Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 108
Gas Hydrates: Frozen Methane - Friend or Foe?
Tohidi, B., and Anderson, R.
Teaching Earth Sciences, 29, 18-21 (2004).
At low temperatures and high pressures, gases such as methane and CO2 can combine with water to form solid, ice-like crystalline compounds known as gas hydrates. Within the hydrate structure, water molecules are organised into hydrogen-bonded cages that enclose the 'guest' gas molecules, forming a solid stable at temperatures well above the melting point of water ice. Three common hydrate structures have been discovered (named structures I, II, and H), each consisting of a different combination of 5 types of water cages, as shown in Figure 1. When hydrate melts, or dissociates, liquid water and a large volume of gas - up to 164m3 for 1m3 of gas hydrate - is produced. In the oil and gas industry, gas hydrates are a major economic and safety concern, causing the blockage of pipelines, processing facilities and safety equipment. Although the formation of hydrates in gas pipelines was first reported in the 1930's, it was not until the 1970's that it was recognized that very large quantities of gas hydrate (mainly methane hydrate) occur naturally in sediments of the subsea continental slopes, and the subsurface of Arctic permafrost regions. Since this discovery, global interest in naturally occurring gas hydrates grown steadily, with research expanding rapidly over the past decade. Important factors driving research include both post positive and negative aspects. Gas hydrates have potential as major strategic energy reserve (current estimates of 1-5x1015 m3 methane at standard conditions), and may provide a novel means for CO2 disposal through sequestration. However, it is now recognised that a close relationship may exist between gas hydrates and subsea slope stability, and in addition to posing a potential immediate danger to deepwater drilling installations, pipelines and subsea cables, there are further important long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release, and global climate change.

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