Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications



Abstract 012
Phase Equilibria in the Presence of Saline Water Systems and Its Application to the Hydrate Inhibition Effect of Produced Water
Tohidi, B., Danesh, A., Burgass, R. W., and Todd, A. C.
SPE 28884, The European Petroleum Conference, London, 25-27 October (1994).

 

The application of extended subsea gathering networks and transportation of unprocessed wellstreams are serious consideration for reducing field development and operational costs. These lines will convey a cocktail of multi-phase fluids, including produced water, which is a mixed electrolyte solution, and liquid and gaseous hydrocarbons. A good knowledge of the behaviour of these complex systems is essential in the confident and economical design and operation of associated fields, pipelines and processing facilities. A major concern in the North Sea is that the above subsea pipelines and process facilities are prone to hydrate formation, giving rise to pipeline blockage, operational problems, and safety concerns. For efficient and economical pipeline design and operation, phase equilibria as well as the boundary of hydrate formation, with and without salts and other chemical inhibitors, must be precisely known. Produced water, due to the presence of salts, has a hydrate inhibiting effect and hence reduces the requirement in the consumption of expensive inhibitors. Available numerical models have limitations in type of calculations and range of application for real reservoir fluids in the presence of salts and/or chemical inhibitors. This paper presents a prediction model based on a rigorous thermodynamic approach in which an equation of state is combined with a modified Debye-Hückel electrostatic term, with only on adjustable parameter for the water-rich phase. The water-salt parameter has been determined by using available vapour pressure depression data of single electrolyte solutions. Nine common salts, such as NaCl, KCl, CaCl2, Na2SO4, NaF, KBr, MgCl2, SrCl2, BaCl2 and their mixtures, have been modelled. Although the main objective of this work is aimed at subsea gathering networks and transmission lines, the model is capable of predicting vapour pressure and freezing point of saline systems as well as the hydrate inhibition effect of single and mixed electrolyte solutions on reservoir fluids. Different sets of experimental data covering a wide range of temperature (250-600 K) and salt concentrations (close to saturation) have been used in model validation. The model predictions are compared with data generated in this laboratory and those in literature and good agreement is demonstrated. The model is also compared with four commercially available packages.

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