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Saga Petroleum, PO Box 490, N-1301 Sandrika, Norway
Three heterogeneous models of a channel sandbody of braided stream origin were constructed by combining reservoir data from the middle reservoir unit of the Lower Jurassic Statfjord Formation in the Snorre Field in the Norwegian Sector of the North Sea, with facies and geometry data from an exposed analogue channel sandbody in the Pennsylvanian-Permian Maroon Formation, Colorado. Permeability classes corresponding to those in the Statfjord Formation were distributed according to facies variation in the analogue sandbody and to the recorded vertical distribution in the Middle Statfjord sandbodies. Relative permeability and capillary pressure curves were made for individual permeability classes. Two-dimensional (yz-plane) reservoir simulation of waterflooding was carried out with injection at the down-dip end and production at the up-dip end of the models, which measured 104 m x 8.75 m and had a structural dip of 8°.
The results were compared with simulations on two reference models, one of a homogeneous sandbody and one with the permeability zonation organized in parallel layers with decreasing permeability from base to top. All models had the same volume-weighted arithmetic mean permeability. The 104 m long basic models were also compared with scaled-up versions measuring 312 m and 936 m, respectively.
The simulations show that variation in internal heterogeneity has considerable impact on recovery efficiency. Compared with the homogeneous reference model, the heterogeneous models had reduced recovery efficiencies at water breakthrough of 11% to 33%, depending on the type and degree of heterogeneity, and a corresponding reduction in mean effective permeability of 9% to 42%. This study also demonstrated the importance of using capillary pressure data in simulations of this type.
Capillary pressure allows imbibition into the low permeability rocks during waterflooding and explains the higher recovery efficiency on simulation runs on the scaled-up models compared to the short basic models. The effect of capillary pressure also results in higher recovery efficiency in runs with low flow rate than in runs with high rate. Similar simulation studies, also in 3-D, should be performed on various types of internal heterogeneities in complex fluvial reservoirs.
