Permeability of marine gas hydrate-bearing sediments considering effective stress and particle size distribution: A theoretical model

Due to heterogeneity of solid particle size, occurrence of gas hydrates in pore space, evolution of pore structures influenced by the changes in effective stress and temperature, the accurate determination of permeability of gas hydrate-bearing sediments (GHBS) remains a great challenge, leading to the unclear flow law in marine gas hydrate reservoirs. In this work, a theoretical permeability model considering effective stress, local thermal stimulation, hydrate saturation, and its occurrence pattern is proposed to predict the permeability for GHBS with heterogeneous particle size distribution. Specifically, by inputting the particle size distribution curve, initial hydrate saturation and relative density of GHBS, the natural-state pore diameter distribution curves of GHBS can be calculated. Then, by combining the calculated pore diameter distribution curve, thermal expansion theory, Hooke's law and bi-fractal theory, the theoretical permeability model for GHBS is established. Subsequently, the proposed permeability model is thoroughly validated against groups of available experimental data, which indicates the validity of this model. Furthermore, effects of several crucial parameters (e.g., particle size distribution, hydrate saturation, initial porosity, effective stress, and temperature increment, etc.) on the permeability of GHBS are quantitatively revealed through parameter sensitivity analysis. Compared with former permeability models, the permeability prediction accuracy of the proposed model is improved by 5-13%. This model can not only provide accurate permeability predictions of GHBS with heterogeneous particle size distribution, but also reveal more mechanisms affecting the process of fluid flow in GHBS, helping to promote the safe, efficient and stable exploitation of hydrate resources.