HR: 11:50h
AN: OS22A-07    [Abstracts]
TI: Micromechanics of Hydrate-Bearing Sediments by Grain-Scale Simulations
AU: Silin, D
EM: silin@patzek.berkeley.edu
AF: University of California at Berkeley, 431 Davis Hall University of California at Berkeley, Berkeley, CA 94720, United States
AU: * Holtzman, R
EM: holtzman@berkeley.edu
AF: University of California at Berkeley, 431 Davis Hall University of California at Berkeley, Berkeley, CA 94720, United States
AU: Patzek, T
EM: patzek@patzek.berkeley.edu
AF: University of California at Berkeley, 431 Davis Hall University of California at Berkeley, Berkeley, CA 94720, United States
AB: Dissociation of gas-hydrates in marine sediments converts the solid hydrate structure into liquid water and gas. Weakening of the solid skeleton causes a reduction of the elastic moduli. The increased pore pressure reduces the effective stress. As a consequence, a point of fracturing or fluidizing of the sediment can be reached. If such events occur, seafloor subsidence and landslides can severely damage off-shore infrastructure. We seek to quantify the impact of hydrate dissociation on the strength of hydrate-bearing sediments. The sediment weakening can be attributed to the reduction of the elastic moduli as hydrates become liquid and gas. We calculate these moduli using numerical simulations of deformations of a random disordered pack of spherical grains. Our model is discrete, accounting for the interactions between individual grains by calculating the loads which develop at each contact. We use a quasi-static approach by presenting deformation as a sequence of equilibrium configurations of the grain pack. Each configuration is characterized by the minimum of the total mechanical work in the pack. We find this minimum numerically, using a modified conjugate-gradient algorithm. In natural sediments, the distribution of hydrates in the pore space is a result of geologic history of hydrate formation. It can be affected, among other factors, by the saturations of gas and water, by the pressure and temperature, and by the pore geometry. There is a big uncertainty regarding the actual hydrate distribution. Therefore, we consider three different models: (a) pore-filling hydrate grains, (b) small amounts of hydrates in the pore bodies, and (c) small amounts of hydrate forming cement bonds at the contacts. To model dissociation, we first reduce the volume of solid hydrate. Then, we change the effective stress by imposing a macroscopic strain at the boundary, using poroelastic constitutive relations. Our simulations reveal the microscopic mechanisms that lead to the nonlinear, path-dependent stress-strain relations which are inherent to granular media. The consequences of dissociation are different for different models of hydrate saturation. The weakening of hydrate- bearing sediments due to the dissociation is captured in our simulations as a reduction in macroscopic moduli.
DE: 0545 Modeling (4255)
DE: 0560 Numerical solutions (4255)
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting