Studies on micromorphology and permeability of hydrate-bearing porous media

The micromorphology of methane hydrates determines the permeability of hydrate-bearing sediments. In this study, the morphological characteristics of ice and methane hydrates before and after dissociation were measured using a Scanning Electron Microscope (SEM). Then, the relationship between micromorphology and permeability was developed using a mathematical model. The results showed that ice and methane hydrates differ in their shapes, lengths, and during sublimation, ice and methane hydrates could be accurately separated in the SEM images. The ice had hexagonal, cubic, and spherical shapes, with lengths of 1.0-10.0 & mu;m, and its surfaces were irregularly shrunk during sublimation. Methane hydrates had cores and block shapes, with lengths of 5.0-150.0 & mu;m, and were divided into grain-coating and pore-filling methane hydrates in a saturation of 15 %. The former was formed with cores, whereas the latter was formed with cores and blocks. During hydrate dissociation in glass beads, homogenous micropores with an average diameter of 0.20 & mu;m were formed on their surfaces, which served as gas release channels. Micropores were special characteristics of methane hydrates during dissociation. Thus, surface porosity was used to determine the quantity of micropores. The surface porosity was 15 % at the hydrate saturation of 15 %. A weighted permeability reduction model was developed considering both grain-coating and pore-filling methane hydrates in the sediments, the curves of which was provided by determining the weight coefficients and permeability reduction exponents in the model. By comparing experimental permeability data with the curves, the weight coefficients, permeability reduction exponents, and growth habits of methane hydrate in the sediments could be obtained.