Resolving apparent contradictions in equilibrium measurements for clathrate hydrates in porous media

A conceptual model for hydrate equilibrium in media with broad pore-size distributions is used to interpret methane hydrate equilibrium experiments in silica gel pores with nominal radii of 2, 5, and 7.5 nm, While the hydrate equilibrium pressure is an intensive property, it is demonstrated that the presence of a broad pore-size distribution in a porous medium causes the experimental pressures to appear extensive. It is observed that the sample size, the headspace volume, the temperature at which the experiments are started, and the amount of hydrate allowed to dissociate to establish equilibrium at this initial temperature all affect the observed pressure at any given temperature. The pore-volume distribution of the porous medium (not the nominal pore radius) and the percent conversion of pore-water to hydrate also affect the measured equilibrium pressures. As a result, measured equilibrium pressures at a specific temperature for identical media can be significantly different, while those for different media may be superposed. These seeming inconsistencies, their causes, and interpretations are examined and explained. Apparently fundamental problem,,; in experimental data are removed when the number of degrees of freedom is properly included in the interpretation.