Internally Consistent Correlations For Predicting Phase Compositions For Use In Reservoir Composition Simulators
Authors: Rowe Allen McGhee 
Published: December 2007

INTERNALLY CONSISTENT CORRELATIONS FOR PREDICTING PHASE COMPOSITIONS FOR USE PREDICTING PHASE COMPOSITIONS FOR USE IN RESERVOIR COMPOSITION SIMULATORS

Abstract
Simple, internally consistent correlations are presented to predict the phase compositions of presented to predict the phase compositions of volatile oils and gas condensates. They are based on K-factors correlated as a function of temperature, pressure, and convergence pressure. The correlations pressure, and convergence pressure. The correlations and methods include:a simple K-factor algorithm,a method for calculating critical temperatures,a critical pressure correlation,a vector solution technique to the convergence pressure calculation that assures internal consistency with the K-factor algorithm, anda method that establishes the convergence pressure as a function of the methane content.
This method simplifies the convergence pressure calculation in numeric compositional simulators.
A total of 74 critical states of complex hydrocarbon mixtures were used in developing the critical pressure correlation. The K-factor algorithm was developed using 14 representative mixtures. The combined K-factor, convergence pressure calculation was then used to predict the pressure calculation was then used to predict the saturation pressures of 10 reservoir fluids, with an average absolute error of 193 psi.
Three constant pressure, constant temperature phase envelopes were predicted and compared phase envelopes were predicted and compared favorably with envelopes published in the literature. These predictions demonstrate the internal consistency between the convergence pressure calculation and K-factor algorithm in the vicinity of the critical state. Thus overcomes a major limitation in current convergence pressure procedures.

Introduction
The prediction of recovery by nonequilibrium fluid displacement from reservoirs requires correlations that compensate for the composition effect on fluid properties. These correlations must be well behaved functions that satisfy the following five criteria.The correlations must be internally consistent.The correlations cannot involve discontinuities.The correlations must be reliable, reasonably accurate, and well behaved.The correlations must not require excessive computer time.They must satisfy the following boundary conditions at the critical state.
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(2)
(3)

INCONSISTENCIES IN THE CONVERGENCE PRESSURE, K-FACTOR CALCULATION PRESSURE, K-FACTOR CALCULATION
In using K-factors correlated as a function of temperature, pressure, and convergence pressure, trial-and-error calculations are required. First, a convergence pressure is assumed. The K-factors are selected from an appropriate correlation, and the flash calculation performed. The results are used to calculate the convergence pressure. This procedure is repeated until the assumed and calculated convergence pressure agree to within a specified tolerance.
One major problem exists with this approach the convergence pressure calculation involves correlations independent of the K-factor correlation.