Excess pore pressure resulting from methane hydrate dissociation in marine sediments: A theoretical approach

Abstract

[1] This study quantifies the excess pore pressure resulting from gas hydrate dissociation in marine sediments. The excess pore pressure in confined pore spaces can be up to several tens of megapascals due to the tendency for volume expansion associated with gas hydrate dissociation. On the other hand, the magnitude of excess pore pressure in wellconnected sediment pores is generally smaller, depending primarily on the hydrate dissociation rate and the sediment permeability. Volume expansion due to gas hydrate dissociation in well-connected pore spaces is related via Darcy's law to an increase in pore pressure and its gradient in sediment, which drives an additional upward fluid flow through the sediment layer overlying the gas hydrate dissociation area. The magnitude of this excess pore pressure is found to be proportional to the rate of gas hydrate dissociation and the depth below seafloor and inversely proportional to sediment permeability and the depth below sea level. The excess pore pressure is the greatest at low initial pressures and decreases rapidly with increasing initial pressure. Excess pore pressure may be the result of gas hydrate dissociation due to continuous sedimentation, tectonic uplift, sea level fall, heating or inhibitor injection. The excess pore pressure is found to be potentially able (1) to facilitate or trigger submarine landslides in shallow water environments, (2) to result in the formation of vertical columns of focused fluid flow and gas migration, and (3) to cause the failure of a sediment layer confined by low-permeability barriers in relatively deep water environments.Citation: Xu, W., and L. N. Germanovich (2006), Excess pore pressure resulting from methane hydrate dissociation in marine sediments: A theoretical approach,

A transfer function for the prediction of gas hydrate inventories in marine sediments

“…They have attracted increasing interest in marine geosciences for various reasons: (i) the use of GH as additional energy source (e.g. Bohannon, 2008;Hester and Brewer, 2009), (ii) the climate effect of melting GH and CH 4 -release into sea water and the atmosphere induced by seafloor warming (e.g Dickens et al, 1995;Kennett et al, 2003;Milkov, 2004;Reagan and Moridis, 2009), and (iii) the potential of dissociating GH triggering slope failure events (Xu and Germanovich, 2006).…”

Thermally induced evolution of phase transformations in gas hydrate sediment

“…Generally, 1 m 3 of NGH may release 164 m 3 methane gas and 0.8 m 3 of water at 1 atm at normal temperature. Then a large excess pore pressure will result in a strength decrease of the sediment if the released gas diffuses slowly [5,6].During exploitation of gas hydrate or gas and oil in the deep sea, high-temperature pipes will pass through GHS and make the temperature of GHS increase. Accordingly, NGH in sediment will dissociate and the GHS and cap-rock become unstable.…”

“…Generally, 1 m 3 of NGH may release 164 m 3 methane gas and 0.8 m 3 of water at 1 atm at normal temperature. Then a large excess pore pressure will result in a strength decrease of the sediment if the released gas diffuses slowly [5,6].…”

Modeling Slope Instability as Shear Rupture Propagation in a Saturated Porous Medium

“…This is an appealing approach for studying failure in the submarine environment, where slopes are often too shallow for failure to be explained by infinite slope or limit equilibrium analyses. In this environment, processes affecting pore pressure such as local fluid flux (Dugan and Flemings 2000) or methane hydrate dissociation (Xu and Germanovich 2006) have been proposed as mechanisms for initiating failure. The works of Palmer and Rice, and Puzrin and Germanovich, considered the shear strength on the surface to degrade with the amount of slip, assumed that the length of the rupture was much greater than its depth and that the stress-strain relationship of the overlying sediments was linearly elastic until passive or active failure.…”