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

Abstract

The formation of hydrates is a major issue in the oil and gas industry. Injection of electrolytes is considered one of the best ways to prevent its formation. A thermodynamic model is developed from the Klauda and Sandler model, PRSV EoS, and Pitzer's correlation to study the phase equilibria of methane hydrate in chloride and bromide electrolyte solutions. The model employs the parameters reported by Pitzer and Mayorga to calculate the Debye−Huckel coefficient and the second and third virial coefficients at 25 °C. The %AAD in predicted equilibrium pressures from experiments is 3.70 when the hydrate formed in NaCl, KCl, MgCl 2 , and CaCl 2 solutions, whereas it is 2.33 in NaBr, KBr, MgBr 2 , CaBr 2 , and ZnBr 2 solutions. The inhibition effect of any electrolyte on methane hydrate formation at 1 wt % is small, slowly increases at 3 wt %, and then pronounces at 5, 10, and 15 wt %. The cage occupancies have been found to increase and the activity of water has been found to decrease with the increase in electrolytes concentration. The freezing point depression of water in the presence of the strongest inhibitor MgCl 2 has been observed to be −12.82 °C, whereas in the presence of the weakest inhibitor ZnBr 2 it is −4.22 °C, both at 15 wt % concentration. Based on the phase equilibrium results and the freezing point depression of water, the order of the methane hydrate inhibition effect among all of the electrolytes studied here is MgCl 2 > NaCl

Formulating Noncovalent Interactions for Gas Hydrates with Electrolytes: A New Approach of Stability Analysis

“…These estimates are complemented with local composition models like universal quasi-chemical (UNIQUAC), − nonrandom two-liquid (NRTL), electrolyte-NRTL (eNRTL), and nonelectrolyte-NRTL-nonrandom factor (N-NRTL-NRF) , to describe the HLVE. Others have relied on solution theories, like the Pitzer activity coefficient model ,− or mean spherical approximation, , to achieve the same.…”