Thermodynamic Modeling of Methane Hydrate Phase Equilibria in Chloride and Bromide Solutions

The formation of hydrates is a major issue in the oiland gas industry.Injection of electrolytes is considered one of the best ways to preventits formation. A thermodynamic model is developed from the Klaudaand Sandler model, PRSV EoS, and Pitzer's correlation to studythe phase equilibria of methane hydrate in chloride and bromide electrolytesolutions. The model employs the parameters reported by Pitzer andMayorga to calculate the Debye-Huckel coefficient and the secondand third virial coefficients at 25 & DEG;C. The %AAD in predictedequilibrium pressures from experiments is 3.70 when the hydrate formedin NaCl, KCl, MgCl2, and CaCl2 solutions, whereasit is 2.33 in NaBr, KBr, MgBr2, CaBr2, and ZnBr2 solutions. The inhibition effect of any electrolyte on methanehydrate formation at 1 wt % is small, slowly increases at 3 wt %,and then pronounces at 5, 10, and 15 wt %. The cage occupancies havebeen found to increase and the activity of water has been found todecrease with the increase in electrolytes concentration. The freezingpoint depression of water in the presence of the strongest inhibitorMgCl(2) has been observed to be -12.82 & DEG;C, whereasin the presence of the weakest inhibitor ZnBr2 it is -4.22 & DEG;C, both at 15 wt % concentration. Based on the phase equilibriumresults and the freezing point depression of water, the order of themethane hydrate inhibition effect among all of the electrolytes studiedhere is MgCl2 > NaCl > CaCl2 > KCl> NaBr >ZnCl2 > MgBr2 > KBr > CaBr2 > ZnBr2.