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Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

Simulation for P-T-Sh Variations during Marine NGH Depressurization by COMSOLYe Chen1,2, Ning He1, Hui Wang1, Xu Li1, Yuan Yao1 and Yonghai Gao2
1.Offshore Oil Engineering Co. Ltd, Tianjin, China
2. China University of Petroleum (East China), Qingdao, Shandong, China

ABSTRACT

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 ormore of them, Russia, America, Canada, Japan and China have alreadycarried 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 feasibilityof hydrate extraction.

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 todescribe the heat and mass transfer in layers, and a trial-calculation caseis carried out using COMSOL Multiphysics. The results show that themass transfer in the porous medium may be always faster than the heattransfer due to depressurization. It is also found that the phasetransitions closer to wellbore may have a negative impact on pressuretransfer. These findings have reference value for promoting hydratecommercial exploitation.

Table 1. Merits of hydrate development methods
MethodAdvantage

KEY WORDS: numerical simulation, natural gas hydrate, pressure
field, temperature field, saturation field, depressurization, COMSOLINTRODUCTION
Natural gas hydrate (NGH) is an alternative green energy attracting
attentions all over the world (Makogon, 1982, 2007). When the ambientconditions are appropriate, especially within the high enough pressure
and the low enough temperature, water molecules can be connected toeach other by hydrogen bonds, constructing cages to capture large
amounts of small molecular gas such as methane (dominate componentwith proportion of 98% or more), ethane and etc (Khurana, 2017). Thengas hydrate is generated. Besides the ambient conditions, the normal
formation of NGH-bearing reservoirs in nature also requires sufficienthydrocarbon 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 havebeen explored through various NGH Exploration and Development
Research Programs, and the scientific evaluation indicates that thepotential reserve of this clean energy resource can afford human society
utilization for more than 1000 years (Zhang, 2012).

Limit

DEP

Easy, cheap, less demand

SlowTS

Fast, safe

Huge energy loss

CI

Convenient for adjustment

High cost, pollution

CR

Greenhouse gas utilization

Immature technologySF

Low engineering risk

Immature technology

Generally, the daily average gas production rates of marine NGHbearing trial-production cases are much higher than those of onshore
NGH-bearing trial production cases (Wu, 2021). What’s more, thepotential hydrate reserves are also huge in deep sea (Michael, 2016).
Therefore, it is believed that the sea will be one of the main fronts forgas hydrate trial-production in the short term. Specific details of few
marine NGH-bearing trial-production cases are listed in Tab.2, showingthat depressurization, the easiest and cheapest development methodwith least demands, is the most popular one or the core one adopted foroffshore operation (Zhang, 2014).
However, even the maximum daily gas production rate of all marineNGH-bearing trial-production cases is still far from the threshold ofcommercial extraction. Since the offshore operation is always harder toexecute, requiring more auxiliary equipment and technology supports,
current production amount of gas obtained from NGH cannot afford theenormous upfront input costs. In addition, certain exploration modes onland cannot be completely transferred to offshore due to technical

Series of development methods, including depressurization (DEP),
thermal stimulation (TS), chemical inhibitor injection (CI), CO2597