Microstructural controls on elastic anisotropy of finely laminated Mancos Shale (original) (raw)
Related papers
Elastic wave velocity evolution of shales deformed under uppermost crustal conditions
Journal of Geophysical Research: Solid Earth
Conventional triaxial tests were performed on a series of samples of Tournemire shale along different orientations relative to bedding (0°, 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 80 MPa, and at strain rates ranging between 3 × 10 −7 s −1 and 3 × 10 −5 s −1. During each experiment, P and Swave elastic velocities were continuously measured along many raypaths with different orientations with respect to bedding and maximum compressive stress. This extensive velocity measurement setup allowed us to highlight the presence of plastic mechanisms such as mineral reorientation during deformation. The evolution of elastic anisotropy was quantified using Thomsen's parameters which were directly inverted from measurement of elastic wave velocity. Brittle failure was preceded by a change in P wave anisotropy, due to both crack growth and mineral reorientation. Anisotropy variations were largest for samples deformed perpendicular to bedding, at the onset of rupture. Anisotropy reversal was observed at the highest confining pressures. For samples deformed parallel to bedding, the P wave anisotropy change is weaker.
Impact of fabric, microcracks and stress field on shale anisotropy
Geophysical Journal International, 2006
Few data are available on shales in terms of seismic to ultrasonic properties and anisotropy, although all are important with regards to imaging problems often encountered in such lithologies. Additionally, mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions. The results presented here are derived from a set of experiments run on Muderong Shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response. This shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy, both likely the result of the presence of fluid-filled cracks in a low-permeability porous medium. Changes in velocity and V p /V s ratios are dependent on both stress and smectite content. S-wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress (<20 MPa) is less sensitive to stress change than P-wave velocity. V p /V s ratios are noted to increase in this shale up to 20 MPa effective stress, then decrease slightly due to stress-induced loss of interlayer water in smectite. Intrinsic anisotropy comes from composition, a strong compaction fabric and the presence of microfractures; changes to ultrasonic anisotropy are the result of the magnitude of the stresses, their orientation with respect to the fractures and the degree of stress anisotropy.
The elastic anisotrophy of shales
Journal of Geophysical Research, 1994
Shales constitute about 75% of the clastic fill of sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure. The elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients Wlmn in an expansion of the clay-particle orientation distribution function in generalized Legendre functions. Application is made to the determination of the anellipticity of shales. For transverse isotropy the anellipticity quantifies the deviation of the P wave slowness curve from an ellipse and is shown to depend on a single coefficient W400 in the expansion of the clay-particle orientation distribution function. If W400 is small, the anellipticity may be neglected, as is apparently the case for a near-surface late Tertiary shale studied by Winterstein and Paulson. Strongly aligned clay particles result in a positive value of W400 and a positive anellipticity, in agreement with the majority of field measurements. However, less well ordered shales could have a significantly positive second moment W200 but only a small positive or even negative value of W400. For such shales the anellipticity would be small or negative despite a preferred alignment of clay particles in the bedding plane. Numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given.
Geophysical Prospecting, 2005
Shales are a major component of sedimentary basins, and they play a decisive role in fluid flow and seismic-wave propagation because of their low permeability and anisotropic microstructure. Shale anisotropy needs to be quantified to obtain reliable information on reservoir fluid, lithology and pore pressure from seismic data, and to understand time-to-depth conversion errors and non-hyperbolic moveout. A single anisotropy parameter, Thomsen's δ parameter, is sufficient to explain the difference between the small-offset normal-moveout velocity and vertical velocity, and to interpret the small-offset AVO response.
Effect of grain scale alignment on seismic anisotropy and reflectivity of shales
Geophysical Prospecting, 2004
The elastic properties and anisotropy of shales are strongly influenced by the degree of alignment of the grain scale texture. In general, an orientation distribution function (ODF) can be used to describe this alignment, which, in practice, can be characterized by two Legendre coefficients. We discuss various statistical ODFs that define the alignment by spreading from a mean value; in particular, the Gaussian, Fisher and Bingham distributions. We compare the statistical models with an ODF resulting from pure vertical compaction (no shear strain) of a sediment. The compaction ODF may be used to estimate how the elastic properties and anisotropy evolve due to burial of clayey sediments.
An in situ estimation of anisotropic elastic moduli for a submarine shale
Journal of Geophysical Research, 1994
Direct arrival times and slownesses from wide-aperture walkaway vertical seismic profile data acquired in a layered anisotropic medium can be processed to give a direct estimate of the phase slowness surface associated with the medium at the depth of the receivers. This slowness surface can, in turn, be fit by an estimated transversely isotropic medium with a vertical symmetry axis (a "TIV" medium). While the method requires that the medium between the receivers and the surface be horizontally stratified, no further measurement or knowledge of that medium is required. When applied to data acquired in a compacting shale sequence (here termed the "Petronas shale") encountered by a well in the South China Sea, the method yields an estimated TIV medium that fits the data extremely well over 180 ø of propagation angles sampled by 201 source positions. The medium is strongly anisotropic. The anisotropy is significantly anelliptic and implies that the quasi-shear mode should be triplicated for off-axis propagation. Estimated density-normalized moduli in units of km2/s 2) for the Petronas shale are All = 6.99 -+ 0.21, A33 = 5.53 _ 0.17, A55 = 0.91 _+ 0.05, and A13 -2.64 _+ 0.26. Densities in the logged zone just below the survey lie in the range between 2200 and 2400 kg/m 3 with an average value close to 2300 kg/m3. 21,659 21,660 MILLER ET AL.' ESTIMATION OF ANISOTROPIC ELASTIC MODULI
GEOPHYSICS, 2016
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 depende...
A multiscale methodology for the analysis of velocity anisotropy in organic-rich shale
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 unsatu-rated 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 mi-crocracks 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.