Carbonate reservoirs have become popular targets for rock property research escpecially rock physics in recent years, mostly at laboratory studies. Gassmann’s fluid substitution in clastic reservoirs have been widely used, but the applicability of Gassmann’s equation in carbonates is still debatable. There are many research in this subject using laboratory experiments that concluded Gassmann’s theory is not applicable in carbonates due to shear modulus variability. In addition, there is a previous work which summarized that Gassmann’s relationship is applicable to carbonate reservoirs. We present an experiment to evaluate the Gassmann’s applicability for carbonate reservoirs using well log data. Well log is processed by the petrophysical analysis and the shear wave log is predicted using Xu-Payne model of clay-carbonate mixtures. Our results show the acoustic velocities and density logs are able to detect the fluid changes in carbonates, especially if the porosity is relatively high. We also observed that the shear modulus is constant with fluid changes. The AVO response for different fluid scenarios are investigated quantitatively to understand the amplitude changes with different fluid cases.
Author : Y. F. Yushendri, A. S. Praptono