A fractal model for the relative permeability prediction of hydrate-bearing sediments

Abstract

Natural gas hydrates, ice-like crystals composed of water and natural gas, are widely distributed in marine sediments along the continental margin and permafrost regions. Natural gas hydrates are of great significance as a future energy resource, and have been officially authorized as a new kind of mineral in China. For now, however, the gas production rate of all the existing technologies is far below the commercial criterion. The permeability of hydrate-bearing sediments is a critical parameter that determines the economic feasibility of gas production from hydrate deposits, and it is one of the basic parameters needed for varieties of numerical simulators. However, most of the existing theoretical models for the permeability prediction are lack of quantitative descriptions of the pore space for fluids flow. In this work, a fractal theory based theoretical model is proposed to predict the permeability of hydrate-bearing sediments. In the proposed model, the pore space for fluids flow is equivalent to a bundle of capillary tubes with their diameters obeying the fractal scaling law. A hydrate saturation dependent area fractal dimension is applied to describe how the capillary tube diameters evolve during hydrate dissociation. The theoretically predicted curves are compared with published experimental data to verify its feasibility, and sensitivity analyses on model parameters are performed. The results suggest that the proposed theoretical model can describe the hydrate saturation and pore-scale behavior dependent permeability of hydrate-bearing sediments. The maximum diameter of the pore for fluids flow which is affected by the pore-scale behavior of gas hydrate is the key influence parameter determining the permeability of hydrate-bearing sediments. The proposed model is of great potential for engineering applications.