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Compressional seismic wave velocities have been used with much success in defining structural features likely to serve as hydrocarbon reservoirs. Recent advances in data collection and processing and, as we will describe, a greater understanding of the factors governing wave propagation in rocks indicate much additional information regarding the in-situ temperature, degree of saturation, and pore pressure may be obtained if data are available on seismic attenuation and shear velocity in addition to compressional velocity.
Both velocity and attenuation are strongly temperature dependent. In dry sandstones, velocity increases rapidly with temperature to 100°C then levels off. Vp/Vs increases with tempeature. Shear and extensional attenuations increase rapidly with temperature from a Q of about 90 at room temperature to over 500 at 225°C. Shear attenuation is less than extensional attenuation below 200°C; the opposite is true above 200°C. In rock with a small amount of free water, thermal fracturing of rocks becomes dominant above 170°C decreasing velocities because of the increased porosity and increasing attentuation due to additional surface area in contact with water. In partly saturated rocks at elevated temperatures (200°C) attenuation is high in b lk compression and about half as great in shear. In fully saturated rocks, shear attenuation is high and bulk attenuation is low. Compressional velocity is greater in a fully saturated rock than in a dry or partly saturated rock. Shear velocity is less in a saturated rock than in a dry rock. Velocity and attenuation roughly follow an effective stress law.
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