9.: Hydrates

Hydrates are crystalline material formed between light hydrocarbon molecules and water in which the gas molecules become trapped in cages formed by hydrogen-bonded water molecules. Gas hydrates resemble ice, but, unlike ice, they can form at higher temperatures. This is because the presence of gas molecules provides extra attraction and, hence, stability, thus helping to fix the position of the water molecules and enabling freezing at higher temperatures.

Hydrate prevention is a key flow-assurance focus area, especially in deepwater fields. The conditions required to form hydrates are

Hydrate formation can block pipelines or wellbores, thereby stopping production or injection, so the conditions under which hydrates form need to be understood. Hydrate equilibrium curves can be calculated for a given composition using pressure/volume/temperature equation-of-state software, and hydrate prevention can be achieved by operating outside the thermodynamic hydrate formation envelope (temperature and pressure). Although there might be options to achieve this—such as by providing heat or insulation—there will be many cases in which this is not possible. In such cases, the injection of thermodynamic inhibitors such as methanol or glycol or the use of kinetic hydrate inhibitors will be required.