Velocities and quality factors of sedimentary rocks at low and high effective pressures

Abstract

Techniques for measuring acoustic velocities and quality factors (Q) of sedimentary rocks in the laboratory are sufficiently accurate to establish closely fitting regression curves which represent the variation of these acoustic properties with pressure. Ultrasonic P- and S-wave measurements of velocity (Vp, Vs) and Q (Qp, Qs) were made on 16 different water-saturated sandstone samples at various effective pressures using an ultrasonic reflection technique, and the data were fitted using a simple regression equation containing a constant, plus linear and exponential pressure-dependent terms. The redundancy of the linear term in the regression of Q data suggests that microcracks alone govern the pressure variation of Q, whereas velocities are additionally dependent on another mechanism, consistent with the changes in elastic moduli caused by intergranular compression (causing small reductions in porosity). The implied rates of microcrack closure with increasing effective pressure in all samples were found to be comparable. The regression equations describe the data closely enough to enable the extrapolation of acoustic properties to both low and high effective pressures. This is of practical use, since laboratory techniques can produce uncertain results at low pressures, due to coupling problems and low signal strengths, whilst high pressures require large confining vessels. Low-pressure and high-pressure data are applicable to shallow seismic surveys and hydrocarbon exploration, respectively.