Phase boundary of CH4, CO2, and binary CH4-CO2 hydrates formed in NaCl solutions

The equilibrium phase boundary of gas hydrates divides the phase diagram of the gas-water system into two zones, i.e., one with hydrates and the other without hydrates. Motivations to investigate the phase boundary of gas hydrates formed by CH4, CO2, and their mixture in saline water stem from its foundation roles in e.g., flow assurance, energy recovery, and CO2 sequestration. In this work, a thermodynamic model has been developed to improve the prediction accuracy of the equilibrium phase boundary of CH4, CO2, and binary CH4-CO2 hydrates in both pure and saline water. A different set of reference properties are employed in this newly developed model. A database collecting the most up to date measured phase boundary of gas hydrates have been newly generated, containing 728 data points with the temperature ranging from 261.8 K to 300 K, and the concentration of NaCl solutions up to 24.12 wt%, which is used for parameter regression and model validation. The impacts of temperature, NaCl, and gas composition on the phase boundary of gas hydrates are well captured by the developed model. A convergence trend of the phase boundary curves of the binary CH4-CO2 hydrates in the high-temperature range is observed and explained with the Clausius-Clapeyron equation. The overall average absolute relative deviation (AARD) of the calculated phase boundary pressure of the 728 points is 4.42%. The performance of the newly developed model is satisfactory considering the discrepancies among measured data from various sources. (C) 2020 Elsevier Ltd.

Automated summary

A new thermodynamic model was developed to improve the prediction accuracy of the equilibrium phase boundary of CH4, CO2, and binary CH4-CO2 hydrates in water. By utilizing a newly generated database, the study successfully captured the impacts of temperature, NaCl concentration, and gas composition on the phase boundary of gas hydrates. Overall, the performance of the newly developed model is satisfactory.