A Gibbs free energy model of phase equilibrium including gas hydrates

Gas hydrates is shorthand for clathrate hydrates of natural gas, which are solid, crystalline molecular complexes formed from mixtures of water and low molecular weight compounds. This work presents a model of the Gibbs Free Energy of Mixing (GFEM) of hydrates, used in the framework of multiphase equilibrium calculations by means of the minimization of the GFEM. Such an approach was previously used for phase equilibrium calculations involving the three states of matter - namely, solid, liquid and vapor [1] - and has been improved in this work to also account for the hydrate phase. The GFEM for the hydrate phase has been expressed as suggested by Englezos and Bishnoi [2], who focused on the phase behavior of the methane-water system with respect to gas hydrate formation. The GFEM for the hydrate phase combines the fugacity of the guest component and the chemical potential of water in the hydrate phase, calculated using the model first presented by van der Waals and Platteeuw [3]. In this work, the analytic fourth-order Equation of State (EoS) proposed by Yokozeki [4] has been used for the fugacity in the solid, liquid and vapor phases. This EoS offers the advantage of describing the three states of matter simultaneously with the same Equation of State, without using a different approach for the solid phase [5,6]. The proposed model, with properly regressed parameters, has been applied to the systems methane-water and hydrogen sulfide-water, which are important in industrial practice in many hydrocarbon reservoirs, especially in Alberta [7]. In order to show the reliability of the proposed approach, which can be easily extended to multicomponent systems, the results of calculations for the hydrate formation pressures are compared with both the experimental data available in the literature and the results obtained using commercially available hydrate prediction programs that make use of different approaches.