Compressional-To-Shear-Wave Velocity Ratio in Organic Shales and Acoustic Dispersion in Low Permeability Unconventional Reservoir Rocks

In situ P-wave and S-wave velocity measurements in a variety of organic-rich shales exhibit compressional-to-shear-wave velocity ratios that are significantly lower than lithologically similar fully brine-saturated shales having low organic content. It has been hypothesized that this drop could be explained by the direct influence of kerogen on the rock frame and/or by the presence of free hydrocarbons in the pore space. Theoretical bounding equations, using pure kerogen as an end-member component without associated gas, indicate that kerogen reduces both the P-wave and S-wave velocities but does not in general reduce their ratio. The theoretical modeling is consistent with ultrasonic measurements on organic-shale core samples that show no dependence of velocity ratios on kerogen volume alone. Sonic-log measurements of compressional and shear-wave velocities in seven organic-rich shale formations deviate significantly from the Greenberg-Castagna empirical brine-saturated shale trend towards lower velocity ratios. In these formations, and on core measurements, Gassmann fluid substitution to 100% brine saturation yields velocity ratios consistent with the Greenberg-Castagna velocity trend for fully brine-saturated shales, despite the high organic content. These measurements, as well as theoretical modeling, all suggest that the velocity-ratio reduction in organic shales is best explained by the presence of free hydrocarbons.

The limitation of the Greenberg-Castagna shear-wave velocity prediction method when applied to organic-rich shales has been resolved, by modifying the original Greenberg-Castagna algorithm. The modified workflow accurately predicts shear-wave velocity for seven organic-shale formations with appreciable solid organic matter to within ±1% percent mean error.

For a number of low-permeability well-lithified shales, utilizing laboratory measurements on dry and fully brine-saturated samples as well as comparing to log data and theoretical modeling, we find no statistically significant intrinsic dispersion from seismic to sonic and laboratory-measurement frequencies due to fluid effects. At in situ stress conditions, the Gassmann zero-frequency P-wave velocity prediction for a Permian-basin sample was within 0.2% to 2.2% of the measured velocity on the brine-saturated sample at ultrasonic frequency. Based on the Biot-Gassmann model, the characteristic frequency occurs at about 10^10 Hz. Applying a squirt-flow model also predicts a transition to the high-frequency regime occurring at about 10^9 Hz.