Excess Pore Pressure and Slope Failures Resulting From Gas-Hydrates Dissociation and Dissolution
Authors: Sultan Nabil 
Published: October 2008

Abstract
Parameters affecting gas hydrate formation include temperature, pore pressure, gas chemistry, and pore-water salinity. Any change in the equilibrium of these parameters may result in dissociation (gas-hydrate turns into free gas/water mixture) and/or dissolution (gas-hydrate becomes mixture of water and dissolved gas) of the gas hydrate. While, gas-hydrate dissociation at the base of the Gas Hydrate Occurrence Zone (GHOZ) is often considered as a major cause of sediment deformation and submarine slope failures the consequence in terms of pore pressure and sediment deformation of the dissolution of the gas hydrate at the top of the GHOZ remains neglected. In this study, we quantify and compare the excess pore pressure resulting from gas hydrate dissociation and dissolution. Based on theoretical development it is demonstrated that excess pore pressure and shear discontinuities generated by hydrate dissociation is unlikely to be a hazardous factor. In natural environment, the excess pore pressure generated by hydrate dissociation is bounded by the gas hydrate stability law inducing for a natural temperature increase a limited amount of excess pore pressure and limited shear discontinuities at the base of the GHOZ. On the other hand, we show that under natural temperature changes hydrate dissolution at the top of the gas hydrate stability zone, which can occur at a regional scale, is a hazardous process that can lead to catastrophic landslides.

Introduction
For the last 3 decades, several authors have raised serious concerns regarding the possible link between gas hydrate and submarine slope failures. McIver1 was amongst the first authors to speculate about this possible link. In the McIver conceptual model1, the excess pore pressure generated by hydrate dissociation and the sediment shear strength decreases (lost of hydrate playing the role of cementing agent between sediment grains) are the two key factors in the slope failure mechanism. The causal factor of the hydrate dissociation in McIver1 model is the continuous sedimentation, which induces the upward migration of the base of the Gas Hydrate Stability Zone (GHSZ). Afterwards, Kvenvolden2 has stated that an upward movement of the bottom of the GHSZ due to an increase of bottom water temperature may accelerate the process of slope failures associated to hydrate dissociation. Different authors have later developed several other hypotheses and theories supposing all that gas hydrate dissociation may lead to important excess pore pressure and lead to sediment deformations and slope instabilities3,4,5,6,7. In the meantime, many large submarine landslides have been described worldwide in areas where gas hydrate occurrence was proved or suspected9,10,11,12,13. Paull and co-authors6 have proposed that gas-hydrate is the main cause of the increased frequency of sea-floor slumping on continental margins containing gas hydrates during sea-level lowstands. While authors seem to agree about the association between gashydrate dissociation and submarine slope failures, few theoretical and mathematical works were developed to define accurately the mechanism associated to the slope failure process.