Compressional and shear wave velocities relationship in anisotropic organic shales

Abstract

Understanding the relationship between compressional and shear wave velocities and their anisotropic characteristics in self-resourcing reservoir shales is of considerable interest for petrophysical, geophysical, and geomechanical applications of unconventional resources exploration and production. We compile laboratory measurements on the ultrasonic velocity of known shale reservoirs worldwide. Their compressional and shear wave velocities propagating in the vertical and horizontal directions can be characterized by a linear model with the coefficients of determination (R2) close to 0.9. The P-to-S-wave velocity ratio in the vertical direction is overall higher than that in the horizontal direction. The P-to-S-wave velocity ratio decreases with decreasing P-wave velocity due to the increasing organic matter content and hydrocarbon-filled porosity. Mineralogy influences the relationship between compressional and shear wave velocities to a minor degree. Carbonate-rich shales have a higher P-to-S-wave velocity ratio than silica-rich shale when the P-wave velocity is lower than 4 km/s and a lower ratio when the P-wave velocity is higher than 5 km/s. By employing anisotropic fluid substitution, we justify that the model derived from the laboratory measurements can apply to shale reservoirs containing gas or volatile oil. We apply the linear models to predict the shear wave velocity in two vertical wells of shale reservoirs. The predictions of shear wave velocity in the targeted reservoirs have less than 3% errors, demonstrating that the proposed linear models, to the first order, can predict shear wave velocity in unconventional shale reservoirs.