  The microscopic mechanisms involved in the formation/dissociation of methane
hydrate confined at the nanometer scale are unraveled using advanced molecular
modeling techniques combined with a mesoscale thermodynamic approach. By means
of atom-scale simulations probing coexistence upon confinement and free energy
calculations, phase stability of confined methane hydrate is shown to be
restricted to a narrower temperature and pressure domain than its bulk
counterpart. The melting point depression at a given pressure, which is
consistent with available experimental data, is shown to be quantitatively
described using the Gibbs--Thomson formalism if used with accurate estimates
for the pore/liquid and pore/hydrate surface tensions. The metastability
barrier upon hydrate formation and dissociation is found to decrease upon
confinement, therefore providing a molecular scale picture for the faster
kinetics observed in experiments on confined gas hydrates. By considering
different formation mechanisms -- bulk homogeneous nucleation, external surface
nucleation, and confined nucleation within the porosity -- we identify a
crossover in the nucleation process; the critical nucleus formed in the pore
corresponds either to a hemispherical cap or a bridge nucleus depending on
temperature, contact angle, and pore size. Using the classical nucleation
theory, for both mechanisms, the typical induction time is shown to scale with
the pore surface to volume ratio and, hence, the reciprocal pore size. These
findings for the critical nucleus and nucleation rate associated to such
complex transitions provide a mean to rationalize and predict methane hydrate
formation in any porous media from simple thermodynamic data.
