Time-lapse seismic monitoring of reservoirs is based on changes in fluid saturation and pressure due to production causing observable changes in seismic response. The Gassmann equation can be used to estimate changes in the bulk modulus of the reservoir for given changes in bulk moduli of dry rock and fluids. Gas, oil and water bulk moduli are approximated using published results and commonly available petroleum reservoir data. The lower and upper limits of the fluid mixture bulk modulus are calculated from the saturation-weighted harmonic and arithmetic averages, respectively. The change in the dry bulk and shear moduli due to changes in effective pressure is approximated for sandstones using published data. Estimated changes in the bulk modulus, shear modulus and fluid-saturated density are used to calculate new compressional and shear wave velocities. Percentage changes in velocities and acoustic impedance can be inspected for significance. Finally, synthetic shot gathers are generated which can be compared for changes in seismic attributes such as reflection coefficient, AVO effects or frequency changes.
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