Effects of porosity and clay content on wave velocities in sandstones

Abstract

The ultrasonic compressional [Formula: see text] and shear [Formula: see text] velocities and first‐arrival peak amplitude [Formula: see text] were measured as functions of differential pressure to 50 MPa and to a state of saturation on 75 different sandstone samples, with porosities ϕ ranging from 2 to 30 percent and volume clay content C ranging from 0 to 50 percent, respectively. Both [Formula: see text] and [Formula: see text] were found to correlate linearly with porosity and clay content in shaly sandstones. At confining pressure of 40 MPa and pore pressure of 1.0 MPa, the best least‐squares fits to the velocity data are [Formula: see text] and [Formula: see text]. Deviations from these equations are less than 3 percent and 5 percent for [Formula: see text] and [Formula: see text], respectively. The velocities of clean sandstones are significantly higher than those predicted by the above linear fits (about 7 percent for [Formula: see text] and 11 percent for [Formula: see text]), which indicates that a very small amount of clay (1 or a few percent of volume fraction) significantly reduces the elastic moduli of sandstones. For shaly sandstones we conclude that, to first order, more sensitive to the porosity and clay content than is [Formula: see text]. Consequently, velocity ratios [Formula: see text] and their differences between fully saturated (s) and dry (d) samples also show clear correlation with the clay content and porosity. For shaly sandstones we conclude that, to first order, clay content is the next most important parameter to porosity in reducing velocities, with an effect which is about 0.31 for [Formula: see text] to 0.38 for [Formula: see text] that of the effect of porosity.