Water retention model for hydrate-bearing sediments considering pore structure evolution

Abstract

The water retention behaviour of hydrate-bearing sediments (HBS) is crucial for evaluating gas production efficiency and sediment response during methane hydrate exploitation from reservoirs. Effects of hydrate on the pore size distribution (PSD) are not explicitly considered in existing models, although the PSD governs the water retention behaviour of HBS. Nuclear magnetic resonance (NMR) data reveal that, with increasing hydrate saturation, the porosity fraction for larger pores decreases significantly, whereas the porosity fraction of smaller pores changes only slightly. Based on these observations, this study proposed a new equation for modelling the PSD evolution with increasing hydrate saturation. Subsequently, by incorporating this PSD evolution equation into the van Genuchten model, a new model was developed to describe the constitutive relationship between water saturation and suction across a wide range of hydrate saturation. Similarly, the proposed PSD evolution equation was applied to other water retention functions, such as those proposed by Fredlund–Xing, to simulate the water retention behaviour of HBS. Model validation against experimental data shows strong agreement between the calculated and measured results. The model successfully captures the key characteristics of water retention in HBS, including variations in air-entry pressure, adsorption/desorption rates, and residual water saturation with hydrate saturation.