Vp/Vs in unconsolidated oil sands: Shear from Stoneley

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Abstract
The SDT-A sonic tool was tested in a borehole in the Orinoco heavy oil belt, eastern Venezuela. The sonically slow reservoir consists of unconsolidated quartz sand interbedded with shale. Full‐waveform analysis yields both compressional and shear slownesses. We calculated the shear‐wave slowness from the Stoneley slowness; compressional and Stoneley slownesses were determined using modified semblance techniques. The compressional velocity is relatively fast in the heavy oil pay zone compared to the remainder of the well. Heavy oil (8 API) possesses a finite rigidity at sonic frequencies, and the rigidity of the hydrocarbon adds to the stiffness of the poorly consolidated sand. The sand would not otherwise yield such a high velocity. Compressional and shear velocities of samples from eight whole cores were measured in the laboratory, and the core velocities were found to be consistent with the logs. Especially encouraging is the agreement of the laboratory shear with the shear log derived from Stoneley. The ratio of the compressional‐to‐shear velocities, [Formula: see text], is sensitive to fluid saturation and rock fabric. The oil sands have a [Formula: see text] of less than 2.5. The shales in the well have a [Formula: see text] of greater than 2.5. We found that water‐saturated formations are governed by Biot’s theory, while oil sands are better described by scattering theory. A third arrival has been identified as a leaky compressional mode trapped in the borehole. The velocity of the mode is dominated by the slowness of the borehole mud.

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