Deterioration of Performance of Mixing Rules in Phase Behavior Modeling of High-Density Reservoir Fluids

Abstract

The mixing rules are used in the cubic equations of state to determine the values of the attractive force parameter, a, and the repulsive force parameter, b, mixtures. The mixing rules are applied here to reservoir fluids. It was discovered that parameter a should not be treated as a constant since it varied significantly with pressure. It was therefore regressed by two straight lines, and the resulting equation of state gave a very good fit to PVT data of reservoir fluids.

Model Development of a Suitable Equation of State for Multicomponent Multiphase Systems: Application to Crude Oil Phase Stability Requirements

“…The authors believe that the error propagated by the assumption that parameter 'a' is a constant for a multi component mixture, is responsible for the deficiencies of equations of state such that tuning and sometime elaborate adjustments are then required. From previous works by Babalola and Susu [18], plots for the three wells have been shown to exhibit significant variations of parameter 'a' with pressure. Each of these plots was regressed to obtain a best fit.…”

“…The work of Babalola and Susu [18], in addressing this issue, showed mathematically that when the variable parameter 'a' curve is regressed by using two straight lines for a reservoir fluid sample from an oil well-one straight line for pressures below bubble point pressure and another for pressures above it, the need for tuning would be automatically eliminated. They inferred that the regression by the straight lines intrinsically 'tunes' the EOS model for more accurate performance.…”

“…This was found appropriate for representative paraffinic, naphthenic and aromatic compounds. The EOS having been developed for a pure substance has been modified to improve its generality and hence can be applied to binary, ternary, and even multi component mixtures [12], [13], [18]. For an EOS to be applied to a mixture, the parameters 'a' and 'b' (which are characteristic of pure substances) are calculated for mixtures using mixing rules.…”

“…Perdersen et al [22] discussed some methods of tuning equations of state each of which requires a specific set of large amounts of data and could be quite involving and expensive. A much simpler alternative to tuning termed the 'variable parameter a approach' (VPAA) has been proposed by Babalola and Susu [18]; in this approach, the parameter 'a' of the Peng-Robinson Equation of State (PR-EOS) is taken no longer as a constant but as a variable.…”