Velocity dispersion between ultrasonic and seismic frequencies in brine-saturated reservoir sandstones

Ultrasonic P- and S-wave velocities have been measured on 44 specimens from core samples of relatively clean sandstones recovered from hydrocarbon reservoirs. Ten specimens have porosities less than 10%, and 34 have porosities in the range 20-30%. Velocities were measured with the specimens in both their dry and brine-saturated states, under hydrostatic effective stresses to 60 MPa for the low-porosity specimens and to 40 MPa for those of the high-porosity set. Gassmann's poroelastic theory is found to account for changes in velocity for the low-porosity set at 60 MPa effective stress when the dry specimens are fully saturated with brine. The velocities used for dry specimens in Gassmann's theory must, however, be those measured after the dry specimens have first adsorbed a small quantity of moisture. For saturated specimens at effective stresses of 40 MPa or less, local flow phenomena due to the presence of open microcracks are assumed to be responsible for the measured Velocities being appreciably higher than those predicted theoretically.