Phase Equilibrium of Methane Hydrates in the Presence of MgBr2, CaBr2, and ZnBr2 Aqueous Solutions

Hydrate formation in natural gas transportation pipelines and during completion and workover operations is considered to be a serious issue. CaBr2 and ZnBr2 salts are widely used to design high-density completion fluids for completion of oil and gas wells; however, information on the hydrate phase equilibria in the presence of these salts is missing. In this study, a three-phase liquid water-hydrate-vapor (L-H-V) equilibrium conditions for methane hydrate in aqueous solutions of MgBr2, CaBr2, and ZnBr2 at 0.1, 0.25, 0.5, 0.75, 1, 3, 5, 10, and 15 wt % in the temperature and pressure range of 276.5-285.4 K and 3.65-9.56 MPa have been reported using the isochoric pressure search method. A total of 129 phase equilibrium experiments have been performed. At low concentrations of the salt, that is, 0.1-0.75 wt %, the thermodynamic inhibition was found to be low and close to the methane hydrate equilibrium conditions in pure water. The inhibition effect is found to increase beyond 1 wt %, becomes significant after 3 wt % salt concentration, and is found to increase with an increase in the salt concentration. In accordance with the hydrated radius and ionic weight, the inhibition effect of salts on the methane hydrate phase equilibria is in the order of MgBr2 > CaBr2 > ZnBr2. The heat of dissociation (evaluated using the Clausius-Clapeyron equation) for methane hydrates in aqueous salt systems is found to be in the order of ZnBr2 > CaBr2 > MgBr2. This study will be useful to design efficient completion fluids for completion and workover operations for onshore and offshore oil and gas production operations.