熱採收法於第三型天然氣水合物儲集層之數值模擬研究

• Main Authors: 黃奕鈞, Huang, Yi-Jyun
• Other Authors: 資源工程學系, 謝秉志, Hsieh, Bieng-Zih
• Format: Thesis
• Language: English
• Published: 2017
• Subjects: 天然氣水合物; 熱採收法; 數值模擬; Gas hydrate; Thermal recovery; Numerical simulation; Ice; permafrost
• Online Access: http://ir.lib.ncku.edu.tw/handle/987654321/174172; http://ir.lib.ncku.edu.tw/bitstream/987654321/174172/1/index.html

天然氣水合物是一種固體、冰晶狀的化合物，由天然氣分子包覆在水分子的晶格中。天然氣水合物存在環境需求為高壓低溫的狀態下，因此能在永凍層、深海沉積層發現到天然氣水合物。第三型天然氣水合物儲集層為單一水合物層被上下蓋岩層封閉，下方並無自由氣層與水層存在。 一旦溫壓環境超出相圖的平衡區，水合物就會發生溶解，釋放出氣體分子與水。因此誘使水合物分解的方法有降壓法、熱採法以及化學藥劑注入法。雖然目前降壓法是公認最有潛力的生產方法，然而，在第三型水合物儲集層中固態水合物飽和度較高，地層其他可流動流體(水)的飽和度就較低。地層流動性越低，壓力傳導就越不容易，進而使得降壓法的效率降低。因此，本研究目的為應用熱採收法在第三型水合物儲集層中，探討不同熱源供應下的熔解行為與機制，並比較各熱源下的生產效率。 本研究使用的是由CMG公司開發的STARS模擬器，STARS模擬器具有耦合熱力學、多相流體流動、岩石力學與地球化學的能力。並經過前人研究，驗證STARS具備能模擬天然氣水合物地層的技術。 本研究中，一對800公尺長的水平井設計在70%的水合物地層中。不同熱源供應設計為電熱設施、熱蒸氣注入(300 °C)、溫水注入(85 °C). 在生產初期，由於注入流體能直接貫入水合物區，因此熱蒸氣、溫水注入法的產氣量較高。然而，當注入流體前鋒貫穿至生產井，使得注入流體迅速產出，導致熱流在地層停滯時間縮短，進而影響熔解驅動力，最終氣產量較不樂觀。計算採收因子可得電熱設施為32.22%、熱蒸氣注入法為14.2%以及溫水注入法為26.04%。應用三對水平井在四方圈合案例上，採用電熱設施與40%壓降可得15億立方公尺的天然氣量，相當於38.66%採收因子。 Gas hydrates are solid ice-like clathrate compounds in which molecules of gas (primarily methane) are trapped within the crystal structure of molecules of water Gas hydrates form and exist when the water pressure is high and the temperature is low Consequently gas hydrates can be found in seabeds permafrost and deep oceanic sediment A Class-3 gas hydrate reservoir has no free gas or water layer below only a single hydrate-bearing layer with an upper and lower burden Once the pressure or temperature is outside the gas hydrate equilibrium zone the gas hydrate dissociates There are three methods for inducing gas hydrate dissociation: depressurization thermal recovery and inhibition Currently depressurization is the most promising recovery technique; its driving force depends upon propagating a pressure disturbance However in a Class-3 gas hydrate reservoir with high gas hydrate saturation relatively low effective permeability caused by a low initial mobile phase that can cause poor depressurization The purpose of this study is to apply the thermal method in the Class-3 gas hydrate reservoir The present study therefore used three different heat sources (steam injection warm water injection and a heater) and then compared the performances of these different mechanisms of heat transfer dissociation behaviors and production The STARS numerical thermal and advanced processes reservoir simulator was used (Computer Modelling Group Ltd ) in this study STARS couples thermal conduction multiphase fluid flow rock mechanics and geochemistry and has been validated for gas hydrate simulation by the National Energy Technology Laboratory (NETL) USA Initial hydrate saturation was set at 70% and 800 meters of dual horizontal wells were designed in the center The upper well was the producer and the lower well was the injector The three different heat sources used were the Heater Method steam injection (SI) at 300 �C and warm water injection (WWI) at 85 �C Injection rates of enthalpy were equal SI and WWI had higher gas production rates in the early stage because of the advection of hot fluids (more efficient heat transfer mechanism than radiation) but both had problems when hot fluids from the injector were recovered in the producer Once this breakthrough occurred the driving force weakened The Heater Method yielded a continuously stable gas production rate and the dissociation front was more like the layer-up The recovery factors were 32 22% for the Heater Method 14 2% for the SI method and 26 04% for the WWI method In a case study of Four-Way-Closure-Ridge three pairs of horizontal wells were used Using the Heater Method and with a 40% pressure decline the recovery factor was 38 66% (1 522 billion standard cubic meters [BSCM])