Micromechanics of HydrateBearing Sediments by GrainScale Simulations
Abstract
Dissociation of gashydrates 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 offshore infrastructure. We seek to quantify the impact of hydrate dissociation on the strength of hydratebearing 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 quasistatic 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 conjugategradient 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) porefilling 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, pathdependent stressstrain 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.
 Publication:

AGU Fall Meeting Abstracts
 Pub Date:
 December 2007
 Bibcode:
 2007AGUFMOS22A..07S
 Keywords:

 0545 Modeling (4255);
 0560 Numerical solutions (4255);
 3004 Gas and hydrate systems