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Time-Lapse Simultaneous AVO Inversion of the Widuri Field, Offshore Southeast Sumatra
Reservoir characterization of the Widuri field, offshore SE Sumatra, is complicated by complex lithology and difficulties in predicting changes in fluid distribution during production. In 2000 a monitor 3D seismic data set of the Widuri field was acquired and processed to take advantage of seismic AVO effects. The base line 1991 seismic data set was reprocessed in parallel. The primary reason for the monitor data set was to observe the reservoir pressure depletion state around a water-flooding project.Furthermore there was a need to improve the characterization of the reservoir sands and to develop a better understanding of the fluid movement over time. To support AVO based reservoir characterization the base line and monitor seismic data were processed to 3 partial angle stacks. These were subsequently inverted to acoustic and shear impedance using a global, Simultaneous AVO Inversion algorithm. Resultant multi-parameter impedance based reservoir characterization from well log data and the impedance inversion results show that sands and coals in the reservoir interval can now be discriminated. Previously, reliable interpretation of these lithologies away from well control from poststack seismic data was infeasible as the acoustic impedance of these lithologies overlap. To investigate and characterize fluid movement, a novel workflow involving joint Simultaneous AVO Inversion of baseline and monitor seismic data was applied. The results from the time-lapse AVO inversion confirm the hypothesis based on production history analysis that aquifer support comes from the SouthEast. The same technique is successfully used to map more specific features related to depletion. As a result of production, increased water cut was observed at most of the wells. In addition to this, pressure dropped below bubble point with gas coming out of solution, which accumulated at structural highs. Both factors influence production and are potential drilling risks for newly planned wells.
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