Monitoring Pore Fluid Changes Using Time-lapse Attenuation

Attenuation is a desirable seismic attribute for monitoring hydrocarbon reservoirs as it is far more sensitive to changes in pore saturation and pressure than P- and S-wave velocities. Here, we demonstrate how a time-lapse estimate of attenuation may be less sensitive to coherent spectral contamination, outline two methods of estimating time-lapse attenuation, and demonstrate their use on a prestack time-migrated dataset. Prestack time migrated data from two vintages (2001 and 2007) situated adjacent to a gas/water injector are analysed. Prestack time-shifts from cross-correlation between vintages are calculated to account for changes in velocity and coefficients from the cross-correlation are used to wieght the data. A single value of 1/Q for each vintage is then calculated and differenced. A value of the change in attenuation is also calculated directly between the vintages of data. Results suggest an increase in attenuation and hence an increase in pressure or gas saturation near to the injection site.