Effect of C7+ Properties on Equation-of-State Predictions

Summary This work studies the effect of heptanes-plus (C7+) characterization on equation-of-state (EOS) predictions. Both the Peng-Robinson EOS (PREOS) and a modified Redlich-Kwong EOS are used. Several characterization schemes found in the literature are used in the study. Six reservoir fluids with extended C7+ analysis have been chosen, representing a broad range of compositions and PVT data. First, the study deals with methods for estimating specific gravities and boiling points of petroleum fractions, two properties usually required by critical property correlations. EOS predictions are made by using critical properties based on specific gravities and boiling points measured experimentally and estimated with correlations. Next, a review of critical-property correlations is given, including a review of commonly used correlations. Four correlations are chosen to study the effect of critical properties on EOS predictions. Results indicate that relatively small differences in critical properties and acentric factor can result in significant differences in EOS predictions. Finally, a study is made of the effect that adjustments in C7+ critical properties have on EOS predictions; adjustments of EOS constants (Oa and Ob) and binary interaction coefficients also are studied. The influence of individual and combined adjustments to C7+ properties are illustrated graphically. They provide qualitative guidelines that can be used when matching experimental PVT data with an EOS. Introduction Recently there has been an increasing interest in EOR methods. One result has been the need to develop numerical models for simulating EOR processes involving complex phase behavior. Most of these models use an EOS to predict vapor/liquid equilibrium (VLE) and volumetric phase behavior. A typical problem with using an EOS is the difficulty in describing petroleum fractions constituting C7+. These fractions significantly affect EOS predictions, as has been reported in the literature. Usually only limited C7+ data are available to describe C7+ fractions. Even so, estimates of critical properties - critical pressure, critical temperature, and acentric factor - are required by most EOS's. Several correlations exist for estimating critical properties of petroleum fractions. Most are empirical equations giving a best fit of graphical correlations based on experimental data. Boiling point and specific gravity usually are required by critical property correlations. Several methods exist for estimating specific gravity and boiling point of petroleum fractions when only C7+ properties are known. Four of these have been used in our study. Several methods have been tried to improve C7+ characterization by approximating the chemical makeup of petroleum fractions. The most common approach assumes that petroleum fractions are composed of three hydrocarbon groups: paraffins (P), naphthenes (N), and aromatics (A). Several methods exist for estimating PNA content of petroleum fractions. 1–3 Three aspects of C7+ characterization have been studied:methods for estimating specific gravity and boiling point of petroleum fractions,correlations for estimating critical pressure, critical temperature, and acentric factor, andmanual adjustment of critical properties, EOS constants (Oa and Ob), and binary interaction coefficients (particularly between methane/CO2 and C7+ fractions). Reservoir fluids used in this study were chosen on the basis of availability of distillation data for C7+ fractions and the type of PVT measurements reported. Six fluids have been chosen from the literature, including fluids ranging from a lean gas to a black oil. PVT measurements have been reported at more than one temperature for some of the mixtures, which allows the study of temperature dependence of binary interaction coefficients between methane and C7+ fractions. Two cubic EOS's are used to make PVT predictions. PREOS4 was chosen because it is widely accepted in the industry and it generally yields better liquid-density estimates than the comparable Soave-Redlich-Kwong5 (SRK) equation. As a representative of the Redlich-Kwong6 family, Yarborough's7 version of the modified Zudkevich-Joffe-Redlich-Kwong8 (ZJRK) equation was chosen. Results from this work suggest that C7+ characterization has a significant influence on EOS predictions of reservoir fluid behavior. It is difficult to make objective conclusions about which C7+ characterization schemes are best. A different approach to characterization is needed. One possible alternative would be to calculate critical properties of petroleum fractions such that measured values of specific gravity and boiling point are force-fit by the EOS.