In the Gassmann assumptions it has been stated that pore fluid will not modify the elastic properties of the rocks. Thus, this theory predicts that shear modulus is not affected by the fluid saturation and will remain constant. In this study, the shear modulus didn’t remain constant during fluid saturation and during the transformation from gas-water saturation state to water saturation state; changes were detected to be around 2 percent. Because of this change in the shear modulus, it makes one hesitate to utilize the Gassmann theory to calculate the velocity. With the calculated and the experimental values of the shear modulus being variable, it can be concluded that Gassmann theory is not accurate for estimating the compressional wave velocity in a saturated state.
The research found that the bulk modulus values obtained from the laboratory experiments show a very weak compatibility with the values calculated using Gassmann formulas. One of the probable reasons for this could be an influence of the transformation of the rock skeleton by brine (salt water). The overall conclusion from these studies is that, for samples saturated in brine at low pressures, good agreement exists between bulk modulus values calculated from Gassmann formula and experimental values obtained in the laboratory. While at high pressures when the rock is very strongly influenced by pressure, this agreement is nonexistent. This change was about 9% at low pressures and at high pressures, these differences reached up to 23%. Therefore, one should contemplate before utilizing Gassmann equation.
2.6 Acknowledgment
Authors extended their appreciation to Petroleum Engineering Department, Curtin University of Technology, Australia that provided the authors the opportunity to utilize the laboratory core flooding system. Also, the authors thank and appreciate very much Prof. Vamegh Rasouli, Dr. Amin Nabipour, Dr. Mohammad Sarmadi and Dr. Mohsen Ghasemi for their help in conducting these experiments. Finally, the authors are very grateful and extend their deepest appreciation to the respected faculty of Petroleum Engineering and Geophysics departments, Curtin University of Technology that cooperated in the design, manufacturing and installation of the transducers.
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