Natural Gas is a major energy source and approximately 22% of the global energy consumption is in the form of Natural Gas. Natural Gas is obtained from Oil Reservoirs, Shale Gas, Tight Gas, Coal Beds, and from Natural Gas Hydrates (NGH) i.e. Methane Hydrates present at sea beds.
As of now, Methane Hydrate deposits are not being exploited commercially, but Japan conducted a six day offshore production test in the Pacific Ocean in March 2012. The test produced a total of 120,000 cubic meters, or 20,000 cu m/day, of gas from Methane Hydrate, and the Japanese government is encouraging research into hydrates for alternative energy source. As Methane Hydrate is converted to gas, so Natural Gas too can be converted to Methane Hydrate for storage and transportation. Conversion of Natural Gas into Hydrates provide considerable storage and transportation advantages compared to Liquefied Natural Gas (LNG), as Hydrates remain stable when stored at temperatures in the range of -15°C to -5°C, at atmospheric pressure whereas LNG needs to be stored at -162°C. Also, one litre of fully saturated Methane Clathrate contains around 169 litres of methane gas at 0°C and 1 atm, thus requires less volume for storage. For experimental purposes, Methane Clathrate has been successfully produced in stirred vessels at pressures from 2 to 6 MPa and in the temperature range of 0 to 20°C (Gudmundsson et al., 1994).
In this technical paper, characteristics & properties of Hydrates are to be discussed and extended to provide insights as an economical storage alternative for Natural Gases, which can be easily transported and used when required, and as reserves for withstanding situations of large energy demands such as in very cold conditions due to climatic changes. First we start with characteristics of Hydrates and then we will move on to production.
Characteristics and Properties of Hydrates
Gas Hydrates or Clathrate Hydrates are crystalline solids, its unit cell consisting of a gas molecule (Methane in context of Natural Gases) trapped in a cage like structure formed by water molecules surrounding it. Gas hydrates are produced at temperatures above the freezing point of water, because the gas molecule stabilises the crystal structure, and the gas hydrate formed is white like snow. High pressure conditions are also required for hydrate formation.
Gas Hydrates are non-stoichiometric, which allows variation in their composition. Hydrate number of Gas Hydrate refers to ratio of number of water molecules to number of gas molecules, and it depends on many factors including gas composition, pressure and temperature.
Three structures have been identified for gas hydrates based on the structure of cage of water molecules formed around the gas molecule and these are- sI, sII and sH. These structures consist of several types of cages, where each cage has the potential to contain one gas molecule. A cage is made of several water molecules held together by hydrogen bonds. The...