This paper describes the results of an integrated study to assess redevelopment options in a mature field with over 40 years of production. The field has been produced under gas cap drive and partly under an internal gas injection scheme to increase oil recoveries. The objective of the study was to confirm the effectiveness and possible expansion of the gas injection scheme and to assess merits of waterflooding.

A two phased modeling approach was adopted and a coarse Phase 1 model for the entire reservoir stack was built using existing interpretations and data available from previous studies while seismic reinterpretation and petrophysical reevaluation to feed the detailed Phase 2 models was ongoing. The Phase 1 model helped to understand the plumbing between the sands and fault blocks and the key parameters impacting field level pressure and cumulative fluid history matches. In Phase 2, two separate models were built for groups of reservoirs at a much finer vertical scale to address sand specific saturation changes and individual well performance.

A structured experimental design (ED) methodology using SUM (Shell proprietary software) was used to explore the full uncertainty space for potential history matches and use calibrated models with the remaining uncertainties to generate a range of forecasts. With the large number of reservoirs involved even a modest number of uncertainties resulted in a large number of ED parameters. The number of parameters and their associated uncertainties was partly narrowed down and constrained in Phase 1 allowing focusing on detailed reservoir specific parameters in Phase 2. In parallel with and complementing the simulation work, conventional contact mapping integrating the historical pressure, water cut and GOR data was carried out to locate potential areas with remaining oil.

This paper will highlight how the phased modeling approach facilitated early integration of reservoir and well performance data for detailed static model construction, ways of managing uncertainty in a brown field with multiple reservoirs, large number of wells and more than 40 years of production history, and finally how traditional reservoir assessment techniques can complement reservoir modeling work.

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