Phase behavior of high-pressure CH4-CO2 hydrates in NaCl solutions

Abstract

Understanding the phase behavior of CH4-CO2 hydrates formed in NaCl solutions is important for gas recovery from hydrate reservoirs, CO2 sequestration, and flow assurance. The observations of CH4-CO2 hydrates formed in NaCl solutions are limited at high-pressure and high-temperature conditions. In this work, a high-pressure experimental apparatus is built to investigate the phase behavior of CH4-CO2 hydrates. The interfacial phenomenon is observed during hydrate formation and dissociation. The phase boundary of CH4-CO2 hydrates is determined by using the isochoric pressure-search method with pressure above 10 MPa. A novel theoretical model is developed to calculate the phase boundary pressure of hydrates by implementing a modified PR EoS for the fluid phases, the van der Waals-Platteeuw model for the hydrate phase, and the Pitzer model for the effect of ions. It is revealed that the roughness of water–gas interface is greatly increased at the initial hydrate formation stage due to the occurrence of fine pillars at the interface, which is explained by two hypotheses, i.e., the non-equilibrium state of gas and water phases and the non-uniform lateral growth of hydrates at the interface. The roughness decreases as time goes by during hydrate formation owing to the thickness growth of the hydrate film. The deviation of the calculated phase boundary pressure from the 86 collected experimental data points of CH4-CO2 hydrates formed in pure water is 3.7%, and is 5.3% for the 24 collected phase boundary data points of CH4-CO2 hydrates formed in the NaCl solutions, which proves the reliability of the newly developed model. The measured phase boundary data points of CH4-CO2 hydrates formed in NaCl solutions in this work extend the phase boundary pressure up to 61.99 MPa and the temperature to 295.09 K, which have filled the blank of the database in the high-pressure and high-temperature region. The deviations of the calculated phase boundary pressure from these two groups of measured ones are 8.9% and 6.2% respectively.