Abstract
Sonic travel time of compressional wave is generally used as porosity tool for given lithology. Introducing shear wave travel time is very helpful in determining mechanical rock properties. It is found that compressional wave is sensitive to the saturating fluid type. The use of the ratio of compressional wave velocity to shear wave velocity, Vp/Vs, is a good tool in identifying fluid type. The fact that compressional wave velocity decreases and shear wave velocity increases with the increase of light hydrocarbon saturation, makes the ratio of Vp/Vs more sensitive to change of fluid type than the use of Vp or Vs separately. Field examples are given to identify fluids type (water, oil and gas) using the Vp/Vs ratio. Field examples have shown that shear travel time decreases while compressional travel time increases when the water saturated points become gas or light oil saturated points in the studied sections. The decrease of shear travel time (increase of shear wave velocity) is due to the decrease of density and the absorption of deformation by free gas in pores. The increase of compressional travel time (decrease of compressional wave velocity) is due to the decrease of bulk modulus of reservoir rocks which compensates the decrease of rock density.
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