Effect of gas saturation on P-wave velocity in tight sandstone

By measuring the variation of the P- and S-wave velocities of tight sandstone samples under water saturation, it was confirmed that with the decrease in water saturation, the P-wave velocity first decreased and then increased. The variation in velocity was influenced by the sandstone's porosity. The commonly used Gassmann equation based on fluid substitution theory was studied. Comparing the calculated results with the measured data, it was found that the Gassmann equation agreed well with the measured data at high water saturation, but it could not explain the bending phenomenon of P-wave velocity at low saturation. This indicated that these equations could not accurately describe the relationship between fluid content and rock acoustic velocity. The reasons for this phenomenon were discussed through Taylor's expansion. The coefficients of the fitting formula were calculated and verified by fitting the measured acoustic velocity changes of the cores. The relationship between P-wave velocity and saturation was discussed, which provides experimental support for calculating saturation using seismic and acoustic logging data.