Estimation of effective pressure and water saturation by viscoelastic inversion of synthetic time-lapse seismic data for a gas sandstone reservoir

Summary Inversion of synthetic seismic data for a viscoelastic reservoir model produces estimates of changes in effective pressure (Pe) and water saturation (Sw). Application is to analysis of time-lapse seismic data for a gas sandstone reservoir. To generate synthetic time-lapse viscoelastic seismic data, reservoir properties [porosity (�-), clay content, permeability (k), Pe and Sw] are used to define the corresponding viscoelastic seismic properties [compressional and shear velocities (Vp and Vs), density (�! ) and compressional and shear quality factors (Qp and Qs)] using petro-seismic modeling. It is assumed that porosity, clay content and permeability do not change during production and are known. Full wavefield inversion is performed for different production scenarios, on synthetic viscoelastic time-lapse seismic data, using a grid search refined by a conjugate gradient solution. Estimation of unique values of Pe and Sw from two independent data (such as Vp/Vs and Qs/Qp) is feasible, if the other reservoir properties are known. Noisy data produces a range of solution values. The composite effect of ®Pe and ®Sw is approximately equal to the sum of the two separate effects. Seismic time-lapse changes are less sensitive to the pressure change than to changes in saturation. For the reservoir properties (�- , clay content, k), and the dominant seismic frequency used in the example, �! and Qs/Qp are most sensitive to Sw, Vp is most sensitive to Pe, and Vs, Qp and Qs and Vp/Vs are sensitive to both Sw and Pe.