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海洋环境中天然气水合物层是理想的毛细管封闭层,游离气被抑制在水合物层下,游离气层的气体压力随气体聚集和气层厚度的增加而升高,当气压超过封闭层的毛细管力时,游离气会克服毛细管进入压力、刺入上伏封闭层孔隙空间,毛细管封闭作用随之消失,从而形成水合物下伏游离气向海底的渗漏.通过对该过程进行的数值模拟计算表明:渗漏气体是以活塞式驱动上伏沉积层中的孔隙水向海底排出,水合物稳定带内流体渗漏速度随水流柱高度的减小而增加,当水流阻抗大于相应沉积层段的静岩压力时,沉积层将转变为流沙,流沙沉积被海流移除后便在海底留下凹陷麻坑.麻坑形成后流体运移通道演化为气体通道,气体快速排放.麻坑深度主要取决于游离气层的厚度和水合物封闭层(底界)的深度,而与沉积层的渗透率无关,麻坑深度一定程度上指示了渗漏前水合物层下伏游离气层的资源量.对布莱克海台海底麻坑深度的数值模拟计算表明,形成4m深的海底麻坑需要至少22m厚的游离气层.
The gas hydrate layer in the marine environment is an ideal capillary sealing layer. The free gas is suppressed under the hydrate layer. The gas pressure in the free gas layer increases with gas accumulation and gas layer thickness. When the gas pressure exceeds the capillary force of the sealing layer , The free gas will overcome capillary pressure and pierce into the pore space of the upper sealing layer, and the capillary sealing will disappear, forming the leakage of free gas under the hydrate to the sea floor.Through the numerical simulation of the process, : Leakage gas is driven by piston-driven sediment layer pore water to the sea floor discharge, hydrate stability zone fluid leakage rate decreases with the height of the water column increases, when the water flow impedance is greater than the corresponding sedimentary section of the static When the rock pressure is changed, the sedimentary layer will change into quicksand, and the sediments will be left in the seafloor after the sediment is removed by the sea currents.The fluid migration pathway evolves into gas channel after the formation of the pit, and the gas is discharged rapidly.The depth of the pit is mainly determined by The thickness of the free gas layer, and the depth of the hydrate seal (bottom boundary), regardless of the permeability of the sedimentary layer, which, to a certain extent, indicates the presence of the pre-leach hydrate layer The amount of free gas layer The numerical simulation of the depth of the seabed pit at the Blake basin shows that a 4m-deep subsea gneiss requires a minimum of 22m thick free gas layer.