Phase equilibrium data for clathrate hydrates formed in the systems of urea plus (methane or carbon dioxide) plus water

Subsea methane hydrates are expected as a potential natural gas resource. To develop gas production methods for the subsea methane hydrates, flow assurance technologies are necessary. Urea is one of the environmentally-friendly thermodynamic hydrate inhibitors (THIs) which is expected to be applicable for subsea methane hydrate systems. To evaluate its performance as a THI, a set of phase equilibrium data are necessary. In this study, we report phase equilibrium data for systems of urea + methane + water and urea + carbon dioxide + water in the range of aqueous compositions of urea up to its solubility limits. The present data for urea + methane + water ranged in pressures from 5 to 13 MPa and in urea mass fractions from 0.05 to 0.5. The comparison with the literature found that the inhibition effect of urea is a few kelvin weaker than that of methanol in the present measurement range. With the dense urea solutions, the system reached four phase equilibrium, i.e., solid urea-aqueous solution-gas-hydrate. At 13 MPa, approximately 13 K of inhibition temperature was obtained with 0.500 of feed mass fraction where solid urea was precipitated from the aqueous phase. In the urea + carbon dioxide + water systems, urea also worked as a THI. The maximum inhibition temperature was approximately 10 K with 0.400 of urea mass feed fraction at the four phase equilibrium. Based on the present equilibrium data and seafloor conditions of the subsea methane hydrates, it was found that urea can be used for the both systems of the methane gas production and the hydrate based carbon capture and storage.