A Practical Model for the Effect of Salinity on Gas Hydrate Formation

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Abstract This paper is based on a previously developed thermodynamic model for gas hydrates and hydrate inhibition. The model uses a cubic equation of state for the fluid phases and parameters have already been determined for the following inhibitors: methanol, MEG, DEG and TEG. The paper describes the extension of the model to include the effect of salinity in produced water or sea water on hydrate formation. To ensure the model would be of practical value, it was designed with the following characteristics: it should be simple to use requiring as input no more than a typical ion analysis table from a laboratory report. It should be based on a cubic equation of state suitable for engineering calculations and the model should operate reliably at temperatures and pressures normally encountered in oil and gas production. The model represents the ionic components in water by a single salt pseudocomponent of the equation of state. The physical properties of the pseudocomponent were set by regressing them to experimental data for sodium chloride solutions. Results will be presented to show that the model can simultaneously represent the lowering of the hydrate dissociation temperatures, the depression of the freezing point of water and the reduction in the water vapour pressure (osmotic coefficient). As sodium chloride is usually the dominant component in produced water or sea water, other salts are handled on a sodium chloride equivalent basis, so that only one salt pseudocomponent is needed for practical calculations. Data for the effect of natural waters supports the use of this approximation. In practice, a hydrate inhibitor may be added to the water phase so it is important that the inhibition model can give accurate predictions in the presence of saline solutions. This has been confirmed by investigating the salting-out effect for methanol and results are shown for salt-water-methanol mixtures. Practical ways of accessing and applying the model are summarised. These include using the model as a standalone computer program, accessing the model via a spreadsheet or using the model as an object code library or as a dynamic link library.