Rock Physics, Geomechanics and Rock Properties in Shales — Where are the Links? (original) (raw)
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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.
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.
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.
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.
Microstructure and Micromechanics of Shale Rocks: Case Study of Marcellus Shale
Facta Universitatis, Series: Mechanical Engineering
Shale rocks play an essential role in petroleum exploration and production because they can occur either as source rocks or caprocks depending on their mineralogical composition and microstructures. More than 60% of effective seals for geologic hydrocarbon bearing formations as natural hydraulic barriers constitute of shale caprocks. The effectiveness of caprock depends on its ability to immobilize fluids, which include a low permeability and resilience to the in-situ formation of fractures as a result of pressurized injection. The alteration in sealing properties of shale rocks is directly related to the differences in their mineralogical composition and microstructure.Failure of the shale starts with deterioration at micro/nanoscale, the structural features and properties at the micro/nanoscale can significantly impact the durability performance of these materials at the macroscale, therefore, study at micro/nanoscale becomes necessary to get better understanding of the hydraulic ...