Semi-Analytical Study of Production of Gas Hydrates and Their Techno-Economic Uses
Authors: Kurawle Irfan Badruddin , Kaul Mohit , Amin Zoaib , Kulkarni Nikhil Jagadish 
Published: November 2008

Gas Hydrates being projected as the next most important source of energy are object of immense research activity. The study relates to an analytical review of the conditions for the Natural Clathrate formation, production and their use as a source of fuel and gas transportation systems. We look at various production techniques for extraction of vast gas reserves inhibited in these resources. The problem of transportation of natural gas using pressurized hydrates for transportation is one of the highlights.

We would like to emphasize the concept of replacement of Methane by Carbon-di-Oxide in the hydrate structure for the purpose of production of free methane from the hydrate bearing zones. The penetration of Carbon-Di-Oxide in the reservoir would be at a rate sufficient for the continuous production and injection of Carbon-di-Oxide and its entrapment helping in reducing global warming effects considering the large area the gas hydrates cover around the world. Transportation of gas in the form of hydrates would help in reducing the huge costs involved in the installation of pipelines and their maintenance and other problems encountered during transportation With the Oil and Natural gas reserves depleting and transportation of the natural gas giving rise to maintenance and installation costs it would be worthwhile to take a look at new production methods for hydrate hydrocarbon production.

Introduction

Hydrates are solid crystalline compounds formed by non-stoichiometric relations between the host and the guest molecules. The host molecules in most cases being water (although other liquids like Freon-12 are also known to form hydrates) forming cage like structures encompassing the guest molecules which include a variety of known compounds like methane, ethane, carbon-di-oxide. There are three known structures of gas hydrates which exist in nature S-I, S-II and the rare H structure with each structure having its own non-stoichiometric relation between the host and the guest molecules. Structure I gas hydrates contain 46 water molecules per unit cell arranged in 2 dodecahedral voids and 6 tetrakaidecahedral voids, which can accommodate at most 8 guest molecules up to 5.8 Angstroms in diameter. Structure I allows the inclusion of both methane and ethane but not propane. Structure II gas hydrates contain 136 water molecules per unit cell arranged in 16 dodecahedral voids and 8 hexakaidecahedral voids, which can also accommodate up to 24 guest molecules, but to a larger diameter of 6.9 Angstroms. This allows inclusion of propane and iso-butane in addition to methane and ethane. The rare Structure H gas hydrates, which contain 34 water molecules per unit cell arranged in 3 pentagonal dodecahedral voids, 2 irregular dodecahedral voids, and 1 icosahedral void, can accommodate even larger guest molecules such as iso-pentane.1 The type of structure and their geometry defines the various properties of the substructure of the formation as the gas hydrates are known to have a substantial role in the mechanical strength of the formations they are found.

Gas hydrates occur in regions where the conditions are ideal for their development and can rapidly form or deteriorate depending upon shift of phase equilibrium for their formation and hence the study of various equilibrium conditions for gas hydrates is essential to produce natural gas commercially from them. These studies help us in deciding the production mechanisms and the design criteria for the production systems. Study of different equilibrium conditions leads to a common understanding of three basic requirements for hydrate formation and stability.