We obtained the complete set of dynamic elastic stiffnesses for a suite of “shales” representative of unconventional reservoirs from simultaneously measured P- and S-wave speeds on single prisms specially machined from cores. Static linear compressibilities were concurrently obtained using strain gauges attached to the prism. Regardless of being from static or dynamic measurements, the pressure sensitivity varies strongly with the direction of measurement. Furthermore, the static and dynamic linear compressibilities measured parallel to the bedding are nearly the same whereas those perpendicular to the bedding can differ by as much as 100%. Compliant cracklike porosity, seen in scanning electron microscope images, controls the elastic properties measured perpendicular to the rock’s bedding plane and results in highly nonlinear pressure sensitivity. In contrast, those properties measured parallel to the bedding are nearly insensitive to stress. This anisotropy to the pressure dependency of the strains and moduli further complicates the study of the overall anisotropy of such rocks. This horizontal stress insensitivity has implications for the use of advanced sonic logging techniques for stress direction indication. Finally, we tested the validity of the practice of estimating the fracture pressure gradient (i.e., horizontal stress) using our observed elastic engineering moduli and found that ignoring anisotropy would lead to underestimates of the minimum stress by as much as 90%. Although one could ostensibly obtain better values or the minimum stress if the rock anisotropy is included, we would hope that these results will instead discourage this method of estimating horizontal stress in favor of more reliable techniques.

We evaluate acoustic scattering and Wave Induced Fluid Flow (WIFF) mechanisms as a source of velocity dispersion in experimental compressional P- and shear S- waves. The latter was carried out by implementing the Self Consistent (SC) approach and the Chapmańs squirt flow model over a data set consisting of frequency- and pressure dependent velocities obtained by Sothcott, McCann, and O'Hara (2000, The influence of two different pore fluids on the acoustic properties of reservoir sandstones at sonic and ultrasonic frequencies. 70th SEG Meeting, Expanded Abstracts, 1883–1886) on brine saturated Clashach sandstones. Modelled velocities are further used to estimate attenuation values for both P- and S- waves. Error estimation between experimental data and theoretical modelling suggest that WIFF is the main mechanism in defining dispersion and attenuation on the studied samples.

This work presents a non-linear Self-Consistent (SC) micromechanics method to model the observed physical elastic properties of a terrigenous formation with the purpose to obtain its depth mineral volume fractions profile. In this approach, it is first assumed that the observed physical elastic properties obtained from well logs, such as the density $$\rho_{o}$$ρ o and the elastic compressional $$Vp_{o}$$V p o and shear $$Vs_{o}$$V s o velocities, are a non-linear relationship of the unknown mineral volume fractions $$\alpha$$α . Then, a gradient descent algorithm is implemented to seek for those volume fractions $$\alpha$$α for which differences between modelled and observed physical elastic properties are minimum. It is assumed that quartz, calcite and clay are the main comprising minerals of the formation. Obtained volume fractions profile follow the same general trends to those estimated by implementing the Linear Least-Squares Inversion LLSI method which is widely used in petrophysical analysis to obtain mineral concentrations from density $$\rho_{o}$$ρ o , photoelectric effect $$Pe_{o}$$P e o and compressional slowness $$\Delta tp_{o}$$Δ t p o well logs. Results also show that calcite and clay volume fractions from these two methods are highly correlated while quartz volume fractions show low correlation. Further comparison between clay concentrations from SC method with clay concentrations calculated from direct measurements of gamma ray GR well logs used as a guideline also exhibits high correlation. These results suggest that the SC method is better suited to obtain clay and calcite volume fractions rather than quartz volume fractions. However, SC method can provide with insights about the general distribution of quartz along the borehole.