A multiscale methodology for the analysis of velocity anisotropy in organic-rich shale

Abstract

Changes in the sources of velocity anisotropy and their relative magnitude as maturation progresses in organic-rich shale are still incompletely characterized in the rock-physics literature. As a result of the increasing importance of organic-rich shale as unconventional reservoirs, a more thorough understanding of the elastic behavior of shale is needed. We have formulated a comprehensive, multiphysics, multiscale experimental methodology for the characterization of the intrinsic (syn-lithification) and extrinsic (postlithification) factors contributing to velocity anisotropy. Application of this methodology to unsaturated samples also enabled the characterization of the shale frame for fluid substitution modeling. The methodological framework was then tested on a set of five naturally matured organic-rich shale samples. In this experimental methodology, we combined classical rock-physics measurements, e.g., ultrasonic velocity and emergent high-resolution imaging techniques, such as X-ray diffraction (XRD), scanning electron microscopy, confocal laser scanning microscopy, and X-ray microtomography to better characterize the heterogeneous and microstructurally complex shale at all scales. The use of XRD-based lattice-preferred orientation measurements in conjunction with conventional ultrasonic velocity experiments confirmed that the degree of alignment of the mineral matrix governed the intrinsic anisotropy of organic-rich shale. The closure of soft, crack-like porosity, as identified from axial strain data, was identified as the extrinsic source governing the pressure sensitivity of velocity anisotropy. We determined, for the set of samples included in this study, that the intrinsic anisotropy was the dominant source of anisotropy at all confining pressures. Indeed, at low confining pressures, the opening of microcracks contributed no more than 30% of the total velocity anisotropy. Applying these results to saturated rocks at depth indicated that, for these shales, the extrinsic, crack-based sources, will contribute no more than 30% of the shale anisotropy in situ.

On the evolution of the elastic properties of organic-rich shale upon pyrolysis-induced thermal maturation

“…Although this increase in pressure-sensitivity is indicative of compliant features aligned with the bedding plane, the greater magnitude decrease in V SH ð90°Þ relative to V SV ð90°Þ (Figure 20) and lack of observed aligned cracking (Figures 13 and 14) indicate that microcracks are not the source of this pressure-sensitivity. We lack the necessary X-ray microtomography data to resolve any possible aligned features within the sample; however, the magnitude of the pressure-sensitivity (AE0.04) is very low relative to previously studied pyrolyzed samples (Allan et al, 2014(Allan et al, , 2015 indicating that microcracking within the sample, if any, is negligible.…”

“…A schematic representation of the P-and S-wave phases recorded during acoustic velocity experiments after Allan et al (2015). (a and c) Vertically propagating P-and S-waves, (b) horizontally propagating P-wave, and two horizontally propagating Swaves -(d) one polarized perpendicular to bedding and (e) the other polarized parallel to bedding.…”

“…Again, the Green River sample shows the greater degree of misalignment (Table 2). To remove the effect of the widely variable clay content of the two samples, we compare the "average degree of alignment" in each sample as in Allan et al (2015) and equation 1, where CPO is the set of maximum (001) poles for each mineral and χ is the set of corresponding volume fractions:…”

“…When open, and sufficiently coaligned, these microcracks may alter the elastic anisotropy of the shale. The slow deposition from suspension and burial-induced compaction of clay platelets typically generates a strongly anisotropic rock frame in shales, which is preserved in the absence of bioturbation (Kaarsberg, 1959;Jones and Wang, 1981;Vernik and Nur, 1992;Johnston and Christensen, 1995;Hornby, 1998;Wang, 2002;Vasin et al, 2013;Allan et al, 2015), although induced, coaligned microcracks can significantly alter this intrinsic anisotropy (Vernik, 1993;Johnston and Christensen, 1995;Dewhurst and Siggins, 2006;Allan et al, 2014). The magnitude of the crack-induced alteration of elastic anisotropy will depend on the number of cracks, the degree of alignment, and their aperture as a function of pressure, i.e., whether they are open or closed cracks.…”

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

“…8 Besides, the scanning electron microscopy (SEM) technique has been commonly used for the microstructure determination of shales. 9 The planes perpendicular and parallel to the bedding were analyzed by SEM, showing some patchy kaolinite and acicular illite with preferred orientations parallel to the bedding.…”