Experimental study of water saturation effect on acoustic velocity of sandstones

This study investigated the effect of water saturation on P-wave and S-wave velocity of gas-water saturated sandstones under atmospheric and high confining pressure. The measured samples were artificial sandstones of the same composition and shale-free. Different porosity was the dominating parameter that influenced the elastic property. Measured results were compared with Gassmann fluid substitution combined with three different mixed fluids effective bulk modulus models. The results demonstrated that with the decrease of porosity and permeability the gas-water mixed fluids were more likely to be inhomogeneous in the pores of sandstone. P-wave velocity trend was gradually close to the Gassmann-Hill boundary and away from Gassmann-Wood boundary, and the Gassmann-Brie model with an adjustable e-coefficient was more practical to estimate P-wave velocity of sandstone. S-wave velocity decreased linearly with the increase of water saturation. With the increase of confining pressure, the overall change in velocity induced by saturation was gradually reduced, and rock compacting may cause the velocity-saturation trend become close to the low porosity case.