Estimating Production Induced Stress Changes from 4D Finite Offset Timeshifts

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Significant stress changes are generated when producing reservoirs compact due to large reductions in the reservoir pore-pressure. These stress changes are not confined to the reservoir. They are redistributed to surrounding formations, modifying the formation velocity and thicknesses. These changes manifest themselves as timelapse time differences on 4D stacks. Various authors (Hatchell et al, 2003; Barkved et al, 2005) have used geomechanical modelling to explain these 4D timeshifts, thereby gaining valuable insight into the behaviour of the subsurface around compacting reservoirs. This has been accomplished by assuming a simple relationship between thickness and velocity changes. The model is reiterated using updated relationships until a match is obtained with the observed 4D timeshifts. We present an approach in which the 4D time differences are measured on prestack data. Without relying on assumed relationships between velocity and thickness changes, we use the additional finite offset information combined with raytracing and linear least squares techniques to derive the thickness and velocity changes. These resulting velocity and thickness changes combined with density and pore-pressure well data are then converted to stress and strain changes. Hence, the technique should close the loop between seismic 4D time differences and geomechanical stress and strain changes.