The presence of stiffness contrasts at scales larger than the typical pore sizes but smaller than the predominant seismic wavelengths can produce seismic attenuation and velocity dispersion in fluid‐saturated porous rocks. This energy dissipation mechanism is caused by wave‐induced fluid pressure diffusion among the different components of the probed geological formations. In many cases, heterogeneities have elongated shapes and preferential orientations, which implies that the overall response of the medium is anisotropic. In this work, we propose a numerical upscaling procedure that permits to quantify seismic attenuation and phase velocity considering fluid pressure diffusion effects as well as generic anisotropy at the sample's scale. The methodology is based on a set of three relaxation tests performed on a 2‐D synthetic rock sample representative of the medium of interest. It provides a complex‐valued frequency‐dependent equivalent stiffness matrix through a least squares procedure. We also derive an approach for computing various poroelastic fields associated with the considered sample in response to the propagation of a seismic wave with arbitrary incidence angle. Using this approach, we provide an energy‐based estimation of seismic attenuation. A comprehensive numerical analysis indicates that the methodology is suitable for handling complex media and different levels of overall anisotropy. Comparisons with the energy‐based estimations demonstrate that the dynamic‐equivalent viscoelastic medium assumption made by the numerical upscaling procedure is reasonable even in the presence of high levels of overall anisotropy. This work also highlights the usefulness of poroelastic fields for the physical interpretation of seismic wave phenomena in strongly heterogeneous and complex media.

The deformations caused by an acoustic wavefield in subsurface rock can induce fluid flow within hydraulically interconnected mesoscopic fractures, from one fracture into the other. The viscous friction associated with this squirt-type fluid flow parallel to the fracture walls results in energy dissipation and velocity dispersion. We have developed a quasi-static hydromechanical approach that is suitable for simulating squirt-type flow in the mesoscopic scale range and microscopic squirt flow. Our approach couples Navier-Stokes equation with Hooke’s law to describe the laminar flow of a viscous compressible fluid in conduits embedded in an elastic solid background. Results from the proposed method were compared with those obtained with Biot’s equations for a model containing interconnected mesoscopic fractures embedded in a background of very low porosity and permeability. Despite significant differences in the flow and dissipation spatial patterns, we have observed an essentially perfect agreement of the attenuation and modulus dispersion characteristics predicted by the two approaches. The difference in the flow and dissipation spatial patterns are associated with the “upscaling” inherent to Biot’s equations and, correspondingly, with differing boundary conditions at the fracture walls. Our results demonstrate that the proposed hydromechanical approach can provide additional insights on the physics of squirt-type flow in the mesoscopic and microscopic scale ranges.

The degree of connectivity of fracture networks is a key parameter that controls the hydraulic properties of fractured rock formations. The current understanding is that this parameter does not alter the effective elastic properties of the probed medium and, hence, cannot be inferred from seismic data. However, this reasoning is based on static elasticity, which neglects dynamic effects related to wave-induced fluid pressure diffusion (FPD). Using a numerical upscaling procedure based on the theory of quasi-static poroelasticity, we provide the first evidence to suggest that fracture connectivity can reduce significantly velocity anisotropy in the seismic frequency band. Analyses of fluid pressure fields in response to the propagation of seismic waves demonstrate that this reduction of velocity anisotropy is not due to changes of the geometrical characteristics of the probed fracture networks, but rather related to variations of the stiffening effect of the fracture fluid in response to FPD. These results suggest that accounting for FPD effects may not only allow for improving estimations of geometrical and mechanical properties of fracture networks, but may also provide information with regard to the effective hydraulic properties.

In fractured rocks, the amplitudes of propagating seismic waves decay due to various mechanisms, such as geometrical spreading, solid friction, displacement of pore fluid relative to the solid frame, and transmission losses due to energy conversion to reflected and transmitted waves at the fracture interfaces. In this work, we characterize the mechanical properties of individual fractures from P wave velocity changes and transmission losses inferred from static full‐waveform sonic log data. The methodology is validated using synthetic full‐waveform sonic logs and applied to data acquired in a borehole penetrating multiple fractures embedded in a granodioritic rock. To extract the transmission losses from attenuation estimates, we remove the contributions associated with other loss mechanisms. The geometrical spreading correction is inferred from a joint analysis of numerical simulations that emulate the borehole environment and the redundancy of attenuation contributions other than geometrical spreading in multiple acquisitions with different source‐receiver spacing configurations. The intrinsic background attenuation is estimated from measurements acquired in the intact zones. In the fractured zones, the variations with respect to the background attenuation are attributed to transmission losses. Once we have estimated the transmission losses associated with a given fracture, we compute the transmission coefficient, which, on the basis of the linear slip theory, can then be related to the mechanical normal compliance of the fracture. Our results indicate that the estimated mechanical normal compliance ranges from 1 × 10−13 to 1 × 10−12 m/Pa, which, for the size of the considered fractures, is consistent with the experimental evidence available.

In interconnected microcracks, or in microcracks connected to spherical pores, the deformation associated with the passage of mechanical waves can induce fluid flow parallel to the crack walls, which is known as squirt flow. This phenomenon can also occur at larger scales in hydraulically interconnected mesoscopic cracks or fractures. The associated viscous friction causes the waves to experience attenuation and velocity dispersion. We present a simple hydromechanical numerical scheme, based on the interface‐coupled Lamé–Navier and Navier–Stokes equations, to simulate squirt flow in the frequency domain. The linearized, quasi‐static Navier–Stokes equations describe the laminar flow of a compressible viscous fluid in conduits embedded in a linear elastic solid background described by the quasi‐static Lamé–Navier equations. Assuming that the heterogeneous model behaves effectively like a homogeneous viscoelastic medium at a larger spatial scale, the resulting attenuation and stiffness modulus dispersion are computed from spatial averages of the complex‐valued, frequency‐dependent stress and strain fields. An energy‐based approach is implemented to calculate the local contributions to attenuation that, when integrated over the entire model, yield results that are identical to those based on the viscoelastic assumption. In addition to thus validating this assumption, the energy‐based approach allows for analyses of the spatial dissipation patterns in squirt flow models. We perform simulations for a series of numerical models to illustrate the viability and versatility of the proposed method. For a 3D model consisting of a spherical crack embedded in a solid background, the characteristic frequency of the resulting P‐wave attenuation agrees with that of a corresponding analytical solution, indicating that the dissipative viscous flow problem is appropriately handled in our numerical solution of the linearized, quasi‐static Navier–Stokes equations. For 2D models containing either interconnected cracks or cracks connected to a circular pore, the results are compared with those based on Biot's poroelastic equations of consolidation, which are solved through an equivalent approach. Overall, our numerical simulations and the associated analyses demonstrate the suitability of the coupled Lamé–Navier and Navier–Stokes equations and of Biot's equations for quantifying attenuation and dispersion for a range of squirt flow scenarios. These analyses also allow for delineating numerical and physical limitations associated with each set of equations.