Permeability Model for Hydrate-Bearing Sediments with Various Hydrate Pore Morphologies under Cyclic Loading and Local Thermal Stimulation Conditions

Abstract

Permeability evolution within the reservoir is a predominant factor governing the gas production rate and efficiency of the natural gas hydrate. During the development of natural gas hydrates, the properties and pore structure of hydrate-bearing sediments (HBSs) undergo significant alterations under dynamic loads, exhibiting distinct characteristics compared to those under static loads. Therefore, investigating the permeability evolution of HBSs under dynamic loads is fundamental to safe and efficient development. In this study, a theoretical model for HBS permeability under cyclic loading and local thermal stimulation is established, considering heterogeneous particle arrangement and diverse hydrate pore morphologies (pore-filling, wall-coating, grain-cementing hydrates, and their hybrids). Validation against publicly available experimental data confirms both the validity and accuracy of the proposed model. The results indicate that cyclic loading induces irreversible permeability damage, which exhibits a gradual decline with continued cycling. Moreover, an increase in the initial hydrate saturation leads to a decrease in the proportion of pore-filling hydrate in HBS pores, while that of grain-cementing hydrate increases. Additionally, the correlations between permeability and key physical parameters (such as effective stress, temperature increment, compression index, swelling index, and thermal expansion coefficient) during elastoplastic deformation and local thermal stimulation are quantitatively analyzed based on sensitivity analysis. The proposed model facilitates accurate prediction of HBS permeability evolution during exploitation and establishes a theoretical foundation for the efficient extraction of natural gas hydrates.