness at low frequencies and mud slowness at high frequencies. In slow formations, estimation of low frequency asymptote of the leaky mode is one of common ways to extract formation compressional slowness. It is therefore important to understand its dispersive behavior.

In this paper, we investigated behavior of the leaky compressional mode with an LWD sonic tool. The main structure of an LWD sonic tool is a rigid drill collar and it occupies a large fraction of a borehole cross-section area. The dispersion behavior of the leaky compressional mode is significantly depending on a series of factors. The formation and mud properties are, of course, primary factors affecting the dispersion, e.g. the formation compressional and mud slowness values are very different with an 8.25 inch LWD sonic tool in 12.25 inch borehole, the dispersive part tends to become invisible due to high attenuation. The presence of the tool also has a strong influence on the slowness dispersion and attenuation. This paper evaluates the effect of those factors and their implications on the sonic tool design and processing techniques.

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