Computation in Gas Hydrate Formation
Authors: Mussumeci Alexandre 
Published: December 2007

A new method to predict incipient gas hydrate formation in systems containing an aqueous phase in the absence or presence of inhibitors is proposed. proposed. The method which is based on the statistical termodynamic model of van der Waais and Platteeuw for clathrates is implemented in a computational codes

Results of several simulations are analysed and compared with experimental data from 67 systems of the literature and Petrobras fields, obtaining good agreement between calculated and observed dissociation pressures, chiefly in multicomponent hydrocarbon systems. pressures, chiefly in multicomponent hydrocarbon systems. Owing to its good performance, the method can be applied as a robust tool in process design or in field operations in which gas hydrate equilibrium calculations are necessary.

Introduction

Gas hydrates (or simply hydrates) are ice-like crystalline compounds that form when water and gas come into contact, normally at low temperatures and high pressures. Hydrates belong to a peculiar group of substances known as clathrates, which are compound containing two or more components joined hot by an ordinary chemical bond but by the complete enclosure of one type of molecule inside a suitable structure formed by another molecule.

Since the discovery of the first hydrate, studies on these compounds have been purely academic and thus restricted to the scientific community. With the development of the US petroleum industry starting in the thirties, the study of hydrate formation gained momentum, particularly when Hammerschimidt proved that the pipeline clogging problems typically encountered during winter months were cased by the formation of hydrates rather than of ice, as had been thought previously. Innumerous researchers soon began studying this phenomenon and produced a number of predictive methods based on theories ranging from simple empirical predictive methods based on theories ranging from simple empirical correlations to the detailed microscopic study of intermolecular forces.

Due to Brazil's climate, specifically to its high temperatures, hydrate formation in petroleum facilities had never presented a serious problem. However, with the steady advance of production activities into problem. However, with the steady advance of production activities into deeper and deeper waters, the possibility of low seafloor temperatures causing hydrates to form inside pipelines has increased substantially.

Although the literature presents a number of predictive methods, at the time that these were proposed they were only compared against experimental data on simple gas systems. Their application to more complex gas systems, such as natural gas, may lead to serious errors.

The purpose of this study was to optimize process design and operational procedure concerned with the formation of natural gas hydrates, by providing reliable predictions of the conditions under which the phenomenon will occur and which include the presence of inhibitors. A new predictive method based on van der Waais and Platteeuw's theory is proposed and implemented in FORTRAN ANSI 77 computational code. The proposed and implemented in FORTRAN ANSI 77 computational code. The results of several simulations are analyzed and compared with the experimental data on multicomponent mixtures available in the literature or obtained in Brazilian oil fields.

SOME ASPECTS OF THE CRYSTALLINE STRUCTURE OF HYDRATES

Hydrates crystallize into two types of structure, known as I and II.