Effect of fracture fill on frequency‐dependent anisotropy of fractured porous rocks

Abstract

In fractured reservoirs, seismic wave velocity and amplitude depend on frequency and incidence angle. Frequency dependence is believed to be principally caused by the wave‐induced flow of pore fluid at the mesoscopic scale. In recent years, two particular phenomena, i.e., patchy saturation and flow between fractures and pores, have been identified as significant mechanisms of wave‐induced flow. However, these two phenomena are studied separately. Recently, a unified model has been proposed for a porous rock with a set of aligned fractures, with pores and fractures filled with two different fluids. Existing models treat waves propagating perpendicular to the fractures. In this paper, we extend the model to all propagation angles by assuming that the flow direction is perpendicular to the layering plane and is independent of the loading direction. We first consider the limiting cases through poroelastic Backus averaging, and then we obtain the five complex and frequency‐dependent stiffness values of the equivalent transversely isotropic medium as a function of the frequency. The numerical results show that, when the bulk modulus of the fracture‐filling fluid is relatively large, the dispersion and attenuation of P‐waves are mainly caused by fractures, and the values decrease as angles increase, almost vanishing when the incidence angle is 90° (propagation parallel to the fracture plane). While the bulk modulus of fluid in fractures is much smaller than that of matrix pores, the attenuation due to the “partial saturation” mechanism makes the fluid flow from pores into fractures, which is almost independent of the incidence angle.

CO2‐Brine Substitution Effects on Ultrasonic Wave Propagation Through Sandstone With Oblique Fractures

“…The properties of the pore fluids and their mobility through the porous media complicate the distinction between pore fluid distribution and rock fabric in fractured media. Kong et al (2017) remarked that the fracture‐filling fluid can disguise P wave dispersion and attenuation effects due to fracture orientation, when its modulus is much smaller than that of the pore‐filling fluid. Amalokwu et al (2015) found that fractures obliquely oriented with respect to wave propagation can lead to S wave velocity anisotropy (SWVA) drop with increasing water saturation, because V S 2 (sensitive to fluid compressibility reduction) decreases, while V S 1 , considered independent of the pore fluid (Tillotson et al, 2012), remains constant.…”

Fluid pressure diffusion effects on the excess compliance matrix of porous rocks containing aligned fractures

“…We would get the opposite result if we adjust the aforementioned parameters in a way that renders the fractures stiffer. Concerning the properties of the pore fluid, Kong et al (2017) showed that, when the fractures are saturated with a very compressible fluid, such as gas, and the background rock is fully saturated with water, seismic attenuation may also be important. Although the analytical derivation presented in our work could be used to study the discrepancies between the TLM and LST approaches in such context, the corresponding analysis is beyond the scope of this paper and will be the subject of future work.…”

Modeling the effects of fracture infill on frequency-dependent anisotropy and AVO response of a fractured porous layer

“…Plenty of rock physics theories that have been derived specifically to explore the frequency dependence in the elastic stiffness tensors press the main challenges in carrying out seismic AVO modeling analysis, principally porosity-anisotropic velocity transform, velocity-pressure relation, fluid saturation and velocity prediction. In this study, an extended poroelastic theory based on Carcione et al (2013) and Kong et al (2017) was utilized for the determination of frequency-dependent full elastic stiffness tensors. Additionally, the introduced theory allows us to account for fluid-sensitive attenuation and velocity dispersion attributes in the frame of anisotropic viscoelasticity.…”

“…where i indicates either the P-, the SV-or the SH-wave mode. Precise forms of the above relationships are also presented in Kong et al (2017) and He et al (2020).…”

“…And then we investigate numerically the influences of abnormally high levels of wave attenuation and velocity dispersion on seismic reflection signatures. In this study, the effect of velocity dispersion and attenuation was determined using an equivalent effective medium theory based on the mesoscopic wave-induced fluid flow concept of Norris (1993) (please also read Brajanovski et al 2005;Kong et al 2017) for fractured porous rocks, in which the pores and fractures are filled with two different fluids. Using a frequency-domain propagator matrix algorithm that allows anisotropic elastic tensors, in this presented work we perform our calculations of seismic AVO responses from a hydrocarbon reservoir with complex lithology (Guo et al 2015a, b;He et al 2018).…”