冷泉形成的数值模拟研究

海底冷泉形成的一种可能机制是海平面下降引起天然气水合物的分解.本文基于对冷泉渗漏特征的分析,建立了二维轴对称模型,利用有限元方法定量分析了南海区域海平面下降对冷泉形成的影响.结果表明,末次冰盛期(26.5~19.0ka BP)海平面下降引起的冷泉活动可以持续到现在,但是从水合物停止分解至今,超孔隙压力的极值在持续减小,而流体向海底的渗漏达西速度先快速减小、然后缓慢减小.同时发现,流体向海底的渗漏达西速度与管状通道的渗透率、通道周围介质的渗透率以及通道的半径等有关,估计目前的冷泉活动还可以持续10000年以上.海平面下降引起的天然气水合物分解,可能是影响全球气候变化的一个重要因素.

Numerical simulation on the formation of cold seepage

Cold seepage, stemmed from the decomposition of natural gas hydrate below the seafloor and controlled by the seawater pressure gradient, is mainly composed of water, hydrocarbons, hydrogen sulfide or carbon dioxide, and its temperature is similar to sea water. As an important prospecting area for hydrate resource in South China Sea, a plurality of cold seepage areas have been found in recent years, but it seems still unclear such as the exact time, dimensions, trigger mechanism of gas hydrates decomposition, and its relevance to climate change and sea level change in most parts of the South China Sea. In order to quantitatively study the effect of sea-level falling on the formation of cold seepage, the dynamic finite element model of hydrate decomposition was established based on the analysis of the characteristic of cold seepage to simulate the seepage Darcy velocity on different permeability conditions and the changes of excess pore pressure caused by hydrate decomposition with time.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.