Recent Publications

Laboratory micro-seismic signature of shear faulting and fault slip in shale

posted Nov 29, 2017, 11:39 PM by IARP Admin   [ updated Nov 29, 2017, 11:39 PM ]

Authors: Joel Sarout, Yves Le Gonidec, Audrey Ougier-Simonin, Alexandre Schubnel, Yves Guéguen, David N. Dewhurst

Link to the Elsevier repossitory: http://www.sciencedirect.com/science/article/pii/S0031920116302709

Abstract:

This article reports the results of a triaxial deformation experiment conducted on a transversely isotropic shale specimen. This specimen was instrumented with ultrasonic transducers to monitor the evolution of the micro-seismic activity induced by shear faulting (triaxial failure) and subsequent fault slip at two different rates. The strain data demonstrate the anisotropy of the mechanical (quasi-static) compliance of the shale; the P-wave velocity data demonstrate the anisotropy of the elastic (dynamic) compliance of the shale. The spatio-temporal evolution of the micro-seismic activity suggests the development of two distinct but overlapping shear faults, a feature similar to relay ramps observed in large-scale structural geology. The shear faulting of the shale specimen appears quasi-aseismic, at least in the 0.5 MHz range of sensitivity of the ultrasonic transducers used in the experiment. Concomitantly, the rate of micro-seismic activity is strongly correlated with the imposed slip rate and the evolution of the axial stress. The moment tensor inversion of the focal mechanism of the high quality micro-seismic events recorded suggests a transition from a non-shear dominated to a shear dominated micro-seismic activity when the rock evolves from initial failure to larger and faster slip along the fault. The frictional behaviour of the shear faults highlights the possible interactions between small asperities and slow slip of a velocity-strengthening fault, which could be considered as a realistic experimental analogue of natural observations of non-volcanic tremors and (very) low-frequency earthquakes triggered by slow slip events.


Stress-dependent permeability and wave dispersion in tight cracked rocks: Experimental validation of simple effective medium models

posted Nov 29, 2017, 11:37 PM by IARP Admin   [ updated Nov 29, 2017, 11:38 PM ]

Authors: Joel Sarout, Emilie Cazes, Claudio Delle Piane, Alessio Arena, Lionel Esteban

Link to the AGU repository: http://onlinelibrary.wiley.com/doi/10.1002/2017JB014147/full

Abstract:

We experimentally assess the impact of microstructure, pore fluid, and frequency on wave velocity, wave dispersion, and permeability in thermally cracked Carrara marble under effective pressure up to 50 MPa. The cracked rock is isotropic, and we observe that (1) P and S wave velocities at 500 kHz and the low-strain (<105) mechanical moduli at 0.01 Hz are pressure-dependent, (2) permeability decreases asymptotically toward a small value with increasing pressure, (3) wave dispersion between 0.01 Hz and 500 MHz in the water-saturated rock reaches a maximum of ~26% for S waves and ~9% for P waves at 1 MPa, and (4) wave dispersion virtually vanishes above ~30 MPa. Assuming no interactions between the cracks, effective medium theory is used to model the rock’s elastic response and its permeability. P and S wave velocity data are jointly inverted to recover the crack density and effective aspect ratio. The permeability data are inverted to recover the cracks’ effective radius. These parameters lead to a good agreement between predicted and measured wave velocities, dispersion and permeability up to 50 MPa, and up to a crack density of ~0.5. The evolution of the crack parameters suggests that three deformation regimes exist: (1) contact between cracks’ surface asperities up to ~10 MPa, (2) progressive crack closure between ~10 and 30 MPa, and (3) crack closure effectively complete above ~30 MPa. The derived crack parameters differ significantly from those obtained by analysis of 2-D electron microscope images of thin sections or 3-D X-ray microtomographic images of millimeter-size specimens.



The elastic anisotropy of clay minerals

posted Aug 15, 2016, 8:36 AM by IARP Admin

Colin M. Sayers1 and Lennert D. den Boer2
1Schlumberger, Houston, Texas, USA. E-mail: csayers@slb.com.
2Schlumberger, Calgary, Alberta, Canada. E-mail: lboer@slb.com.

The layered structure of clay minerals produces large elastic anisotropy due to the presence of strong covalent bonds within layers and weaker electrostatic bonds in between. Technical difficulties associated with small grain size preclude experimental measurement of single-crystal elastic moduli. However, theoretical calculations of the complete elastic tensors of several clay minerals have been reported, using either first-principle calculations based on density functional theory or molecular dynamics. Because of the layered microstructure, the elastic stiffness tensor obtained from such calculations can be approximated to good accuracy as a transversely isotropic (TI) medium. The TI-equivalent elastic moduli of clay minerals indicate that Thomsen’s anisotropy parameters ϵϵ and γγ are large and positive, whereas δδ is small or negative. A least-squares inversion for the elastic properties of a best-fitting equivalent TI medium consisting of two isotropic layers to the elastic properties of clay minerals indicates that the shear modulus of the stiffest layer is considerably larger than the softest layer, consistent with the expected high compliance of the interlayer region in clay minerals. It is anticipated that the elastic anisotropy parameters derived from the best-fitting TI approximation to the elastic stiffness tensor of clay minerals will find applications in rock physics for seismic imaging, amplitude variation with offset analysis, and geomechanics.

http://library.seg.org/doi/abs/10.1190/geo2016-0005.1



Measurements of elastic and electrical properties of an unconventional organic shale under differential loading

posted Jul 27, 2015, 12:43 AM by IARP Admin

1Colorado School of Mines, Department of Geophysics, Golden, Colorado, USA. E-mail: wfwoodruff@gmail.commprasad@mines.edu.
2Colorado School of Mines, Department of Geophysics, Golden, Colorado, USA and Université Savoie Mont Blanc, ISTerre, CNRS, UMR CNRS, Le Bourget du Lac, France. E-mail: arevil@mines.edu
3Colorado School of Mines, Department of Geophysics, and Department of Petroleum Engineering, Golden, Colorado, USA.
4University of Texas at Austin, Department of Petroleum and Geosystems Engineering, Austin, Texas, USA. E-mail: cverdin@austin.utexas.edu.

We have developed an experimental approach to simultaneously measure the stress dependence of ultrasonic wave velocities at 1 MHz, and therefore the components of the undrained elastic stiffness tensor, as well as the components of the complex conductivity tensor in the frequency range from 100 mHz to 10 Hz. We performed the experiments on a cylindrical core sample from the Haynesville Formation (porosity of approximately 0.08, bound water excluded, and clay content, mostly illite, approximately 30–40 wt%). We performed experiments under controlled confining and pore fluid pressures, achieving differential pressure states representative of autochthonous reservoir conditions. Directional measurements were made using independent acquisition arrays (piezoelectric crystals and nonpolarizing electrodes) distributed azimuthally on the core sample external surface, the bedding plane being along the axis of the cylindrical core sample. Ultrasonic waveforms were recorded on a high-resolution oscilloscope, and complex impedance spectra were recorded with a four-electrode acquisition system using an impedance meter with precision of 0.1 mrad. Experiments were repeated under drained and undrained conditions, over loading and unloading sequences for fully water-saturated conditions. Measurements show strong stress dependence on ultrasonic and complex conductivity measurements, which can be ascribed to the opening and closing of cracks within the samples. The data were used to estimate the anisotropic electrical and elastic effective pressure coefficients of the core sample, resulting in effective stress coefficients smaller than one for both elastic and electrical properties. These effective stress coefficients were also smaller for the electrical and acoustic properties once the cracks have closed. The anisotropy ratio for the components of the complex conductivity tensor was on the order of 30, whereas it was only 2 for the compressional components of the stiffness tensor (C11/C33).



The effect of systematic diagenetic changes on the mechanical behavior of a quartz-cemented sandstone

posted May 5, 2015, 12:35 PM by Sirikarn Narongsirikul   [ updated Jul 12, 2015, 8:38 AM by IARP Admin ]

Jennie E. Cook, Laurel B. Goodwin, David F. Boutt, and Harold J. Tobin - A key goal of petrophysical studies of sandstones is to relate common field measurements, particularly seismic or sonic velocities, to parameters defining the rock’s mechanical and hydrologic characteristics. These include elastic and inelastic mechanical properties, porosity, and permeability. We explored relationships among these properties in variably quartz-cemented, mature arenites of the St. Peter Sandstone with porosities ranging from 9% to 25%. In a previous paper, we described microstructural changes accompanying progressive quartz cementation and related porosity and permeability reduction in this sample suite. Here, we report ultrasonic velocities ultrasonic velocities, dynamic and static elastic properties, confined compressive strength, and tensile strength. Analyses of these data demonstrated that factors controlling permeability also fundamentally determined the elastic and inelastic mechanical properties. We found that the number of grain contacts, or bonds, per number of grains viewed in the thin section (bond-to-grain ratio [BGR]) was a key predictive parameter of the mechanical and hydrologic properties. Although the contact length and number of contacts correlated well with the mechanical behavior, statistical analyses showed that BGR was a better predictor of strength, elastic stiffness, and fluid transport properties than was the contact length. The BGR provided a measure of the pore throat occlusion that reduced permeability and the connectivity of the grain framework that stiffened and strengthened the rock. Because porosity and BGR were typically well correlated, porosity was a more quickly and easily measured proxy for BGR in this case. However, our analysis showed that it was the microstructural changes associated with porosity loss rather than porosity loss per se that largely controlled the properties of interest. Thus, consideration of BGR as well as the relative strengths of grains and bond type (cement, pressure solution) for different compositions of sandstone and cement may constructively form the basis for comparative studies of other more complex sandstones.


Read More: http://library.seg.org/doi/abs/10.1190/geo2014-0026.1

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