Low‐frequency anisotropy in fractured and layered media

Pang, Shuo; Stovas, Alexey

doi:10.1111/1365-2478.12833

Cited by 4 publications

(3 citation statements)

References 32 publications

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“…Based on Chapman's theory, we can obtain the series coefficients that control the behaviour of velocity and anisotropy parameter dispersion in the fractured medium at low frequencies (Pang and Stovas ). Then, we consider this intrinsic dispersion in a layered model and derive the frequency‐dependent PP and PS reflection coefficients at the elastic–anelastic surface.…”

Section: Discussionmentioning

confidence: 99%

“…To obtain the fracture-induced velocity dispersion and anisotropy dispersion at low frequencies, we expand the elements of the effective stiffness tensor in the series in frequency (Pang and Stovas 2019) c i j (ω) = a (0) i j + a (1) i j iω + a (2) i j ω 2 .…”

Section: A P P E N D I X : F R E Q U E N C Y -D E P E N D E N T V E Lmentioning

confidence: 99%

“…The elastic medium is considered to be shale, and the anelastic medium is considered to be sandstone with aligned horizontal fractures described by Chapman's model (Chapman ). Based on the series coefficients that control the behaviour of velocity dispersion and anisotropy parameter dispersion at low frequencies (Pang and Stovas ), we add the intrinsic dispersion into the layered model. We extend the conventional AVO equations for vertical transverse isotropy (VTI) medium into frequency domain and derive equations for frequency‐dependent AVO attributes and reflection coefficients at an interface between elastic shale layer and anelastic sandstone.…”

Section: Introductionmentioning

confidence: 99%

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Frequency‐dependent PP and PS reflection coefficients in fractured media

Pang

Stovas

2019

Geophysical Prospecting

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A B S T R A C TWhen a porous layer is permeated by mesoscale fractures, wave-induced fluid flow between pores and fractures can cause significant attenuation and dispersion of velocities and anisotropy parameters in the seismic frequency band. This intrinsic dispersion due to fracturing can create frequency-dependent reflection coefficients in the layered medium. In this study, we derive the frequency-dependent PP and PS reflection coefficients versus incidence angle in the fractured medium. We consider a two-layer vertical transverse isotropy model constituted by an elastic shale layer and an anelastic sand layer. Using Chapman's theory, we introduce the intrinsic dispersion due to fracturing in the sand layer. Based on the series coefficients that control the behaviour of velocity and anisotropy parameters in the fractured medium at low frequencies, we extend the conventional amplitude-versus-offset equations into frequency domain and derive frequency-dependent amplitude-versus-offset equations at the elastic-anelastic surface. Increase in fracture length or fracture density can enlarge the frequency dependence of amplitude-versus-offset attributes of PP and PS waves. Also, the frequency dependence of magnitude and phase angle of PP and PS reflection coefficients increases as fracture length or fracture density increases. Amplitude-versus-offset type of PP and PS reflection varies with fracture parameters and frequency. What is more, fracture length shows little impact on the frequencydependent critical phase angle, while the frequency dependence of the critical phase angle increases with fracture density.

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Section: Discussionmentioning

confidence: 99%

Section: A P P E N D I X : F R E Q U E N C Y -D E P E N D E N T V E Lmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Frequency‐dependent PP and PS reflection coefficients in fractured media

Pang

Stovas

2019

Geophysical Prospecting

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Frequency-dependent anisotropy in partially saturated porous rock with multiple sets of mesoscale fractures

Pang

Stovas

Xing

2021

Geophysical Journal International

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Summary Accurate modeling of the frequency-dependence of seismic wave velocity related to fracture system and fluid content is crucial to the quantitative interpretation of seismic data in fractured reservoirs. Both meso-scale fractures and patchy saturation effects can cause significant velocity dispersion and attenuation in the seismic frequency band due to wave-induced fluid flow (WIFF) mechanism. Considering the coupled impact of ‘meso-scale fractures’ and ‘patchy saturation’, we derive expressions for the frequency-dependent anisotropy in partially saturated porous rock containing two fracture sets with different orientations, sizes and connectivities. Especially, we simplify the rock-physics model as an orthorhombic (ORT) media by assuming the meso-scale fractures to be orthogonal and give the explicit expressions for frequency-dependent elastic constants. Finally, we give the expressions for the frequency-dependent phase velocity in patchy saturated and fractured ORT media and investigate the effect of patchy saturation on P-wave velocity at different polar and azimuth angles. In this paper, we investigate the effects of fluid saturation and fluid pressure on frequency-dependent velocities and Thomsen anisotropy parameters. Also, the effect of the relative permeability is very noticeable. The relaxation frequency can be lower in partially saturated fractured rocks compared with the fully saturated case, which makes the rock have a larger stiffness. The non-monotonic relationships between frequency-dependent anisotropy and fluid saturation add complexity to seismic forward modeling and inversion in reservoirs with complex fracture patterns.

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Seismic characterization of fractured reservoirs with elastic impedance difference versus angle and azimuth: A low-frequency poroelasticity perspective

Pan

Zhang

et al. 2021

GEOPHYSICS

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PP-wave azimuthal elastic impedance (EI) data inverted from azimuthal prestack seismic reflection data can be theoretically and practically used for fluid- and fracture-property discrimination in naturally porous and fluid-saturated fractured reservoirs within the exploration seismic frequency band. From the perspective of low-frequency Biot-Gassmann theory, Russell’s fluid indicator can be stably applied to fluid identification in hard-rock fractured reservoirs. Based on the low-frequency anisotropic poroelasticity theory, we first derive an azimuthal PP-wave reflection coefficient and an azimuthal elastic impedance equation in terms of Russell’s fluid indicator (a combination of acoustic impedances and dry-rock velocity ratio without the effect of squirt flow in seismic frequency range) and two fracture indicators (i.e., normal and tangential fracture weaknesses). Using azimuthal differences in elastic impedance data, we then propose a three-step inversion method of EI difference versus angle and azimuth. It can be used accurately to estimate the fluid and fracture indicators, and characterize the sensitive properties of saturated fractured porous reservoirs. Noisy synthetic data are used to verify the stability and robustness of the proposed hierarchal impedance inversion approach. A real data set acquired over a gas-bearing fractured reservoir is also employed to obtain the sensitive indicators of fluid and fracture properties, which agree well with the well log data. Thus, the proposed hierarchal inversion approach provides a more straight-forward and efficient manner to characterize the porous and fluid-saturated fractured reservoirs.

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Low‐frequency anisotropy in fractured and layered media

Cited by 4 publications

References 32 publications

Frequency‐dependent PP and PS reflection coefficients in fractured media

Frequency‐dependent PP and PS reflection coefficients in fractured media

Frequency-dependent anisotropy in partially saturated porous rock with multiple sets of mesoscale fractures

Seismic characterization of fractured reservoirs with elastic impedance difference versus angle and azimuth: A low-frequency poroelasticity perspective

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