Fluid effect on elastic properties of carbonate and implication for CO2 geologic sequestration monitoring

Laboratory measurement of the elastic properties of carbonates saturated with different fluids is crucial for characterizing reservoirs and monitoring CO2 sequestration. However, it is challenging to distinguish the effects of different fluids. We measure eight carbonate samples under varying pressure and fluid saturation conditions and conduct quantitative analysis. The measurements indicate that water saturation can increase P-wave velocity (VP), whereas oil and CO2 saturation may lead to an increase, decrease, or no change in VP. Meanwhile, all fluid saturation reduces the S-wave velocity (VS). The velocities of the dolomite samples are higher and less pressure sensitive than that of limestone samples. Four attributes are adopted to distinguish the fluid effect, including elastic moduli, impedance, VP/VS ratio, and a newly designed fluid discrimination factor F, which is obtained by multiplying Pwave modulus and square value of density change. Among the attributes, the F factor performs the best as it achieved evident and consistent results for all the samples. Rock-physics models are used to simulate elastic behaviors with the differential effective medium model predicting VS the best. The Gassmann's equation correctly indicates positive or negative changes in rock velocity after fluid saturation even though its prediction does not exactly match the data. This study highlights the impact of various pore fluids on the carbonate elastic properties through laboratory measurement and theoretical modeling and introduces a novel fluid identification factor, providing insights for reservoir seismic characterization and CO2 sequestration monitoring.