No abstract

Carbon dioxide injection for methane replacement from hydrate reservoirs is considered as one of the effective techniques for both methane production and carbon dioxide sequestration. To understand the gas exchange process, a thermodynamic model based on the classical fugacity approach has been utilized to predict the phase behavior and the cage occupancies of both binary CH4+CO2 and ternary CH4+N2+CO2 mixed gas hydrate systems in three phase (L–H−V) equilibria. A total of 256 experimental phase equilibrium data points on binary CH4+CO2, and ternary CH4+N2+CO2 gas hydrate systems have been selected from various literatures. For the ternary system, various compositions of CH4+N2+CO2 gas mixtures with varying N2/CO2 gas ratio, i.e., 0.13, 0.19, 0.27, 0.33, 0.37, 0.5, 0.98, 2.6, and 4, and varying CH4 compositions, i.e., 20, 50, 60, 70, 80, and 90%, have been considered. The absolute average deviations (%AAD) in the predicted equilibrium pressures with the experimental results are observed to be within 2.9% and 6.4% for binary and ternary mixed hydrate systems, respectively. For CH4+CO2 mixed hydrate systems, the average distribution coefficient of methane has been found to be 2.06, which indicates that the CH4 molecules are selectively replaced by CO2 molecules preferentially from large cages. For the ternary CH4+N2+CO2 mixed hydrate system, the N2/CO2 ratio in small cages of the hydrate is found to be almost 20 times larger than that in the large cages revealing the capacity of N2 and CO2 molecules to replace most of the CH4 molecules from small and large cages of the hydrate, respectively. The N2/CO2 ratio in small and large cages is temperature independent at low N2/CO2 gas ratio, which becomes temperature dependent at higher ratios, i.e., 2.6 and 4. From this study, we conclude that the injection of N2/CO2 gas mixture having ∼1:3 ratio is a good choice for enhancing the production of methane gas and carbon dioxide sequestration deep into the hydrate reservoirs.

Hydrate formation in natural gas transportation pipelines and during completion and workover operations is considered to be a serious issue. CaBr 2 and ZnBr 2 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 threephase liquid water−hydrate−vapor (L-H-V) equilibrium conditions for methane hydrate in aqueous solutions of MgBr 2 , CaBr 2 , and ZnBr 2 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 MgBr 2 > CaBr 2 > ZnBr 2 . 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 ZnBr 2 > CaBr 2 > MgBr 2 . This study will be useful to design efficient completion fluids for completion and workover operations for onshore and offshore oil and gas production operations.