Measurement and Computation of Near-Critical Phase Behavior of a C1/nC24 Binary Mixture

Abstract

Summary Equations of state (EOS) satisfactorily predict phase behavior and densityof reservoir fluids outside the retrograde and near-critical regions. Predictions in these regions, however, are an unresolved problem. A first stepto improve the understanding of retrograde and near-critical regions would beto measure the phase behavior of well-defined binary hydrocarbon mixtures ofvery different molecular sizes. This paper reports measurements on a C1/nC24binary mixture with a wide retrograde and critical region. Measured dewpointsare in the range of 12,000 to 15,500 psia [82.74 to 106.8 MPa]. Retrogradeliquid volumes and gas densities at three different temperatures were measured. Liquid phase compositions and densities also were measured on a selectivebasis. Conventional use of the EOS failed to predict the phase behavior of themodel binary system. The data will be useful in modifying the EOS in theretrograde and near-critical regions. Introduction Some in-situ reservoir fluids, and the fluids of a multiple-contact-miscibleprocess for an injected gas and reservoir oil can exhibit complex phasebehavior. The degree of complexity depends on the distance to the criticalpoint and the spread in volatility of the components. Reservoir fluidphase-behavior computations with two-constant and other simple EOS haveadvanced to the stage that phase-behavior predictions away from the criticalregion are quite reliable. For predictions away from the critical region arequite reliable. For such fluid systems, the use of binary interactioncoefficients and appropriate properties is sufficient for reliable vapor/liquidequilibria (VLE) prediction. The concept of volume translations Will improvethe volumetric predictions for equations that have a density-predictiondeficiency. Use of a constant shift parameter with a simple mixing rule hasproved adequate for the concept of volume translation. 3.4 The VLE andvolumetric computations for reservoir fluid systems in the retrograde andnear-critical regions, however, are a real challenge. Unlike simplemulticomponent systems, where the critical region could be small, multicomponent fluids with a large spread of volatility for the components showan expanded critical region. For some complex reservoir fluids, the criticalregion may extend over a wide pressure range. In the following, we review someof the attempts pressure range. In the following, we review some of theattempts to address the problem of phase behavior prediction in the criticalregion. The deficiency of pure component density prediction from the EOS in thecritical region was recently alleviated when Mathias et al. extended thevolume-translation idea of Peneloux et al. They suggested an additionalcorrection term in the vicinity of the critical point. This correction term isgiven by (1) The last term on the right side is the additional correction term in thecritical region. This form of correction forces the equation to give truecritical volume. When applied to a two-parameter EOS, the proposal of Mathiaset al. makes it superior to a three-parameter EOS. Three-parameter equationsgive a substance-dependent critical compressibility factor but not the truecritical compressibility factor. Mathias et al. also extended their proposal tomixtures by a simple mixing rule. While their modification of thevolume-translation concept enhances density prediction of pure components inthe critical region, similar prediction of pure components in the criticalregion, similar enhancement for mixtures is questionable. We will later discussthis point. Chou and Prausaitz also proposed a volume-translation procedure, similar to that of Ref. 5, for improving density predictions for both purecomponents and mixtures in the critical region. In addition to the modifiedvolume translation, Chou and Prausnitz suggested a procedure to account fordensity fluctuations near the critical point. They observed that for somebinary mixtures, density fluctuations are often masked by the influence ofbinary interaction coefficients. Recently, Danesh et al. 7 showed that the Mathias et al. procedure cannot be extended to mixtures. They proposed avolume-translation distance parameter, which is a function of the equilibriumratio of the lightest and the heaviest components. The compositional dependencychanges the expression for the fugacity if the proposals of Refs. 5 and 7 areincorporated into the phase-behavior proposals of Refs. 5 and 7 areincorporated into the phase-behavior calculations. Danesh et al. attributed theunreliability of phase- behavior predictions in the critical region to theunreliability of phase- density predictions. In our view, thecomposition-prediction deficiency is the main reason for the unreliability ofphase-behavior predictions in the critical region. Another approach to overcomethe difficulty in computing near- critical phase behavior for reservoir fluidsystems is the tuning of the parameters in the EOS. The parameters could betuned by non- linear regression to minimize the error between EOS-calculatedvalues and data. This technique may result in unphysical constants and maycause large errors outside the range of adjustment. In an alternative approach, Slot-Peterson emphasized the need to adjust only the binary interactioncoefficients for a specific reservoir fluid. His proposal is based on theassumption that binary interaction coefficients between two given components indifferent fluids are not the same. He reasons that because of interference fromthe surrounding molecules, the interaction between specific heavy and lightmolecules modeled by binary interaction coefficients is not necessarily thesame in different fluid systems. For this approach, data such asconstant-mass-expansion experiments are required to determine binaryinteraction coefficients. Slot-Peterson also observed that binary interactioncoefficients are temperature dependent. The dependency seems to be lesspronounced and more random than the compositional dependency. The literaturereviewed above gives an idea how far we are from complete predictive capabilityfor near-critical reservoir fluids with EOS. The question raised regarding thedependence of the variation of binary interaction coefficients on specificfluid systems is of a fundamental nature. Both experimental and theoreticalresearch work will be needed to resolve the problems of phase-behaviorprediction in the e and near-critical regions and to phase-behavior predictionin the e and near-critical regions and to bring predictive capability to the EOS. In this paper, we report the results of our experimental research work ona complex binary system. The fluid pair was selected after detailed and carefulconsideration of our ultimate objective of bringing full predictive capabilityto EOS. After the presentation of data, existing computational methods will beexamined with our data. SPERE P. 271