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Xu, Wenyue1 
(1) Georgia Institute of Technology, Atlanta, GA

ABSTRACT: Response of Marine Gas Hydrate Systems to Environmental Changes - Role of Hydrate Dissociation

We consider methane gas hydrate system as a three-component (water, methane, salt) four-phase (liquid, gas, hydrate, halite) system. Pressure, temperature and salinity of pore fluid constrain the stability of gas hydrate and affect phase transition processes via their effects on methane solubility and fluid density and enthalpy. A method, which employs pressure, enthalpy, salinity and methane content as independent variables, is developed to calculate phase transition processes (not just the stability) of the three-component four-phase system. It is further incorporated into a finite difference simulator to carry out numerical modeling of marine gas hydrates. 
Environmentally introduced changes in pressure and temperature can affect the stability of gas hydrate in marine sediment. Processes considered in this study to introduce gas hydrate dissociation in marine sediment include continuous sedimentation, sea level drop and increasing seafloor temperature. Calculations indicate that dissociation of gas hydrates may lead to a build-up of excess pore pressure in marine sediment. The most significant factor constraining the magnitude of over-pressure is sediment permeability. The rate of sedimentation and pressure or temperature change at seafloor affects the degree and rate of gas hydrate dissociation and, hence, also the degree of excess pore pressure. The buildup of excess pore pressure may contribute to sub-marine slope instability, seafloor sediment failure, formation of mud volcanoes and pockmarks, potential vulnerability of engineering structures, and the risk to drilling and production.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.