Sensitivity of AVA reflectivity to fluid hydrocarbon properties in porous media
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The sensitivity of transmission and reflection coefficients to varying physical parameters of a hydrocarbon fluid is examined. In particular, earlier work on generalized fluid properties is extended to consider realistic hydrocarbon properties at in-situ conditions. I also expand previous studies of P-P reflectivity, including Biot theory fluid effects, to include P-Sv mode-converted reflections. The goal is to understand how the reflection coefficients change as individual fluid parameters, such as density or viscosity, are varied, which fluid parameters have the greatest affect on the transmission and reflection coefficients, and what actual hydrocarbon and reservoir properties are involved. There is no sensitivity to a fluid or hydrocarbon parameter if the transmission and reflection coefficients do not change as that parameter is varied. The sensitivity analysis is further refined by calculating partial derivates of the reflection coefficients with respect to fluid viscosity. Special attention is given to viscosity because viscosity may act as a proxy for permeability as well as partial gas-saturation in common reservoir sands. To this end I am able to quantitatively describe the degree of gas-saturation and its influence on reflection coefficients. This might provide a technique to lessen the uncertainty encountered in the fizz-gas risk phenomenon. In the gas-oil interfaces, I find that the changes in the P-P reflectivity to be as much as 1.3% for a decrease in oil viscosity corresponding to a change from 1 to 99% gassaturation. In the oil-brine interfaces, the changes in the P-P reflectivity are estimated to be a maximum of 2.6% for a decrease in oil viscosity corresponding to a change from 1 to 99% gas-saturation. Changes in P-SV reflectivity are negligible (<1%) for both models at typical seismic frequencies (~100 Hz).