岩石力學機制對於天然氣水合物產層降壓生產之影響－台灣西南海域四方圈合海脊水合物好景區案例研究

為了使台灣能達到低碳排放的目標，未來的能源政策主要仰賴潔淨能源，其中包含再生能源與低碳能源。除了發展再生能源(如太陽能、風能)之外，台灣將增加天然氣發電的使用來達到低碳排放的目標。在台灣西南海域曾發現的大量天然氣水合物資源將成為台灣能源自主的新契機。因此，近年來台灣政府也在水合物的探勘調查上做了許多準備工作。天然氣水合物的生產方式對於水合物蘊藏量的評估是個很重要的議題。迄今，降壓生產是普遍被認可最具工程經濟開採天然氣水合物的方式。在此工法中，地層的壓降程度是造成水合物分解效率好壞的重要指標，但是當地層孔隙壓力降低使水合物分解時，也會造成地層岩石受到更多的應力並可能發生地層形變。本研究的主要目的是以台灣西南海域四方圈合海脊為研究範圍，研究第三型水合物礦區利用降壓生產時產生的岩石力學影響。研究使用的數值模擬軟體為STARS，STARS 是一套可模擬多相流體流動、岩石力學反應、熱對流及化學反應的油層模擬軟體。該軟體也經由美國國家科學實驗室認可，可以模擬天然氣水合物的形成與分解。本研究的範圍是台灣西南海域四方圈合海脊，此天然氣水合物礦區是由擬海底反射 (Bottom Simulating Reflectors, BSRs) 所發現的。研究所使用的地層參數主要類比自鄰近海域資料及日本水合物生產案例資料。由於模擬使用的地質資料及岩石力學資料由類比而得，因此在本研究中也將這些參數進行參數敏感度分析，以了解各種地層參數對於水合物生產開發的影響。在不同降壓程度的案例中，當壓力降為60%、70%或75%時，所模擬出的採收率為2.5%、13.5%及20.5%。當地層水合物初始飽和度為30%、40%、50%或60%時，所得到的採收率也有所差異，結果分別是35.1%、25.9%、24.5%及16.7%。研究結果中也指出，當地層滲透率越佳時，所得到的採收率會越高；毛細壓力對於採收率的影響在這個研究中較不明顯；生產井的位置則是因為重力因素，所以設置在高區的生產井會有較好的採收效果。在岩石力學影響的結果中，可以觀察到岩石力學機制對於水合物的生產量有所影響。當地層楊氏模數較大或泊松比較小時，在海床表面上所反映的沉陷量較少。在本研究案例中，在四方圈合海脊案例中觀察到的沉陷量約為1 公尺。

關鍵字

敏感度分析；沉陷量；擬海底反射

並列摘要

The future energy policy of Taiwan will rely heavily on the clean energy, including renewable energy and low-carbon energy, to meet the target of mitigating CO_2 emission. In addition to developing the renewable energies like solar and wind resources, Taiwan will increase the natural gas consumption to obtain enough electrical power with low-carbon emission. The vast resources of gas hydrates recognized in southwestern offshore Taiwan make a great opportunity for Taiwan to have its own energy resources in the future. Therefore, Taiwan put significant efforts on the evaluation of gas hydrate reserves recently. Production behavior of natural gas dissociated from gas hydrate deposits is an important issue to the hydrate reserves evaluation. The depressurization method is a useful engineering recovery method for gas production from a class-3 type hydrate deposit. The dissociation efficiency will be affected by the pressure drawdown disturbance. However, when the pore pressure of hydrate deposits is depressurized for gas production, the rock matrix will surfer more stresses and the formation deformation might be occurred. The purpose of this study was to investigate the effects of geomechanical mechanism on the gas production from a class-3 hydrate deposit using depressurization method. The case of a class-3 type hydrate deposit of Four-Way-Closure Ridge was studied. In this study a reservoir simulator, STARS, was used. STARS is a multiphase flow, heat transfer, geo-chemical and geo-mechanical mechanisms coupling simulator which is capable to simulate the dissociation/reformation of gas hydrate and the deformation of hydrate reservoirs and overburdens. The simulating ability of STARTS simulator was validated by duplicating the hydrate comparison projects of National Energy Technology Lab. The study target, Four-Way-Closure (FWC) Ridge hydrate deposit, was discovered by the bottom simulating reflectors (BSRs). The geological parameters were collected from the geological and geophysical studies and the geo-mechanical data were analogized from Japan's hydrate production case. The sensitivity analyses on geological and geo-mechanical parameters were conducted in this study. The case of different pressure drop showed that the recovery factor (RF) was 2.5%, 13.5% and 20.5% when the pressure drop of 60%, 70% and 75% from the initial reservoir pressure was used respectively. Based on the case of pressure drop of 75% (from the initial reservoir pressure), the RF was 35.1%, 25.9%, 20.5% and 16.7% when the initial hydrate saturation of 30%, 40%, 50% and 60% was assumed respectively. The greater formation permeability is, the better gas recovery will be. The capillary pressure had a minor affection on the gas production in this case study. The best well location was at the upper layer because of the gravity effect. For the effects of the geo-mechanics, we observed that the rock mechanisms had impacts on the final cumulative gas production. With the larger the Young's Modulus or the smaller the Poisson's Ratio, the smaller the subsidence on the seabed can be. Our simulation results showed that the seabed subsidence in FWC Ridge was about 1 meter during the production period.

並列關鍵字

Sensitivity Analysis ； Subsidence ； Bottom Simulating Reflectors

國際替代計量

岩石力學機制對於天然氣水合物產層降壓生產之影響－台灣西南海域四方圈合海脊水合物好景區案例研究

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