Hydrate dissociation in chemical potential gradients: theory and simulations

Kinetic stability limits of hydrate reservoirs partially exposed to the ocean floor were studied with respect to methane concentration in surrounding seawater. The implications of low chemical potential of seawater-dissolved methane and the corresponding lack of thermodynamic stability of hydrates were analyzed. The minimum concentrations of methane in seawater necessary to achieve equilibrium between methane-containing seawater and methane hydrate were calculated. Nucleation theory was used to estimate the critical size and shape of the dissociating front as a function of methane concentration in the seawater, for concentration regions outside the stability limits for hydrate. The critical shape of the dissociating front was predicted to be flat and thin. Estimated induction times to reach critical size for dissociated layer vary from in the order of seconds for mole-fraction methane as low as 1.0 x 10(-8) to 3 days for a seawater mole-fraction of methane a magnitude lower than the necessary seawater methane concentration for equilibrium with hydrate. The theoretical predictions were compared with molecular dynamics investigations of model methane hydrate-free water and methane hydrate-brine systems under different temperatures and pressures. (C) 2003 Elsevier B.V. All rights reserved.