Fracture connectivity can reduce the velocity anisotropy of seismic waves

Abstract

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.

Sensitivity of Seismic Attenuation and Phase Velocity to Intrinsic Background Anisotropy in Fractured Porous Rocks: A Numerical Study

“…Consequently, there is a given frequency for which, V P (45 ∘ ) = V P (0 ∘ ) = V P (90 ∘ ) and the P wave phase velocity is virtually isotropic. These results are in agreement with the findings of Rubino et al () who showed that the level of phase velocity anisotropy produced by the presence of fractures is strongly affected by the degree of hydraulic connectivity of the fracture network. Moreover, they found that, in the case of low‐permeability formations and fully connected fractures, the regime where V P (45 ∘ ) ∼ V P (0 ∘ ) ∼ V P (90 ∘ ) prevails in the seismic frequency band.…”

“…The maximal S wave attenuation occurs at an angle of incidence of ∼45°, and it is particularly strong in the FF‐WIFF regime. As pointed out by Rubino et al (), this is related to the fact that the induced pressures are positive and negative in the horizontal and vertical fractures, respectively, and hence, the fluid pressure gradient between connected fractures is particularly large.…”

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

“…In a series of papers, Rubino et al (2013Rubino et al ( , 2014Rubino et al ( , and 2017 found that besides FB-WIFF, WIFF also occurs within connected mesoscopic fractures (FF-WIFF), which can have a significant influence on the dispersion, attenuation, and anisotropy of seismic waves. Since this fluid flow critically depends on the connectivity degree of the probed fracture network, these results suggest the possibility to detect fracture connectivity and, hence, to quantify the effective permeability of fractured formations using seismic data.…”

Seismic Signatures of Fractured Reservoirs: Theory Versus Numerical Simulations

“…Besides the FB-WIFF, in the recent studies of Rubino et al (2013Rubino et al ( , 2014Rubino et al ( , and 2017, it is found that the WIFF also occurs within the connected fractures (FF-WIFF). It is shown that FF-WIFF can also result in significant seismic dispersion and attenuation.…”