Natural gas hydrate is considered to be a huge, efficient and clean energy. Understanding the mechanism of fluid flow in hydrate-bearing sediments (HBS) plays an important role in guiding hydrate production. This paper constructs the 3D pore structure of HBS considering the influencing factors such as porosity, particle size and pore throat ratio, including theoretical model and natural sediments. The multi-point geostatistical method is used to duplicate the proportion and spatial distribution relationship between phases, and the porous media of HBS under different hydrate growth habits and hydrate saturation is established. Finally, the Lattice Boltzmann Method (LBM) is used to simulate the pore scale seepage.
Natural gas hydrate (gas hydrate for short) is an ice and crystalline cage compound formed by water and natural gas under high pressure and low temperature (Sloan and Carolyn, 2008), which is mainly distributed in marine sediments and permafrost area. The growth habit of gas hydrate in the pore space has the following four modes: coating, pore-filling, skeleton-supported, and particle-contact (Qin et al., 2015; Boswell and Collett, 2011; Lee and Collett, 2009). According to researchers’ estimation, the total resources of gas hydrate in the world converted into methane gas are about 1.8×1016~2.1×1016 m3, with organic carbon reserves equivalent to twice of the world's fossil fuels discovered (coal, oil and natural gas, etc.) (Paull and Dillon, 2001). The amount of marine gas hydrate is very huge, more than 100 times that of terrestrial permafrost (Paull and Dillon, 2001). Therefore, gas hydrate, especially marine gas hydrate, is considered to be a promising energy in the 21st century.
During hydrate extraction, the permeability of HBS is not only influenced by hydrate growth habits and saturation, but also related to the size, shape and distribution of particles and pores, as well as porosity. Absolute permeability is used to measure the fluid flow capacity through the porous medium, which is determined by the connectivity of the medium and the size of the pores. The effect of different hydrate saturation and growth habits on the pore-scale permeability of HBS can be expressed in terms of the normalized permeability, defined by the ratio of the permeability of a porous medium containing hydrates to the permeability of a hydrate-free medium. The normalized permeability can be estimated in the following ways: (i) laboratory experiments based on synthetic HBS (Masuda et al., 1997; Kumar et al., 2010; Seol and Kneafsey, 2011; Ai et al., 2017; Choi et al., 2020); (ii) in situ experiments using remodeled hydrate-bearing samples (Nimblett and Ruppel, 2003; Johnson et al., 2011; Santamarina et al., 2010); (iii) in situ field investigations (Kleinberg, 2003; Collett et al., 2011; Song et al., 2013); (iv) numerical simulations using synthesized or conceptual porous media (Sakamoto et al., 2010; Dai and Seol, 2014; Wang et al., 2015; Kang et al., 2016; Chen et al., 2018; Hou et al., 2018; Ji et al., 2019). In the absence of reliable experimental data and equipment for in situ measurements, numerical methods are more often used to study the permeability properties during hydrate generation and decomposition.
