Pressure Depletion Observations fromTime-Lapse Seismic Data at the Troll Field

摘要

Seismic lines have been repeatedly acquired over thernTroll East gas reservoir in 1997 and 2002. Pressurernobservations from wells together with the field-widernsimulation model give an average pressure drop ofrn15 bar over this five-year period. High seismicrnrepeatability enables time-lapse observations causedrnby this pressure drop, both along the top and beneathrnthe thick gas reservoir. Detailed studies of the toprnreservoir reflection show a 2.0% average decrease ofrnamplitudes between 1997 and 2002. Careful noisernanalyses, including characterization of its spatialrncorrelation, reveal the statistical significance of thisrnnumber. Pull-up is observed beneath the reservoir,rnmost where the gas column is highest. When crossrnplotting gas column height and timeshift beneath therngas accumulation, a well-defined relation is found.rnThis is explained by a velocity increase caused byrnpore pressure drop. The pressure-velocity sensitivityrnobserved in the time-lapse seismic data can then bernquantified. Amplitude dimming along the gas-liquid-contact reflection may be explained by reduced seismic velocities beneath the Troll East gas reservoir, possibly due to formation of free gas in the underlying aquifer and relict oil zone.