Variations of acoustic velocity as function of brine and oil saturation in carbonates

This study aims to improve our understanding of the seismic signature (i.e., variations in acoustic velocity) as a result of variations in the brine and oil saturation. To this scope, we utilized a customized core flooding system and measured P- and S-wave velocity (Vp and Vs respectively) under reservoir pressure and as function of brine and oil saturation during drainage and imbibition, in three carbonate samples. Our results show that both Vp and Vs decrease as oil displaces brine (imbibition) and then increase as brine displaces oil (drainage). The Vp-saturation trend during imbibition followed the lower bound predicted by Gassmann theory assuming uniform fluid distribution, while drainage data followed the upper bound assuming patchy fluid distribution. Since oil production or EOR processes can be represented by the drainage process (i.e., gas or brine displacing oil), patchy fluid distribution should be considered in Gassmann theory when performing feasibility studies to predict changes in Vp as function of saturation changes. Despite the success of Gassmann theory in predicting Vp here, the results obtained for Vs show that Gassmann theory fails significantly to predict Vs as function of saturation. This suggests that the constant-shear-modulus condition in Gassmann is violated as fluid saturation changes.