Research advances in gas-water relative permeability of hydrate-bearing sediments

Gas-water relative permeability is a key parameter in the trial production of natural gas hydrate. Measurement and evaluation of the parameter of the hydrate reservoir is essential for enhancing gas production efficiency and realizing the industrialization of natural gas hydrate production. This paper provides a review on the research progress in the relative permeability of hydrate-bearing sediments. It is summarized from three aspects, i.e. experimental measurement, numerical simulation, and model establishment. It is found that the unsteady-state method is widely employed in the permeability measurements for hydrate-bearing sediment. The relative permeability curve suggests that higher hydrate saturation will cause lower water relative permeability at a given water saturation, but the variation of gas relative permeability is complicated; the pore-structure of the sediment changes along with the hydrate saturation, which further results in the changes of gas-water relative permeability. Numerical simulation mostly calculates relative permeability with a pore network model or a water retention curve to explore hydrate growth habits and pore-filling characteristics and the effect of various factors such as particle size, hydrate saturation, wettability, and interface tension and their influence on gas-water relative permeability. A newly developed relative permeability model for hydrate-bearing media considering the influences of the capillarity and pore-size distribution (referred to as RPHCP) is introduced in this work, which shows obvious advantages in simulating the multi-phase flow of hydrate-bearing sediments and explaining the changes of hydrate saturation compared to other models. RPHCP model is recommended to overcome the defect of the high cost of computation of RPHCP model to achieve precise modeling of multiphase flow in hydrate-bearing sediments.