Abstract
Abstract. Understanding the properties of cracked rocks is of great importance in scenarios involving CO2 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.
Highlights
Wave propagation is controlled by the effective rock properties
The ciCjon stiffness matrix precisely belongs to the tetragonal symmetry class, while the ciDjis stiffness matrix has all diagonal components different from each other; it represents the orthorhombic symmetry class
The overall anisotropy of the model mainly increases due to squirt flow between the cracks, so the crack connectivity increases the overall anisotropy of the model towards low frequencies
Summary
Wave propagation is controlled by the effective rock properties. Wave velocity and attenuation can be estimated from seismic data in scenarios such as seismic exploration, seismology, borehole measurements and tomography. Rock physics could be used to estimate different rock properties, such as mineral composition, elastic moduli, the presence of a fluid, and pore space connectivity (and permeability) from seismic measurements. In activities including nuclear waste disposal, CO2 geological sequestration, hydrocarbon exploration and production, geothermal energy production, and seismotectonics, a quantification of the fluid content, porosity and permeability of rocks are of great interest. All these activities can benefit from rock physics studies, and that is why cracked rocks have been under intensive studies during the last decades
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