An Accurate Method for Extending the Analysis of C7+

Abstract

Members SPE-AIME Abstract The main objective of this paper is to present an accurate method for extending the molar distribution of the heptanes-plus fraction. The method originated from studying the extended analysis of ten natural gas-condensate mixtures. A FORTRAN subroutine to perform this computation is presented, along with a graphical form of the proposed method. Introduction Equations of state have generally been recognized by the petroleum industry as important analytic expressions used in describing the thermodynamics and behavior of complex hydrocarbon mixtures. An important consideration in using equations of state (EOS) is the difficulty in characterizing the heavy fractions of hydrocarbon mixtures. These fractions, usually lumped as heptanes-plus (C7+), are difficult to define without an extended molar analysis of the plus-fraction. A conventional laboratory hydrocarbon plus-fraction. A conventional laboratory hydrocarbon analysis of a gas-condensate system usually reports the composition of the system (methane through heptanes-plus), molecular weight and density of the plus fraction. Several authors have indicated the well necessity of splitting the C7+ fraction into a number of components for accurate retrograde calculations. Firoozabadi and Katz demonstrated the use of the extended analysis of C7+ with the Peng-Robinson equation of state to accurately predict the behavior of crude oil and gas condensate mixtures. Whitson, in a pioneering paper, noted that lighter systems such as condensate exhibit exponential molar distributions while heavier systems show a left skewed distribution. The author proposed that a probability function (three parameter gamma distribution) can be used to characterize both types of behavior by adjusting a single parameter in the distribution function. Whitson demonstrates excellent agreement between data and his models calculation of the extended analysis given by Hoffman. This paper presents a simple and practical method for extending the molar distribution of heptanes-plus fraction from the normally available physical measurements of C7+, i.e. molecular weight and physical measurements of C7+, i.e. molecular weight and density. To establish the validity of the procedure two types of comparisons were used through-out this study. One is between two sets of calculated results, obtained from the proposed model and other models. The second comparison is between experimental data with extended analysis of C7+ and predicted molar distribution resulting from the method. The Peng-Robinson Equation of State The details of the development of the Peng-Robinson equation of state are given elsewhere. A Peng-Robinson equation of state are given elsewhere. A summary of the equation is presented here for convenience. The equation has the form .........................(1) p. 223