Shale Dynamic Properties & Anisotropy under Triaxial Loading (original) (raw)
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.
A Study of the Elastic Behaviour Presented by Different Types of Sedimentary Rocks
Revista Brasileira de Geofísica, 2015
ABSTRACT. The purpose of this paper is to study the elastic behavior of the different kinds of sedimentary rocks from outcrops of the Middle-West region of the United States of America, among those, Berea sandstone from Bedford formation (Ohio), Indiana limestone from Salem formation (Indiana) and Silurian dolomite from Thornton formation (Illinois). To do so, it has been made in the Laboratório de Física de Rochas from Cenpes (Centro de Pesquisas e Desenvolvimento da Petrobras ), measurements of porosity, density and elastic wave propagation velocity presented by each type of studied rock. The wave propagation velocities were estimated by measuring the transit time ultrasonic pulses transmitted through the samples. From the results obtained, it was possible to compare the measured velocities with predictions from theoretical models, as well as to observe correlations between the petrophysical properties of rocks and its seismic behavior. Understanding these correlations helps to im...
SN Applied Sciences
Different geomechanical properties such as elastic modulus/deformation modulus, Poisson's ratio (υ), uniaxial compressive strength, shear strength properties, tensile strength and point load index are widely used for rock mass characterisation in geological and geotechnical engineering. However, there are no such direct methods by which these properties can be obtained in the laboratory or in situ, without following time-consuming and a laborious procedure. Thus, ultrasonic technique, an indirect method, was found reliable to determine these properties in rocks. Many researchers have studied the correlation between the compressional wave velocity (Vp) and the geomechanical properties of sedimentary rocks such as sandstones and carbonates. However, these correlations have not considered the change in rock mineralogy, porosity and saturation conditions when deriving relationships. Hence, in this study, the authors attempt to understand the variability of Vp with the help of a fragmented analysis of major mineral constituents, porosity variation and saturation conditions for rock mass classification based on Vp. A review of the existing studies on the relationship between the rock properties and Vp has been used to perform this analysis. The resulting template can be a basis for interpreting more realistic lithology-based geomechanical behaviours and thus highlights the importance of an integrated study involving geological, petrophysical and engineering data. The results derived from fragmented analysis indicate that with an increase in quartz content and a subsequent decrease in feldspar content in sandstones, Young's modulus (E) and Vp increase. With an increase in porosity from 2 to 40%, there is a decrease in E and Vp values for all types of saturation scenarios (water, gas, brine and oil saturated) for sandstones and carbonates.
Stress dependency of elastic properties of shales: The effect of uniaxial stress
2011
Understanding seismic anisotropy in shales is important for quantitative interpretation of seismic data, 4D monitoring and pore pressure prediction. Along with intrinsic anisotropy caused by preferred mineral orientation that is common in shales, anisotropic stress is an important factor that affects shale elastic response. While variations of elastic coefficients with anisotropic stress have been the subject of experimental studies, theoretical insight is still largely lacking. Here we suggest a new model that allows parameterization of the stress dependency of elastic coefficients of shales under anisotropic stress conditions. We show that the parameterization requires four parameters, namely, specific tangential compliance of a single crack, the ratio of normal to tangential compliances, characteristic pressure and a crack orientation anisotropy parameter. These parameters can be estimated from experimentally measured stress sensitivity of elastic coefficients in shales to isotropic stress.
SHARPP Consortium - Rock Physics & Petrophysics in Shales
CSIRO and Curtin University have developed a proposal for investigating the links between rock physics, petrophysics and micro-to-macro structure in conventional shales. The research programme will take an experiment-to-theory, lab-to-field approach to evaluate petrophysical properties of conventional shales and their effect on seismic data interpretation, stress field and pore pressure prediction and sealing capacity. This project will employ theoretical and phenomenological methods to build generic petrophysical models of shales. The project will also involve a systematic experimental characterisation of elastic properties of shales at a wide frequency range from 1Hz to 1 MHz, including stress dependency and pore pressure dependency of these properties. The experimental results will then be used to develop, verify and calibrate rock physics, petrophysics model of conventional shales in order to improve scientific understanding of seismic and petrophysical response of conventional shales and/or shale/sand sequences.
Elastic-wave velocities in sandstones with non-load-bearing clay
Geophysical Research Letters, 1999
The elastic moduli (and acoustic velocities) of shaley sandstones strongly depend on the amount of clay as well as on its position among sand grains. In many cases clay may fill pores without noticeably affecting the stiffness of the rock. Such clays are non-load-bearing. Here, the stiffness of the rock is determined primarily by the load-bearing quartz frame. We examine several data sets and show that in this case a simple unique relation exists between the acoustic velocity and the porosity of the load-bearing frame of the rock. This porosity can be calculated as the sum of the effective porosity (total porosity minus microporosity inside clay) and the volumetric fraction of clay in the rock. Also, it is the sum of the total porosity and the volumetric fraction of solid clay particles in the rock.
Stress-induced anisotropy in brine saturated shale
Geophysical Journal International, 2011
This 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
GEOPHYSICS, 2011
Stress dependency and anisotropy of dynamic elastic properties of shales is important for a number of geophysical applications, including seismic interpretation, fluid identification, and 4D seismic monitoring. Using Sayers-Kachanov formalism, we developed a new model for transversely isotropic (TI) media that describes stress sensitivity behavior of all five elastic coefficients using four physically meaningful parameters. The model is used to parameterize elastic properties of about 20 shales obtained from laboratory measurements and the literature. The four fitting parameters, namely, specific tangential compliance of a single crack, ratio of normal to tangential compliances, characteristic pressure, and crack orientation anisotropy parameter, show moderate to good correlations with the depth from which the shale was extracted. With increasing depth, the tangential compliance exponentially decreases. The crack orientation anisotropy parameter broadly increases with depth for most...
Strength anisotropy of shales deformed under uppermost crustal conditions
Journal of Geophysical Research: Solid Earth
Conventional triaxial tests were performed on three sets of samples of Tournemire shale along different orientations relative to bedding (0 ∘ , 45 ∘ , and 90 ∘). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 160 MPa, at strain rates ranging between 3 × 10 −7 s −1 and 3 × 10 −5 s −1. This allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure. Results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength, with 45 ∘ orientation being the weakest. We fit our results with a wing crack micromechanical model and an anisotropic fracture toughness. We found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals (between 0.2 and 0.3) and values of K Ic comparable to that already published in the literature. We also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms. Although brittle failure was systematically observed, stress drops and associated slips were slow and deformation always remained aseismic (no acoustic emission were detected). This confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events.
SPE/CSUR Unconventional Resources Conference, 2015
In a vertically transverse isotropic (VTI) medium, accurate prediction of the vertical and horizontal Young's moduli (E) and Poisson's ratios (ν) is crucial to predicting minimum horizontal stress (σhmin) and hence selecting drilling mud, cement weights, and perforation locations. Fully characterizing geomechanical properties of VTI shale requires five independent stiffness coefficients: C33, C44, C66, C11, and C13. In a vertical well, C33 and C44 are directly calculated from the velocity of the vertically propagating P- and S- waves, while C66 is estimated from the Stoneley wave velocity. To obtain C11 and C13, an empirical model must be employed. This study integrates laboratory mechanical and sonic measurements to evaluate the ANNIE and modified-ANNIE models and extend the dynamic-to-static conversion equation. Laboratory static and dynamic geomechanical methods were applied to multiple core materials extracted at different depths from a target shale play. The dynamic ela...