本研究目的是要研究天然氣與二氧化碳混合之壓力-體積-溫度或相態變化行為。並研究在凝結油氣層注入二氧化碳(或二氧化碳沖排)所能增產之油氣或凝結油。
本研究主要分為基礎模式研究及現場案例分析兩部分，在基礎模式研究中主要進行：(1)特定天然氣PVT資料分析，(2)不同比例的二氧化碳與特定天然氣混合之PVT變化分析，(3)基礎一維線性二氧化碳沖排模式之增進油氣採收研究。在現場案例分析中主要進行：(1)現場天然氣PVT資料分析，(2)不同比例的二氧化碳與現場天然氣混合之PVT變化分析，(3)現場三維數值模式二氧化碳沖排之增進油氣採收研究。
本研究所獲得的主要結果為：(1)當二氧化碳混合到天然氣中，在P-T相圖之兩相區域會內縮，混合氣體之臨界壓力與臨界溫度將會降低，有部分的凝結油會再蒸發而變成氣相，二氧化碳注入的比例增加，P-T相圖的內縮程度加大。(2)由本研究二氧化碳沖排機制之基礎研究可知，地層因生產造成壓力下降至露點壓力以下，導致地層凝結油生成，造成油氣生產效率降低。利用二氧化碳沖排法可有效增加天然氣層之氣體與凝結油採收。(3)在二氧化碳沖排法增進油氣採收之案例研究中，本研究選定已經進入生產末期之Y氣田為目標研究礦區，利用一口二氧化碳注入井進行二氧化碳沖排而研究增進油氣採收。在本案例中，凝結油氣層之初級採收階段，天然氣最終採收率約為77%，而凝結油最終採收率約為43%；在二氧化碳沖排階段，設定不同的二氧化碳生產監測準則下，模擬計算可得之天然氣增加的採收率為2%~10%，凝結油之增加的採收率為0.6%~8%。

he purpose of this research is to study PVT behavior of natural gas before and after mixing with the injection of CO2, and to investigate improved gas recovery for a specific gas reservoir though carbon dioxide flooding.
A theortical study with a 1-D linear model and a case study with a 3-D model are conducted in this study.The major works in the theoretical study are: (1) studying PVT behavior of a natural gas sample, (2) studying the PVT behavior of natural gas mixing with different portions of CO2, (3) studying production characteristics during carbon dioxide flooding in 1-D linear model, using simulation method. The major works in the case study are: (1) studying the PVT behavior of a nature gas sample from the production well of a specific gas reservoir, (2) studying the PVT behavior of the sampled natural gas mixing with different portions of CO2, (3) studying the improved gas recovery of the specific reservoir from the carbon dioxide flooding, using reservoir simulation method.
The results obtained from the study are: (1) when the carbon dioxide is mixed with nature gas, the critical pressure and the critical temperature of the mixture have decreased. The two-phase envelop in P-T diagram has shrank, and partial condensate revaporizes from liquid (oil) phase to gas phase. The more percentage of carbon dioxide mixed with natural gas, the less the area of the two-phase region in the P-T diagram. (2) during production period, the condensate in the reservoir drops when reservoir pressure is lower than dew point pressure, thus the performance of gas production decreases. In the carbon dioxide flooding study of 1-D linear model, the results show that the injected CO2 displaces the movable natural gas and repressurizes the reservoir. Some of the condensate has revaporized because of injecting CO2. The extra gas and oil recovery are 21% and 5% respectively, when carbon dioxide flooding in 1-D model. (3)From the results of case study of a depleted gas reservoir in Y- gas field, to study the improved gas recovery from carbon dioxide flooding in one CO2 injection well, the gas and oil (condensate) primary recovery are 77% and 43% respectively. And extra gas recovery is 2% to 10%, extra oil (condensate) recovery is 0.6% to 8% obtain rate if the carbon dioxide flooding has different carbon dioxide monitoring criteria of 5 mol% to 80 mol% CO2.

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