Production and transportation of unprocessed wellstreams is economically an attractive option and may significantly enhance the development of many marginal oil and gas fields. These pipelines may however be prone to hydrate formation which potentially can block the pipe and lead to serious operational problems. These can be avoided by either preventing hydrate formation or allowing the formation of hydrates, but preventing their aggregation and transporting them as slurry.
The first approach is the current practice in the industry and it can be more cost effective by determining the hydrate phase boundary more reliably. in the oil and gas industry, n-butane is generally regarded as the heaviest hydrate forming compound, and anything heavier than that as a non-hydrate former. Oil and gas condensate systems contain a significant amount of intermediate/heavy hydrocarbon compounds which recently have been found to be hydrate formers. The water produced with oil also contains dissolved salts which inhibit hydrate formation to some extent. If the formation of hydrates is to be avoided, information on the effect of heavy compounds and salts on the hydrate boundary is required.
Recent studies suggest that some chemicals at low concentrations may modify the growth of hydrate crystals, preventing their aggregation and hence allowing their transportation as slurry. if the use of crystal growth modifiers are to be considered, it is required to determine the amount of hydrates to be transported as a slurry.
This paper reviews briefly the effect of electrolyte solutions, benzene, and methyl cyclo-pentane (MCP) on the hydrate free zone and concentrates on new methods and equipment for measuring the amount and composition of different phases in hydrate forming conditions. An in house numerical model was successfully used for prediction of the hydrate free zone and compositional data. The developed model can be used as an engineering tool to determine the hydrate free zone for transportation of unprocessed well streams in sub-sea pipelines and gathering networks and also the amount of hydrates to be transferred as slurry where the use of growth modifiers is considered as an option.
Gas hydrates are inclusion compounds in which certain compounds stabilise the cages formed by hydrogen bonded water molecules under favourable conditions of pressure and temperature. For hydrates to remain stable, a minimum fraction of these cavities have to be filled with guest molecules. The most common hydrate structures are those of structure I and ii, where each structure is composed of a certain number of large and small cavities formed by water molecules. For a molecule to enter a cavity, its size should be smaller than a certain value. Large molecule guests which can enter only a limited number of large cavities require smaller "help gas" molecules to mainly fill some smaller cavities sufficiently to stabilise hydrate crystals. Gas hydrates have been reviewed in depth by Sloan.
The application of extended subsea gathering networks and transportation of unprocessed wellstreams are amongst favourable options for reducing field development and operational costs. These lines will convey a cocktail of multiphase fluids, including mixed electrolyte produced water, and liquid and gaseous hydrocarbons. A good knowledge of the behaviour of these complex systems is essential for confident and economical design and operation of associated fields, pipelines and processing facilities.