Microstructure evolution and dynamic permeability anisotropy during hydrate dissociation in sediment under stress state

Clarifying the hydrate dissociation behavior, microstructure evolution, and corresponding seepage property evolution of hydrate reservoirs is essential for dissert area prediction, single-well production ability evaluation and the development of reasonable exploitation strategies. In this study, depressurizing dissociation was conducted on a hydrate-bearing sandy specimen using X-ray CT under triaxial stress, and the results show that hydrate dissociation first starts from the hydrate-gas interface preferentially, and then, some cavities would be formed among the patchy clusters. Later, when the hydrate particle dissociates to a sufficiently small size, the dissociated water gathers on the surface of the hydrate, wrapping the hydrate in the water phase, which prevents contact between the hydrate and gas phases. Additionally, obvious subsidence was observed after hydrate dissociation even the isotropic stress is 1 MPa. In addition, there is a linear relationship between the volume and area for each hydrate particle, while it is the intercept not the slope depends on the hydrate saturation. With decreasing hydrate saturation, the pore system characteristics would vary significantly. Correspondingly, the dynamic permeability of the hydrate-bearing sediment exponentially increases with decreasing hydrate saturation and is anisotropic due to the inhomogeneity of hydrate dissociation, and it could be predicted via electrical conductivity.

Automated summary

In this study, depressurizing dissociation was conducted on a hydrate-bearing sandy specimen using X-ray CT under triaxial stress. The results show that hydrate dissociation starts from the hydrate-gas interface and then results in the formation of cavities among patchy clusters. As the hydrate particle dissociates to a smaller size, the dissociated water gathers on the surface of the hydrate, preventing contact between the hydrate and gas phases. Subsidence is observed even at a low isotropic stress of 1 MPa after hydrate dissociation. Furthermore, the pore system characteristics vary with decreasing hydrate saturation, and the permeability of the hydrate-bearing sediment increases exponentially with decreasing hydrate saturation and is anisotropic due to the inhomogeneity of hydrate dissociation, which can be predicted via electrical conductivity.