利用紅外線熱像儀釐定利氏海底熱流探針之熱容量及偵測沉積物中之天然氣水合物的探討

Abstract

利用地熱的方法探測天然氣水合物，主要是根據海床之溫度梯度與天然氣水合物的穩定曲線的交點推估天然氣水合物的穩定帶底部（Base of Gas Hydrate Stability Zone；BGHS）的深度，以及天然氣水合物在減壓及升溫的情況下解離，產生急速吸熱之異常現象而偵測它的存在。一般學界廣泛地採用了利氏海底熱流探針（Lister-type marine heat probe）測量海床原位（in situ）之溫度梯度及熱導係數，然而從探針所取得之資料做迴歸計算求取海床之溫度梯度及熱導係數時，必須假定探針之熱容量為已知，但是目前並無一有效的方法來釐定探針的熱容量，僅能根據組成的材料來推估熱容量，因此估算出來的熱容量可大可小，範圍高達±20 %以上，實際上我們發現熱容量對求解上述參數的結果影響極大，尤其是溫度梯度，當然也就影響了BGHS深度的推估。因此我們提出了兩種方法，其一是將漆包線纏繞在探針表面，並觀察漆包線隨溫度而改變的電阻；其二是使用紅外線熱像儀（Infrared thermal camera）直接掃描探針表面溫度的變化。經實驗測試的結果就準確度及方便性而言，紅外線掃描法遠優於漆包線纏繞法，所測出來熱容量的標準偏差僅在±2 %以內，較一般估計值的偏差範圍縮小了十倍。 另外，將天然氣水合物從海床下取出至海面時，由於升溫及減壓的結果會使天然氣水合物解離而快速吸熱，產生熱導係數及低溫異常的情形，此時如使用紅外線熱像儀進行掃描即可快速研判有無天然氣水合物，估計從海床表面採集沉積物至船上的時間約需兩個小時，實驗結果顯示，顆粒狀的天然氣水合物與沉積物的比例即使非常少，經過兩個小時後仍然能發現與周圍沉積物的溫度尚有1 ℃左右的溫差。 在預定鑽井採取天然氣水合物的各測站中，四個測站顯示出熱導係數相當高並會隨時間而快速下降的異常，其中之三站在紅外線熱像中有低溫異常的出現，而且兩種異常現象出現的位置極為接近，尤其是在測站KP-7-1，出現了沉積物中含有天然氣水合物的所有前述的熱異常現象，應該是最有可能含有天然氣水合物的地點。

From the study of gas hydrate related thermal phenomena; it is possible to estimate the base of gas hydrate stability zone (BGHS) based on the intersection of the temperature gradient and the gas hydrate stability boundary curve and detect the gas hydrate within the sediment via negative temperature anomaly induced from the dissociation of gas hydrate during temperature raising and depressurizing. The Lister-type marine heat probe has been extensively utilized to measure in situ temperature, temperature gradient and thermal conductivity. However, the difficulty is we can only assume the thermal capacity of the probe when the data are regressed to calculate the temperature, temperature gradient and thermal conductivity of the seabed sediments, as there are currently no methods to precisely determine the probe’s heat capacity. We can only make very rough estimates of the heat capacity based on the building materials of the probes. In general, the estimated range could vary up to ±20 %. Yet we have come to understand that the heat capacity affects the calculated results greatly, especially the temperature gradient, which would further influence the estimated depth of BGHS. Here we have developed two alternative methods. One is to wrap the probe with enameled wire and observe its resistance corresponding to the temperature change. The other is to use infrared camera to scan the temperature change on the probe. After experimenting with both methods, we have found the infrared scanning method to be far better than the enameled wire method in both accuracy and convenience. Furthermore, the standard deviation of heat capacity measured by the infrared scanning method is within ±2 % which is about one order better than conventional estimate. Due to temperature raising and depressurizing as we transport the gas hydrate from the seabed toward the sea surface, it will dissociate and rapidly absorb the surrounding heat, which would produce anomalies in thermal conductivity and negative temperature. According to our experiments, even when the sediment is bearing very little granulated gas hydrate, we can still detect the negative temperature anomaly of about 1 °C after two hours which is normally the time it takes to carry the sediment from seabed to the coring vessel. In the planed drilling sites for gas hydrate investigation, four of the sites have show anomalies in thermal conductivity. Amongst them, three of the sites have also shown negative temperature anomalies in infrared thermal images. The locations of these two related anomalies were in very close proximity. Particularly at site KP-7-1, we have detected every thermal anomaly indicating the presence of gas hydrate within the sediment.