Analysis and Modeling of an Extensive Data Material for the Compositional Variation with Depth in a North Sea Reservoir

Abstract Detailed compositional data is presented for 43 fluid samples taken at different depths in a 359 m North Sea reservoir fluid column with a gas cap on top and an underlying oil zone. The data enables the depth of the gas-oil contact to be determined quite accurately. A common equation of state model has been developed for the fluids and depth gradient simulations carried out. In the gas zone, gravitational forces can explain most of the observed compositional variation. In the oil zone, the compositional variation is larger than can be explained by the gravitational effect. There is a need to also account for the heavy hydrocarbons orienting themselves towards the warmer bottom zone and lighter hydrocarbons towards the colder top zone. The Haase model, which is based on the theory of irreversible thermodynamics, is used to describe how the vertical temperature gradient affects the compositional gradient. It requires knowledge of the absolute partial molar enthalpies of the individual components, of which it thanks to the extensive data material has been possible to give a good estimate. With the presented model concept, it is possible to represent the compositional gradient over the entire fluid column including the depth of the gas-oil-contact and the variations in gas/oil ratio and saturation point with depth. The determined model parameters are also expected to be applicable to other petroleum reservoirs.