What kinds of rock properties are undergoing changes during field production (related to 4D seismic)?
As the reservoir is produced, the fluid saturations change (water replaces oil, gas comes out of solution, etc). The fluid pressures also change. These processes are slightly different at locations across (through) the reservoir. The combination of porous rock plus pore fluids determines the acoustic velocities. Small changes in velocity are associated with saturation changes, and these have an impact on the seismic signal. There are also changes in acoutic velocities due to the pore pressure changes. This is related to changes in the effective stress (see Appendix). What we have come to realise is that the changes in fluid pressure change both the reservoir and the surrounding rocks, so there are effects that occur at long distance. Resolving the causes of the observed seismic changes is not a simple task!
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