The Attaka field, located northeast of the Mahakam delta, offshore East Kalimantan, Indonesia, consists of complex arrangements of distributary/tidal channels and tidal/barrier bars. Over the last 18 years, the field has produced about 460 million STB of oil and 700 billion SCF of gas. Despite the mature status of the field, lateral correlation of the various sand bodies is uncertain. This paper describes how reservoir simulation is helping to characterize the field architecture. Matching the production history with simple reservoir models is providing a means for combining geological and engineering data and is serving as a catalyst for synergism.

A simple approach is described for constructing a large multilayered model in a complex geological setting. Models of single reservoirs are developed and a preliminary history match is obtained by use of pseudo functions. Because most completions involve commingled production from two or more reservoirs, several reservoir models are combined to refine the history match. Finally, all reservoir models are stacked to form a large-scale field model.

The approach may be classified as adaptive deterministic. As more history matching studies are completed, a more unified set of generic pseudo functions which describe the various reservoir facies is obtained. Classification of rock types and facies is based on gamma ray logs. History matching is guided by descriptive use of gamma ray log fence diagrams and by analogy to the modern Mahakam delta. The history matching process is directed to define facies distributions and reservoir connectivity.

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