Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 014
Hydrates Formed in Unprocessed Wellstreams
Tohidi, B., Danesh, A., Burgass, R. W., and Todd, A. C.
SPE 28478, SPE 69th Annual Conference and Exhibition, New Orleans, USA, 25-28 September (1994).
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 hydrate 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 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.

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