1. . 
For a hygroscopic material such as a clay or polymer to absorb water. Hydration is the first stage of clay-water (or polymer-water) interaction. When dry bentonite is stirred into water, hydration is observed as swelling.
2. . 
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.
4. . 
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.