ABSTRACT:

The phenomenon of variation of fluid properties with depth was investigated in many of the Gulf of Suez and Western Desert fields of Egypt. This phenomenon primarily exists in reservoirs with large closures (large thickness or high dip angle, even for thin formations) where the gravitational forces have a significant effect on the overall composition of the system over the geological time.1,2,3 The phenomenon often exists also in high shrinkage oils, volatile oils4 and rich gas condensates as well as in black oils. Such compositional grading can have significant influences on various aspects of reservoir development. This must be taken into account during estimation of the stock-tank oil initially in place of such fields and well/reservoir behavior evaluation. This paper presents an engineering evaluation for validating PVT lab analyses of many fluid samples collected at different depths from several reservoirs of different fields. As a result of this evaluation, the valid samples were supporting fluid property variation with depth where its properties can be correlated as a linear function of depth.Two methods have been used for correlating the fluid properties with depth. The first method relies only on the experimental PVT data provided by the laboratories while the second method uses an equation of state to predict the fluid properties from the experimentally determined compositional analyses. The second method make use from the advantage that in this particular case the equation of state parameters assigned to each hydrocarbon and pseudo-hydrocarbon component within the oil column should be constant for all valid samples. In this method, the most accurate reservoir fluid composition for one of the valid samples was used to predict the reservoir fluid composition and its corresponding PVT properties at different bottomhole locations. The prediction procedure is a combination of stream blending and flash liberation processes to estimate the reservoir fluid properties at shallower and deeper depths from the depth of the selected sample

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