An effective reservoir management program is designed to optimize reservoir performance by assuring maximum economic return and recovery over the life of the reservoir. Such a program requires a continuing process of engineering and geologic study. Numerical reservoir simulation models provide a powerful tool for analyzing the production history of a reservoir and for predicting future performance under a variety of possible operating methods. Experience has shown that an accurate reservoir description is essential to the success of any numerical simulation study and to the development of an effective reservoir management plan1,2 .

The mechanics of conducting a numerical simulation of reservoir performance are essentially the same for either a carbonate or sandstone reservoir. The most important difference in modeling these two general reservoir types involves the conceptual approach required to describe the lateral and vertical continuity of flow units within the reservoir. The continuity and internal characteristics of sandstones are controlled primarily by the original depositional system and environment, with usually only minor post-depositional changes. In carbonates, the distribution of porosity and permeability and the continuity of reservoir-quality units can be determined either by the original depositional environment, by post-depositional diagenetic changes, or, most commonly, by a combination of these factors.

Carbonate reservoir rock types are formed in a variety of depositional settings and show great diversity in size and form, ranging from reefs covering one or two square kilometers to extensive carbonate banks covering thousands of square kilometers3 . The common characteristic of carbonate reservoirs is the extreme heterogeneity of porosity types and permeability distribution which results from the complex interaction of the physical, biological, and chemical processes that form these rocks. In addition, carbonate rocks are particularly susceptible to post-depositional diagenetic changes. Some of the more important diagenetic processes which act to alter the original rock texture in carbonates are dolomitization, recrystallization, cementation, and leaching or solution.

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