Velocity dispersion between ultrasonic and seismic frequencies in brine‐saturated reservoir sandstones

Geophysics ◽  
2002 ◽  
Vol 67 (1) ◽  
pp. 254-258 ◽  
Author(s):  
Michael S. King ◽  
J. Robert Marsden
Keyword(s):  
High Porosity ◽  
S Wave ◽  
Local Flow ◽  
Effective Stresses ◽  
Wave Velocities ◽  
Flow Phenomena ◽  
Seismic Frequencies ◽  
Reservoir Sandstones ◽  
Poroelastic Theory ◽  
Low Porosity

Ultrasonic P‐ and S‐wave velocities have been measured on 44 specimens from core samples of relatively clean sandstones recovered from hydrocarbon reservoirs. Ten specimens have porosities less than 10%, and 34 have porosities in the range 20–30%. Velocities were measured with the specimens in both their dry and brine‐saturated states, under hydrostatic effective stresses to 60 MPa for the low‐porosity specimens and to 40 MPa for those of the high‐porosity set. Gassmann's poroelastic theory is found to account for changes in velocity for the low‐porosity set at 60 MPa effective stress when the dry specimens are fully saturated with brine. The velocities used for “dry” specimens in Gassmann's theory must, however, be those measured after the dry specimens have first adsorbed a small quantity of moisture. For saturated specimens at effective stresses of 40 MPa or less, local flow phenomena due to the presence of open microcracks are assumed to be responsible for the measured velocities being appreciably higher than those predicted theoretically.

The relation between seismic P‐ and S‐wave velocity dispersion in saturated rocks

Geophysics ◽  
1994 ◽  
Vol 59 (1) ◽  
pp. 87-92 ◽  
Author(s):  
Gary Mavko ◽  
Diane Jizba
Keyword(s):  
Wave Velocity ◽  
Large Scale ◽  
Velocity Dispersion ◽  
Seismic Velocity ◽  
Ultrasonic Velocities ◽  
S Wave ◽  
Local Flow ◽  
Wave Velocities ◽  
Shear Compliance

Seismic velocity dispersionin fluid-saturated rocks appears to be dominated by tow mecahnisms: the large scale mechanism modeled by Biot, and the local flow or squirt mecahnism. The tow mechanisms can be distuinguished by the ratio of P-to S-wave dispersions, or more conbeniently, by the ratio of dynamic bulk to shear compliance dispersions derived from the wave velocities. Our formulation suggests that when local flow denominates, the dispersion of the shear compliance will be approximately 4/15 the dispersion of the compressibility. When the Biot mechanism dominates, the constant of proportionality is much smaller. Our examination of ultrasonic velocities from 40 sandstones and granites shows that most, but not all, of the samples were dominated by local flow dispersion, particularly at effective pressures below 40 MPa.

Nonlinear behavior of soil sediments identified by using borehole records observed at the Ashigra Valley, Japan

1995 ◽  
Vol 85 (6) ◽  
pp. 1821-1834
Author(s):  
Toshimi Satoh ◽  
Toshiaki Sato ◽  
Hiroshi Kawase
Keyword(s):  
Shear Strain ◽  
Main Part ◽  
Nonlinear Behavior ◽  
Strong Motion ◽  
S Wave ◽  
Ground Shaking ◽  
Wave Velocities ◽  
The One ◽  
Soil Sediments ◽  
Strong Ground

Abstract We evaluate the nonlinear behavior of soil sediments during strong ground shaking based on the identification of their S-wave velocities and damping factors for both the weak and strong motions observed on the surface and in a borehole at Kuno in the Ashigara Valley, Japan. First we calculate spectral ratios between the surface station KS2 and the borehole station KD2 at 97.6 m below the surface for the main part of weak and strong motions. The predominant period for the strong motion is apparently longer than those for the weak motions. This fact suggests the nonlinearity of soil during the strong ground shaking. To quantify the nonlinear behavior of soil sediments, we identify their S-wave velocities and damping factors by minimizing the residual between the observed spectral ratio and the theoretical amplification factor calculated from the one-dimensional wave propagation theory. The S-wave velocity and the damping factor h (≈(2Q)−1) of the surface alluvial layer identified from the main part of the strong motion are about 10% smaller and 50% greater, respectively, than those identified from weak motions. The relationships between the effective shear strain (=65% of the maximum shear strain) calculated from the one-dimensional wave propagation theory and the shear modulus reduction ratios or the damping factors estimated by the identification method agree well with the laboratory test results. We also confirm that the soil model identified from a weak motion overestimates the observed strong motion at KS2, while that identified from the strong motion reproduces the observed. Thus, we conclude that the main part of the strong motion, whose maximum acceleration at KS2 is 220 cm/sec2 and whose duration is 3 sec, has the potential of making the surface soil nonlinear at an effective shear strain on the order of 0.1%. The S-wave velocity in the surface alluvial layer identified from the part just after the main part of the strong motion is close to that identified from weak motions. This result suggests that the shear modulus recovers quickly as the shear strain level decreases.

Jacobian matrix for the inversion of P- and S-wave velocities and its accurate computation method

2010 ◽  
Vol 54 (5) ◽  
pp. 647-654 ◽  
Author(s):  
FuPing Liu ◽  
XianJun Meng ◽  
YuMei Wang ◽  
GuoQiang Shen ◽  
ChangChun Yang
Keyword(s):  
Jacobian Matrix ◽  
Computation Method ◽  
S Wave ◽  
Wave Velocities ◽  
Accurate Computation

Shallow velocity structure and poisson's ratio at the Tarzana, California, strong-motion accelerometer site

1996 ◽  
Vol 86 (6) ◽  
pp. 1704-1713 ◽  
Author(s):  
R. D. Catchings ◽  
W. H. K. Lee
Keyword(s):  
Urban Areas ◽  
Velocity Structure ◽  
Strong Motion ◽  
P Wave ◽  
S Wave ◽  
Near Surface ◽  
Velocity Models ◽  
Wave Velocities ◽  
Poisson's Ratios ◽  
Poisson’S Ratios

Abstract The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P-and S-wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P-wave velocities range from 0.9 to 1.65 km/sec, and S-wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S-wave low-velocity zone (LVZ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface (BGS). Our velocity model is consistent with the near-surface (<30 m) P- and S-wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S-wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.

A dynamic elastic model for squirt-flow effect and its application on fluid-viscosity-associated velocity dispersion in reservoir sandstones

Geophysics ◽  
2020 ◽  
Vol 85 (4) ◽  
pp. MR201-MR212
Author(s):  
Zhi-Qiang Yang ◽  
Tao He ◽  
Chang-Chun Zou
Keyword(s):  
Velocity Dispersion ◽  
Pore Space ◽  
Pore Fluid ◽  
Quantitative Prediction ◽  
Elastic Model ◽  
Fluid Viscosity ◽  
Porous Rocks ◽  
S Wave ◽  
Flow Effect ◽  
Reservoir Sandstones

Velocity dispersion is a common phenomenon for fluid-charged porous rocks and carries important information on the pore structure and fluid in reservoir rocks. Previous ultrasonic experiments had measured more significant non-Biot velocity dispersion on saturated reservoir sandstones with increasing pore-fluid viscosity. Although wave-induced local squirt-flow effect could in theory cause most of the non-Biot velocity dispersion, its quantitative prediction remains a challenge. Several popular models were tested to predict the measured velocities under undrained conditions, but they either underestimated the squirt-flow effect or failed to simultaneously satisfy P- and S-wave velocity dispersions (especially for higher viscosity fluids). Based on the classic double-porosity theory that pore space is comprised of mainly stiff/Biot’s porosity and minor compliant porosity, an effective “wet frame” was hypothesized to account for the squirt-flow effect, whose compliant pores are filled with a hypothesized fluid with dynamic modulus. A new dynamic elastic model was then introduced by extending Biot theory to include the squirt-flow effect, after replacing the dry-frame bulk/shear moduli with their wet-frame counterparts. In addition to yielding better velocity predictions for P- and S-wave measurements of different fluid viscosities, the new model is also more applicable because its two key tuning parameters (i.e., the effective aspect ratio and porosity of compliant pores) at in situ reservoir pressure could be constrained with laboratory velocity measurements associated with pore-fluid viscosities.

scholarly journals Characterization of the Petrophysical Properties of the Timahdit Oil Shale Layers in Morocco

Geosciences ◽  
2020 ◽  
Vol 10 (9) ◽  
pp. 337
Author(s):  
Hanane Sghiouri El Idrissi ◽  
Abderrahim Samaouali ◽  
Younes El Rhaffari ◽  
Salah El Alami ◽  
Yves Geraud
Keyword(s):  
Thermal Conductivity ◽  
Organic Matter ◽  
Thermal Diffusivity ◽  
Oil Shale ◽  
Velocity Ratio ◽  
Organic Matter Content ◽  
Matter Content ◽  
S Wave ◽  
Wave Velocities ◽  
The Impact

In this work, we study the variability of the lithological composition and organic matter content of samples were taken from the different layers M, X and Y of the Timahdit oil shale in Morocco, in order to experimentally analyze the impact of this variability on petrophysical measurements. The objective of this study is to predict the properties of the layers, including their thermal conductivity, thermal diffusivity, porosity and P and S wave velocities. The results of the study of the impact of the organic matter content of the samples on the petrophysical measurements show that, regardless of the organic matter content, thermal conductivity and diffusivity remain insensitive, while P and S wave velocities decrease linearly and porosity increases with increasing organic matter content. On the other hand, the study of the organic matter variability content is consistent with the velocity ratio, so can be used as an organic matter indicator of the layers. Conductivity and thermal diffusivity are almost invariant to the variability of the organic matter.

Local and global fluid-flow effects on dipole flexural waves

Geophysics ◽  
2020 ◽  
Vol 85 (1) ◽  
pp. D1-D11
Author(s):  
Elliot J. H. Dahl ◽  
Kyle T. Spikes
Keyword(s):  
Fluid Flow ◽  
Low Frequency ◽  
Summation Method ◽  
Sensitivity Analyses ◽  
Flexural Wave ◽  
Biot Theory ◽  
S Wave ◽  
Local Flow ◽  
Flow Effects ◽  
Wave Induced

Wave-induced fluid flow (WIFF) can significantly alter the effective formation velocities and cause increasing waveform dispersion and attenuation. We have used modified frame moduli from the theory of Chapman together with the classic Biot theory to improve the understanding of local- and global-flow effects on dipole flexural wave modes in boreholes. We investigate slow and fast formations with and without compliant pores, which induce local flow. The discrete wavenumber summation method generates the waveforms, which are then processed with the weighted spectral semblance method to compare with the solution of the period equation. We find compliant pores to decrease the resulting effective formation P- and S-wave velocities, that in turn decrease the low-frequency velocity limit of the flexural wave. Furthermore, depending on the frequency at which the local-flow dispersion occurs, different S-wave velocity predictions from the flexural wave become possible. This issue is investigated through changing the local-flow critical frequency. Sensitivity analyses of the flexural-wave phase velocity to small changes in WIFF parameters indicate the modeling to be mostly sensitive to compliant pores in slow and fast formations.

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