天然氣水合物資源量大且分佈於全世界各地，也是一種潔淨的非傳統油氣資源，降壓生產法(Depressurization)是開採天然氣水合物的其中一種方法，被視為是有效可行的方法。至今已經有許多小尺度的實驗利用降壓生產法觀察天然氣水合物的分解機制並進一步觀察其現象，然而，實驗通常需要耗費長時間使水合物生成並觀察分解行為。在經過比對驗證的條件下，數值模擬法是一種快速且方便的工具可以用來預測水合物的分解行為。因此，本研究的目的為利用數值模擬建立二維圓柱模型模擬水合物在實驗室尺度下的降壓分解實驗，並探討各項參數對於水合物分解行為的影響。
本研究採用國外水合物降壓分解實驗案例，仿照實驗反應釜建立數值模擬模型，透過敏感度分析調查各個不確定參數對水合物分解行為的影響，接著利用歷史調諧找出最佳的擬合參數。並且將本研究結果與前人研究利用TOUGH+HYDRATE模擬器結果比較與討論。最後，將此數值模擬模組應用於台灣科技大學研究團隊的水合物分解實驗。
所獲得的主要結論為：(1) 建立天然氣水合物之數值模擬模型，且利用國外實驗案例進行數值模擬計算與實驗數據擬合，證明其可行性。(2) 水合物的化學反應參數(活化能、反應速率常數)對反應速率的影響劇烈，而流動參數(相對滲透率、不可移棲氣飽和度)則影響氣體流動行為。(3) 與前人模擬結果比較顯示，本研究計算之溫度壓力結果較前人結果平滑，但兩者在溫度結果皆略高於實驗資料。(4) 在台科大實驗中，即使水合物不是在孔隙介質中分解，可以藉由某些假設使數值模擬計算結果與實驗結果吻合。

Natural gas hydrates are clean and vast unconventional resources that exist around the world. Depressurization is one of their production mechanisms and is considered an effective way to produce natural gas from hydrates. To date, some experiments have been conducted in other laboratories to investigate the mechanism of hydrate dissociation. However, it is worth having the comparative results from a simulation study. The purpose of this study is to develop a two-dimensional axisymmetric numerical model to simulate the behavior of hydrate dissociation in a lab-scale depressurization experiment. This work also investigates the effects of the flow characteristics, hydrate reaction parameters, and basic properties of hydrate.
In this study, the CMG-STARS numerical model is set up to simulate a lab-scale depressurization experiment chosen from the literature. The numerical grids are discretized based on the vessel size used in the laboratory. To understand which parameters are likely to seriously affect the dissociation behavior, we perform a sensitive analysis of the flow properties, hydrate reaction parameters, and thermal properties. The numerical simulation runs were done to match the experimental data by adjusting the sensitive parameters. Furthermore, the simulation results from CMG-STARS and from TOUGH+HTDRATE, which has been used in other studies, are compared. Finally, the verified module is applied to an experiment conducted by a research team from National Taiwan University of Science and Technology (NTUST).
The major findings from this study are: (1) The numerical model is developed for simulating a depressurization experiment from the literature, and it is verified by matching the experimental data. (2) Hydrate reaction parameters such as activation energy and intrinsic rate constant have a significant effect on the dissociation rate. Moreover, the flow parameters, e.g., relative permeability and irreducible gas saturation, are crucial for gas production. (3) The numerical results are compared with the results of another numerical simulation from the literature. Both works have a minor experimental data difference in the temperature profile but our study shows smoother results. (4) Although the hydrate formation and dissociation are done in a hollow vessel without porous media filled in, the numerical results still can model the behavior on the assumption that methane, water, and hydrate are distributed uniformly in the vessel.

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