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Exxon Production Research Company, PO Box 2189, Houston TX 77001, USA
An increasing percentage of remaining reserves in mature Brent Group producing fields is found in the Rannoch Formation, a lower shoreface succession characterized by low permeabilities, abundant mica and stratabound carbonate cements. The high contrast in permeability between the Rannoch and overlying Etive formations has resulted in severe water overrunning of oil-bearing Rannoch sandstones in a number of fields. Remaining Rannoch reserves will be difficult to recover and will require sophisticated reservoir management techniques. Differences in log correlation style which in the past may have been dismissed as inconsequential may now be important to consider in field depletion planning.
The objective of this study was to assess whether, in a high net-to-gross shoreface section such as the lower Brent Group at Cormorant Field, a sequence-based correlation would result in a significant difference in reservoir model behaviour, particularly in oil displacement patterns in lower shoreface sandstones of the Rannoch Formation. The modelled interval includes the prograding shoreline succession of the Rannoch (lower shoreface), Etive (upper shoreface-strandplain) and Ness (coastal plain) formations. There is a significant amount of facies interfingering between the Etive and Rannoch within the modelled area. Permeability models were constructed from both sequence-based and lithostratigraphic well-log correlations and used as input for 2-D reservoir simulations.
The results show a divergence in oil displacement behaviour between the two models, depending upon the degree of permeability anisotropy assumed. The principal reason for the difference lies in how the Rannoch layers are correlated. In the lithostratigraphic model, the Rannoch is treated as a tabular unit with layer boundaries parallel to the formation boundaries. Water injection into the upper Rannoch sweeps the upper Rannoch across the length of the model. In contrast, the sequence-stratigraphic approach treats the Rannoch-Etive as a series of clinoforms with layers offlapping toward the north in the direction of progradation. In this scenario, water injected into the upper Rannoch in the downdip area migrates parallel to layering into the lower Etive in the updip direction. The oil-bearing upper Rannoch in the updip areas correlates downdip to low-permeability facies which do not take water on injection; thus, the upper Rannoch remains unswept.
Predicted recovery efficiencies from the Rannoch lower shoreface sandstones are consistently lower in the sequence stratigraphic model, and more closely match observed reservoir behaviour at Cormorant Field than predictions based on lithostratigraphic correlation. This study suggests that, even in sand-rich systems such as the Rannoch-Etive, a sequence-based approach to reservoir zonation can improve predictions of reservoir performance.
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