Davide Gei - Profile on Academia.edu (original) (raw)

Papers by Davide Gei

Research Square (Research Square), Feb 8, 2024

We model the acoustics of rocks partially saturated with hydrogen. We compare the acoustic proper... more We model the acoustics of rocks partially saturated with hydrogen. We compare the acoustic properties of H 2 (hydrogen), CO 2 (carbon dioxide) and CH 4 (methane) and those of rocks saturated with these gases and brine. The gas properties are obtained from the Peng-Robinson equation of state, and we consider the pressure-temperature conditions based on a linear basin modeling with a constant geothermal gradient. The bulk and shear moduli of the dry rock are obtained with the Krief model, while the Athy equation is used to calculate the porosity as a function of depth. Mesoscopic attenuation and velocity dispersion due to fluid effects is quantified using the Johnson model. The viscoelastic Cole-Cole model is used to describe the velocities and attenuation predicted by the Johnson model when synthetic seismograms with an equivalent viscoelastic rheology are to be calculated. In this case, the P-wave velocity as a function of gas saturation is obtained with the Gassmann equation and an effective fluid modulus based on the Brie equation.

Geophysical Journal International, Apr 6, 2016

The present study evaluates the capacity of the Boom Clay as a host rock for disposal purposes, m... more The present study evaluates the capacity of the Boom Clay as a host rock for disposal purposes, more precisely its seismic characterization, which may assess its long-term performance to store radioactive wastes. Although the formation is relatively uniform and homogeneous, there are embedded thin layers of septaria (carbonates) that may affect the integrity of the Boom Clay. Therefore, it is essential to locate these geobodies. The seismic data to characterize the Boom Clay has been acquired at the Kruibeke test site. The inversion, which allowed us to obtain the anisotropy parameters and seismic velocities of the clay, is complemented with further information such as log and laboratory data. The attenuation properties have been estimated from equivalent formations (having similar composition and seismic velocities). The inversion yields quite consistent results although the symmetry of the medium is unusual but physically possible, since the anisotropy parameter is negative. According to a timedomain calculation of the energy velocity at four frequency bands up to 900 Hz, velocity increases with frequency, a behaviour described by the Zener model. Then, we use this model to describe anisotropy and anelasticity that are implemented into the equation of motion to compute synthetic seismograms in the space-time domain. The technique is based on memory variables and the Fourier pseudospectral method. We have computed reflection coefficients of the septaria thin layer. At normal incidence, the P-wave coefficient vanishes at specific thicknesses of the layer and there is no conversion to the S wave. For example, calculations at 600 Hz show that for thicknesses of 1 m the septarium can be detected more easily since the amplitudes are higher (nearly 0.8). Converted PS waves have a high amplitude at large offsets (between 30 • and 80 • ) and can be useful to identify the target on this basis. Moreover, we have investigated the effect of septaria embedded in the Boom Clay with several simulations, by considering a lateral partial continuity of the calcareous thin inclusions. The simulations with layers of calcareous material show continuity of the reflections even when the percentage of carbonate within the layer is very small (5-15 per cent), while for low content of the calcareous material, isolated septaria boulders generate diffraction events. We have also simulated the stacked seismic section obtained from processing of the field data. The matching between the field and synthetic sections is acceptable.

A matrix-fracture-fluid decoupled PP reflection coefficient approximation for seismic inversion in tilted transversely isotropic media

Geophysics, Sep 30, 2022

Fracture-induced azimuthal anisotropy of seismic waves has useful applications in the characteriz... more Fracture-induced azimuthal anisotropy of seismic waves has useful applications in the characterization of hydrocarbon reservoirs as well as the overburden. Existing theories face problems estimating the fracture-weakness parameters, identifying the saturating fluid, and constraining the depth model building. To overcome these problems, we have adopted an azimuthal amplitude variation with angle/offset inversion for the estimation of these parameters and identification of the fluid. First, we define more intuitive fracture and fluid indicators based on rock physics, identifying the fluid by decoupling the fracture weakness parameters. Then, we derive a “rock matrix-fracture-fluid” decoupled PP-wave reflection coefficient approximation of a weakly tilted transversely isotropic medium by using a perturbation matrix and scattering theory. Compared with the conventional fracture weakness-based approximation, our method incorporates the fracture density and the fluid indicator. The inversion test finds that our approximation is effective.

Seismic Rock Physics of Gas-Hydrate Bearing Sediments

Springer eBooks, 2022

Influence of capillary pressure on CO2 storage and monitoring

SUMMARY Sequestration of CO2 in geological formations is one of the solutions to mitigate the gre... more SUMMARY Sequestration of CO2 in geological formations is one of the solutions to mitigate the greenhouse effect. We are interested in analyzing the influence of capillary pressure on CO 2 injection, storage and monitoring in saline aquifers. To do so, we present a methodology integrating numerical simulation of CO2- brine flow and seismic wave propagation. Besides, we build a suitable geological model that includes mudstone layers and fractures. The simultaneous flow of CO 2 and brine in porous media is described by the Black-Oil formulation, which applies a simplified thermodynamic model. Capillary pressure is represented as a potential function of CO 2 saturation. The wave propagation is simulated using a viscoelastic model that includes attenuation and dispersion effects due to mesoscopic scale heterogeneities. The fluid simulator properly models the CO2 injection, obtaining accumulations below the mudstone layers as injection proceeds. We are able to identify the time-lapse distribution of CO2 from the synthetic seismograms, which show the typical pushdown effect. When capillary pressure is higher, CO 2 upward migration is slower and thicker zones of CO2 accumulations are obtained. Numerical examples show the effectiveness of this methodology to detect the spatio-temporal distribution of CO 2 and to make long term predictions.

On the Normal-Incidence Reflection Coefficient in Porous Media

Surveys in Geophysics, May 5, 2021

We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to app... more We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to approximations reported by several authors, based on open-pore boundary conditions at a plane interface between two porous media. The approximations correspond to low frequencies. Two of them are derived from the low-frequency Biot theory below the Biot characteristic frequency, but the results show significant differences much below the Biot frequency. Then, we extend the Geertsma–Smit equations by including the high-frequency viscodynamic operator (i.e., the full-frequency range Biot theory), showing that there are additional substantial differences at the high-frequency range. Use of this latter expression is required to honor the physics in the whole frequency range. We further generalize the Geertsma–Smit equations to the case of general boundary conditions other than the open-pore interface. At the seismic band, it is shown that the lossless (elastic) expression based on the Gassmann P-wave impedance is the reflection coefficient to use for practical applications. It is inferred that interpretations based on the frequency dependency of these approximations can be misleading, since this dependency does not provide a suitable description of the physics.

Geophysical Research Letters, Mar 1, 2010

Finely layered media behaves as an anisotropic medium at long wavelengths. If the constituent med... more Finely layered media behaves as an anisotropic medium at long wavelengths. If the constituent media are anelastic, Q-anisotropy of qP, qSV and SH waves can be described by a generalization of Backus averaging to the lossy case. To test the theory, we introduce a novel method to obtain the complex and frequency-dependent stiffnesses from numerical simulations of oscillatory (harmonic) tests based on a space-frequency domain finite element method. We apply the methodology to a periodic sequence of shale and limestone and compute the quality factor and wave velocities as a function of frequency and propagation direction.

Amplitude variation with angle and azimuth inversion to estimate fracture properties in shale-gas reservoirs

Geophysical and Geocryological Investigation of Active Layer Along The North Russian Railway (Khanovey, Russia)

Engineering and Mining Geophysics 2021, 2021

The injection of CO 2 into a saline aquifer induces changes in pore pressure and fluid saturation... more The injection of CO 2 into a saline aquifer induces changes in pore pressure and fluid saturation, which in turn induce variations in the petrophysical properties of the storage site. Thus, numerical modeling of CO 2 sequestration combining multiphase fluid flow and wave propagation simulators requires determining the time steps at which the flow parameters (porosity and absolute permeability) need to be updated during the simulation of CO 2 injection. For this purpose, this work presents a sensitivity analysis of the seismic response of the Utsira formation (where CO 2 is being injected) due to variations in its petrophysical properties. A multiphase fluid flow simulator is used to determine the spatio-temporal distribution of CO 2 and brine during injection. The porosity and absolute permeability are assumed to be dependent of saturation and pore pressure. In the wave propagation simulator the Lamé parameters include effects of mesoscopic losses due to the presence of CO 2 in the pore space. The numerical experiments allow to define the time step at which the flow parameters need to be updated to obtain accurate seismic images of the spatial distribution of CO 2 after injection, with a more precise definition of the zone where the pushdown effect is observed.

Frontiers in Earth Science

P-wave anelasticity (attenuation and dispersion) of hydrate-bearing sediments depends on several ... more P-wave anelasticity (attenuation and dispersion) of hydrate-bearing sediments depends on several factors, namely the properties of the mineral components, hydrate content and morphology, and fluid saturation. Anelasticity is analyzed with a triple-porosity model (stiff pores, clay micropores and hydrate micropores), by considering hydrate as an additional solid skeleton. We relate the hydrate volume ratio, porosity and radii of the hydrate inclusion and clay mineral to the P-wave velocity and attenuation. The model takes wave-induced local fluid flow (mesoscopic loss) at the grain contacts into account. The results are compared with those of a double-porosity and load-bearing models, and verified with well-log data from Offshore Drilling Program sites 1247B and 1250F, and data reported in Nankai Trough, Japan. Model results and data show a good agreement.

On the Normal-Incidence Reflection Coefficient in Porous Media

Surveys in Geophysics

We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to app... more We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to approximations reported by several authors, based on open-pore boundary conditions at a plane interface between two porous media. The approximations correspond to low frequencies. Two of them are derived from the low-frequency Biot theory below the Biot characteristic frequency, but the results show significant differences much below the Biot frequency. Then, we extend the Geertsma–Smit equations by including the high-frequency viscodynamic operator (i.e., the full-frequency range Biot theory), showing that there are additional substantial differences at the high-frequency range. Use of this latter expression is required to honor the physics in the whole frequency range. We further generalize the Geertsma–Smit equations to the case of general boundary conditions other than the open-pore interface. At the seismic band, it is shown that the lossless (elastic) expression based on the Gassmann P-wave impedance is the reflection coefficient to use for practical applications. It is inferred that interpretations based on the frequency dependency of these approximations can be misleading, since this dependency does not provide a suitable description of the physics.

Windowless Q‐factor tomography by the instantaneous frequency

Geophysical Prospecting, 2020

ABSTRACTThe estimation of the Q factor of rocks by seismic surveys is a powerful tool for reservo... more ABSTRACTThe estimation of the Q factor of rocks by seismic surveys is a powerful tool for reservoir characterization, as it helps detecting possible fractures and saturating fluids. Seismic tomography allows building 3D macro‐models for the Q factor, using methods as the spectral ratio and the frequency shift. Both these algorithms require windowing the seismic signal accurately in the time domain; however, this process can hardly follow the continuous variations of the wavelet length as a function of offset and propagation effects, and it is biased by the interpreter choice. In this paper, we highlight some drawback of signal windowing in the frequency‐shift method, and introduce a tomographic approach to estimate the Q factor using the complex attributes of the seismic trace. We show that such approach is particularly needed when the dispersion is broadening the waveforms of signals with a long wave‐path. Our method still requires an interpretative event picking, but no other para...

Seismic Rock Physics of Gas-Hydrate Bearing Sediments

World Atlas of Submarine Gas Hydrates in Continental Margins, 2022

Capture and storage of Carbon dioxide in aquifers and reservoirs is one of the solutions to mitig... more Capture and storage of Carbon dioxide in aquifers and reservoirs is one of the solutions to mitigate the greenhouse effect. Geophysical methods can be used to monitor the location and migration of the gas in the underground. To perform this task properly, a suitable geological model is important, which simulates the geometry and petro-elastical properties of the different formations. In this work we integrate numerical simulators of CO2-brine flow and seismic wave propagation to model and monitor CO2 storage in saline aquifers. We also build a petrophysical model of a shaly sandstone based on porosity and clay content and considering the variation of properties with pore pressure and fluid saturation. The pressure map before the injection of CO2 is assumed to be hydrostatic for which a reference porosity map is defined. The permeability is assumed to be anisotropic and is obtained from first principles as a function of porosity and grain sizes. The density is the usual arithmetic av...

Data‐Driven Design of Wave‐Propagation Models for Shale‐Oil Reservoirs Based on Machine Learning

Journal of Geophysical Research: Solid Earth, 2021

The exploration and exploitation of shale oil is an important aspect in the oil industry. Seismic... more The exploration and exploitation of shale oil is an important aspect in the oil industry. Seismic properties and well‐log data are essential to establish wave‐propagation models. Specifically, the description of wave dispersion and attenuation under complex geological conditions needs proper lithological and petrophysical information. This complex physical mechanism has to be considered if a traditional modeling approach is adopted. In this sense, machine learning (ML) techniques provide new possibilities for this purpose. We compare two deep‐neural‐network (DNN)‐based wave propagation models. In the first (pure data‐driven), a DNN is trained to connect seismic attributes, such as wave velocities, to multivariate functions of rock‐physics properties. By training DNNs with different initial parameters, the uncertainty of the proposed method can be quantified. The second method assumes the form of the wave equations. Then, the elastic constants of the constitutive relations are predicted by DNNs. The resulting dynamical equations describe the dispersion and attenuation and wavefield simulations can be performed to obtain more information. On the basis of a test, the two kinds of wave‐propagation models yield acceptable estimations of the seismic properties, with the second approach showing a broader application because the DNN is trained without S wave data. The methodologies illustrate that the new wave‐propagation model based on ML has high precision and can be general in terms of rheological description.

We simulate the effects of diagenesis, cementation and compaction on the elastic properties of sh... more We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastical theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a 5th-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for by using Athy equation and the Hertz-Mindlin model. The petro-elastic models ...

Geophysical Journal International, 2021

SUMMARY We analyse the concepts of instantaneous frequency (IF) and quality factor (IQ). It is ve... more SUMMARY We analyse the concepts of instantaneous frequency (IF) and quality factor (IQ). It is verified that the time-averaged IF is equal to the centroid of the signal energy of the spectrum and that the centroid of the signal spectrum is equal to the IF at the peak of the signal envelope. The latter property can be used to obtain the frequency shift required by tomographic methods. Then, we analyse the two-tone stationary Mandel signal in the lossless and lossy cases. The IQ is not infinite in the lossless case, although its reciprocal average vanishes, and the lossless and lossy IFs at the peak of the signal envelope are the same, whereas the IQ at this peak depends on the amplitudes and quality factors of the tones. The IQ of a propagating Ricker wavelet has a singularity at the peak of the envelope, which shows a shift in the lossy case, related to the velocity dispersion. We consider a lossy layer described by the Zener model. Varying its thickness implies a large variation in...

CO2 storage at the Aztbach-Schwanenstadt gas field: a seismic monitoring feasibility study

First Break, 2008

ABSTRACT

The Journal of the Acoustical Society of America, 2020

P-wave conversion to slow diffusion (Biot) modes at mesoscopic (small-scale) inhomogeneities in p... more P-wave conversion to slow diffusion (Biot) modes at mesoscopic (small-scale) inhomogeneities in porous media is believed to be the most important attenuation mechanisms at seismic frequencies. This study considers three periodic thin layers saturated with gas, oil, and water, respectively, a realistic scenario in hydrocarbon reservoirs, and perform finite-element numerical simulations to obtain the wave velocities and quality factors along the direction perpendicular to layering. The results are validated by comparison to the Norris-Cavallini analytical solution, constituting a cross-check for both theory and numerical simulations. The approach is not restricted to partial saturation but also applies to relevant properties in reservoir geophysics, such as porosity and permeability heterogeneities. This paper considers two cases, namely, the same rock skeleton and different fluids, and the same fluid and different dry-rock properties. Unlike the two-layer case (two fluids), the resul...

Research Square (Research Square), Feb 8, 2024

We model the acoustics of rocks partially saturated with hydrogen. We compare the acoustic proper... more We model the acoustics of rocks partially saturated with hydrogen. We compare the acoustic properties of H 2 (hydrogen), CO 2 (carbon dioxide) and CH 4 (methane) and those of rocks saturated with these gases and brine. The gas properties are obtained from the Peng-Robinson equation of state, and we consider the pressure-temperature conditions based on a linear basin modeling with a constant geothermal gradient. The bulk and shear moduli of the dry rock are obtained with the Krief model, while the Athy equation is used to calculate the porosity as a function of depth. Mesoscopic attenuation and velocity dispersion due to fluid effects is quantified using the Johnson model. The viscoelastic Cole-Cole model is used to describe the velocities and attenuation predicted by the Johnson model when synthetic seismograms with an equivalent viscoelastic rheology are to be calculated. In this case, the P-wave velocity as a function of gas saturation is obtained with the Gassmann equation and an effective fluid modulus based on the Brie equation.

Geophysical Journal International, Apr 6, 2016

The present study evaluates the capacity of the Boom Clay as a host rock for disposal purposes, m... more The present study evaluates the capacity of the Boom Clay as a host rock for disposal purposes, more precisely its seismic characterization, which may assess its long-term performance to store radioactive wastes. Although the formation is relatively uniform and homogeneous, there are embedded thin layers of septaria (carbonates) that may affect the integrity of the Boom Clay. Therefore, it is essential to locate these geobodies. The seismic data to characterize the Boom Clay has been acquired at the Kruibeke test site. The inversion, which allowed us to obtain the anisotropy parameters and seismic velocities of the clay, is complemented with further information such as log and laboratory data. The attenuation properties have been estimated from equivalent formations (having similar composition and seismic velocities). The inversion yields quite consistent results although the symmetry of the medium is unusual but physically possible, since the anisotropy parameter is negative. According to a timedomain calculation of the energy velocity at four frequency bands up to 900 Hz, velocity increases with frequency, a behaviour described by the Zener model. Then, we use this model to describe anisotropy and anelasticity that are implemented into the equation of motion to compute synthetic seismograms in the space-time domain. The technique is based on memory variables and the Fourier pseudospectral method. We have computed reflection coefficients of the septaria thin layer. At normal incidence, the P-wave coefficient vanishes at specific thicknesses of the layer and there is no conversion to the S wave. For example, calculations at 600 Hz show that for thicknesses of 1 m the septarium can be detected more easily since the amplitudes are higher (nearly 0.8). Converted PS waves have a high amplitude at large offsets (between 30 • and 80 • ) and can be useful to identify the target on this basis. Moreover, we have investigated the effect of septaria embedded in the Boom Clay with several simulations, by considering a lateral partial continuity of the calcareous thin inclusions. The simulations with layers of calcareous material show continuity of the reflections even when the percentage of carbonate within the layer is very small (5-15 per cent), while for low content of the calcareous material, isolated septaria boulders generate diffraction events. We have also simulated the stacked seismic section obtained from processing of the field data. The matching between the field and synthetic sections is acceptable.

A matrix-fracture-fluid decoupled PP reflection coefficient approximation for seismic inversion in tilted transversely isotropic media

Geophysics, Sep 30, 2022

Fracture-induced azimuthal anisotropy of seismic waves has useful applications in the characteriz... more Fracture-induced azimuthal anisotropy of seismic waves has useful applications in the characterization of hydrocarbon reservoirs as well as the overburden. Existing theories face problems estimating the fracture-weakness parameters, identifying the saturating fluid, and constraining the depth model building. To overcome these problems, we have adopted an azimuthal amplitude variation with angle/offset inversion for the estimation of these parameters and identification of the fluid. First, we define more intuitive fracture and fluid indicators based on rock physics, identifying the fluid by decoupling the fracture weakness parameters. Then, we derive a “rock matrix-fracture-fluid” decoupled PP-wave reflection coefficient approximation of a weakly tilted transversely isotropic medium by using a perturbation matrix and scattering theory. Compared with the conventional fracture weakness-based approximation, our method incorporates the fracture density and the fluid indicator. The inversion test finds that our approximation is effective.

Seismic Rock Physics of Gas-Hydrate Bearing Sediments

Springer eBooks, 2022

Influence of capillary pressure on CO2 storage and monitoring

SUMMARY Sequestration of CO2 in geological formations is one of the solutions to mitigate the gre... more SUMMARY Sequestration of CO2 in geological formations is one of the solutions to mitigate the greenhouse effect. We are interested in analyzing the influence of capillary pressure on CO 2 injection, storage and monitoring in saline aquifers. To do so, we present a methodology integrating numerical simulation of CO2- brine flow and seismic wave propagation. Besides, we build a suitable geological model that includes mudstone layers and fractures. The simultaneous flow of CO 2 and brine in porous media is described by the Black-Oil formulation, which applies a simplified thermodynamic model. Capillary pressure is represented as a potential function of CO 2 saturation. The wave propagation is simulated using a viscoelastic model that includes attenuation and dispersion effects due to mesoscopic scale heterogeneities. The fluid simulator properly models the CO2 injection, obtaining accumulations below the mudstone layers as injection proceeds. We are able to identify the time-lapse distribution of CO2 from the synthetic seismograms, which show the typical pushdown effect. When capillary pressure is higher, CO 2 upward migration is slower and thicker zones of CO2 accumulations are obtained. Numerical examples show the effectiveness of this methodology to detect the spatio-temporal distribution of CO 2 and to make long term predictions.

On the Normal-Incidence Reflection Coefficient in Porous Media

Surveys in Geophysics, May 5, 2021

We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to app... more We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to approximations reported by several authors, based on open-pore boundary conditions at a plane interface between two porous media. The approximations correspond to low frequencies. Two of them are derived from the low-frequency Biot theory below the Biot characteristic frequency, but the results show significant differences much below the Biot frequency. Then, we extend the Geertsma–Smit equations by including the high-frequency viscodynamic operator (i.e., the full-frequency range Biot theory), showing that there are additional substantial differences at the high-frequency range. Use of this latter expression is required to honor the physics in the whole frequency range. We further generalize the Geertsma–Smit equations to the case of general boundary conditions other than the open-pore interface. At the seismic band, it is shown that the lossless (elastic) expression based on the Gassmann P-wave impedance is the reflection coefficient to use for practical applications. It is inferred that interpretations based on the frequency dependency of these approximations can be misleading, since this dependency does not provide a suitable description of the physics.

Geophysical Research Letters, Mar 1, 2010

Finely layered media behaves as an anisotropic medium at long wavelengths. If the constituent med... more Finely layered media behaves as an anisotropic medium at long wavelengths. If the constituent media are anelastic, Q-anisotropy of qP, qSV and SH waves can be described by a generalization of Backus averaging to the lossy case. To test the theory, we introduce a novel method to obtain the complex and frequency-dependent stiffnesses from numerical simulations of oscillatory (harmonic) tests based on a space-frequency domain finite element method. We apply the methodology to a periodic sequence of shale and limestone and compute the quality factor and wave velocities as a function of frequency and propagation direction.

Amplitude variation with angle and azimuth inversion to estimate fracture properties in shale-gas reservoirs

Geophysical and Geocryological Investigation of Active Layer Along The North Russian Railway (Khanovey, Russia)

Engineering and Mining Geophysics 2021, 2021

The injection of CO 2 into a saline aquifer induces changes in pore pressure and fluid saturation... more The injection of CO 2 into a saline aquifer induces changes in pore pressure and fluid saturation, which in turn induce variations in the petrophysical properties of the storage site. Thus, numerical modeling of CO 2 sequestration combining multiphase fluid flow and wave propagation simulators requires determining the time steps at which the flow parameters (porosity and absolute permeability) need to be updated during the simulation of CO 2 injection. For this purpose, this work presents a sensitivity analysis of the seismic response of the Utsira formation (where CO 2 is being injected) due to variations in its petrophysical properties. A multiphase fluid flow simulator is used to determine the spatio-temporal distribution of CO 2 and brine during injection. The porosity and absolute permeability are assumed to be dependent of saturation and pore pressure. In the wave propagation simulator the Lamé parameters include effects of mesoscopic losses due to the presence of CO 2 in the pore space. The numerical experiments allow to define the time step at which the flow parameters need to be updated to obtain accurate seismic images of the spatial distribution of CO 2 after injection, with a more precise definition of the zone where the pushdown effect is observed.

Frontiers in Earth Science

P-wave anelasticity (attenuation and dispersion) of hydrate-bearing sediments depends on several ... more P-wave anelasticity (attenuation and dispersion) of hydrate-bearing sediments depends on several factors, namely the properties of the mineral components, hydrate content and morphology, and fluid saturation. Anelasticity is analyzed with a triple-porosity model (stiff pores, clay micropores and hydrate micropores), by considering hydrate as an additional solid skeleton. We relate the hydrate volume ratio, porosity and radii of the hydrate inclusion and clay mineral to the P-wave velocity and attenuation. The model takes wave-induced local fluid flow (mesoscopic loss) at the grain contacts into account. The results are compared with those of a double-porosity and load-bearing models, and verified with well-log data from Offshore Drilling Program sites 1247B and 1250F, and data reported in Nankai Trough, Japan. Model results and data show a good agreement.

On the Normal-Incidence Reflection Coefficient in Porous Media

Surveys in Geophysics

We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to app... more We compare the exact normal-incidence PP reflection coefficient [Geertsma–Smit expression] to approximations reported by several authors, based on open-pore boundary conditions at a plane interface between two porous media. The approximations correspond to low frequencies. Two of them are derived from the low-frequency Biot theory below the Biot characteristic frequency, but the results show significant differences much below the Biot frequency. Then, we extend the Geertsma–Smit equations by including the high-frequency viscodynamic operator (i.e., the full-frequency range Biot theory), showing that there are additional substantial differences at the high-frequency range. Use of this latter expression is required to honor the physics in the whole frequency range. We further generalize the Geertsma–Smit equations to the case of general boundary conditions other than the open-pore interface. At the seismic band, it is shown that the lossless (elastic) expression based on the Gassmann P-wave impedance is the reflection coefficient to use for practical applications. It is inferred that interpretations based on the frequency dependency of these approximations can be misleading, since this dependency does not provide a suitable description of the physics.

Windowless Q‐factor tomography by the instantaneous frequency

Geophysical Prospecting, 2020

ABSTRACTThe estimation of the Q factor of rocks by seismic surveys is a powerful tool for reservo... more ABSTRACTThe estimation of the Q factor of rocks by seismic surveys is a powerful tool for reservoir characterization, as it helps detecting possible fractures and saturating fluids. Seismic tomography allows building 3D macro‐models for the Q factor, using methods as the spectral ratio and the frequency shift. Both these algorithms require windowing the seismic signal accurately in the time domain; however, this process can hardly follow the continuous variations of the wavelet length as a function of offset and propagation effects, and it is biased by the interpreter choice. In this paper, we highlight some drawback of signal windowing in the frequency‐shift method, and introduce a tomographic approach to estimate the Q factor using the complex attributes of the seismic trace. We show that such approach is particularly needed when the dispersion is broadening the waveforms of signals with a long wave‐path. Our method still requires an interpretative event picking, but no other para...

Seismic Rock Physics of Gas-Hydrate Bearing Sediments

World Atlas of Submarine Gas Hydrates in Continental Margins, 2022

Capture and storage of Carbon dioxide in aquifers and reservoirs is one of the solutions to mitig... more Capture and storage of Carbon dioxide in aquifers and reservoirs is one of the solutions to mitigate the greenhouse effect. Geophysical methods can be used to monitor the location and migration of the gas in the underground. To perform this task properly, a suitable geological model is important, which simulates the geometry and petro-elastical properties of the different formations. In this work we integrate numerical simulators of CO2-brine flow and seismic wave propagation to model and monitor CO2 storage in saline aquifers. We also build a petrophysical model of a shaly sandstone based on porosity and clay content and considering the variation of properties with pore pressure and fluid saturation. The pressure map before the injection of CO2 is assumed to be hydrostatic for which a reference porosity map is defined. The permeability is assumed to be anisotropic and is obtained from first principles as a function of porosity and grain sizes. The density is the usual arithmetic av...

Data‐Driven Design of Wave‐Propagation Models for Shale‐Oil Reservoirs Based on Machine Learning

Journal of Geophysical Research: Solid Earth, 2021

The exploration and exploitation of shale oil is an important aspect in the oil industry. Seismic... more The exploration and exploitation of shale oil is an important aspect in the oil industry. Seismic properties and well‐log data are essential to establish wave‐propagation models. Specifically, the description of wave dispersion and attenuation under complex geological conditions needs proper lithological and petrophysical information. This complex physical mechanism has to be considered if a traditional modeling approach is adopted. In this sense, machine learning (ML) techniques provide new possibilities for this purpose. We compare two deep‐neural‐network (DNN)‐based wave propagation models. In the first (pure data‐driven), a DNN is trained to connect seismic attributes, such as wave velocities, to multivariate functions of rock‐physics properties. By training DNNs with different initial parameters, the uncertainty of the proposed method can be quantified. The second method assumes the form of the wave equations. Then, the elastic constants of the constitutive relations are predicted by DNNs. The resulting dynamical equations describe the dispersion and attenuation and wavefield simulations can be performed to obtain more information. On the basis of a test, the two kinds of wave‐propagation models yield acceptable estimations of the seismic properties, with the second approach showing a broader application because the DNN is trained without S wave data. The methodologies illustrate that the new wave‐propagation model based on ML has high precision and can be general in terms of rheological description.

We simulate the effects of diagenesis, cementation and compaction on the elastic properties of sh... more We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastical theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a 5th-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for by using Athy equation and the Hertz-Mindlin model. The petro-elastic models ...

Geophysical Journal International, 2021

SUMMARY We analyse the concepts of instantaneous frequency (IF) and quality factor (IQ). It is ve... more SUMMARY We analyse the concepts of instantaneous frequency (IF) and quality factor (IQ). It is verified that the time-averaged IF is equal to the centroid of the signal energy of the spectrum and that the centroid of the signal spectrum is equal to the IF at the peak of the signal envelope. The latter property can be used to obtain the frequency shift required by tomographic methods. Then, we analyse the two-tone stationary Mandel signal in the lossless and lossy cases. The IQ is not infinite in the lossless case, although its reciprocal average vanishes, and the lossless and lossy IFs at the peak of the signal envelope are the same, whereas the IQ at this peak depends on the amplitudes and quality factors of the tones. The IQ of a propagating Ricker wavelet has a singularity at the peak of the envelope, which shows a shift in the lossy case, related to the velocity dispersion. We consider a lossy layer described by the Zener model. Varying its thickness implies a large variation in...

CO2 storage at the Aztbach-Schwanenstadt gas field: a seismic monitoring feasibility study

First Break, 2008

ABSTRACT

The Journal of the Acoustical Society of America, 2020

P-wave conversion to slow diffusion (Biot) modes at mesoscopic (small-scale) inhomogeneities in p... more P-wave conversion to slow diffusion (Biot) modes at mesoscopic (small-scale) inhomogeneities in porous media is believed to be the most important attenuation mechanisms at seismic frequencies. This study considers three periodic thin layers saturated with gas, oil, and water, respectively, a realistic scenario in hydrocarbon reservoirs, and perform finite-element numerical simulations to obtain the wave velocities and quality factors along the direction perpendicular to layering. The results are validated by comparison to the Norris-Cavallini analytical solution, constituting a cross-check for both theory and numerical simulations. The approach is not restricted to partial saturation but also applies to relevant properties in reservoir geophysics, such as porosity and permeability heterogeneities. This paper considers two cases, namely, the same rock skeleton and different fluids, and the same fluid and different dry-rock properties. Unlike the two-layer case (two fluids), the resul...