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Time-Lapse seismic analysis has been applied to two 3-D seismic surveys acquired over the Central North Sea hulmar Held -- a pre-production survey shot in 1977, reprocessed in 1987, and a 1992 survey. The Upper Jurassic reservoirs in the field have been under production since 1982. Water is the main drive mechanism, supported by flank injection. Although the field is currently at over yi)7o water cut, there are inrm drilling opportunities. hetropnysicai analyses for huimar indicate that water replacing on win result in an increase in seismic impedance. In addition, a pressure decline of about 1000 psi during the time between the two seismic surveys will result in a further impedance increase. These impedance changes are observed between the two seismic surveys. In order to overcome inherent differences in the seismic data due to acquisition and processing differences, the data are equalized and then inverted to obtain impedance which is then averaged between the top of the reservoir and the position of the original oil-water contact. Differences in averaged impedance between the 1977 and 1992 surveys clearly show the effects of water influx and pressure decline. The changes observed in the seismic data are overall consistent with predictions obtained from a full-field, history-matched flow simulation as well as production data. Differences in details may suggest areas of bypassed oil. However, data quality is not sufficient to serve as the sole basis for drilling decisions.

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