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Realistic well-productivity calculations based on geologic models are an important aid in predicting field performance. For the Leman field in the North Sea, such models have been used to predict production potentials of untested wells and to judge the danger of water coning.
The Leman field reservoir rock in the Permian Rotliegendes Sandstone is 180 to 270 m thick. The major producing unit is composed of giant eolian cross-bed sets with an average thickness of 4.5 m. The orientation of the foreset-lamina dip is remarkably uniform. The variation of the bottomset zones underlying the spoon-shaped cross-bed sets caused a very heterogeneous permeability distribution. The heterogeneity is enhanced by variations in clay content and diagenesis.
No data were available on the length/width/thickness ratio of giant eolian cross-bed sets formed by transverse dunes. Outcrop studies in the Canyon de Chelly (Arizona) have been carried out to gather information on the geometry of this type of cross-bed sets. The large horizontal extent of the cross-bed sets (length approximately 200 times the thickness), combined with the low permeability of the associated bottomsets, indicates that water coning will be minimal. Initial well behavior is probably controlled by the properties of the thickest, more permeable cross-bed sets.
Furthermore, some pairs of wells may be interconnected via continuous, fairly permeable beds because the average well spacing is less than the average cross-bed set length (900 m). Log correlations tend to confirm this conclusion.
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