Coupled THMC modeling of dissociation induced deformation of gas hydrate bearing media

Abstract

Methane gas recovery from hydrate bearing sediment (HBS) by depressurization causes significant pore pressure decrease, loss of solid load bearing hydrate from pores and small temperature drop. The decrease in pore pressure increases the effective compressive stress arising from overburden load on the HBS and thus causes the settlement of the HBS. The reduction of medium strength by the loss of load bearing hydrate and thermal contraction also influences the deformation of HBS. We have developed a THMC solver to simulate the complex geomechanics problem along with hydrate phase change kinetics, non-isothermal multiphase flow in porous media, and the alteration of porosity, permeability and bearing capacity. The deformation behavior of the hydrate-bearing media depends on the host sediment and hydrate morphology, effective porosity and critical porosity. We have included the influence of loss of load bearing hydrate and compaction due to increase of effective compressive stress as the result of pore pressure decrease, fraction of hydrate morphology, effective porosity and critical porosity on the effective medium bulk and shear moduli using a modified effective medium model. The effects of hydrate saturation decrease and effective compressive pressure increase on the deformation of HBS are thoroughly investigated. We have performed numerical simulations to understand the deformation behavior for different porosities and matrix supporting hydrate fractions. Our results show that the deformation is about 20% less when the fraction of matrix supporting hydrate increases from 0.1 to 0.4 because of increased soil bonding.