Biot dispersion for P- and S-wave velocities in partially and fully saturated sandstones

Ultrasonic compressional- and shear-wave velocities have been measured on 3 4 samples of sandstones from hydrocarbon reservoirs. The sandstones are all of low clay content, high porosity, and cover a wide range of permeabiliti es. They were measured dry and brine-saturated under hydrostatic effective stresses of 10, 20 and 40 MPa. For eight of the sandstones, ultrasonic velo city measurements were made at different partial water saturations in the r ange from dry to fully saturated. The Gassmann-Biot theory is found to acco unt for most of the changes in velocities at high effective stress levels w hen the dry sandstones are fully saturated with brine, provided the lower v elocities resulting when the dry sandstone initially adsorbs small amounts of moisture are used to determine the elastic properties of the 'dry' sands tone. At lower effective stress levels, local flow phenomena due to the pre sence of open microcracks are assumed to be responsible for measured veloci ties higher than those predicted by the theory. The partial saturation resu lts are modelled fairly closely by the Gassmann-Biot theory, assuming heter ogeneous saturation for P-waves.