Natural gas hydrates are solid crystalline compounds formed by the chemical combination of natural gas and water under pressure at temperature considerably above the freezing point of water. In the presence of free water, hydrates will form when the temperature is below a certain degree (hydrate temperature).
Gas hydrate crystals resemble ice or wet snow in appearance but do not have ice's solid structure, are much less dense, and exhibit properties that are generally associated with chemical compounds. The main framwork of their structure is water; the hydrocarbon molecule occupies the void space in a crystalline network held together by chemically weak bonds with the water. The water framework is ice-like; unlike ice, however, it has void space and a network structure.
Hydrate formation is often confused with condensation, and the distinction between the two must be clearly understood. Condensation of water from a natural gas under pressure occurs when the temperature is at or below the dew point at that pressure. Free water obtained under such conditions is essential to formation of hydrates which will occur at or below the hydrate temperature at the same pressure. Hence, the hydrate temperature would be less than or equal to the dew point temperature.
During the flow of natural gas, it becomes necessary to define, and thereby avoid, conditions that promote the formation of hydrates. This is essential since hydrates may choke the flow string, surface lines, and other equipment. Hydrate formation is the flow string resutls in a lower value for measured wellhead pressure. In a flow rate measuring device, hydrate formation results in lower flow rates. Excessive hydrate formation may also completely block flow lines and surface equipment.
The conditions that tend to promote the formation of natural gas hydrates are:
1. Natural gas at or below its water dew point with liqdui water present
2. Temperature below the "hydrate formation" temperature for the pressure and gas composition considered.
3. High operating pressure which increase the "hydrate formation" temperature.
4. High viscosity or agitation through piping or equipment.
5. Presence of a small "seed" crystal of hydrate.
6. Presence of H2S or CO2 is conductive to hydrate formation since these acid gases are more soluble in water than hydrocarbons.
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