RIASSUNTO
Abstract
New, cost-effective, and environmentally acceptable hydrate inhibitors that allow multiphase fluids to be transported untreated over long distances have been under increasing investigation by the oil and gas industry.
Replacing traditional hydrate inhibitors such as methanol and glycols with a new generation of hydrate inhibitors can lead to very substantial cost savings, not only for the reduced cost of the new inhibitor but also in the transportation costs and the size of the injection, pumping and storage facilities. Thus it is possible to redesign production facilities on a smaller scale. Such a change-over in hydrate inhibitor technology may also be an environmental requirement in the near future.
We will present an overview of patent and other literature on new hydrate inhibitors, including specific chemicals that have been investigated. Our own results on some of these published chemicals will be given and their applicability compared. We will also present data on new inhibitors developed at RF, as well as improved methods of evaluating their activity. A discussion of the relative merits of the two main types of new hydrate inhibitor, ""kinetic inhibitors"" and ""antiagglomerators"", will be given.
Introduction
The cost of preventing hydrate formation either by pipeline insulation or by using thermodynamic inhibitors (THIs) such as methanol can be an important economic factor to consider when developing small to medium fields.
Kinetic inhibitors (KIs) have been proposed as an alternative chemical treatment to THIs. However, the main drawback with KIs is their limited activity. The best KIs published to date appear only to have applications down to ca. 7-8 C subcoolings into the hydrate region if one is considering multiphase transportation over a reasonable distance. This subcooling effect is equivalent to using ca. 15-18 wt.% methanol. However, the KIs discovered to date are far from structurally optimised, and there are probably other classes of KI that remain to be discovered that are effective at higher degrees of subcooling. Hence, research on KIs is justified on this basis. Even so, improvements to be made in KI activity may still not good enough for some applications at extremely low temperatures or high pipeline pressures. Therefore, as the oil industry needs to control hydrate formation at very high subcoolings, a third method of chemical treatment is being developed. These types of chemicals are called Anti-Agglomerators (AAs).
Besides chemical treatment and the use of pipeline insulation, other hydrate control methods include using a jacket of hot water injected from the platform side, and using electric heating cable wrapped around the pipe. Neither technique has a track record yet.
Hydrate inhibitors and their Application
The three chemical methods for preventing hydrates from blocking pipelines can be summarised as follows:
Thermodynamic Inhibitors (THI). These are added at high concentrations (10-60 wt.%) and alter the chemical potential of the aqueous or hydrate phase so that the hydrate dissociation curve is displaced to lower temperatures or higher pressures. Examples are methanol and ethylene glycol.
Kinetic Inhibitors (KI). These are added at low concentrations (<1 wt.%) and do not affect the thermodynamics of hydrate formation. However, they do delay hydrate nucleation and/or crystal growth. When the inhibition mechanism breaks down and fast, auto-catalytic hydrate formation begins, the hydrate agglomerates rapidly and can form a plug.
Anti-Agglomerators (AA). These are also added at low concentrations (< 1 wt.%) and prevent the agglomeration of hydrates so that all the hydrate crystals are transportable and do not build up in the pipe.
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