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Detailed geologic studies have become increasingly important especially for secondary- and tertiary-recovery projects. Success or failure of such projects is strongly dependent on a proper understanding of reservoir heterogeneity. In virtually every case, the reservoir has been found to be more complex than originally envisaged. In some reservoirs, complexity has cast serious doubts on viability of the project as initially formulated.
With development of modern computers capable of handling much larger numbers of grid blocks, different scales of heterogeneity can be simulated realistically. It is possible to simulate complex fluvial channel architecture over an interval of several hundred feet, as well as the effect on reservoir performance of crossbedding of meter scale or less.
Today's reservoir geologist is faced with the challenge of quantifying his conceptual sedimentologic models more precisely. At higher levels of reservoir heterogeneity, probabilistic techniques to predict reservoir configuration between available wells appear promising. Reservoir modeling of individual sandstone bodies is dependent on modern analogs or outcrop-derived prototypes. Textural criteria should be taken into account in the selection of prototypes, as grain size ultimately controls shale barrier distribution and sedimentary structures. Cementation is an important form of reservoir heterogeneity commonly neglected in reservoir modeling. In the case of small-scale reservoir heterogeneity as manifested in cores, carefully planned core analysis programs are required to measure th potential anisotropy caused by sedimentary structures and to determine accurately in-situ properties resulting from clay-mineral diagenesis or overburden stress.
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