We show that attenuation of high‐resolution interwell seismic and acoustic waves based on velocity dispersion analysis relates to fluid‐flow effects in fractured and shale–sand sequence formations at the Buena Vista Hills reservoir, California. Fractured low quality factor (Q‐factor) zones in the Brown Shale and Antelope Shale reservoir intervals in the Monterrey Formation correlate with a system of fractures having permeabilities of 2.5 to 5 md. Vertical fractures oriented at azimuths from 0° to 30° are detected in the frequency range of 1 to 10 kHz. We establish that a poroelastic model based on the Biot/squirt‐flow (BISQ) mechanism can be used to relate the low Q‐factor zones in the Brown and Antelope Shales. Because the Brown Shale has no sands, we use it to evaluate a fracture system's response to attenuation. We adapt the BISQ mechanism to simulate fluid flow in fracture‐induced anisotropy, which provides flow properties parallel and perpendicular to fractures in the siliceous shale formations. The model assumes that the principal axes of the stiffness tensor are aligned with the axes of the permeability and squirt‐flow tensors. We simulate the fracture system by assuming that (1) a squirt‐flow length on the order of centimeters represents fluid flow in fractures and (2) a squirt‐flow length <1 mm represents flow in low‐permeability shales. Two types of fractures at the site are joint‐like tectonic fractures and sigmoidal vein fractures. Their fracture permeability (approximately 5 md) and squirt‐flow lengths (between 1 and 2 cm) predict a Q‐factor of about 20 that fits the observed Q‐factor in the Brown Shale. We find that fractures associated with squirt‐flow lengths ≥3 cm are sensitive to horizontal attenuation for a frequency range of 120 to 1000 Hz. In addition, the horizontal Q‐factor derived from sonic and crosswell data is about five times less than the vertical Q‐factor associated with waves originating from the earth's surface.
DISPERSION ANALYSIS OF BULK ACOUSTIC WAVES IN PIEZOELECTRIC MEDIA DISCRETIZED BY ENERGY-ORTHOGONAL FINITE ELEMENTS
