Impact of gas arising from gas hydrate decomposition on the effective permeability of bottom sediments of the Arctic shelf: a laboratory simulation

Abstract

Relevance. The necessity to investigate the mechanism of gas (methane) flow occurring during decomposition of gas hydrate formations within the layers of bottom sediments. This process is evident through substantial releases of methane bubbles in extensive shallow regions of the Russian Arctic shelf and has the potential to alter the equilibrium of atmospheric methane, a critical greenhouse gas. Aim. To quantitatively investigate, within the context of gas transport in bottom sediments, the impact of free and bound gas within the pores on the nonlinearity of filtration flows. Methods. The model samples with filtration properties similar to those of the bottom rocks of the East Siberian Arctic shelf. During the experiments, a specific amount of gas was injected into the saturated model sample, followed by the measurement of its effective permeability during a slow decrease in pore pressure gradient. The obtained curve was interpreted within the framework of the threshold gradient model. Results. The experiments yielded curves showing the dependency of the threshold gradient magnitude on gas saturation for several samples. It was found that the threshold gradient linearly increases with the growth in gas fraction within the pore space. Already at a gas fraction of approximately 0.02, this value in the experiments reached 0.01 MPa/m, corresponding to the hydrostatic pressure gradient in water. This suggests that even areas with relatively low gas saturation may be impermeable to convective fluid flow. This possibility should be considered when creating models for bubble gas transporting through the rock layers of bottom sediments. Furthermore, the existence of gas-saturated zones with threshold gradients can significantly impact the vertical profile of pore pressure and lead to a reassessment of the depth of gas hydrate stability zones.

Keywords

• permeability
• threshold pressure gradient
• flow nonlinearity
• gas hydrate decomposition
• Arctic shelf