Time-lapse timeshifts refer to the differences in two-way seismic travel times that are frequently observed in the analysis of time-lapse seismic surveys. One source of timeshifts originates inside the reservoir interval as a result of changes in the pore-fluid properties that alter the seismic velocity. Another is from changes in seismic velocity and layer thickness that occur both inside and outside of the reservoir as a result of reservoir compaction and stress and strain redistribution in the surrounding formations. Timeshifts induced by changes in fluid properties are always zero above the top reservoir reflection event and constant below the base of the reservoir. These fluidinduced timeshifts can be significant (for example, when gas is released as an oil passes through bubble point) and are routinely calculated using Gassmann or similar theories and are not the focus of this paper. The compaction-induced timeshifts have opposite gradients on the inside and outside of the reservoir. Within the reservoir, the reduction in layer thickness and the expected increase in seismic velocity will reduce the seismic travel time across these layers. Outside the reservoir, the decrease in reservoir thickness is exactly balanced by surface subsidence and rock expansion. The expanding overburden produces increased layer thickness and slower seismic velocities that increase the seismic travel times. Observations on real time-lapse seismic data over compacting reservoirs show that the positive timeshifts that accrue in the overburden are larger than the negative timeshifts that accrue inside the reservoir (the sign convention chosen is that positive timeshifts result when the seismic travel time increases). The amount of overburden elongation cannot exceed the amount of reservoir compaction. So if the change in velocity were simply proportional to the change in vertical strain, the reduction in travel time through the reservoir would exceed the increase in travel time though the overburden. The net effect would be a negative timeshift below the reservoir. Instead positive timeshifts are observed below compacting reservoir indicting velocity reduction per unit elongation strain significantly exceeds the velocity increase per unit contraction strain. Using simple models of the velocity-strain response it is shown that time-lapse timeshifts are proportional to the stretching of the overburden layers and that this is highly correlated with the reservoir compaction. The net result is that time-lapse timeshifts are a good measurement of the reservoir compaction.


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