Solid-fluid equilibria modelling in wax, hydrate and combined wax-hydrate forming systems
Mahabadian, Mohammadreza Ameri
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Waxes and hydrates formation are two major flow assurance challenges, imposing considerable costs for prevention and, in worst case scenario, pipeline blockage removal and deferred production. Employing remediation and prevention schemes for hydrate and wax related problems necessitates knowledge of their formation conditions as well as their amount. The main focus of this work is thermodynamic modelling of phase equilibria in systems prone to waxes, hydrates and combined wax−hydrate formation. Study of these complex mixtures requires the development of a robust multiphase flash calculation algorithm capable of identifying the correct number and nature of the phases in equilibrium. Such an algorithm is devised in this work based on the Gibbs free energy minimization concept. The algorithm is first applied to complex hydrate forming systems and then extended to combined wax-hydrate forming mixtures, enabling investigation of the mutual interactions between hydrates and waxes from the thermodynamics viewpoint. The new algorithm is fast and is capable of showing complex behaviours in hydrate and wax forming systems including stability of several wax phases or more than one hydrate structure at equilibrium conditions. In this work, an integrated thermodynamic model coupling three highly accurate schemes, i.e., the cubic plus association equation of state, UNIQUAC activity coefficient model and van der Waals and Platteeuw approach−to describe the non-idealities of the fluids, paraffinic solids (waxes) and hydrates, respectively−is implemented. Furthermore, the formation of waxes in high-pressure condition is thoroughly investigated, especially for highly asymmetric condensate-like systems. Accordingly, a modified thermodynamic model is presented for wax formation in high-pressure systems. Comparing with experimental solid-fluid equilibrium data of synthetic mixtures, the integrated model presents excellent agreement which demonstrates the reliability of the approach. Finally, the method available for the extension of the integrated model−which was based on synthetic mixtures−to real oil systems and especially for wax formation, are evaluated. Based on the analysis presented the best model is chosen and used for illustrating the combined wax-hydrate precipitation in a real crude oil.