A Novel Model for Permeability of Hydrate-Bearing Sediments Considering Sand Particle Packing Arrangements and Hydrate Occurrence Patterns

The permeability of hydrate reservoirs constitutes a crucial seepage parameter in natural gas hydrate (NGH) exploration and development. Influenced by both the pore structure of the host sediment and the distribution pattern of hydrates, it governs fluid flow behaviour within sediment pores and determines the extraction efficiency of methane hydrates. Consequently, it is frequently selected as a key indicator for evaluating hydrate extraction performance. Existing empirical and non-empirical models (primarily based on the classical Kozeny-Carman equation, parallel capillary models, and Kozeny grain model) have enhanced the accuracy of permeability modelling predictions to varying degrees. However, they fail to account for the complexity of sediment pore structures and the diversity of hydrate occurrence patterns. Therefore, they cannot clearly identify the pore-scale influencing factors and interaction mechanisms responsible for the evolution of permeability variations. Furthermore, the presence of empirical parameters limits the large-scale application of existing models. In this work, a quantitative relationship between the permeability of hydrate-bearing sandy sediments and key factors was established through theoretical modelling based on the equivalent transformation relationship between granular media and tortuous capillaries. This yields a novel permeability model accounting for sand particle packing arrangements and hydrate occurrence patterns. Subsequently, the validity and accuracy of the newly established model are validated using test data from laboratory synthetic samples and in-situ core sediment. Finally, through sensitivity analysis of the quantitative relationships between model parameters, the primary controlling factors determining the order of magnitude of permeability of hydrate-bearing sediments and their evolution characteristics with respect to hydrate saturation were elucidated. The research findings indicate that compared to other existing models, proposed model provides favourable predictions for permeability data of both laboratory-synthesized samples and in-situ core sediment. As hydrate saturation increases, the permeability of various types of hydrate-bearing sediments exhibit non-linear decline. Moreover, variations in hydrate occurrence patterns (pore wall-coating, pore centre-occupying) lead to two distinct trends in permeability evolution. Initial porosity and sand grain size are key factors determining permeability magnitude and evolution characteristics. When sand grain size and initial porosity increased by a fold of four (from 100 μm to 400 μm) and 1.8 times (from 25.95% to 47.64%) respectively, permeability increase by approximately 16-fold and 22.7-fold respectively. The change in sand grain arrangement and the increased distance between adjacent grains resulted in enhanced porosity, leading to increases in permeability of approximately 37.0% and 39.1% respectively. The ranking of key factors governing the permeability characteristics of hydrate-bearing sediments is as follows: ϕ0 > Dsp > Sh-PCO > Sh-PWC > β > lsp. This paper establishes a model linking multiple parameters to the microscopic structure coupled with host sediment-hydrate interactions, accurately quantifying the relationship between sand grain size, initial porosity, particle packing arrangement, hydrate saturation, and permeability of hydrate-bearing sediments. This not only enhances the understanding of permeability evolution mechanisms but also improves the predictive accuracy for permeability measurements of both laboratory-synthesized samples and in-situ core sediment. It lays a theoretical foundation for fundamental physical property research and optimization of development strategies for hydrate reservoirs in the South China Sea.