Dynamic seismic signatures of saturated porous rocks containing two orthogonal sets of fractures: theory versus numerical simulations

Self Cite

We estimate the seismic attenuation and velocity dispersion induced by fluid pressure diffusion in a saturated porous medium containing a sparse distribution of aligned slit fractures. This is done by solving the scattering problem for a single crack and using Waterman‐Truell scattering approximation for a distribution of cracks. This approach gives a numerical solution for the entire frequency range and analytical solutions for the low‐ and high‐frequency limits. Analysis of the results shows that the characteristics of the seismic dispersion and attenuation for the slit cracks are similar to known results for penny‐shaped crack. At high frequencies, the attenuation and dispersion are controlled by the crack density regardless of the crack geometry. At low frequencies, due to the different geometries of the slit and penny‐shaped cracks, the characteristics of seismic dispersion and attenuation are different. For both the slit and penny‐shaped crack case, the slope of the frequency dependence attenuation of factor (inverse quality factor) Q−1 tends to be 1. Yet the low‐frequency asymptotic solutions for these two cases are somewhat different. For penny‐shaped cracks, the Q−1(ω) on the bilogarithmic scale has a straight line asymptote with slope 1, so that the attenuation factor at low frequencies is proportional to the frequency ω. However, for slit cracks, the asymptotic low‐frequency solution contains a logarithmic function of ω, and no such asymptote exists. At the same time, similarly to the penny‐shaped crack case, the solution for compressional velocity in the low‐frequency limit is consistent with the anisotropic Gassmann theory.

Section: Discussionsupporting

confidence: 91%

Section: Analytical Solutionsupporting

confidence: 88%

Seismic Dispersion and Attenuation in Saturated Porous Rock With Aligned Slit Cracks

Guo

et al. 2018

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“…When subjected to stresses, fractures can more easily close, which results in the increase of rock elastic moduli (e.g., Glubokovskikh et al, ; Mavko et al, ; Shapiro, ). Due to these important effects of fractures on rock hydraulic and elastic properties, it is of great interest to detect and characterize fractures in many disciplines, such as earth and environmental sciences, and underground engineering, among many others (e.g., Guo et al, ; Guo, Rubino, Barbosa, et al, ; Guo, Rubino, Glubokovskikh, et al, ; Guo, Shuai, et al, ; Huo & Gong, ; Lisjak et al, ; Liu et al, ; Nelson, ; Neuzil, ).…”

Section: Introductionmentioning

confidence: 99%

“…Due to the fact that fracture sizes are usually much smaller than the seismic wavelength, it is difficult to image fractures directly on seismic profiles. For this reason, seismic attributes are often used in fracture detection and characterization (e.g., Bakulin et al, , , ; Gao, ; Guo et al, ; Guo, Rubino, Glubokovskikh, et al, ; Guo, Shuai, et al, ; Guo, Rubino, Barbosa, et al, ; Maultzsch et al, , ; Tsvankin, ). Under the in situ stress field, parallel fractures are often developed in the direction of the maximum stress (e.g., Schoenberg & Sayers, ).…”

Section: Introductionmentioning

confidence: 99%

Effective Elastic Properties of Rocks With Transversely Isotropic Background Permeated by Aligned Penny‐Shaped Cracks

Guo

Han

et al. 2019

Self Cite

Seismic characterization of fractures is of great importance in many disciplines, for which rock physics models provide the basis to link fracture properties to seismic attributes. However, to date, most rock physics models neglect the background anisotropy that may exist in many fractured formations. Hence, in this work, we developed a theoretical model for rock effective elastic properties with penny-shaped cracks embedded in the transversely isotropic (TI) background medium. We first derived analytical solutions for the case with dry or fluid-filled penny-shaped cracks parallel to the isotropic plane of TI background medium. Further, the results were extended to the case with cracks inclined to the isotropic plane with any angles. We then studied, based on the developed theoretical model, two tight sand samples with TI and isotropic background, respectively, to illustrate effects of background anisotropy on effective elastic properties of fractured rocks. The results show that the background anisotropy has significant influence on P and S wave velocity anisotropy, as well as on shear wave splitting. The background anisotropy can either increase or decrease P and SH wave velocity anisotropy depending on crack inclination angles, whereas it has relatively small influence on SV wave velocity anisotropy. To further illustrate the important effects of background anisotropy, we compared theoretical predictions to ultrasonic measurements on a synthetic fractured sandstone sample with TI background medium. The results show notable improvement of the agreement between theoretical predictions and experimental results after considering background anisotropy.

“…Wave attenuation plays an important role in the study of the physical properties of underground rocks, owing to its strong relation to lithology, mineral composition, grain size, degree of cementation, fluid saturation, temperature, pressure, etc. (Casula & Carcione, 1992; Guo, Rubino, et al, 2018; Müller et al, 2010; Wu, 1985). Seismic attenuation can be due to intrinsic loss or scattering at small‐scale heterogeneities and can be estimated directly from ultrasonic measurements (Adam et al, 2009; Borgomano et al, 2017; Chapman et al, 2016; Frankel & Clayton, 1986; Hu et al, 2017; Klimentos, 1990; Zhang et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Coda and Intrinsic Attenuations From Ultrasonic Measurements in Tight Siltstones

2020

We have performed ultrasonic measurements on tight siltstone samples to obtain the P wave coda and intrinsic attenuations using a single isotropic scattering model and the spectral ratio method.The results indicate that attenuation is predominantly caused by scattering. Intrinsic attenuation increases with gas saturation and permeability and decreases with grain size, whereas coda attenuation is stronger under partial saturation conditions and increases with grain size. Porosity and mineral composition do not have significant effects on attenuation. The triple-porosity and scattering models are used to simulate the intrinsic and scattering attenuations, respectively. The results show that the intrinsic attenuation is predominately caused by wave-induced fluid flow between microcracks/micropores and intergranular pores. The model incorporating the scattering effects of microcracks underestimates the scattering loss in the coda, which may thus be better explained by heterogeneities in the clustering structure of grains. Key Points:• Coda and intrinsic attenuations in siltstones are estimated with a single isotropic scattering model and the spectral ratio method • Ultrasonic P wave coda attenuation in oil-water partially saturated siltstones is stronger than that at the full saturations • Coda and intrinsic attenuations depend on permeability, grain size, and skewness but generally do not depend on mineral content