Abstract

This work presents the first equation-of-state (EOS) algorithm to computeminimum miscibility pressures (MMP) in hydrocarbon systems exhibitingthree-phase (L-L-V) equilibria. Previous efforts have been limited to systemsinvolving only two-phase equilibria. This work is particularly applicable tolow-temperature reservoirs since they often exhibit L-L~V phase behavior. Acomparison of two- and three-phase algorithm results shows that the two phasealgorithm may over predict the MMP in cases where three phase equilibria isrealized. The results of a CO2 - West Texas oil system show that theerror by the two-phase algorithm is significant and the three-phase algorithmoffered here should be used.

Introduction

Minimum miscibility pressure (MMP) is an important screening criteria fortarget miscible flood reservoirs. Several investigators1–3 havepresented methods to calculate MMP's using equations-of-slate (EOS). Alimitation of all these works is that they assume two hydrocarbon phaseequilibria and cannot be used to compute the MMP in systems exhibiting threehydrocarbon phase equilibria. This limitation is significant insofar as manylow-temperature reservoirs subject to miscible flooding exhibit L-L-V phasebehavior.4,5

This work prepares for a deeper understanding of developed miscibility andoffers a generalized EOS MMP algorithm which accounts for both two- andthree-phase equilibria. The new algorithm is equally applicable to minimumenrichment calculations even though our remarks here are limited to MMPcalculations. We illustrate the new algorithm by considering two examples. Theexamples compare MMP predictions using both two- and three phase algorithms andwe show that the two-phase algorithm often over predicts the MMP if the MMPoccurs within a pressure range where three-phase equilibria appears or flashsolution multiplicities exist.

Theory
Two-Phase Algorithm

Past EOS MMP algorlthms1–3 basically employ variations of the mixingprocedure shown in Fig. 1. Fig. 1a shows a ternary diagram at a fixedtemperature and pressure and illustrates the mixing process whereby miscibilitydevelops by a vaporizing mechanism. Fig. 1 considers three pure components:CO2, an intermediate hydrocarbon species CI, and a heavyhydrocarbon species CH. Figure 1considers a case where the drive gasis composed of 100% CO2and the oil is composed of a mixture of CI and CH(denoted by point I).

The mixing process proceeds as follows, see Fig. 1a. The drive gas and oil Iare mixed in such a proportion to yield overall mixture M1. Mixture M1 equilibrates into vapor and liquid compositions denoted by points V1 and L1. To simulate the vaporizing process, theresulting vapor phase is repeatedly contacted by fresh reservoir oil. As such, vapor V1 is contacted by Oil1 and overall mixture M2results. Mixture M2 equilibrates into vapor V2 and liquid L2. This mixing process continues until either miscibility developsor immiscibility results. The vapor phase compositions resulting from thismixing process are sometimes referred to as "gas-forward" contacts.

Miscibility develops for a specified drive gas and oil at a specifiedtemperature and pressure it the mixing process eventually yields a single-phasemixture, see Fig. 1a. Alternatively, immiscibility results it an unchangingvapor composition develops, see Fig. 1b.

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