Seismic Dispersion and Attenuation in Fluid‐Saturated Carbonate Rocks: Effect of Microstructure and Pressure

Borgomano, Jan V. M.; Pimienta, Lucas Xan; Fortin, Jérôme; Guéguen, Yves

doi:10.1029/2019jb018434

Cited by 45 publications

(49 citation statements)

References 99 publications

(162 reference statements)

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“…The bulk moduli and attenuation of brine saturated and oil saturated sample are put together using the apparent frequency f * = f * η fluid / η brine , where the η fluid and η brine are the fluid and brine viscosities, respectively (Borgomano et al, 2019; Pimienta et al, 2015). The bulk modulus and shear modulus versus the apparent frequencies are shown in Figure 7.…”

Section: Results and Analysismentioning

confidence: 99%

Dispersion and Attenuation of Elastic Wave Velocities: Impact of Microstructure Heterogeneity and Local Measurements

et al. 2020

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The variation of the seismic properties with frequency of a brine-saturated sandstone sample with a porosity of 22% was measured using a forced oscillation apparatus. Two pairs of orthogonal biaxial strain gauges were pasted at different locations at the length center of the sample. In the frequency range of 1-300 Hz, using these two pairs of strain gauges, we observed experimentally the same global flow but different local flows (squirt flow). Therefore, the observation of the local flow is influenced by the position of the strain gauges in contrast to the global flow. Indeed, local flow is strongly influenced by the microstructure (structure of the grain contacts and microcracks). In particular, we show that although the sample is homogeneous in terms of porosity and crack density, it is not the case in terms of crack aspect ratio, which may slightly vary along the sample. A 3D diffusion model coupled with a simple squirt model was built to further interpret the data. These results show that the wave-induced fluid flows occur at different scales and controls the dispersion and attenuation of saturated rocks.

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Section: Results and Analysismentioning

confidence: 99%

Dispersion and Attenuation of Elastic Wave Velocities: Impact of Microstructure Heterogeneity and Local Measurements

et al. 2020

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“…The measured bulk modulus at pressures 3.5 (solid circles) and 5 MPa (red diamonds) is significantly smaller than that for the undrained condition (horizontal line) as estimated from the Gassmann theory at pressure 3.5 MPa. This indicates that at these pressures, the (Borgomano et al, 2019…”

Section: Comparisons With Forced Oscillation Measurementsmentioning

confidence: 87%

“…Indeed, from pressure 3.5 (solid circles) to 20 MPa (magenta triangles), the increase in the bulk modulus is almost 22 GPa, which is much larger than for the dry modulus or the undrained modulus of the water-saturated sample (about 11 GPa) (see Fig. 8c of Borgomano et al, 2019). Similarly to the case of the Coquina, this suggests that the rock has been significantly altered by nonmechanical interaction with glycerin, and hence, the measurements for glycerin saturated samples cannot be modeled using pressure dependency of the dry moduli.…”

Section: 1029/2019jb019297mentioning

confidence: 93%

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Modeling the Effect of Pressure on the Moduli Dispersion in Fluid‐Saturated Rocks

Sun

Gurevich

2020

JGR Solid Earth

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Laboratory experiments of ultrasonic velocities of fluid‐saturated rocks are often much higher than the predictions of the Gassmann theory. This difference is usually attributed to the velocity dispersion caused by fluid pressure relaxation between pores of different shapes and orientation. This paper proposes a simple model to characterize pressure and frequency effects on the elastic moduli of fluid‐saturated rocks in a broad frequency range. The proposed model incorporates micromechanics of wave‐induced fluid pressure relaxation at grain contacts (between crack‐like contacts and stiff pores) into pressure dependency of elastic moduli. Previously, the pressure dependency of the velocities or elastic moduli was ascribed to the progressive closure of cracks with the increasing effective pressure and expressed as an integral of crack compliance over the range of aspect ratios. For isolated cracks, this compliance is a function of crack geometry only. For cracks hydraulically connected to stiff pores, this crack compliance can be replaced by a frequency‐dependent solution of the micromechanical problem of fluid pressure relaxation between a single crack and surrounding pores. The resulting equation expresses the bulk and shear moduli of the fluid‐saturated rock as functions of both pressure and frequency. Furthermore, if pressure‐dependent moduli of both dry and fluid‐saturated moduli are known, the aspect ratio distribution can be obtained from the pressure dependency of the dry moduli, and then the saturated moduli can be computed with no adjustable parameters. The model predictions show reasonable agreement with laboratory data measured using ultrasonic and forced oscillation techniques.

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“…The consequent friction between particles of the viscous fluid dissipates energy. Squirt flow evidence at seismic and sonic frequencies were shown in laboratory experiments in which glycerine‐saturated samples of Fontainebleau and Berea sandstones, as well as of limestones, were submitted to forced oscillations by Pimienta et al (), Subramaniyan et al (), Borgomano et al (), and Chapman et al ().…”

Section: Introductionmentioning

confidence: 97%

Squirt Flow in Cracks with Rough Walls

et al. 2020

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We explore the impact of roughness in crack walls on the P wave modulus dispersion and attenuation caused by squirt flow. For that, we numerically simulate oscillatory relaxation tests on models having interconnected cracks with both simple and intricate aperture distributions. Their viscoelastic responses are compared with those of models containing planar cracks but having the same hydraulic aperture as the rough wall cracks. In the absence of contact areas between crack walls, we found that three apertures affect the P wave modulus dispersion and attenuation: the arithmetic mean, minimum aperture, and hydraulic aperture. We show that the arithmetic mean of the crack apertures controls the effective P wave modulus at the low-and high-frequency limits, thus representing the mechanical aperture. The minimum aperture of the cracks tends to dominate the energy dissipation process and, consequently, the characteristic frequency. An increase in the confining pressure is emulated by uniformly reducing the crack apertures, which allows for the occurrence of contact areas. The contact area density and distribution play a dominant role in the stiffness of the model, and in this scenario, the arithmetic mean is not representative of the mechanical aperture. On the other hand, for a low percentage of minimum aperture or in the presence of contact areas, the hydraulic aperture tends to control the characteristic frequency. Analyzing the local energy dissipation, we can more specifically visualize that a different aperture controls the energy dissipation process at each frequency, which means that a frequency-dependent hydraulic aperture might describe the squirt flow process in cracks with rough walls. Key Points:• We solve the quasi-static linearised Navier-Stokes equations coupled to elasticity equations • Seismic attenuation due to squirt-flow is strongly affected by the roughness of the crack walls • The minimum and the hydraulic apertures significantly affect the energy dissipation process presented a comparison between numerical results and an analytical model for squirt flow. In general, accepted analytical models should reproduce the equations of Gassmann (1951) in the low-frequency limit (Chapman et al., 2002). The reason is that at the relaxed state for undrained boundary conditions (low-frequency limit), the time of a half period of a passing wave allows for fluid pressure to equilibrate through FPD. At the unrelaxed state (high-frequency limit), the fluid pressure has no time to equilibrate during a half period of a passing wave and the elastic properties of the saturated material are predicted by the formulation of Mavko and Jizba (1991), which assumes that no FPD occurs during the passage of the wave. At intermediate frequencies, FPD occurs inside the cracks during the passage of the wave and part of its energy is dissipated. Nevertheless, all analytical solutions assume smooth walls for the cracks despite the fact that crack walls in rocks have been observed to present complex profiles including wall roughnes...

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Seismic Dispersion and Attenuation in Fluid‐Saturated Carbonate Rocks: Effect of Microstructure and Pressure

Cited by 45 publications

References 99 publications

Dispersion and Attenuation of Elastic Wave Velocities: Impact of Microstructure Heterogeneity and Local Measurements

Dispersion and Attenuation of Elastic Wave Velocities: Impact of Microstructure Heterogeneity and Local Measurements

Modeling the Effect of Pressure on the Moduli Dispersion in Fluid‐Saturated Rocks

Squirt Flow in Cracks with Rough Walls

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