Effect of capillary term parameters on the thermodynamic modeling of methane hydrate formation in porous media

Abstract

According to the solid-solution theory, the stability of gas hydrates is related to the activity of water. Geometrical constraints decrease the activity of water. Therefore, differences in phase behavior calculations of gas hydrates in porous media from gas hydrates in the bulk, stem from activity of water calculations. In this work, the effects of different parameters describing capillary forces on the results of methane-hydrate formation modeling are studied. Two different thermodynamic models are applied for calculations. The first model assumes a single-size porous media and the equality of chemical potential. The second model considers the pore size distribution and the equality of water fugacity as the equilibrium criteria. It is shown that the value of surface tension proposed by Uchida and co-workers yields the most accurate predictions compared to experimental results. In addition, values of shape factors (assuming a fixed wetting angle) are optimized using experimental data. The optimized values are correlated to the hydrate formation temperature. To validate the optimized shape–factor parameters, the correlated values are used for an additional set of experimental data, which leads to reduced errors. If the single-size pore modeling is used, percent of average absolute deviation (AAD%) decreases from 7.83% to 4.59% in LW–H–V region and from 19.58% to 15.71% and 7.03% in the I–H–V region for perfect wetting contact and complete non-wetting contact, respectively. If the model which considers the pore size distribution (PSD) is used, AAD% decreases from 9.98% to 5.79%. The results of this study show that at temperatures below the temperature of quadruple point, assumption of a cylindrical contact leads to much more accurate predictions in the I–H–V region even when the pore size distribution is not considered.