Micromechanics of Hydrate Dissociation in Marine Sediments by Grain-Scale Simulations
byHoltzman Ran, Dmitriy B. Silin, Tadeusz W. Patzek
Holtzman, R., Silin, D. B., and Patzek, T. W., Paper SPE 114223: “Micromechanics of Hydrate Dissociation in Marine Sediments by Grain-Scale Simulations,” presented at the 2008 SPE Western Regional and Pacific Section AAPG Joint Meeting held in Bakersfield, California, U.S.A., 31 March-2 April 2008.
We seek to quantify the impact of hydrate dissociation on the strength of hydrate-bearing sediments. Dissociation of gas-hydrates in marine sediments converts the solid hydrate structure into liquid water and gas. Together with the associated pore pressure increase, this process reduces the stiffness of the sediments, which may fracture or be fluidized. If sediment failure occurs, seafloor subsidence and landslides can severely damage offshore infrastructure. To evaluate the mechanical properties of a sediment sample, we simulate loading of a disordered pack of spherical grains by incremental displacements of its boundaries. The deformation is described as a sequence of equilibrium configurations. Each configuration is characterized by a minimum of the total potential energy. This minimum is computed using a modification of the conjugate gradient algorithm. We verify our model against published data from experiments on glass beads. Our simulations capture the nonlinear, path-dependent behavior of granular materials observed in experiments. Hydrates are modeled as load-bearing solid particles within the pores. To simulate the consequences of dissociation, we reduce the solid fraction by shrinking the hydrate grains. The effect of the associated excess pore pressure is modeled by isotropic compression of the solid grains, and reduction in macroscopic effective stress. Weakening of the sediment is quantified as a reduction of the effective elastic moduli.