A comparative study of the anisotropic dynamic and static elastic moduli of unconventional reservoir shales: Implication for geomechanical investigations

Abstract

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.

Laboratory experiments simulating poroelastic stress changes associated with depletion and injection in low‐porosity sedimentary rocks

“…The derived effective stress coefficient for ultrasonic velocities (dynamic) in the same specimens [ Ma and Zoback , ] was found to differ from that by the static volumetric strain shown in this study. Recent studies on dynamic and static properties in the same shale specimens also confirmed the fundamental differences between the two physical quantities and their sensitivities to stresses [ Meléndez‐Martínez and Schmitt , ; Ong et al , ]. Although the transmitting waves through the rock also induce strain (by vibration), the amount of dynamic deformation is a few orders smaller than the strain produced by the static loading [cf.…”

Investigating the Onset of Strain Localization Within Anisotropic Shale Using Digital Volume Correlation of Time‐Resolved X‐Ray Microtomography Images

“…Both the stress field and the evolving fracture network geometry influence the anisotropy of the strength (e.g., Sarout & Guéguen, ) and the elasticity (e.g., Sviridov et al, ) of deforming shales. Higher lithostatic stresses tend to reduce porosity, which increases the uniaxial compressive strength (e.g., Donath, ) and compressional wave velocity (e.g., Dewhurst & Siggins, ; Melendez‐Martinez & Schmitt, ; Ong et al, ; Sarout & Guéguen, ; Zadeh et al, ). Consequently, elastic wave velocity measurements have been used to infer the geometries of evolving porosity and fracture networks in shales (e.g., Baird et al, ).…”

Static and dynamic pressure sensitivity anisotropy of a calcareous shale

“…; Cheadle, Brown, and Lawton ; Homand et al . ; Melendez Martinez and Schmitt ). With regard to multi‐faced polyhedral, the measurements of Nara et al .…”

“…Examination of equation in light of the symmetry of S together with the requirement that there be at most five independent moduli demands ν 21 = ν 12 , ν 13 /ν 31 = E 1 /E 3 , and μ 13 = μ 31 . Melendez Martinez and Schmitt () review a number of the relationships between equations , , and useful for converting between stiffnesses, compliances, and engineering moduli; we include these in Appendix A for the sake of completeness.…”