Variation of reservoir properties over production time frames is a complex superposition of pore pressure and saturation changes. For this reason, time-lapse (4D) seismic projects to monitor reservoir changes are always subject to the nonuniqueness of interpretation, even if the quality of the 4D signal is good and the magnitude of stress/saturation change is strong. While saturation variations can be adequately handled by Gassmann's theory, stress changes due to pore-pressure depletion within the reservoir are less well understood and multiple conceptual models exist to account for this effect on seismic rock properties.

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