In this research, the variations of critical physical fields in marine NGH-bearing layers during depressurization are numerical simulated. Considering the mutual effects, a set of coupled model is established to describe the heat and mass transfer in layers, and a trial-calculation case is carried out using COMSOL Multiphysics. The results show that the mass transfer in the porous medium may be always faster than the heat transfer due to depressurization. It is also found that the phase transitions closer to wellbore may have a negative impact on pressure transfer. These findings have reference value for promoting hydrate commercial exploitation.
Natural gas hydrate (NGH) is an alternative green energy attracting attentions all over the world (Makogon, 1982, 2007). When the ambient conditions are appropriate, especially within the high enough pressure and the low enough temperature, water molecules can be connected to each other by hydrogen bonds, constructing cages to capture large amounts of small molecular gas such as methane (dominate component with proportion of 98% or more), ethane and etc (Khurana, 2017). Then gas hydrate is generated. Besides the ambient conditions, the normal formation of NGH-bearing reservoirs in nature also requires sufficient hydrocarbon generation and trap geological structure (Paull, 2001). According to these formation requirements, it is speculated that gas hydrate can exist under the surface of both 30% land and 70% sea, mainly concentrated in polar tundra and submarine continental slope (Michael, 2003). Up to now, hundreds of NGH-bearing layers have been explored through various NGH Exploration and Development Research Programs, and the scientific evaluation indicates that the potential reserve of this clean energy resource can afford human society utilization for more than 1000 years (Zhang, 2012).
Series of development methods, including depressurization (DEP), thermal stimulation (TS), chemical inhibitor injection (CI), CO2 replacement (CR) and solid fluidization (SF) have been gradually proposed to extract hydrate buried underground (Makogon, 1966, Li, 2019). The merits of each method are listed in Tab.1. Using one or more of them, Russia, America, Canada, Japan and China have already carried out several trial-production projects at Messoyakha gas field (Shao, 2016), Mallik tundra (Dallimore, 2002), Nankai Through (Takahashi & Tsuji, 2005) and South China Sea (Ye, 2020), obtaining the flammable gas fixed in crystal cages and confirming the feasibility of hydrate extraction.
