水合物為一種固態的冰狀結晶，水合物分子由一個小型氣體分子(如甲烷及二氧化碳)以及一定比例的水分子組成，其中水分子將會形成晶格狀牢籠，將氣體分子包覆在其中。氣體水合物在高壓、低溫的環境下形成，多存在於高緯度永凍土層以及海洋中的沉積岩層中。根據台灣長年的探勘研究，在台灣西南海域也有天然氣水合物資源賦存。在天然氣儲集層類型中，第一型天然氣水合物儲集層由於在水合物層下方有自由氣層的存在，因此被認為是最具經濟開採價值的生產標的。大多數的海域水合物資源蘊藏在未完全壓密、膠結的鬆軟沉積層中，因此海域天然氣水合物開採計畫可能有造成海床下陷及地層構造破壞等地質災害之疑慮。本研究係想藉由在第一型水合物地層下方之自由氣層注入二氧化碳以達激勵天然氣生產、維持地層壓力、防止水合物層融解而造成蓋岩阻隔能力影響及二氧化碳封存等多重目的。本研究根據台灣西南海域的地層條件，建立一個典型第一型水合物地層模型，並採用CMG STARS油層模擬軟體進行生產模擬研究。 STARS具有多相流體流動、熱力學、岩石力學及化學反應等數值模擬能力，能夠模擬水合物開採的複雜行為，並對水合物礦區開採進行岩石力學評估。本研究對二氧化碳注入策略進行測試及討論，結果顯示二氧化碳的突破時間控制對第一型水合物地層生產表現十分重要，注入地層的二氧化碳越晚達到生產井則能夠產出越多的天然氣。在本研究的測試結果中可以發現延後二氧化碳開始注入的時間以及降低二氧化碳注入壓力對累積天然氣產量有正向的影響，但卻可能造成地層沉陷以及水合物融解的狀況。放寬生產井的二氧化碳含量限制能夠有助於在低風險的條件下延長生產時間並增加產量，但任意放寬限制可能損害礦區生產獲利。

Gas hydrates are solid ice-like component composed of water molecules and small size gas molecules, which exist in the condition of high pressure and low temperature. Due to the stable condition of hydrate, it is an unconventional gas resources that widely spread over deep oceanic sediments and permafrost regions. According to the exploration, there are also gas hydrate resources in southwestern Taiwan. There are 3 classes of gas hydrate deposits. Because of the existent of free gas zone beneath the hydrate layer, the class 1 gas hydrate is considered to be the most profitable production target. Most of the marine hydrate resources are found in unconsolidated sedimentary formation. Therefore, there is a risk of potential geohazard caused by seafloor subsidence and hydrate dissociation during the hydrate deposit production. Targeting at the marine class 1 hydrate deposits, the purpose of this study is to establish a safety operation strategy to produce the gas resource from the free gas zone. The CO2 EGR strategy is applied to stabilize the reservoir pressure preventing seafloor subsidence during the gas production. CMG STARS simulator is used to calculate the reservoir production and the geomechanics behavior of the formation. In this study, different operation strategies are tested and discussed to figure out the relevance between the operation methods and the production performances. The results suggest that the CO2¬ breakthrough control is essential to the application of CO2 EGR strategy. Later CO2¬ breakthrough results in greater gas production. By applying CO2 injection delay or lower injection pressure, the production can be extended and result in higher production with more severe situation of formation subsidence and hydrate dissociation. Allowing more CO2 content in produced gas can benefit the overall production without subsidence.

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