Bulk modulus dispersion and attenuation in sandstones

Geophysics ◽  
2015 ◽  
Vol 80 (2) ◽  
pp. D111-D127 ◽  
Author(s):  
L. Pimienta ◽  
J. Fortin ◽  
Y. Guéguen
Keyword(s):  
Fluid Flow ◽  
Bulk Modulus ◽  
Low Frequency ◽  
Confining Pressure ◽  
S Wave ◽  
Standard Samples ◽  
Stress Strain ◽  
Fontainebleau Sandstone ◽  
Attenuation Characteristic ◽  
Confining Pressures

We report experimental data on the frequency dependence of bulk elastic modulus in porous sandstones. A new methodology was developed to investigate the dispersion/attenuation phenomena on a rock’s bulk modulus [Formula: see text] for varying confining pressures in the range of 1–50 MPa and fluids of varying viscosities (i.e., air, glycerin, and water). This methodology combined (1) ultrasonic (i.e., [Formula: see text]) P- and S-wave velocity measurements, leading to the high-frequency (HF) [Formula: see text], (2) stress-strain measurements from forced periodic oscillations of confining pressure at low-frequency (LF) ranges (i.e., [Formula: see text]), leading to [Formula: see text] and [Formula: see text], and (3) pore-pressure measurement to document the induced fluid-flow in the LF range (i.e., [Formula: see text]). The stress-strain method was first checked using three standard samples: glass, gypsum, and Plexiglas samples. Over the frequency and pressure range of the apparatus [Formula: see text] was stable and accurate and the lowest measurable LF attenuation was [Formula: see text]. The methodology was applied to investigate Fontainebleau sandstone samples of 7% and 9% porosity. The [Formula: see text] and [Formula: see text] exhibited correlated variations, which also correlated with an experimental evidence of frequency-dependent fluid-flow out of the sample. Attenuation peaks as high as [Formula: see text] and [Formula: see text] are measured. The attenuation/dispersion measured under glycerin saturation was compared to Biot-Gassmann predictions. The overall behavior of one sample was consistent with a dispersion/attenuation characteristic of the drained/undrained transition. On the reverse, the other sample exhibited exotic behaviors as the measurements were underestimated by the drained/undrained transition and indicated a direct transition from drained to unrelaxed domain. These different behaviors were consistent with the values of the critical frequencies expected for the drained/undrained (i.e., [Formula: see text]) and relaxed/unrelaxed (i.e., [Formula: see text]) transitions.

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.

scholarly journals Macroscopic and Mesoscopic Mechanical Properties of Mine Tailings with Different Dry Densities under Different Confining Pressures

Geofluids ◽  
2020 ◽  
Vol 2020 ◽  
pp. 1-12
Author(s):  
Zhi-jun Zhang ◽  
Yao-hui Guo ◽  
Ya-kun Tian ◽  
Lin Hu ◽  
Xi-xian Wang ◽  
...  
Keyword(s):  
Numerical Simulation ◽  
Coordination Number ◽  
Mine Tailings ◽  
Confining Pressure ◽  
Simulation Software ◽  
Particle Flow ◽  
Dry Density ◽  
Stress Strain ◽  
High Confining Pressure ◽  
Confining Pressures

Particle flow numerical simulation software (PFC3D) was utilized to establish the consolidated-undrained triaxial compression test numerical models of mine tailings with different dry densities to deeply investigate the macroscopic and microscopic characteristics of mine tailings in a tailing pond in Hunan Province. Comparing the results of the simulation and the laboratory experiment, the mesoscopic parameters of the particle flow numerical simulation were obtained through continuously adjusting the mesoscopic parameter with the higher degree of agreement between the stress-strain curve, the peak strength, and the elastic modulus as the determining standard. The macroscopic and microscopic characteristics of mine tailings were studied from the perspectives of stress-strain, axial strain-volume strain, coordination number, particle velocity vector, and contact force between particles. After numerous numerical tests, it was found that the PFC3D simulation results are consistent with experiment results of the dry density tailing samples under different confining pressures; compared with the high confining pressure, the simulation test results at lower confining pressures were more with that of the laboratory tests; low density and high confining pressure both have inhibitory effect on the dilatancy characteristics of triaxial samples; with the same confining pressure, the dilatancy tendency of low dry density samples is suppressed comparing with the high dry density samples. The initial coordination number of the numerical model is large, which proves that the contact degree of the model is good to some extent.

The influence of confining pressure and water saturation on dynamic elastic properties of some Permian coals

Geophysics ◽  
1993 ◽  
Vol 58 (1) ◽  
pp. 30-38 ◽  
Author(s):  
Gang Yu ◽  
Keeva Vozoff ◽  
David W. Durney
Keyword(s):  
Elastic Properties ◽  
Water Saturation ◽  
Elastic Anisotropy ◽  
Confining Pressure ◽  
S Wave ◽  
Bedding Planes ◽  
Confining Pressures ◽  
Mine Development ◽  
Water Saturated ◽  
Dynamic Elastic Properties

Laboratory measurements are described on Permian coals from Wollongong, New South Wales, Australia related to the dependence of ultrasonic P‐ and S‐wave velocities, attenuation, anisotropy and the dynamic elastic moduli on confining pressure, water saturation, and pore pressure. Five independent stiffness constants are used to represent the elastic anisotropy of the specimens as a function of confining pressure and water saturation. The anisotropy is believed to be controlled mainly by the internal structure of the coals, while the pressure dependence of the constants is controlled mainly by randomly oriented cracks. P‐ and S‐wave dispersions were measured on water‐saturated specimens as confining pressures increased from 2 MPa to 40 MPa. The samples represented cores taken both parallel and perpendicular to bedding planes. Velocities along bedding planes are marginally higher than those across bedding planes. This anisotropy is insensitive to confining pressure. Attenuation was also measured, both normal and parallel to bedding planes, on dry and water‐saturated specimens from 2 MPa to 40 MPa confining pressures. The experimental results show that dynamic elastic properties are potential indicators of the states of stress and saturation in coal seams, and provide necessary information for computer modeling and interpreting seismic surveys carried out to assist mine development.

Effects of fluids and dual-pore systems on pressure-dependent velocities and attenuations in carbonates

Geophysics ◽  
2008 ◽  
Vol 73 (5) ◽  
pp. N35-N47 ◽  
Author(s):  
Remy Agersborg ◽  
Tor Arne Johansen ◽  
Morten Jakobsen ◽  
Jeremy Sothcott ◽  
Angus Best
Keyword(s):  
Fluid Flow ◽  
Confining Pressure ◽  
Substitution Effects ◽  
Fluid Substitution ◽  
Pore System ◽  
S Wave ◽  
Low Velocity ◽  
Wave Velocities ◽  
Three Samples ◽  
Local Fluid Flow

The effects of fluid substitution on P- and S-wave velocities in carbonates of complex texture are still not understood fully. The often-used Gassmann equation gives ambiguous results when compared with ultrasonic velocity data. We present theoretical modeling of velocity and attenuation measurements obtained at a frequency of [Formula: see text] for six carbonate samples composed of calcite and saturated with air, brine, and kerosene. Although porosities (2%–14%) and permeabilities [Formula: see text] are relatively low, velocity variations are large. Differences between the highest and lowest P- and S-wave velocities are about 18% and 27% for brine-saturated samples at 60 and [Formula: see text] effective pressure, respectively. S-wave velocities are measured for two orthogonal polarizations; for four of six samples, anisotropy is revealed. TheGassmann model underpredicts fluid-substitution effects by [Formula: see text] for three samples and by as much as 5% for the rest of the six samples. Moreover, when dried, they also show decreasing attenuation with increasing confining pressure. To model this behavior, we examine a pore model made of two pore systems: one constitutes the main and drainable porosity, and the other is made of undrained cracklike pores that can be associated with grain-to-grain contacts. In addition, these dried rock samples are modeled to contain a fluid-filled-pore system of grain-to-grain contacts, potentially causing local fluid flow and attenuation. For the theoretical model, we use an inclusion model based on the [Formula: see text]-matrix approach, which also considers effects of pore texture and geometry, and pore fluid, global- and local-fluid flow. By using a dual-pore system, we establish a realistic physical model consistently describing the measured data.

scholarly journals Laboratory measurements of low- and high-frequency elastic moduli in Fontainebleau sandstone

Geophysics ◽  
2013 ◽  
Vol 78 (5) ◽  
pp. D369-D379 ◽  
Author(s):  
Emmanuel C. David ◽  
Jérome Fortin ◽  
Alexandre Schubnel ◽  
Yves Guéguen ◽  
Robert W. Zimmerman
Keyword(s):  
Bulk Modulus ◽  
Elastic Moduli ◽  
High Pressures ◽  
Low Frequency ◽  
Frequency Dispersion ◽  
Low Frequencies ◽  
High Frequencies ◽  
Fontainebleau Sandstone ◽  
Wave Velocities ◽  
Water Saturated

The presence of pores and cracks in rocks causes the fluid-saturated wave velocities in rocks to be dependent on frequency. New measurements of the bulk modulus at low frequencies (0.02–0.1 Hz) were obtained in the laboratory using oscillation tests carried out on two hydrostatically stressed Fontainebleau sandstone samples, in conjunction with ultrasonic velocities and static measurements, under a range of differential pressures (10–95 MPa), and with three different pore fluids (argon, glycerin, and water). For the 13% and 4% porosity samples, under glycerin- and water-saturated conditions, the low-frequency bulk modulus at 0.02 Hz matched well the low-frequency and ultrasonic dry bulk modulus. The glycerin- and water-saturated samples were much more compliant at low frequencies than at high frequencies. The measured bulk moduli of the tested rocks at low frequencies (0.02–0.1 Hz) were much lower than the values predicted by the Gassmann equation. The frequency dispersion of the P and S velocities was much higher at low differential pressures than at high pressures, due to the presence of open cracks at low differential pressures.

Experimental Study of Stress-Strain Characteristics of the Fly Ash Blended with Curing Agent

2010 ◽  
Vol 168-170 ◽  
pp. 1934-1942
Author(s):  
Zheng Shen ◽  
Lan Zong ◽  
Xiang Dong
Keyword(s):  
Fly Ash ◽  
Triaxial Compression ◽  
Confining Pressure ◽  
Uniaxial Stress ◽  
Curing Agent ◽  
Compression Tests ◽  
Growth Trend ◽  
Stress Strain ◽  
Confining Pressures ◽  
Triaxial Compression Tests

The stress-strain characteristics of the fly ash blended with curing agent was studied using uniaxial and triaxial compression tests. Curing agent JNS-2 was used as the stabilizing agents in sample preparation. Four curing agent JNS-2 contents of 3%, 6%, 9% and 12% were selected for sample preparation. UU triaxial compression tests were conducted in a range of confining pressures from 100 kPa to 300 kPa. The experimental results obtained from the laboratory tests showed that curing age, mixture ratio, compaction degree and confining pressures had significant influence on the shape of curves. Uniaxial stress-strain test results demonstrated that the latter strength and deformation characteristics of the fly ash blended with curing agent grew little and with the increase of curing agent amount and compaction factor, the curve of uniaxial stress-strain changed significantly. On the other hand, triaxial stress-strain test results indicted that the failure strain showed a partial negative growth trend with the increase of curing agent amount, and the failure stress showed a partial positive growth trend with the increase of curing agent amount. When the curve was at high confining pressure, it showed hardening type, when at low confining pressure it showed softening type.

Effect of fluids and frequencies on Poisson’s ratio of sandstone samples

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. D183-D195 ◽  
Author(s):  
Lucas Pimienta ◽  
Jérôme Fortin ◽  
Yves Guéguen
Keyword(s):  
Frequency Dependence ◽  
Poisson’S Ratio ◽  
Poisson's Ratio ◽  
Fluid Viscosity ◽  
S Wave ◽  
Standard Samples ◽  
Frequency Dependent ◽  
S Waves ◽  
Fontainebleau Sandstone ◽  
Dry Conditions

Poisson’s ratio [Formula: see text] is an important parameter when interpreting measured geophysical and seismic data. For an isotropic medium, it directly relates to the ratio of P- and S-wave velocities. We have measured [Formula: see text] as a function of pressure and frequency in fluid-saturated sandstones. The method of measuring [Formula: see text] was first tested as a function of pressure and frequency using standard samples. The phase shift [Formula: see text] between radial and axial strains was also measured. For all standard samples, such as the linear viscoelastic Plexiglas, the data indicated that [Formula: see text] correlated with [Formula: see text] and related to a dissipation on [Formula: see text]. Then, [Formula: see text] and [Formula: see text] were measured as a function of pressure and frequency for two dry and fluid-saturated Fontainebleau sandstone samples. Under dry conditions, no frequency dependence and very small pressure dependence were observed. Unusual behaviors were observed under fluid-saturated conditions. In particular, [Formula: see text] of one sample indicated a frequency-dependent bell-shaped dispersion under water and glycerin saturation that correlated with peaks in [Formula: see text]. Plotting the measurements as a function of apparent frequency (i.e., normalizing by the fluid viscosity) indicated a good fit between the water- and glycerin-saturated measurements. The bell-shaped dispersion in [Formula: see text] that was observed for one particular sandstone held for all effective pressures. These variations fully correlated with the peaks of [Formula: see text] observed. Our results can be interpreted using fluid flow and effective medium theories in the case of a porous microcracked rock. Drained/undrained and relaxed/unrelaxed transitions have frequency and magnitude of variations that are consistent with the measurements. The rock sample microcrack density strongly affects this frequency dependence. The inferred [Formula: see text] ratio at low effective pressures also indicates a large frequency-dependent bell-shaped dispersion. The parameter [Formula: see text] is a clear indicator of the frequency-dependent dissipation of [Formula: see text] and relates to the attenuation of P- and S-waves.

Dynamic elastic properties of coal

Geophysics ◽  
2010 ◽  
Vol 75 (6) ◽  
pp. E227-E234 ◽  
Author(s):  
Anyela Morcote ◽  
Gary Mavko ◽  
Manika Prasad
Keyword(s):  
Elastic Properties ◽  
High Pressures ◽  
Confining Pressure ◽  
Acoustic Properties ◽  
Coal Rank ◽  
S Wave ◽  
Wide Range ◽  
Confining Pressures ◽  
Intrinsic Anisotropy ◽  
Dynamic Elastic Properties

Laboratory ultrasonic velocity measurements of different types of coal demonstrate that their dynamic elastic properties depend on coal rank and applied effective pressure. In spite of the growing interest in coal beds as targets for methane production, the high abundance in sedimentary sequences and the strong influence that they have on seismic response, little data are available on the acoustic properties of coal. Velocities were measured in core plugs parallel and perpendicular to lamination surfaces as a function of confining pressure up to [Formula: see text] in loading and unloading cycles. P- and S-wave velocities and dry bulk and dry shear moduli increase as coal rank increases. Thus, bituminous coal and cannel show lower velocities and moduli than higher ranked coals such as semianthracite and anthracite. The [Formula: see text] relationship for dry samples is linear and covers a relatively wide range of effective pressures and coal ranks. However, there is a pressure dependence on the elastic properties of coal for confining pressures below [Formula: see text]. This pressure sensitivity is related to the presence of microcracks. Finally, the data show that coal has an intrinsic anisotropy at confining pressures above [Formula: see text], the closing pressure for most of the microcracks. This intrinsic anisotropy at high pressures might be due to fine lamination and preferred orientation of the macerals.

Numerical Simulation of SHPB Test for Concrete under Confining Pressure

2014 ◽  
Vol 580-583 ◽  
pp. 3144-3148 ◽  
Author(s):  
Hua Zhang ◽  
Ao Yu Xie ◽  
Yu Wei Gao
Keyword(s):  
Strain Rate ◽  
Split Hopkinson Pressure Bar ◽  
Dynamic Properties ◽  
Confining Pressure ◽  
Hopkinson Pressure Bar ◽  
Stress Strain ◽  
Confining Pressures ◽  
Stress States ◽  
Pressure Bar ◽  
The Impact

Using the HJC dynamic constitutive model, the Split Hopkinson Pressure Bar (SHPB) impact test with confining pressure for concrete was simulated in the software ANSYS/LS-DYNA. The confining pressure was simulated by applying constant pressure around the specimen. The triangle velocity wave, which has less diffusion, is used as loader in the simulation. The confining pressures used were 0MPa, 2MPa, 4MPa, 8MPa and 16MPa and the stress-strain curves were presented. The influence of confining pressure on the dynamic properties was analyzed by comparing the stress-strain curves of concrete under different stress states. The strain rate decreases sensitively as long as the confining pressure increases. By debugging the impact velocity, the stress-strain curves under the similar strain rate were obtained, which indicate the toughening and reinforcing effect with the increase of confining pressure.

scholarly journals Experimental Study on Shear Strength of Saturated Remolded Loess

2021 ◽  
Author(s):  
Jie Lai ◽  
Yun Liu ◽  
Yuzhou Xiang ◽  
Wei Wang ◽  
Jiangbo Xu ◽  
...  
Keyword(s):  
Shear Strength ◽  
Shear Rate ◽  
Strain Curve ◽  
Confining Pressure ◽  
Cohesive Force ◽  
Dry Density ◽  
Stress Strain ◽  
Loess Area ◽  
Confining Pressures ◽  
Homogeneous Composition

Abstract Loess has the characteristic of macropore, loose structure, homogeneous composition and collapsibility. It is easy to saturate when it encounters heavy rainfall and irrigation, resulting in landslides, roadbed subsidence and dam instability in the loess area. To study the influence of dry density and shear rate on shear strength of saturated remolded loess, an SLB-6A stress-strain controlled triaxial shear penetration tester was used to conduct Consolidated Undrained(CU) test in the Yan'an area. During the test, three variables of shear rate, confining pressure and dry density were controlled. The dry densities of the samples were 1.5g/cm3, 1.6g/cm3 and 1.7g/cm3 respectively. The CU test of the saturated remolded loess at a confining pressure of 100kPa, 150kPa, and 200kPa was performed at a shear rate of 0.04mm/min, 0.08mm/min, 0.16 mm/min, and 0.4mm/min respectively. It is found that the stress-strain curve of saturated remolded loess gradually moves up with the increase of dry density. When the dry density is equal to ρd=1.5g/cm3, the deviatoric stress under different confining pressures there is a tendency to increase first and then decrease with increases of shear rate. When the dry density is equal to ρd=1.6g/cm3 and ρd=1.7g/cm3, the deviational stress under different confining pressures shows the trend of increasing first, decreasing and then increasing with the increase of shear rate, which is different from that at the dry density ρd =1.5g/cm3 at a shear rate v=0.4mm/min. When the dry density ρd=1.5g/cm3, the cohesive force decreases first and then increases with the increase of shear rate. When the dry density ρd=1.6g/cm3 and ρd=1.7g/cm3, the cohesive force first increases at 0.08 mm/min, and then decreases with the increase of shear rate. The cohesion and internal friction angles tend to increase as the dry density increases.

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