Unified models for water permeability in hydrate-bearing sandy soil considering pore morphology evolution

Abstract

The water permeability of hydrate-bearing sediments is of paramount importance for assessing the exploitation efficiency of methane hydrate from reservoirs. It is largely influenced by the interrelated factors of hydrate morphology and saturation. Experimental results revealed that as hydrate saturation increases, the pore morphology shifts from primarily grain-coating to predominantly pore-filling, but this coupling effect between hydrate morphology and saturation on water permeability is often overlooked in existing models. This study aims to model the water permeability of hydrate-bearing sandy soils, considering the evolution of pore morphology with changing hydrate saturation. An eccentric annulus is used to depict the pore structure of pore-filling hydrate, in contrast to the conventional unrealistic concentric annulus geometry. Two new models to describe water relative permeability were derived, each incorporating only a single parameter, assuming that grain-coating and pore-filling hydrates grow at different rates either sequentially or simultaneously. These models were validated using a dataset comprising 29 hydrate-bearing soils, with the hydrate saturations ranging from approximately 0 to 0.9. Comparison between model predictions and experimental data confirmed the good performance of both water permeability models, with low RMSE, MAE and GMV values of around 0.05, 0.03 and 1.28, respectively. Both models were further improved by correlating the two parameters with porosity data, which could ensure a rapid estimation of relative permeability based solely on porosity data without requiring any fitting parameters. Results in this study provide a novel perspective for understanding the impact of hydrate evolution on permeability reduction in hydrate-bearing soils.