A compacting reservoir remains physically connected to the rock surrounding it. Therefore, as the reservoir deforms and compacts, stresses will change and reorient themselves in the reservoir as well as in the overburden, underburden and sideburden, and there will be deformation in these rocks too. It is a well known and extensively described physiological phenomenon that stresses and strains induce changes in seismic velocity. Thus, reservoir deformation induces changes in velocity affecting the seismic wave throughout the basin. For certain combinations of velocity change and overburden thickness, vertical integration of the local traveltime changes may lead to timeshifts of several milliseconds, which can be measured in well-repeated seismic surveys. Timeshift technology has an interesting application in reservoir monitoring: Maps of timeshifts could indicate the areal distribution of reservoir compaction and thus reveal the areal distribution of depletion. Timeshift analysis then has the potential to help determine compartmentalization, locate bypassed oil in undrained compartments, identify new drilling targets and sidetracks, avoid expensive infill wells, and eventually decide when to abandon a field. Promising patterns of timeshifts of up to 15 milliseconds have already been observed in several fields.
We simulated the geomechanical effects of the depletion of stacked oil-saturated reservoir sands. The calculated deformation and stress changes were used to calculate timeshifts as a function of production. Reasonable agreement was obtained between calculated and measured timeshifts. However, the fact that the measured timeshifts resulted from stress perturbation due to the combined effect of several stacked reservoirs complicates their interpretation: It requires a good (i.e. field-data-proven) static and dynamic model for all sands to correctly model their effect on the overall timeshift signal. Only then can the remaining difference between synthetic and field-observed timeshifts be used to monitor and interpret the production performance of a target "problem reservoir" within the stacked pay. Our models also suggest that stress changes in rocks overlying compacting reservoirs are complex and anisotropic, and dependent on proximity and structure of the relatively stiff formations. When these are present, the 3D-variation in stress state, and not the 1D-variation, should be used to compute and analyse timeshifts in 4D-seismic data.