Methane hydrate thermodynamic phase stability predictions in the presence of salt inhibitors and their mixture for offshore operations

Abstract

A thermodynamic model is developed and used in this study to predict the phase equilibrium conditions of a methane -aqueous salt inhibitor -water hydrate system. The modified van der Waals-Platteeuw (vdWP) model is used to calculate the equilibrium condition of the hydrate phase. To compute the fugacities of gas and liquid phases, the Peng-Robinson equation of state (PR-EoS) and a previously developed Pitzer-Mayorga-Zavitsas-hydration model are used. The model predictions are compared to experimental results on methane hydrate phase equilibrium in the presence of an aqueous salt inhibitor. The absolute average relative deviation in predicted methane -aqueous salt inhibitor -water hydrate equilibrium pressure (AARD-P%) with the developed Pitzer-Mayorga-Zavitsas-Hydration model is 3.59% for CH4+NaCl, 1.52% for CH4+KCl, and 2.70% for CH4+ CaCl2. Finally, the hydrate suppression temperature caused by aqueous salt inhibitors and their mixtures on methane hydrate phase stability is calculated, and it is CaCl2>KCl>NaCl in that order. This work's phase equilibrium model demonstrates the potential application of determining methane hydrate equilibrium conditions in the presence of aqueous salt inhibitors and their mixtures used in offshore oil field applications.