Modelling phase equilibria of complicated systems containing petroleum reservoir fluids

Formation of gas hydrates can lead to serious operational, economic and safety problems in the petroleum industry due to potential blockage of oil and gas transmission lines and processing facilities. Thermodynamic inhibitors are widely used to reduce the risks associated with gas hydrate formation. In practice, the aqueous phase in which inhibitors are added already contains electrolytes from either drilling/completion fluids or from formation water. In such mixed inhibitor systems, both co-solvents and strong electrolytes are present in the aqueous phase, making the thermodynamics of these highly non-ideal systems difficult to model. Hydrate/aqueous and hydrocarbon PVTX thermodynamic modelling at Heriot-Watt University (HWHYD) dates back over 25 years. In the latest version of the HWHYD model (2.1), a new approach has been introduced and applied for modelling phase equilibria in systems containing components which can form hydrogen bonds (e.g. water, methanol, ethanol, mono-ethylene glycol, and etc) and hydrocarbon mixtures using a robust general-purpose implementation of the CPA (Cubic Plus Association) model. This work is evaluating the capability of this model for conventional and challenging hydrates calculations, including; (1) gas hydrate in low water content gases, (2) hydrate stability zone in the presence of high concentration of inhibitor(s) or salt(s) and/or high pressure conditions, (3) hydrate stability zone of oil/condensate in the presence of produced water and inhibitors, and (4) prediction of hydrate inhibitor distribution in multiphase systems. The results show that the Heriot-Watt Hydrate (HWHYD 2.1) software predictions are consistently in a good agreement with experimental data. Copyright 2009, Society of Petroleum Engineers.