The effects of gas saturation on the acoustic velocity of carbonate rock

Abstract

Changes in the P-wave and S-wave velocities (Vp and Vs, respectively) and their ratio at different gas saturations in carbonate rock were investigated experimentally in this study. The results showed the following: (1) Both the P-wave velocity and Vp/Vs tended to decrease as the gas saturation increased, while the S-wave velocity marginally decreased or remained constant. These measured results were in agreement with the theoretical calculations of the Gassmann fluid substitution equation. (2) The acoustic velocity was related to the acoustic frequency. Most rock samples showed the tendency of Vp (1000 kHz) ＜ Vp (50 kHz) ＜ Vp (250 kHz) and Vs (1000 kHz) ＜ Vs (50 kHz) ＜ Vs (250 kHz). The primary distribution domain of Vp/Vs also varied with different frequencies and generally showed that Vp/Vs when Sg = 0% was larger than that when Sg = 100%. (3) Considering fully saturated rock as the water zone, it was found that Vp/Vs decreases from its initial value as the gas pressure increases; a sharper decline, indicating a transition from the water zone to the gas zone, was also found. Additionally, the integrated use of a cross-plot analysis of Vp/Vs vs. P-wave impedance was used to more accurately identify the gas zone in carbonate rock. This study provides some helpful references for gas-zone identification in carbonate rock using acoustic logging data in practical applications.