Abstract
As a clean energy source with ample reserves, natural gas hydrate is studied extensively. However, the existing hydrate production from hydrate deposits faces many challenges, especially the uncertain mechanism of complex multiphase seepage in the sediments. The relative permeability of hydrate-bearing sediments is key to evaluating gas and water production. To study such permeability, a set of pore-scale microsimulations were carried out using the Lattice Boltzmann Method. To account for the differences between hydrate saturation and hydrate pore habit, we performed a gas-water multiphase flow simulation that combines the fluids’ fundamental properties (density ratio, viscosity ratio, and wettability). Results show that the Lattice Boltzmann Method simulation is valid compared to the pore network simulation and analysis models. In gas and water multiphase flow systems, the viscous coupling effect permits water molecules to block gas flow severely due to viscosity differences. In hydrate-bearing sediments, as hydrate saturation increases, the water saturation S w between the continuous and discontinuous gas phase decreases from 0.45 to 0.30 while hydrate saturation increases from 0.2 to 0.6. Besides, the residual water and gas increased, and the capillary pressure increased. Moreover, the seepage of gas and water became more tedious, resulting in decreased relative permeability. Compared with different hydrate pore habits, pore-filling thins the pores, restricting the gas flow than the grain-coating. However, hydrate pore habit barely affects water relative permeability.
Highlights
Natural gas hydrate (NGH) is an ice-like, crystalline solid in which gas molecules are trapped within water molecules at low temperatures and high pressures [1, 2]
The simulation results show the evolution of gas/water relative permeability, and their phase distributions vary with water saturation under a certain hydrate saturation and hydrate pore habit
The black spot is the pore network simulation reported by Mahabadi et al [17], the green spot is the analysis model studied by Singh et al [23], and the solid red line is from the current research using the Lattice Boltzmann Method (LBM)
Summary
Natural gas hydrate (NGH) is an ice-like, crystalline solid in which gas molecules are trapped within water molecules at low temperatures and high pressures [1, 2]. Existing hydrate-bearing sediments (HBS) are widely distributed in continental margins and permafrost [3]. It has been estimated that the total carbon amount stored in the hydrates is at the same order of magnitude as all fossil fuels combined [4]. NGH is viewed as a potential energy resource in the future. Hydrate production is a complex process involving phase transition, multiphase flow, mass transfer, heat transfer, and reservoir deformation [5]. Investigations into safe and efficient hydrate production from HBS have been widely carried out
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