HR: 0830h
AN: OS51B-0848    [PDF]
TI: A Pore-Network Study of Methane Clathrate Hydrate Dissociation
AU: Lichtner, P C
EM: lichtner@lanl.gov
AF: Los Alamos National Laboratory, EES-6, LANL, Los Alamos, NM 87545 United States
AB: Clathrate hydrates are very important compounds due to their capacity to store large volumes of gases. They are studied extensively as a possible source of energy, since an enormous reservoir of carbon is deposited in worldwide accumulations of hydrates, containing predominantly methane, both on-shore (under the permafrost), and off-shore (in marine sediments). They are also considered as a possible way to sequester carbon dioxide, as a means to remove it from the atmosphere and in order to reduce its effect in global warming. In this study, a 2-D pore-network simulation based on concepts from Invasion Percolation in a Gradient is introduced to study the isothermal dissociation of methane clathrate hydrate in porous media. We use reported experimental values of porosity and permeability that are found in oceanic sediments to reconstruct a porous medium with similar properties. This is done by varying appropriately the size range of throat radii and the ratio pore/throat radii. The reconstructed porous medium is either fully or partially saturated with hydrate. Two important issues are addressed. First, we examine the stability of the dissociation front of a porous medium fully saturated with hydrate and show that stable fronts (having a characteristic width) are obtained. The front width is similar to the case of Invasion Percolation in a Stabilizing Gradient. Scaling of the front width is obtained as a function of the mean position of the front with exponent 0.57. Second, we examine the patterns formed by the release of methane gas in a partially saturated porous medium after the hydrate dissociates, and investigate how the various patterns are affected by parameters such as the viscous pressure drop applied to the system, the in size range of throat radii, hydrate saturation, and pore size distribution. In order for the methane gas to be produced from the system, the critical gas saturation (the volume fraction of the gas phase at the onset of bulk gas flow) must be reached. For this to occur the produced gas clusters must: either (i) connect to each other forming a sample-spanning gas cluster through which the gas phase can be produced, or (ii) be mobilized by the presence of viscous or buoyancy forces and thus arrive at the producing end of the network (this is a limiting case where continuum models may break down because of the formation of gas bubbles). In order to obtain a better understanding of the production efficiency for the commercial development of hydrate deposits we address the significant question whether the resulting dissociating system (at lower hydrate saturation values) is below or above the critical gas saturation. We also examine scaling of the critical gas saturation as a function of various parameters of the system.
DE: 0999 General or miscellaneous
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting