In this study, we find that variation of Hashin-Shtrikman bounds caused by pore fluid change is governed by the Biot-Gassmann theory. Also, we have shown that it is generally valid to assume that the relative position of the effective moduli of a rock within the Hashin-Shtrikman bounds is not affected by pore fluid change. Thus Hashin-Shtrikman bounds can be used as a valid fluid substitution tool. Comparing with Gassmann fluid substitution, the velocity predicted by Hashin-Shtrikman bounds generally fits better with the measured data since part of the dispersion is included, and it can be used as a more general pore infill substitution tool.

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