Validation of Splitting the Hydrocarbon plus Fraction First step in tuning Equation-of-State
- Ali Abduallh Al-Meshari (Saudi Aramco) | William McCain Jr.
- Document ID
- Society of Petroleum Engineers
- SPE Middle East Oil and Gas Show and Conference, 11-14 March, Manama, Bahrain
- Publication Date
- Document Type
- Conference Paper
- 2007. Society of Petroleum Engineers
- 5.2 Reservoir Fluid Dynamics, 5.5 Reservoir Simulation, 5.2.1 Phase Behavior and PVT Measurements, 5.2.2 Fluid Modeling, Equations of State, 5.8.8 Gas-condensate reservoirs
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Volumetric data and phase behavior of volatile oils and gas condensates are very sensitive to the fluid composition and the properties of the plus fraction. Since the plus fraction of a reservoir fluid has some uncertainty in its measured molecular weight, the equation of state, EOS, is generally not predictive unless its parameters are tuned to match the experimental data. Characterization of the plus fraction, which is important in tuning the EOS, consists of splitting the plus fraction to single carbon number groups, assigning critical pressure, critical temperature and acentric factors to each single carbon number group, SCN, and then grouping single carbon number groups to multiple carbon number groups, MCN.
Direct measurement of the critical pressure, critical temperature and acentric factor of the plus fraction is not practical. Several authors have shown that erroneous predictions and calculations can result if the plus fraction is used directly as one component in EOS calculations and that use of the extended composition of the measured plus fraction will give more accurate PVT predictions for crude oil and gas-condensate mixtures.
To tune EOS three main steps should be followed: (1) Extend the measured composition into SCN, assign critical properties and acentric factor for each SCN, and match saturation pressure using the extended composition (2) Group SCN into pseudo-components, group the properties of the pseudo-components, and rematch the saturation pressure using the grouped composition and (3) Match PVT properties.
The objective of this study is to find an accurate method for splitting the hydrocarbon plus fraction to single carbon number groups and then validate the proposed method by using experimental extended fluid composition of different reservoir fluids. The proposed method was compared to other splitting methods published in the literature.
Reservoir fluids are composed of a lot of different hydrocarbons and non-hydrocarbons such as CO2, N2 and H2S. These hydrocarbon components can be classified as follows:
(a) Defined components which have a well known critical properties and acentric factor.
(b) Trueboiling point (TBP) or single carbon number (SCN) components which have a measures or estimated molecular weight and specific gravity, and whose critical properties are difficult to obtain experimentally.
(c) Heavy ends (residue), (i.e., C7+) which has a measure mole fraction, molecular weight and specific gravity.
A conventional laboratory fluid composition report for reservoir fluids usually reports the mole fraction or mole percent for non-hydrocarbon and for methane through plus fraction, including the molecular weight and specific gravity of the plus fraction.
An EOS requires the values of critical pressure, critical temperature, and acentric factor for each component. There are several correlations in the literature for calculating critical properties and acentric factor for each SCN group. Together with the molecular weight, these properties are sufficient for simpler property prediction models. The liquid density information represented by specific gravity can also be considered a physical constant for many models. For plus fraction, accurate properties can be predicted with accurate representation of the critical properties and acentric factor. Direct measurement of the critical properties for heavy ends is not practical. So, characterization of the heavy ends is required and it consists of three main steps:
(a) Splitting the plus fraction to SCN groups.
(b) Assigning critical properties and acentric factors for each SCN group.
(c) Grouping SCN into multiple carbon number groups, MCN.
Insufficient description for the plus fraction will reduce the accuracy of the EOS calculations. So, a proper description for the plus fraction is important to effectively predict the volumetric data and phase behavior of the reservoir fluids.
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