Firstly, calculate uplifting height of the base of gas hydrate stability zone (BGHS) based on the falling height of sea level, the natural gas hydrate phase equilibrium equation, the hydrostatic pressure formula, the temperature-depth equation of seafloor and the temperature-depth equation of deposition layer. Then establish two-dimensional axisymmetric model according to the characteristic of cold seepage, and set its material properties according to the previous studies on the composition of seafloor sediments and the properties of methane hydrate, methane, and sea water. The timeframe for this study is set as 26500 years, starting from the beginning of the Last Glacial Maximum (26.5 ka BP), and the hydrate decomposition time is set as 7500 years. The computational time step is set as 100 years. The excess pore pressure of upper boundary and lower boundary is set as zero, the right boundary set as impervious, and the left boundary is set as symmetric. Use structured quadrilateral mesh generation method to divide the model, and refine the mesh of hydrate zone and tubular channel. Finally, solve the two-dimensional axisymmetric unsteady seepage flow equation by the finite element method, and quantitatively analyze the influence of the sea-level falling on the formation of cold seepage.
The main results are as following: (1) With hydrate decomposition lasting, the maximum value of excess pore pressure gradually increases, the area moves up, the range of fluid diffusion enlarges, and the highest excess pore pressure in the decomposition zone can even rise to 13180 Pa. But at the same depth, the excess pore pressure of tubular channel is larger than that of the sediment above the decomposition zone, and the range of fluid diffusion around tubular channel changes with depth. After the hydrate decomposition stops, the maximum value of excess pore pressure is smaller and smaller, but the range of fluid diffusion is larger and larger. Finally a funnel is formed at the bottom of the tubular channels, and the range of the funnel increases with time. (2) Just the same, the Darcy velocity in tubular channel increases with time and can rise to m/s during the hydrate decomposition process, decreases with time after the hydrate decomposition stops, and can reach m/s now. Also, the Darcy velocity in tubular channel increases with depth during the hydrate decomposition process, and the effect of depth on the Darcy velocity is very small after the hydrate decomposition stops. (3) The factors that influence the seepage Darcy velocity can be as the followings: (a) The larger the permeability of the tubular channel, the larger the seepage Darcy velocity, but the shorter the seepage duration. (b) The impact from the permeability of the media around the channel on the Darcy velocity changes with time; the larger the permeability of the media, the smaller the Darcy velocity with hydrate decomposition lasting; after the hydrate decomposition stops, the smaller the permeability of the media, the smaller the Darcy velocity. (c) The smaller the radius of channel, the larger the Darcy velocity.
The results based on this model seem to show that cold seepage, caused by sea-level falling in Last Glacial Maximum (26.5~19.0 ka BP), can continue to this day, and even last for more than ten thousand years. But after hydrate decomposition stops, the excess pore pressure continues to decrease over time, and seepage Darcy velocity decreases fast at first then get slow. Also, the seepage Darcy velocity has something to do with the permeability of the tubular channel, the permeability of the media around the channel, and the radius of the channel. Thus, it may be reasonable that the gas hydrate decomposition caused by sea-level falling can be an important factor affecting the global climate change.
海底冷泉是指在压力梯度的影响下,从沉积体中运移和排放出的以水、碳氢化合物、硫化氢或二氧化碳为主要成分、温度与海水相近、并具有一定流速的流体(陈忠等,2007).冷泉中碳氢化合物的来源之一是天然气水合物分解产生的甲烷(邸鹏飞等,2008),所以海底冷泉指示其下方可能存在天然气水 合物.1958年,Emery和Hoggan(Emery and Hoggan, 1958)首次报道了海底冷泉的存在,50多年来,随着海洋调查技术的不断提高,调查范围从浅水区逐渐扩展到半深水和深水区,已经在近岸区、陆架区、大陆坡区、增生复合体区域、大陆边缘的深水区等发现了海底冷泉(栾锡武等,2010).近年来的调查研究表明,我国南海是重要的水合物资源远景区(姚伯初, 1998,2001;张洪涛等,2007),不仅通过钻探获得了水合物实物样品,还发现了大量与冷泉活动有关的地质证据(泥火山、泥底辟、麻坑、海底滑塌、冷泉生物群落和冷泉碳酸盐岩等)(黄永样等,2008).在我国,已经初步确认的近海冷泉区主要有 7 个,东海仅发现冲绳海槽 1个冷泉区,南海海域分布 6 个,包括台西南海域、东沙群岛西南海域、东沙群岛东北海域、神狐海域、南沙海槽和西沙海槽海区(徐翠玲,2013).
甲烷冷泉的重要成因之一是天然气水合物的分解(陈忠等,2008).天然气水合物的稳定性依赖于周围环境的温度和压力,在低温高压环境下稳定存在,但其稳定性可能被海平面变化、或上覆底层海水温度的变化破坏,导致其分解,形成冷泉.
温度对冷泉形成影响的研究主要集中在温度对天然气水合物稳定性的研究上,但温度对水合物稳定性的影响存在一定争议.Macdonal等(1994)通过对墨西哥湾北部陆坡出露的天然气进行原位观测研究认为水合物温度的波动和水合物底下浮力的聚集会导致气体的释放和封闭的交替.但也有研究发现温度对水合物稳定性的影响不明显,Macdonal等(2005)对布什海山进行原位观测发现底层水温度短期的变化对海底水合物的稳定性没有显著影响;Vardaro等(2006)发现水文短期变化和一天为周期的温度和光度显著变化对海底水合物稳定没有显著影响.
海平面下降,深海沉积物所受的静水压力减少,使原本处于稳定域的水合物由于静水压力的降低而失稳分解,将导致赋存其中的天然气水合物快速释 放(Paull et al., 2002; 韩喜球等,2013).Kvenvolden等(1993)认为阿拉斯加北坡发生的大面积滑坡是由冰期水合物释放引起的,当时全球海平面下降约120m.Kortsenshteyn(1970)研究认为,海平面降低120 m,将使水合物的底部抬升约20 m,其结果会使得气水合物溶解并引起洋底滑塌.Maslin等(1998)研究表明出现在35 ka和42~45 ka的两次大冰期,造成海平面下降速率约为15~ 25 m/ka,由此导致的气体水合物分解而诱发的海底滑坡,是这两个时期南美亚马逊海底扇大量沉积物堆积的主要原因.
但是,受深海探测技术条件的限制及现有条件的复杂多变,对与冷泉活动有关的甲烷渗漏的研究才刚刚起步,并且海平面变化对冷泉形成的影响及甲烷渗漏时序变化规律的研究也未全面开展.目前仍不清楚南海大部分地区天然气水合物分解的确切时间、规模、触发机制、及其与气候变化、海平面变化的相关性.为了定量研究海平面下降对冷泉形成的影响,本文基于对冷泉渗漏特征的分析,建立了水合物分解的动态有限元模型,模拟在不同渗透率条件下,冷泉流体的渗流达西速度,以及由水合物分解造成的超孔隙压力随时间的变化. 2 方法原理 2.1 数值模型
天然气水合物的稳定带是指海底以下的特定区域,该区域内的温度和压力处于天然气水合物形成的热力学稳定范围(王淑红等,2005).其底界通常是根据海底温度、地温梯度、 天然气水合物相平衡曲线计算确定的,通常用深度-温度图来确定,其数值在地温曲线和纯水或海水相平衡曲线的交汇点上. Miles(1995)提出了海水中甲烷水合物稳定存在的压力-温度四阶多项式方程:
其中,P为静水压力,单位为MPa,T为温度,单位为℃,a=2.8074023,b=1.559474×10-1,c=4.8275×10-2,d=-2.78083×10-3,e=1.5922×10-4.稳定带底界的静水压力P与海水深度及稳定带厚度的关系为
其中,Patm为大气压,取0.101325 MPa;h、z分别为水深与稳定带厚度,单位为m; g为重力加速度,取9.80 m·s-2;ρsw为平均海水密度,取1035 kg·m-3.Shyu等(1998)在台湾西南部实测的14个海底温度数据,用三阶多项式拟合这些数据得到海底温度-深度方程(宋海斌等,2007):
海底以下沉积层中某一深度的温度 G为地温梯度,取0.037 ℃/m(宋海斌等,2007).
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