Nikos Varotsis

Modelling gas hydrate thermodynamic behaviour: theoretical basis and computational methods

The thermodynamic theory of van der Waals and Platteeuw has served up to now and for many decades as the principal basis for the development of tools for predicting the equilibrium conditions of various gas hydrate systems. Several of these developments are briefly presented in the present paper, with the aim of revealing associated difficulties and possible inherent limitations. After a discussion of thermochemical properties, a general phase equilibrium model is presented in detail and is tested against experimental gas hydrate data of recently reported synthetic mixtures and petroleum reservoir fluids with and without methanol inhibitor. A critical discussion of relevant thermodynamic stability algorithms is also proposed. Finally, areas are indicated where further academic research might aid industrial practice.

Microdistillation: A method combining gas and liquid chromatography to characterize petroleum liquids

Abstract A chromatographic method has been developed for the analysis of petroleum liquids to replace the standard distillation which is used by the analytical laboratories to split the sample into light and heavy fractions, thus permitting the detailed characterization of the former. The microdistillation, which uses only a few microlitres of sample, is based on a dynamic accelerated vaporization of the light components assisted by an overlying flow of an inert gas and it achieves a fast and efficient separation between the residue and the distilled product. As the microdistillation unit is physically attached to the gas analyser, it thereby constitutes a closed system with the latter and allows the vaporized fraction to be characterized directly by gas chromatography. The remaining residue is subsequently characterized by liquid chromatography. The accuracy of the method was tested against synthetic mixtures of known composition and against stock tank oils. The microdistillation can be extended to the analysis of all types of multicomponent mixtures with constituents exhibiting a wide boiling point distribution.

Reservoir fluid characterisation using gas chromatography-mass spectrometry

The compositional characterisation of naturally occurring hydrocarbon mixtures is an integral part of routine petroleum laboratory analysis. Currently, the measurements of the gas and liquid phase compositions require separate gas chromatographic set-ups with analysis times of up to 1 h. These analyses may lack important information because several critical components are not sufficiently separated. This work uses a GC-MS with a quadrupole mass analyser to routinely provide classification of the eluted peaks into alkanes, iso-alkanes, naphthenes and aromatics as well as to determine their concentration. The selected set-up uses a single injection point and a single analytical column to analyse sequentially both gas and liquid samples. Specific calibration procedures have been developed to allow for the variation of the m/z intensities from run to run and for the conversion of peak areas to concentrations. The new extended compositional analysis throws light on the distribution of the different types of molecules in the reservoir fluid and provides accurate physical properties for each cut, thus improving the understanding of the thermodynamic and rheological behaviour of the petroleum fluids.