EFFECT OF OVERPRESSURE ON GAS HYDRATE DISTRIBUTION Gaurav Bhatnagar, Walter G. Chapman, George J. Hirasaki  Department of Chemical & Biomolecular Engineering Rice University 6100 Main St., Houston, TX, 77005 USA Gerald R. Dickens, Brandon Dugan Department of Earth Science Rice University 6100 Main St., Houston, TX, 77005 USA ABSTRACT The effect of overpressure on gas hydrate and free gas distribution in marine sediments is studied using a one-dimensional numerical model that couples sedimentation, fluid flow, and gas hydrate formation. Natural gas hydrate systems are often characterized by high sedimentation rates and/or low permeability sediments, which can lead to pore pressure higher than hydrostatic (overpressure). To quantify the relative importance of these two factors, we define a dimensionless sedimentation-compaction group, sc N , that compares the absolute permeability of the sediments to the sedimentation rate. Higher values of sc N mean higher permeability or low sedimentation rate which generally yield hydrostatic pore pressure. Conversely, lower values of sc N generally create pore pressure greater than hydrostatic. Simulation results show that decreasing sc N not only increases pore pressure above hydrostatic values, but also lowers the lithostatic stress gradient and gas hydrate saturation. This occurs because overpressure results in lower effective stress, causing higher porosity and lower bulk density of the sediment. This leads to higher sediment velocity through the gas hydrate stability zone, thereby reducing the mass accumulation of methane and gas hydrate in the pore space. Effect of overpressure on depth of the gas hydrate stability zone is also studied. Keywords: gas hydrate, overpressure, sedimentation-compaction, numerical modeling  Corresponding author: Phone: +1 713 348 5416 Fax +1 713 348 5478 E-mail: gjh@rice.edu NOMENCLATURE j i c Mass fraction of component i in phase j , l m eqb c Methane solubility at base of GHSZ m D Methane diffusivity in seawater Da Damkohler number g Acceleration due to gravity sc N Sedimentation-compaction group k Absolute sediment permeability 0 k Absolute sediment permeability at seafloor rj k Relative permeability of phase j t L Depth to the base of the GHSZ L  Characteristic depth of compaction Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.