Three-Dimensional Computer Modeling for Reserve Estimation of Norway's Brage Field

Carlton R. Johnson, Scott W. Hopke, Hege Bolas, Knut Furuheim

Three-dimensional computer models of two distinctively different clastic reservoirs in Norway's Brage field were used to appraise several possible field development plans.

The Stratfjord reservoir of Early Jurassic age is a low-sinuosity braided-stream complex characterized by impermeable but discontinuous clay channel fill representing less than 30% of the formation. The Middle Jurassic Fensfjord reservoir is a marine sand with highly continuous calcite-cemented tight sand and siltstone layers, and scattered concretionary lenses only a meter or two thick.

The computer models consist of 912,978 and 1,663,578 uniform tabular cells for the Statfjord and Fensfjord reservoirs, respectively. Each cell contains a value for porosity interpolated from well data. The interpolation is controlled by a stratigraphic framework of two-dimensional grids containing the folded and faulted structure and any unconformities within the reservoir. Using estimated sizes for the channel fill and concretionary lenses and frequency distributions based on a few available wells, these heterogeneities were introduced stochastically as a series of randomly located "wells."

The numerical arrays required by the reservoir simulator are generated directly from the geological models. In this way, the complex reservoir description provided by the geologist is utilized by the reservoir engineer in evaluating field development plans.

Both reservoirs contain undersaturated oil and will be waterflooded to maintain pressure. The discontinuous shaly layers in the otherwise high-quality Statfjord reservoir favorably affect oil recovery by reducing water coning. In the Fensfjord reservoir, predicted oil recovery was higher than we would normally expect from a reservoir of its relatively poor quality; simulation results indicate that most oil production will be from thick, continuous sands lower in the oil reservoir.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.