ABSTRACT
A method of predicting fluid compositional grading with depth is presented. It was designed to enable numerical simulations of this phenomena, now required in compositional model studies: the nature of petroleum fluids today (condensate gases, volatile oils, "critical" fluids) increases this need.
Based on ONSAGER's theory, general relations between compositional grading of a fluid with depth make predictive calculations of this evolution possible.
Our method only takes into account the gravitational effect; it enables to calculate the phase behavior of a fluid everywhere in a column, assuming the stationary state has been reached. The proposed algorithm is an iterative calculation based on the fugacities of each component at different depths; knowing pressure, temperature and overall composition at a reference depth we can predict the compositional grading, gas-oil contact location and even physical state change with depth.
The required conditions to realize such a prediction are discussed.
In addition, a "new" thermodynamic fitting method is suggested.
An example is given for a North Sea real "critical" fluid; it clearly shows that the gravitational segregation alone can largely explain the compositional grading observed in the reservoir.
PVT and compositional simulations of the fluid are based on the PENG-ROBINSON's equation of state1 but this method can be developed with any other equation.
