Effects of pore sizes on dissociation temperatures and pressures of methane, carbon dioxide, and propane hydrates in porous media

Equilibrium conditions of CH4, CO2, and C3H8 hydrates confined in small pores of porous glass were determined. The dissociation temperature of each hydrate at a given pressure shifted lower than that for bulk hydrate; the largest shift for CH4 hydrate was -12.3 K +/- 0.2 K for 4-nm-diameter pores and the shift decreased to only -0.5 K for 100-nm pores. CH4 hydrate experiments at temperatures lower than the quadruple point of 270.6 K in 30-nm porous glass showed no shift of the equilibrium line. All temperature shifts were fitted by the Gibbs-Thomson equation; the best fits for CH4, CO2, and C3H8 hydrates predicted hydrate-water interfacial energies of 1.7(3) x 10(-2) J/m(2), 1.4(3) x 10(-2) J/m(2), and 2.5(1) x 10(-2) J/m(2), respectively. Both type-I hydrates of CH4 and CO2 had interfacial energies within 20% of each other but significantly smaller than the type-H hydrate Of C3H8. Ice formation in the same porous glass fit the Gibbs-Thomson relation with an interfacial energy of 2.9(6) x 10(-2) J/m(2), which is in good agreement with established values. The estimated interfacial tensions between gas hydrates and water were found to be only weakly affected by the kinds of gas. This indicated that the pore effect on the phase equilibrium was mainly due to the water activity change. The wide range of experiments on pore size, temperature, and the kind of gas allowed us to evaluate the validity of previous model predictions for pore effects on gas hydrate stability.