Pressure Depletion Observations from Time-Lapse Seismic Data at the Troll Field

摘要

Seismic lines have been repeatedly acquired over the Troll East gas reservoir in 1997 and 2002. Pressure observations from wells together with the field-wide simulation model give an average pressure drop of 15 bar over this five-year period. High seismic repeatability enables time-lapse observations caused by this pressure drop, both along the top and beneath the thick gas reservoir. Detailed studies of the top reservoir reflection show a 2.0% average decrease of amplitudes between 1997 and 2002. Careful noise analyses, including characterization of its spatial correlation, reveal the statistical significance of this number. Pull-up is observed beneath the reservoir, most where the gas column is highest. When cross plotting gas column height and timeshift beneath the gas accumulation, a well-defined relation is found. This is explained by a velocity increase caused by pore pressure drop. The pressure-velocity sensitivity observed in the time-lapse seismic data can then be quantified. 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.