1. n. [Geology]
An unusual occurrence of hydrocarbon in which molecules of natural gas, typically methane, are trapped in ice molecules. More generally, hydrates are compounds in which gas molecules are trapped within a crystal structure. Hydrates form in cold climates, such as permafrost zones and in deep water. To date, economic liberation of hydrocarbon gases from hydrates has not occurred, but hydrates contain quantities of hydrocarbons that could be of great economic significance. Hydrates can affect seismic data by creating a reflection or multiple.
Synonyms: gas hydrate
2. n. [Production Testing]
Compounds or complex ions that are formed by the union of water with other substances. Hydrates can form in pipelines and in gas gathering, compression and transmission facilities at reduced temperatures and high pressures. Once hydrates are formed, they can plug the pipelines and significantly affect production operations.
Synonyms: hydrate
3. n. [Drilling Fluids]
A crystalline solid consisting of water with gas molecules in an ice-like cage structure. The general term for this type of solid is clathrate. Water molecules form a lattice structure into which many types of gas molecules can fit. Most gases, except hydrogen and helium, can form hydrates. C1 to nC5 hydrocarbons, H2S and CO2 readily form hydrates at low temperature and high pressure. Heavier hydrocarbons may also enter the structure but do not form hydrates by themselves. Gas-cut muds can form hydrates in deepwater drilling operations, plugging BOP lines, risers and subsea wellheads, causing a well-control risk. Gas hydrates are thermodynamically suppressed by adding antifreeze materials such as salts or glycols. A common practice is to use 20 to 23 wt.% NaCl. Nucleation and growth of hydrates can be dynamically inhibited by certain polymers or surfactants. Gas hydrates are found in nature, on the bottom of cold seas and in arctic permafrost regions. Drilling into these can be hazardous, but they offer another source of hydrocarbons for future exploitation.

Reference: Ebeltoft H, Yousif M and Soergaard E: "Hydrate Control During Deep-water Drilling: Overview and New Drilling Fluids Formulations," paper SPE 38567, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5-8, 1997.
Synonyms: gas hydrate