Effects of fracture connectivity on S-wave attenuation caused by wave-induced fluid flow

We studied seismic attenuation of P- and S-waves caused by the physical mechanism of wave-induced fluid flow at the mesoscopic scale. Stress relaxation experiments were numerically simulated by solving Biot’s equations for consolidation of 2D poroelastic media with finite-element modeling. The experiments yielded time-dependent stress-strain relations that were used to calculate the complex moduli from which frequency-dependent attenuation was determined. Our model consisted of periodically distributed circular or elliptical heterogeneities with much lower porosity and permeability than the background media, which contained 80% of the total pore space of the media. This model can represent a hydrocarbon reservoir, where the porous background is fully saturated with oil or gas and the low-porosity regions are always saturated with water. Three different saturation scenarios were considered: oil-saturated (80% oil, 20% water), gas-saturated (80% gas, 20% water), and fully water-saturated media. Varying the dry bulk and shear moduli in the background and in the heterogeneities, a consistent tendency was observed in the relative behavior of the S-wave attenuation among the different saturation scenarios. First, in the gas-saturated media the S-wave attenuation was very low and much lower than in the oil-saturated or in the fully water-saturated media. Second, at low frequencies the S-wave attenuation was significantly higher in the oil-saturated media than in the fully water-saturated media. The P-wave attenuation exhibited a more variable relative behavior among the different saturation degrees. Based on the mechanism of wave-induced fluid flow and on our numerical results, we suggest that S-wave attenuation could be used as an indicator of fluid content in a reservoir. Additionally, we observed that impermeable barriers in the background can cause a significant increase in S-wave attenuation. This suggests that S-wave attenuation could also be an indicator of permeability changes in a reservoir due to, for example, fracturing operations.

Download Full-text

Local and global fluid-flow effects on dipole flexural waves

Geophysics ◽

10.1190/geo2018-0040.1 ◽

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.

Download Full-text

Seismic wave attenuation and modulus dispersion in sandstones

Geophysics ◽

10.1190/geo2015-0342.1 ◽

2016 ◽

Vol 81 (3) ◽

pp. D211-D231 ◽

Cited By ~ 39

Author(s):

James W. Spencer ◽

Jacob Shine

Keyword(s):

Wave Attenuation ◽

High Viscosity ◽

P Wave ◽

S Wave ◽

Local Flow ◽

Shear Moduli ◽

Low Viscosity ◽

Confining Pressures ◽

Wave Induced ◽

Dispersion And Attenuation

We have conducted laboratory experiments over the 1–200 Hz band to examine the effects of viscosity and permeability on modulus dispersion and attenuation in sandstones and also to examine the effects of partial gas or oil saturation on velocities and attenuations. Our results have indicated that bulk modulus values with low-viscosity fluids are close to the values predicted using Gassmann’s first equation, but, with increasing frequency and viscosity, the bulk and shear moduli progressively deviate from the values predicted by Gassmann’s equations. The shear moduli increase up to 1 GPa (or approximately 10%) with high-viscosity fluids. The P- and S-wave attenuations ([Formula: see text] and [Formula: see text]) and modulus dispersion with different fluids are indicative of stress relaxations that to the first order are scaling with frequency times viscosity. By fitting Cole-Cole distributions to the scaled modulus and attenuation data, we have found that there are similar P-wave, shear and bulk relaxations, and attenuation peaks in each of the five sandstones studied. The modulus defects range from 11% to 15% in Berea sandstone to 16% to 26% in the other sandstones, but these would be reduced at higher confining pressures. The relaxations shift to lower frequencies as the viscosity increased, but they do not show the dependence on permeability predicted by mesoscopic wave-induced fluid flow (WIFF) theories. Results from other experiments having patchy saturation with liquid [Formula: see text] and high-modulus fluids are consistent with mesoscopic WIFF theories. We have concluded that the modulus dispersion and attenuations ([Formula: see text] and [Formula: see text]) in saturated sandstones are caused by a pore-scale, local-flow mechanism operating near grain contacts.

Download Full-text

Seismic wave attenuation and dispersion due to wave-induced fluid flow in rocks with strong permeability fluctuations

The Journal of the Acoustical Society of America ◽

10.1121/1.4824967 ◽

2013 ◽

Vol 134 (6) ◽

pp. 4742-4751 ◽

Cited By ~ 14

Author(s):

J. Germán Rubino ◽

Leonardo B. Monachesi ◽

Tobias M. Müller ◽

Luis Guarracino ◽

Klaus Holliger

Keyword(s):

Fluid Flow ◽

Seismic Wave ◽

Wave Attenuation ◽

Seismic Wave Attenuation ◽

Wave Induced ◽

Attenuation And Dispersion

Download Full-text

Seismic wave attenuation and dispersion resulting from wave-induced flow in porous rocks — A review

Geophysics ◽

10.1190/1.3463417 ◽

2010 ◽

Vol 75 (5) ◽

pp. 75A147-75A164 ◽

Cited By ~ 457

Author(s):

Tobias M. Müller ◽

Boris Gurevich ◽

Maxim Lebedev

Keyword(s):

Fluid Flow ◽

Velocity Dispersion ◽

Wave Attenuation ◽

Field Measurements ◽

Theoretical Models ◽

Seismic Attenuation ◽

P Wave ◽

Induced Flow ◽

Wave Induced ◽

Attenuation And Dispersion

One major cause of elastic wave attenuation in heterogeneous porous media is wave-induced flow of the pore fluid between heterogeneities of various scales. It is believed that for frequencies below [Formula: see text], the most important cause is the wave-induced flow between mesoscopic inhomogeneities, which are large compared with the typical individual pore size but small compared to the wavelength. Various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter-scale mesoscopic heterogeneities. Laboratory and field measurements of seismic attenuation in fluid-saturated rocks provide indications of the role of the wave-induced flow. Signatures of wave-induced flow include the frequency and saturation dependence of P-wave attenuation and its associated velocity dispersion, frequency-dependent shear-wave splitting, and attenuation anisotropy. During the last four decades, numerous models for attenuation and velocity dispersion from wave-induced flow have been developed with varying degrees of rigor and complexity. These models can be categorized roughly into three groups ac-cording to their underlying theoretical framework. The first group of models is based on Biot’s theory of poroelasticity. The second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation. Another group of models is derived using the theory of viscoelasticity. Though all models predict attenuation and velocity dispersion typical for a relaxation process, there exist differences that can be related to the type of disorder (periodic, random, space dimension) and to the way the local flow is incorporated. The differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies. In recent years, some theoretical models of wave-induced fluid flow have been validated numerically, using finite-difference, finite-element, and reflectivity algorithms applied to Biot’s equations of poroelasticity. Application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data.

Download Full-text

NUMERICAL ALGORITHM OF SEISMIC ATTENUATION ESTIMATION IN ANISOTROPIC FRACTURED POROUS FLUID-SATURATED MEDIA

Interexpo GEO-Siberia ◽

10.33764/2618-981x-2021-2-2-186-195 ◽

2021 ◽

Vol 2 (2) ◽

pp. 186-195

Author(s):

Mikhail A. Novikov ◽

Vadim V. Lisitsa

Keyword(s):

Fluid Flow ◽

Seismic Wave ◽

Numerical Algorithm ◽

Wave Attenuation ◽

Seismic Attenuation ◽

Staggered Grid ◽

Filling Material ◽

Seismic Wave Attenuation ◽

Saturated Media ◽

Wave Induced

In our work we investigate the effect of transport and elastic properties anisotropy on seismic attenuation due to fracture-to-fracture wave-induced fluid flow using numerical algorithm of estimation of seismic wave attenuation in anisotropic fractured porous fluid-saturated media. Algorithm is based on numerical solution of anisotropic Biot equations using finite-difference scheme on staggered grid. We perform a set of numerical experiments to model wave propagation in fractured media with anisotropic fractured-filling material providing wave-induced fluid flow within interconnected fractures. Recorded signals are used for numerical estimation of inverse quality factor. Results demonstrate the effect of fracture-filling material anisotropy on seismic wave attenuation.

Download Full-text

Attenuation of Seismic Waves in Partially Saturated Berea Sandstone as a Function of Frequency and Confining Pressure

Frontiers in Earth Science ◽

10.3389/feart.2021.641177 ◽

2021 ◽

Vol 9 ◽

Author(s):

Nicola Tisato ◽

Claudio Madonna ◽

Erik H. Saenger

Keyword(s):

Fluid Flow ◽

Wave Attenuation ◽

Confining Pressure ◽

Berea Sandstone ◽

Experimental Conditions ◽

Frequency Dependent ◽

Near Surface ◽

Partially Saturated ◽

Near Surface Geophysics ◽

Wave Induced

Frequency-dependent attenuation (1/Q) should be used as a seismic attribute to improve the accuracy of seismic methods and imaging of the subsurface. In rocks, 1/Q is highly sensitive to the presence of saturating fluids. Thus, 1/Q could be crucial to monitor volcanic and hydrothermal domains and to explore hydrocarbon and water reservoirs. The experimental determination of seismic and teleseismic attenuation (i.e., for frequencies < 100 Hz) is challenging, and as a consequence, 1/Q is still uncertain for a broad range of lithologies and experimental conditions. Moreover, the physics of elastic energy absorption (i.e., 1/Q) is often poorly constrained and understood. Here, we provide a series of measurements of seismic wave attenuation and dynamic Young’s modulus for dry and partially saturated Berea sandstone in the 1–100 Hz bandwidth and for confining pressure ranging between 0 and 20 MPa. We present systematic relationships between the frequency-dependent 1/Q and the liquid saturation, and the confining pressure. Data in the seismic bandwidth are compared to phenomenological models, ultrasonic elastic properties and theoretical models for wave-induced-fluid-flow (i.e., squirt-flow and patchy-saturation). The analysis suggests that the observed frequency-dependent attenuation is caused by wave-induced-fluid-flow but also that the physics behind this attenuation mechanism is not yet fully determined. We also show, that as predicted by wave-induced-fluid-flow theories, attenuation is strongly dependent on confining pressure. Our results can help to interpret data for near-surface geophysics to improve the imaging of the subsurface.

Download Full-text

Seismic attenuation: Effects of pore fluids and frictional‐sliding

Geophysics ◽

10.1190/1.1441276 ◽

1982 ◽

Vol 47 (1) ◽

pp. 1-15 ◽

Cited By ~ 268

Author(s):

Kenneth W. Winkler ◽

Amos Nur

Keyword(s):

Fluid Flow ◽

Strain Amplitude ◽

Seismic Wave ◽

Wave Attenuation ◽

Velocity Ratio ◽

Seismic Attenuation ◽

P Wave ◽

Amplitude Dependence ◽

S Wave ◽

Frictional Sliding

Seismic wave attenuation in rocks was studied experimentally, with particular attention focused on frictional sliding and fluid flow mechanisms. Sandstone bars were resonated at frequencies from 500 to 9000 Hz, and the effects of confining pressure, pore pressure, degree of saturation, strain amplitude, and frequency were studied. Observed changes in attenuation and velocity with strain amplitude are interpreted as evidence for frictional sliding at grain contacts. Since this amplitude dependence disappears at strains and confining pressures typical of seismic wave propagation in the earth, we infer that frictional sliding is not a significant source of seismic attenuation in situ. Partial water saturation significantly increases the attenuation of both compressional (P) and shear (S) waves relative to that in dry rock, resulting in greater P‐wave than S‐wave attenuation. Complete saturation maximizes S‐wave attenuation but causes a reduction in P‐wave attenuation. These effects can be interpreted in terms of wave induced pore fluid flow. The ratio of compressional to shear attenuation is found to be a more sensitive and reliable indicator of partial gas saturation than is the corresponding velocity ratio. Potential applications may exist in exploration for natural gas and geothermal steam reservoirs.

Download Full-text