The compositional analysis of gas condensate reservoir fluids is an integral part of their routine PVT laboratory analysis. Naturally occurring hydrocarbon mixtures contain thousands of components, the presence and concentration of which determine their physical properties and phase behavior during production. Accurate compositional data is the prerequisite for generating reliable Equation-of-State (EoS) based predictions of the PVT and physical properties of the reservoir fluids over a wide range of conditions as well as for quality assurance of the crude.
Unfortunately, no standard methods are available in the petroleum industry for measuring compositions. Different analytical techniques are employed for obtaining the molecular composition of tank oil such as direct injection analysis, simulated distillation by gas chromatography, separation of the sample by fractional distillation, identification of specific compounds by mass spectrometry, component group distributions by High Pressure Liquid Chromatography (HPLC), etc. In addition, several correlations are utilised for defining the physical properties of the single carbon number fractions (SCNF) and of the heavy end fraction. This diversification inevitably leads to differences in the compositions reported by various laboratories which can significantly affect the quality of the predictions provided by the EoS based simulators.
This paper reviews the current analytical techniques and discusses their advantages and disadvantages. The compositional data obtained on gas condensate reservoir fluids by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and fractional distillation are compared and assessed. The effects that the differences in the derived molecular compositions bear on the EoS based predictions of the physical properties of petroleum fluids are illustrated using test cases studied in the PVT lab. These effects are particularly pronounced for the estimation of the retrograde liquid deposit curve, which constitutes a crucial set of data for efficient reservoir management.
Introduction
Petroleum reservoir fluids are composed of many thousands of different components, hydrocarbons and hetero-compounds covering a wide range of boiling points and molecular weights. Gas condensates contain fewer hydrocarbons of heavier molecular weight than oils whilst the heaviest hydrocarbons in a low shrinkage oil can have a molecular weight in thousands.
There are different ways to classify the constituents of the reservoir fluids. Normally the defined components consist of methane through normal pentane, the main non-hydrocarbon constituents (permanent gases) and the heavier than C6 components expressed as Single Carbon Number Fractions. Methane through propane exhibit unique molecular structures; butane can exist as two isomers and pentane as three isomers. For hexanes and heavier, the number of isomers rises exponentially. The hydrocarbons heavier than C6 are grouped as normal, iso, cyclo and aromatic (nica). For many years, the industry has been concerned with the very heavy asphaltenic constituents of crude oil. This paper does not address the specific issues associated with the classification and characterization of these very heavy ends.
