Shale Dynamic Properties & Anisotropy under Triaxial Loading (original) (raw)
Related papers
PHYSICS AND CHEMISTRY OF THE EARTH, 2007
This paper is concerned with the experimental identification of the whole dynamic elastic stiffness tensor of a transversely isotropic clayrock from a single cylindrical sample under loading. Measurement of elastic wave velocities (pulse at 1 MHz), obtained under macroscopically undrained triaxial loading conditions are provided. Further macroscopic (laboratory scale) interpretation of the velocity measurements is performed in terms of (i) dynamic elastic parameters; and (ii) elastic anisotropy. Experiments were performed on a Callovo-Oxfordian shale, Jurassic in age, recovered from a depth of 613 m in the eastern part of Paris basin in France. Moreover, a physically-based micromechanical model is developed in order to quantify the damaged state of the shale under loading through macroscopic measurements. This model allows for the identification of the pertinent parameters for a general transversely isotropic orientational distribution of microcracks, superimposed on the intrinsic transverse isotropy of the rock. It is directly inspired from experimental observations and measurements. At this stage, second-and fourth-rank tensors a ij and b ijkl are identified as proper damage parameters. However, they still need to be explicited in terms of micromechanical parameters for the complex case of anisotropy. An illustration of the protocole of this microstructural data recovery is provided in the simpler case of isotropy. This microstructural insight includes cavities geometry, orientation and fluid-content.
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...
Rock Physics, Geomechanics and Rock Properties in Shales — Where are the Links?
Proceedings of the First Southern Hemisphere International Rock Mechanics Symposium, 2008
Understanding shale behaviour is of increasing importance to the petroleum industry and also impacts on engineering issues such as landslides and hazardous waste disposal. Few data are currently available regarding geomechanical, petrophysical and dynamic elastic properties of shales that have been properly preserved and tested under controlled pore pressure conditions. The research detailed here involves triaxial testing of shales to determine failure envelopes, with ultrasonic measurements taken during the application of differential stress through to failure. Empirical relationships are then derived between the geomechanical properties and more easily (or regularly) measured physical and petrophysical properties such as porosity, clay content, cation exchange capacity and dielectric properties. The dynamic elastic properties of shales and their anisotropy are shown to be significantly impacted by maximum principal stress orientation with respect to microfabric and microfracture orientation.
Static and dynamic pressure sensitivity anisotropy of a calcareous shale
Geophysical Prospecting, 2016
Optimizing the productivity of nonconventional, low-permeability "shale" reservoirs requires detailed knowledge of the mechanical properties of such materials. These rocks' elastic anisotropy is acknowledged but usually ignored due to difficulties in obtaining such information. Here we study in detail the dynamic and static elastic properties of a suite of calcareous mudstones from the nonconventional Duvernay reservoir of Alberta, Canada. The complete set of transversely isotropic elastic constants is obtained from strategically oriented ultrasonic transducers to confining pressures of 90 MPa. Wave speed anisotropies of up to 35% are observed at even the highest confining pressures. Furthermore, the stress sensitivity of the wave speeds, and hence moduli, is itself highly dependent on direction with speeds taken perpendicular to the bedding plane being highly nonlinearly dependent on pressure, whereas those along the bedding plane show, unexpectedly, nearly no pressure dependence. These observations are in qualitative agreement with the preferentially oriented porosity and minerals seen in scanning electron microscope images. These results may be significant to the interpretation of sonic logs and azimuthal amplitude versus offset for principal stress directions, for the concentration of stress within such formations, and for estimation of static engineering moduli from sonic log wave speeds.
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.
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.
The Elastic Properties of Clay in Shales
Journal Of Geophysical Research: Solid Earth, 2018
To accurately characterize shales, rock physics models must account for anisotropic clay minerals. Due to compliant regions between clay platelets, the elastic stiffness of clay in shales is much less than that of clay minerals. The clay in shales can be modeled as anisotropic clay platelets embedded in a softer interparticle region containing clay-bound water.