Determination of the relation between elastic moduli and gas volume in the reservoir zone of a gas field in South of Iran
The purpose of this study is to calculate the elastic moduli and compare their changes due to the presence of gas in Kangan and Upper Dalan Formations in the zone of a gas field in the South of Iran. Compressional wave and shear wave velocities are directly related to the elastic moduli. The measurements of the compressional wave velocity, shear wave velocity, and density are necessary to determine the elastic moduliThe presence of gas in porous rocks significantly affects the acoustic wave velocities and Poisson's ratio. Shear wave slowness need to be determined for the mechanical properties of rock. The compressional wave velocity is sensitive to the fluid types. When water is replaced by gas in the porous media of a reservoir rock, the rock density decreases while there is not a significant change in shear modulus, since fluids do not support shear wave velocity. In this study, compressional and shear wave velocities were calculated using compressional and shear wave slowness derived from the Dipole Shear Sonic Imager (DSI). Also, density values were determined using the density log. Elastic moduli (Bulk modulus, Shear modulus, Young's modulus and Lame parameter), Poisson's ratio and the ratio of K/μ were calculated using the relationship between the acoustic wave velocities and the density. In this study, the presence of the gas volume was confirmed using the ratio of Vp/Vsand corresponding changes in the reservoir zone. Through comparing the gas volumes, the changes in Vp/Vs were investigated. The result of this study indicates that when the volume of gas is increased, the ratio of Vp/Vsis decreased. The ratio of Vp/Vs can be considered as an indicator for the detection of hydrocarbons. In carbonate rocks, the range of Vp/Vsbetween 1. 6 and 2 can show very high gas saturation. The Vp/Vs ratio is more sensitive to the fluids than the shear wave velocity and compressional wave velocity. The changes in elastic moduli were studied by investigating the gas volume changes, and a decrease in the elastic moduli was observed when the volume of gas was increased. In carbonate reservoir rocks, the values of the shear modulus range between 13. 5 GPa and 22 GPa and the values of bulk modulus range between 21 GPa and 45 GPa which indicates a high gas saturation. Also, in gas zones, the reduction of the bulk modulus is much more than the shear modulus. In these reservoirs, the values of λparameter range between 10 GPa and 32 GPa and the values of Young modulus range between 17. 8 GPa and 30. 6 GPa, which shows the high level of the gas volume. In this study, one can observe that when the values of the Lame parameters (μ and λ) are close to each other, it can indicate the presence of gas. Poisson’s ratio and K/μ ratio were compared with the gas volume in the log format. As a result of this comparison, one can indicate that the Poisson’s ratio and the K/μ are reduced by increasing the gas volume. In carbonate rocks, the low ranges of Poisson’s ratio (0. 19 to 0. 3) and the K/μ (1. 27 to 2. 33) are dependent on the gas volume.
Iranian Journal of Geophysics, Volume:9 Issue:4, 2016
34 - 44  
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