scholarly journals Noncontacting benchtop measurements of the elastic properties of shales

Geophysics ◽  
2013 ◽  
Vol 78 (3) ◽  
pp. C25-C31 ◽  
Author(s):  
Thomas E. Blum ◽  
Ludmila Adam ◽  
Kasper van Wijk
Keyword(s):  
Group Velocity ◽  
Elastic Anisotropy ◽  
P Wave ◽  
Laser Source ◽  
Velocity Versus ◽  
P Wave Velocity ◽  
Least Squares Fit ◽  
In Situ Conditions ◽  
Velocity Information ◽  
Attenuation Anisotropy

We evaluated a laser-based noncontacting method to measure the elastic anisotropy of horizontal shale cores. Whereas conventional transducer data contained an ambiguity between phase and group velocity measurements, small laser source and receiver footprints on typical core samples ensured group velocity information in our laboratory measurements. With a single dense acquisition of group velocity versus group angle on a horizontal core, we estimated the elastic constants [Formula: see text], [Formula: see text], and [Formula: see text] directly from ultrasonic waveforms, and [Formula: see text] from a least-squares fit of modeled to measured group velocities. The observed significant P-wave velocity and attenuation anisotropy in these dry shales were almost surely exaggerated by delamination of clay platelets and microfracturing, but provided an illustration of the new laboratory measurement technique. Although challenges lay ahead to measure preserved shales at in situ conditions in the lab, we evaluated the fundamental advantages of the proposed method over conventional transducer measurements.

Velocity anisotropy in shales: A petrophysical study

Geophysics ◽  
1997 ◽  
Vol 62 (2) ◽  
pp. 521-532 ◽  
Author(s):  
Lev Vernik ◽  
Xingzhou Liu
Keyword(s):  
Seismic Anisotropy ◽  
Elastic Anisotropy ◽  
Clay Mineralogy ◽  
Fluid Phase ◽  
Black Shales ◽  
P Wave ◽  
Velocity Versus ◽  
Softening Effect ◽  
Porosity Reduction ◽  
Wide Range

Using ultrasonic velocity and anisotropy measurements on a variety of shales with different clay and kerogen content, clay mineralogy, and porosity at a wide range of effective pressure, we find that elastic anisotropy of shales increases substantially with compaction. The effect is attributed to both porosity reduction and smectite‐ to‐illite transformation with diagenesis. A means of kerogen content mapping using velocity versus porosity crossplot for shales is shown. Matrix anisotropy of shales dramatically increases with kerogen reaching the maximum values of about 0.4 at total organic carbon (TOC)=15–20%. A strong chemical softening effect was found in shales containing even minor amounts of swelling (smectite) clay when saturated with aqueous solution. This effect results in a significant P‐wave anisotropy reduction as compared to dry and oil‐saturated shales. Since mature black shales are normally oil wet, this effect can only have a local significance restricted to the wellbore wall. Accurate measurements of phase velocities, including velocities at a 45° direction to the bedding plane, allow us to immediately calculate elastic stiffnesses and anisotropic parameters. Intrinsic (high pressure) properties of shales display an ε > δ > 0 relation. Introduction of the bedding‐parallel microcracks in overpressured shales results in a δ decrease when fully fluid saturated and a δ increase when partially gas saturated, with a characteristic effect on the shape of the P‐wave velocity surface at small angles of incidence. Filtering the contribution of the intrinsic anisotropy of shales, it is possible to estimate the pore fluid phase, microcrack density, and aspect ratio parameters using seismic anisotropy measurements.

Pyrolysis-induced P-wave velocity anisotropy in organic-rich shales

Geophysics ◽  
2014 ◽  
Vol 79 (2) ◽  
pp. D41-D53 ◽  
Author(s):  
Adam M. Allan ◽  
Tiziana Vanorio ◽  
Jeremy E. P. Dahl
Keyword(s):  
Wave Velocity ◽  
Elastic Anisotropy ◽  
Rock Physics ◽  
Confining Pressure ◽  
Source Rocks ◽  
Acoustic Properties ◽  
P Wave ◽  
Velocity Anisotropy ◽  
Geochemical Parameters ◽  
P Wave Velocity

The sources of elastic anisotropy in organic-rich shale and their relative contribution therein remain poorly understood in the rock-physics literature. Given the importance of organic-rich shale as source rocks and unconventional reservoirs, it is imperative that a thorough understanding of shale rock physics is developed. We made a first attempt at establishing cause-and-effect relationships between geochemical parameters and microstructure/rock physics as organic-rich shales thermally mature. To minimize auxiliary effects, e.g., mineralogical variations among samples, we studied the induced evolution of three pairs of vertical and horizontal shale plugs through dry pyrolysis experiments in lieu of traditional samples from a range of in situ thermal maturities. The sensitivity of P-wave velocity to pressure showed a significant increase post-pyrolysis indicating the development of considerable soft porosity, e.g., microcracks. Time-lapse, high-resolution backscattered electron-scanning electron microscope images complemented this analysis through the identification of extensive microcracking within and proximally to kerogen bodies. As a result of the extensive microcracking, the P-wave velocity anisotropy, as defined by the Thomsen parameter epsilon, increased by up to 0.60 at low confining pressures. Additionally, the degree of microcracking was shown to increase as a function of the hydrocarbon generative potential of each shale. At 50 MPa confining pressure, P-wave anisotropy values increased by 0.29–0.35 over those measured at the baseline — i.e., the immature window. The increase in anisotropy at high confining pressure may indicate a source of anisotropy in addition to microcracking — potentially clay mineralogical transformation or the development of intrinsic anisotropy in the organic matter through aromatization. Furthermore, the evolution of acoustic properties and microstructure upon further pyrolysis to the dry-gas window was shown to be negligible.

Frequency-dependent P-wave anisotropy due to scattering in rocks with aligned fractures

Geophysics ◽  
2020 ◽  
Vol 85 (2) ◽  
pp. MR97-MR105 ◽  
Author(s):  
Junxin Guo ◽  
Boris Gurevich ◽  
Da Shuai
Keyword(s):  
Wave Velocity ◽  
Wave Scattering ◽  
Wave Attenuation ◽  
P Wave ◽  
Frequency Dependent ◽  
Velocity Anisotropy ◽  
P Wave Velocity ◽  
Anisotropy Parameters ◽  
Scattering Anisotropy ◽  
Attenuation Anisotropy

Frequency-dependent P-wave anisotropy due to scattering often occurs in fractured formations, whereas the corresponding theoretical study is lacking. Hence, based on a newly developed P-wave scattering model, we have studied the frequency-dependent P-wave scattering anisotropy in rocks with aligned fractures. To describe P-wave scattering anisotropy, we develop the corresponding anisotropy parameters similar to those for elastic anisotropy. Our results indicate that the P-wave velocity anisotropy parameters [Formula: see text] and [Formula: see text] do not change with frequency monotonically, which is different from that caused by wave-induced fluid flow. Fluid saturation in fractures can greatly decrease the P-wave velocity anisotropy, whose effects depend on the ratio of the fluid bulk modulus to the fracture aspect ratio. The P-wave exhibits elliptical anisotropy for the dry fracture case at low frequencies, but anelliptical anisotropy for the case with fluid-filled fractures. The P-wave attenuation anisotropy parameters [Formula: see text] and [Formula: see text] vanish in the low- and high-frequency limits but reach their maxima at the characteristic frequency when the P-wavelength is close to the fracture length. The influence of fluid on the P-wave attenuation anisotropy is similar to that on the velocity anisotropy. To further analyze frequency-dependent P-wave scattering anisotropy, theoretical predictions are compared with experimental results, which indicate reasonable agreement between them.

scholarly journals No mafic layer in 80 km thick Tibetan crust

2021 ◽  
Author(s):  
Gaochun Wang ◽  
Thybo Hans ◽  
Irina M. Artemieva
Keyword(s):  
Wave Velocity ◽  
Lower Crust ◽  
Bulk Composition ◽  
P Wave ◽  
Indian Plate ◽  
Velocity Versus ◽  
Low Velocity ◽  
Crustal Earthquakes ◽  
Temperature Regimes ◽  
P Wave Velocity

<div> <p>All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. It has been suggested that the lower crust does not need to be basaltic, but until now all seismic observations show high P-wave velocity, which requires that the bulk composition of the lower crust must include at least 20-40% of mafic rocks. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (V<sub>p</sub>>7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (V<sub>p</sub><6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. The calculated typical velocity versus depth curves for different crustal lithologies and temperature regimes imply the composition of the lower crust is felsic. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.</p> </div><p> </p>

VSP traveltime inversion: Near‐surface issues

Geophysics ◽  
2004 ◽  
Vol 69 (2) ◽  
pp. 345-351 ◽  
Author(s):  
Geoff J.M. Moret ◽  
William P. Clement ◽  
Michael D. Knoll ◽  
Warren Barrash
Keyword(s):  
Wave Velocity ◽  
Regularization Parameter ◽  
Velocity Model ◽  
P Wave ◽  
Seismic Profiles ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Shallow Subsurface ◽  
P Wave Velocity ◽  
Velocity Information

P‐wave velocity information obtained from vertical seismic profiles (VSPs) can be useful in imaging subsurface structure, either by directly detecting changes in the subsurface or as an aid to the interpretation of seismic reflection data. In the shallow subsurface, P‐wave velocity can change by nearly an order of magnitude over a short distance, so curved rays are needed to accurately model VSP traveltimes. We used a curved‐ray inversion to estimate the velocity profile and the discrepancy principle to estimate the data noise level and to choose the optimum regularization parameter. The curved‐ray routine performed better than a straight‐ray inversion for synthetic models containing high‐velocity contrasts. The application of the inversion to field data produced a velocity model that agreed well with prior information. These results show that curved‐ray inversion should be used to obtain velocity information from VSPs in the shallow subsurface.

Effect of pressure on 3D distribution of P-wave velocity and attenuation in antigorite serpentinite

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. WA33-WA43 ◽  
Author(s):  
Tomáš Svitek ◽  
Václav Vavryčuk ◽  
Tomáš Lokajíček ◽  
Matěj Petružálek ◽  
Hartmut Kern
Keyword(s):  
P Wave ◽  
Contact Conditions ◽  
Velocity Amplitude ◽  
Velocity Anisotropy ◽  
Wave Velocities ◽  
Spherical Sample ◽  
P Wave Velocity ◽  
Pressure Medium ◽  
Sample Assembly ◽  
Attenuation Anisotropy

We have developed a detailed study on the pressure dependence of P-wave velocities and amplitudes on a spherical sample of antigorite serpentinite from Val Malenco, Northern Italy. Measurements were done at room temperature and hydrostatic pressures up to 400 MPa in a pressure vessel with oil as a pressure medium. The transducer/sample assembly allows simultaneous velocity and amplitude measurements on the spherical sample in 132 independent directions. Three significant directions of the foliated sample were selected to study changes of the directional dependence (anisotropy) of velocity, amplitude, and [Formula: see text]-factor with increasing pressure. Remarkable differences are observed between the changes of velocity and attenuation anisotropy as pressure is increased. Although the velocity anisotropy is quite stable through all pressure levels, the attenuation anisotropy and the [Formula: see text]-factor vary significantly in magnitude and orientation. The variations are probably caused by the closing of microcracks due to acting hydrostatic pressure, so the contact conditions between individual minerals consolidate and the transmitting energy is less attenuating.

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