Prediction of acoustic wave velocities by incorporating effects of water saturation and effective pressure

Acoustic wave velocities play essential roles in interpreting seismic and well logging data for their broad applicability in determining rock elastic properties. Many types of research have demonstrated the effects of porosity and clay fraction on acoustic waves, but few have simultaneously studied the effects of effective pressure and water saturation on the same. To quantify the effects of effective pressure and water saturation on acoustic wave velocities, this research conducts a series of ultrasonic wave transmission tests by using shaly sandstones with various porosities (<0.30) and clay fractions (<0.35). Statistical analysis results show that acoustic wave velocities significantly increase with decreased porosity, decreased clay fraction, and increased effective pressure. The increased water saturation can help enhance P-wave velocity but does not significantly affect S-wave velocity. The variance between P- and S- wave velocities significantly increases with decreased values of porosity and clay fraction. However, it responds positively to effective pressure and water saturation. Meanwhile, the compressional-to-shear wave velocities ratio can be significantly affected by porosity, clay fraction, and water saturation. Combining Wyllie's and Eberhart-Phillips's equations with regression tools, the effects of water saturation and effective pressure on acoustic wave velocities can be respectively quantified by a multivariate function. The findings in this research specifically shed light on responses of acoustic wave velocities to changes in water saturation and effective pressure, and they show potential for precisely interpreting acoustic wave velocities via in-situ geophysical properties of rocks.