Effect of Heavy Hydrate Formers on the Hydrate Free Zone of Real Reservoir Fluids

Abstract

Abstract Recently we have investigated and modelled a number of the heavy hydrate formers, i.e. benzene, cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, isopentane, and 2, 3-dimethylbutane. This work reviews some of the previous findings and investigates the effect of four of the above compounds on the hydrate free zone of real reservoir fluids. The preliminary results show that the effect of the heavy hydrate formers cannot be ignored and in some cases, they reduce the hydrate free zone significantly. Introduction Gas hydrates are inclusion compounds in which certain nonpolar or slightly polar molecules (guest) of a suitable size stabilise the crystalline structure formed by hydrogen bonded water molecules (host) under favourable conditions of pressure and temperature. The known hydrate structures are those of structure I, II, and the newly discovered H, where each structure is composed of a certain number of large and small cavities. For hydrate structures to remain stable, a minimum fraction of their cavities have to be filled with the guest molecules. On the other hand, for a molecule to enter a cavity, its size should be smaller than a certain value. Therefore, large molecules which can enter only a limited number of large cavities require smaller molecules "help gas" to mainly fill some smaller cavities to stabilise hydrate crystals. Gas hydrates have been reviewed by Sloan. One serious concern in the North Sea is that the subsea gathering networks and pipelines are prone to hydrate formation, giving rise to pipeline blockage, operational problems, and other safety concerns. These can be avoided by either of the following two approaches:Preventing the hydrate formation by heating and/or insulating the pipe or by adding chemical inhibitors to operate, with a safety margin, outside the hydrate boundary zone.Allowing the formation of hydrates, but modifying their growth in order to prevent aggregation of hydrate crystals and hence avoiding the blockage by transporting hydrates as slurry. The first approach which is widely accepted and practised in the industry can become more economical by a more reliable determination of the hydrate free zone. In the oil and gas industry, n-butane is generally regarded as the heaviest hydrate forming compound, and anything heavier than it is treated as a non-hydrate former. However, some hydrocarbon compounds heavier than n-butane have an effective van der Waals diameter which theoretically should allow them to enter the large cavities of structure-II gas hydrates. Furthermore, Ripmeester et al have suggested the presence of a third structure, called H, with cavities larger than those of structures- I and II. This would allow the formation of hydrates by even larger molecules in the presence of a help gas. Whilst available models can accurately predict the hydrate equilibria for synthetic and simple mixtures, they are generally optimistic, i.e. under predicting the hydrate zone, for real reservoir fluids4. This could be partly attributed to the presence of heavy hydrate forming compounds in above fluids. Although the structure-II gas hydrates are well known, the first equilibrium data on structure H gas hydrates was reported for methane/adamantane by Lederhos et al. Mehta and Sloan have reported structure H hydrate data for some other heavy compounds with methane. In a recent communication, they presented a thermodynamic model for structure H hydrates and determined the Kihara parameters for four structure H hydrate formers. P. 257