The effect of pore structure complexity and saturation history on the variations of acoustic velocity as function of brine and oil saturation in carbonates

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Time lapse seismic is a major technique that has been successfully implemented to monitor reservoir dynamics (such as changes in fluid saturation) in sandstones but its applicability in carbonate rocks is not well understood yet. This study aims to improve our understanding of the time lapse seismic signature in carbonates oil reservoirs by investigating how acoustic velocity changes as a result of variations in the brine and oil saturation. To this scope, we utilize a core flooding system to perform brine-oil displacement experiment in three reservoir carbonate samples, while monitoring the acoustic velocity as function of brine saturation (Sw) changes. The fluid displacement experiment included injection of oil into the fully brine saturated samples until reaching irreducible brine saturation, followed by injection of brine until reaching the residual oil saturation. We also characterize the pore structure complexity of the samples using nuclear magnetic resonance (NMR) and micro-CT scans to investigate its effect on the velocity-saturation relationship. We finally compare the measured acoustic velocity as function of saturation with the predictions of Gassmann theory while considering both uniform and patchy fluid distribution styles.Our results show that both P- and S-wave velocity (Vp and Vs respectively) decrease as oil displaces brine and then they both increase as brine displaces oil. Nevertheless, the velocity-saturation relationship exhibits a hysteresis: velocities measured at a given saturation during brine injection tend to be higher than those measured during oil injection. Our analysis shows that the Vp-Sw trend during oil injection into brine saturated samples follows the lower bound predicted by Gassmann theory (i.e., uniform fluid distribution), while the brine flooding data follows the upper bound of Gassmann theory (i.e., patchy fluid distribution). The transition from the uniform to patchy fluids distribution seems to occur around the irreducible brine saturation. Since oil production or enhanced oil recovery can be represented by the process of brine displacing oil, our results suggest that patchy fluid distribution should be considered in Gassmann theory when performing feasibility studies to predict changes in Vp as function of saturation changes. This study also highlights the importance of pore structure in controlling the changes of saturation and the corresponding changes in acoustic velocity. Larger changes and hysteresis in the measured velocity as function of saturation seem to correlate with lower pore stiffness (i.e., more micro cracks and less content of intraparticle rounded pores).