A permeability model for gas hydrate-bearing sediments considering the changes in hydrate occurring habits

The permeability of gas hydrate-bearing sediments (GHBS) is an important factor affecting the gas-liquid transport characteristics during the processes of hydrate formation and decomposition, and is often selected as an indicator for evaluating the extraction capacity of natural gas hydrates. The changes in hydrate occurring habits and hydrate saturation in GHBS pores significantly influence GHBS permeability. Existing permeability models are mostly based on a single hydrate occurring habit, making it difficult to consider the impact of changes in hydrate occurring habits on GHBS permeability. Based on the parallel capillary tube models, considering the influence of the varying of hydrate occurring habits on pore structure of GHBS, a mixed occurring habit with grain-coating and pore-filling coexisting is proposed. A logical function with two parameters is proposed to describe how hydrate occurring habits change with saturation, and a permeability model of GHBS considering the variation of hydrate occurring habits is established. The correctness of the model is verified by comparing it with measurement data obtained from laboratory and in-situ permeability tests, and the effectiveness of the model is analyzed by comparing it with existing mathematical models. The results indicate that changes in hydrate saturation during hydrate formation usually lead to changes in hydrate occurring habits, affecting the trend of the GHBS permeability with hydrate saturation. The changes in hydrate occurring habits vary under different formation conditions, and the main characteristics of the changing process are reflected in the critical hydrate saturation corresponding to the transformation of the main occurring habit, as well as the direction and trend of changes. Compared to the existing models, this model can capture the changing characteristics of permeability when the hydrate occurring habit changes, and can better predict measurement data from both laboratory and in-situ permeability tests of GHBS.