HR: 0830h
AN: OS51B-0849    [PDF]
TI: Phase Transition of Methane Gas Hydrate and Response of Marine Gas Hydrate Systems to Environmental Changes
AU: * Xu, W
EM: wenyue.xu@eas.gatech.edu
AF: School of Earth & Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 United States
AB: Gas hydrates, which contain mostly methane as the gas component in marine sediment, are stable under relatively high pressure and low temperature conditions such as those found along continental margins and permafrost regions. Its stability is mostly controlled by in-situ pressure, temperature and salinity of pore fluid. Environmentally introduced changes in pressure and temperature can affect the stability of gas hydrate in marine sediment. While certain changes may enhance the process of gas hydrate formation, we are much more interested in the resultant dissociation processes, which may contribute to sub-marine slope instability, seafloor sediment failure, formation of mud volcanoes and pock marks, potential vulnerability of engineering structures, and the risk to drilling and production. We have been developing models to quantify phase transition processes of marine gas hydrates and to investigate the response of marine gas hydrate systems to environmental changes. Methane gas hydrate system is considered 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. Compared to the great quantity of studies on its stability in the literature, in-depth research on phase transition of gas hydrate is surprisingly much less. A method, which employs pressure, enthalpy, salinity and methane content as independent variables, is developed to calculate phase transition processes of the three-component four-phase system. Temperature, an intensive thermodynamic parameter, is found not sufficient in describing phase transition of gas hydrate. The extensive thermodynamic parameter enthalpy, on the other hand, is found to be sufficient both in calculation of the phase transition processes and in modeling marine gas hydrate systems. Processes considered in this study to introduce gas hydrate dissociation in marine sediment include 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 relationship between gas hydrate dissociation and the stability/failure of marine sediment are to be further discussed.
DE: 1800 HYDROLOGY
DE: 3022 Marine sediments--processes and transport
DE: 3939 Physical thermodynamics
DE: 5139 Transport properties
DE: 8105 Continental margins and sedimentary basins
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting