使用有限元素法修正淺層地溫梯度之地形效應並推估天然氣水合物的底部深度

Abstract

受到表面地形的影響，熱流在靠近海床表面時會產生發散與匯聚的效果，使地層淺部的地溫梯度隨深度而產生變化，測量到的梯度值也無法推算較深處的溫度值。為了能更有效的掌握地層的溫度變化，我們使用ANSYS軟體進行有限元素法來修正這種地形效應，並推算在地形修正後天然氣水合物穩定態底部（ Base of Gas Hydrate Stability Zone；BGHS）所在的深度。 經過初步測試後，我們發現西南海域的地形起伏對地層溫度分佈有高達超過一百公尺的影響，表示西南海域的確有進行地形修正的必要，因此我們嘗試將海研一號786航次的海床溫度實測值來計算地形效應對地溫梯度的影響。 作法是以天然氣水合物蘊藏區的一個地熱測站為基準點，使用有限元素法計算其附近測站的地溫梯度經修正地形效應後的資料，顯示同樣位於山谷沉積處的此一測站，ANSYS計算結果與實測值近似。但同樣經過地形效應修正後的另一測站之地溫梯度，就與實測值有相當大的差距。推測造成差距的原因應是後者之測站下方為隆起構造，基盤構造的突起造成其沉積厚度與先前的兩個測站有所差異，可見地形修正應配合沉積物效應修正才能掌握更準確的地溫梯度變化與推估天然氣水合物穩定態的底部位置。另外我們也利用此法修正中央地質調查所預定鑽井位置kp-4、kp-5-1及kp-5-2之地形效應，修正後各位置所推估的天然氣水合物底部位置（BGHSt）深度分別為311mbsf、298mbsf、350mbsf，此結果可做為天然氣水合物鑽井之參考。 本研究的模型因受限於沉積資料的缺乏，因此並未考量基盤起伏的影響，但實際上地層淺部溫度的變化除了受地形影響外，地層構造與基盤地形、沉積物厚度也是重要的因素，未來若能取得更詳盡的地層構造與各層之熱導係數，修正效果必定會更好。

By the effect of rough topography, heat flow will tend to be preferentially convergent towards valleys or low areas, and divergent from ridges or peaks on the sea floor. Hence, the measured result of the seafloor gradient cannot correctly estimate the real thermal gradient of the sub-bottom. In order to calculate the variation in temperature more efficiently, we use ANSYS software using the finite element method to correct such effects and compute the Base of Gas Hydrate Stability Zone; BGHS. During the preliminary test, we found that the temperature of the sub-bottom deviated for more than one hundred meters due to topography effects in the offshore of southwestern Taiwan. This means that it is necessary to correct for the topography effects. To achieve this, we try to remove the topographic effects on the temperature gradients from cruise No. 786 of the R/V Ocean Researcher I. When applying ANSYS to calculate the temperature gradients, we adopted the data collected from one site as the boundary condition, and corrected the errors that resulted from the topography effect in this area. The calculated seafloor thermal gradient of the other site, situated on a similar sediment deposited depression, is close to the measured value; but, on the third site, the result is significantly different from the measured value. We believe the difference is due to the third site being located on the ridge, thus causing heat to be refracted away from depressions deposited with thick sediments. This example emphasizes that not only the topographic effects, but also the sedimentation is important to the temperature gradient calculation, and therefore the estimate of BGHS. For the application, we use ANSYS to correct the topographic effects on sites KP-4, KP-5-1, and KP-5-2 which are drilling sites for gas-hydrate investigation proposed by the Central Geological Survey. After the correction we obtained, the BGHSt (corrected BGHS) for the three sites are 311, 298, and 350 mbsf, respectively. The results are useful information for borehole drilling reference. The model proposed in this study is limited by the lack of sedimentary data and excludes the basement relief effect. Actually, the sub-bottom thermal gradient variation is affected by the topography, the thickness of the sediments, the basement relief, and the structure of the rock. It will be better if we can get more information about the structures, the thickness of the sediments, and thermal conductivities for each layer.