Azimuth-, angle-and frequency-dependent seismic velocities of cracked
rocks due to squirt flow
Abstract
Understanding the properties of cracked rocks is of great importance in
scenarios involving CO 2 geological sequestration, nuclear waste
disposal, geothermal energy , and hydrocarbon exploration and
production. Developing noninvasive detecting and monitoring methods for
such geological formations is crucial. Many studies show that seismic
waves exhibit strong dispersion and attenuation across a broad frequency
range due to fluid flow at the pore scale known as squirt flow.
Nevertheless, how and to what extent squirt flow affects seismic waves
is still a matter of investigation. To fully understand its angle-and
frequency-dependent behavior for specific geometries, appropriate
numerical simulations are needed. We perform a three-dimensional
numerical study of the fluid-solid deformation at the pore scale based
on coupled Lamé-Navier and Navier-Stokes linear quasistatic equations.
We show that seismic wave velocities exhibit strong azimuth-, angle-and
frequency-dependent behavior due to squirt flow between interconnected
cracks. Furthermore, the overall anisotropy of a medium mainly increases
due to squirt flow, but in some specific planes the anisotropy can
locally decrease. We analyze the Thomsen-type anisotropic parameters and
adopt another scalar parameter which can be used to measure the
anisotropy strength of a model with any elastic symmetry. This work
significantly clarifies the impact of squirt flow on seismic wave
anisotropy in three dimensions and can potentially be used to improve
the geophysical monitoring and surveying of fluid-filled cracked porous
zones in the subsurface.