Validation of Gassmann’s Equation for Static and Dynamic Dry Moduli

In practice, general assumptions of Gassmann’s theory can be violated due to many factors. The complex pore system in carbonates is one of the main factors contributing to the ambiguity of Gassmann’s application for fluid replacement purposes in carbonate rocks. Therefore, it is important to understand the conditions under which Gassmann’s equation is working properly. This paper investigates the effects of using dry static and dry dynamic bulk moduli in Gassmann’s equation to model saturated bulk modulus. Dry static modulus was measured from triaxial compressive test, and dry dynamic modulus was calculated based on ultrasonic tests. Furthermore, the modelled saturated bulk modulus from both dry static and dry dynamic bulk moduli were compared against saturated bulk moduli measured from laboratory tests. Our findings confirmed that Gassmann’s theory gives better results in static condition rather than dynamic condition. Poor prediction of dynamic condition could be attributed to the different pore stiffness in these samples which are magnified due to the fluid-solid related phenomena going toward ultrasonic frequencies.