Stress-induced anisotropy in brine saturated shale

Abstract

S U M M A R YThis paper reports the intrinsic and crack-induced anisotropic properties of a set of preserved, brine-saturated shale samples and their response to external stresses. We used undrained multistage triaxial tests to evaluate how the ultrasonic wave velocities and their anisotropy changed with increasing isotropic and differential stress conditions. In addition, the impact of stress orientation with respect to fabric orientation was evaluated. An array of ultrasonic transducers allowed to measure five independent wave velocities which were used to calculate the elastic properties of the shale. Results indicate that in this shale P-and S-wave velocities vary with stress in a different manner dependent on the maximum principal stress orientation with respect to the fabric. Where the maximum stress is normal to bedding, V pv and V s1 increase monotonically with increasing effective stress. However V ph and V sh decrease during individual loading stages but increase from stage to stage as confining pressure increases. The reverse occurs when the microfabric is parallel to the maximum principal stress. Where the maximum stress is bedding normal, velocity anisotropy decreases as differential stress increases; when maximum stress is fabric parallel, anisotropy increases. Elastic anisotropy is related to the initial composition and the spatial distribution of the different minerals (fabric) in the sediment and the presence of microfractures, while changes in elastic anisotropy result from the applied stresses, their orientation with respect to the rock fabric and the degree of stress anisotropy.

Parameterization of elastic stress sensitivity in shales

“…The suggested method works quite well for predicting anisotropic elastic moduli of shales on the basis of CPD and silt fraction. However, it fails to predict stress dependency of elastic properties of shales (Pervukhina et al, 2008a(Pervukhina et al, , 2008b, which has been observed in many experiments (e.g., Jakobsen and Johansen, 2000;Dewhurst and Siggins, 2006;Delle Piane et al, 2010;Kuila et al, 2011).…”

Layer Caused an Anisotropic Terahertz Response of a 3D-printed Simulative Shale Core

“…5 Ultrasonic measurements under variable saturation and pressure conditions in the laboratory have revealed that anisotropy of elastic wave propagation is caused mainly by the preferential alignment of mineral grains and thin layers. 6,7 Near cracks, the effective density and thickness are clearly different from those at other locations. Such heterogeneity is minimized at high confining pressures upon the closure of cracks.…”

“…Ultrasonic systems have been widely used to study shale anisotropy in the laboratory. 6,7 These studies have mainly focused on the elastic properties of shales, and have revealed that the preferential alignment of clay minerals and cracks is the major factor leading to the intrinsic anisotropy of shales.…”

Development of far-infrared attenuated total reflectance spectroscopy for the mineralogical analysis of shales

“…Quantitative analysis was performed on the XRD data using the commercial package SIROQUANT (Sietronics Pty Ltd). Further details of the XRD method can be found elsewhere [36]. For XRD analysis bulk samples were pre-ground for 15 s in a tungsten carbide mechanical mill to pass through a 0.5 mm sieve.…”