In recent years, 4-D seismic has developed into a sophisticated technique of reservoir monitoring and management relying on the integration of geologic models, static and dynamic properties of reservoir rocks and fluids, and detailed production and pressure field data. Current geophysical and engineering studies tend to emphasize discrimination of clearly time-variant partial fluid saturations and pressures from 4-D data. However, common geological processes such as production induced compaction for the case of unconsolidated to poorly consolidated sediments, and steam-induced mineral precipitation, for the case of steam stimulation projects, cannot be assumed to be time-invariant. Both of these mechanisms may strongly influence porosity, density, and permeability variations during the life of a 4-D project.
Other geological characteristics and processes may not change at small time scales, but they are closely linked to reservoir porosity and permeability magnitudes and distributions, themselves key factors controlling fluid flow and pressures. Hence, successive 3-D seismic images of these geologic parameters will very likely change in a diagnostic fashion during production and depletion of a reservoir. Appropriate examples are the well-known heterogeneity of typical reservoir intervals, the geometry and sealing properties of faults, and the role of hydrocarbons as a control on cementation and porosity development above- and below oil-water contacts. Careful consideration of these factors will make key contributions to the success of 4-D seismic projects.
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