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Abstract

Realistic well productivity calculations based on geologic models are an important aid in predicting field performance. For the Leman field, such models have been used to predict production potentials of untested wells and to judge the danger of water coning at an early stage (1973) of the field production history.

The Leman field is situated in the southern North Sea 48 km offshore from the English coast. The original reserves were about 10.5 Tcf when production started in 1968. The western half of the field is operated by Shell/Esso and the eastern unit by Amoco. At the time of the study, there were 10 platforms with 130 producing wells.

The Leman field reservoir rock is the Permian Rotliegendes Sandstone, which is 180-270 m thick and lies at a depth of about 2,000 m. The major and most productive part of the reservoir is composed of giant aeolian crossbed sets with an average thickness of 4.5 m. The orientation of the forset laminae is remarkably uniform. It is inferred that the laminae represent the lee slope of transverse dunes. The variation in permeability of the foreset laminae and the generally low permeability of the bottomset zones underlying these spoon-shaped crossbed sets cause a very heterogeneous permeability distribution. The heterogeneity is enhanced by variations in grain size and associated authigenic clay content and diagenesis.

No literature data were available on the length/width/thickness ratio of giant aeolian crossbed sets formed by transverse dunes. Outcrop studies in the De Chelly Canyon (Arizona) were thus carried out to gather information on the geometry of this type of crossbed set. The large horizontal extent of the crossbed sets (length approximately 200 x thickness), combined with the low permeability of the associated bottomsets, indicates that water coning will be minimal, an interpretation that has been corroborated by 10 years of production data. Initial well behavior is probably controlled by the properties of the thickest more permeable crossbed sets. Furthermore, some pairs of wells may be interconnected via continuous, fairly permeable beds, because the average well spacing is smaller than the average crossbed set length (900 m). Log correlations tend to confirm this conclusion.

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