Effects of aspect ratio on wave velocities in fractured rocks

Abstract

We present recent results on the effect of crack/pore aspect ratio on seismic wave velocities. Our comparative studies of experimental and theoretical investigations show that the crack/pore aspect ratio may be an important reservoir parameter in fractured and porous rocks. For low-porosity rock, the effect of aspect ratio on velocities at low effective pressure is greater than that of porosity. We found that the change of wave velocities with effective pressure may well be modeled by an extented Biot’s theory with the following relationship between aspect ratio and effective pressure: + where is effective pressure, is the initial aspect ratio at zero effective pressure, is the terminal aspect ratio, and is a constant for a given rock. We determine these model parameters for 4 rocks: granite, limestones (Bedford and Leuders), sandstone, having porosities from 0.65% to 21.7%. Variations in velocity with differential pressure correspond with aspect ratio changes from 0.0015 to 0.125. In low-porosity Casco granite, Vp increases with pressure from 3.3 km/s to 6.4 km/s and is associated with more than an 80 fold increase in aspect ratio from 0.0015 to 0.125. The Bedford limestone is populated with fractures and cracks of aspect ratio 0.0085; the Leuders limestone has an average aspect ratio of 0.03. Gulf Coast sandstone has an average aspect ratio of about 0.05. The model results can be used for high-resolution forward modeling and inversion of reservoir properties and to estimate in situ porosity, fluid content, and pore pressure from seismic data.