水合物层下伏游离气渗漏过程的数值模拟及实例分析

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4 m深的海底麻坑需要至少22 m厚的游离气层.

Numerical computation and case analysis of the venting process of free gas beneath hydrate layer

A hydrate layer is an ideal capillary seal,beneath which free gas is trapped.Gas overpressure increases as gas accumulates and gas column grows.Capillary seals have the property that they fail completely when gas pressure reaches the point that they are invaded by gas,and thereafter they offer little resistance to gas venting.After the seepage is triggered,the venting gas will push the overlying water upward at increasingly higher velocities as the gas "piston" approaches the seafloor. Numerical model shows that as the water velocity increases,the near surface sediments will become quick at a depth that the resistance of water flow exceeds the hydrostatic pressure of the sediment hosting the water flow. These quick sediments can then be removed by bottom ocean currents, leaving a hollow pockmark on the seafloor. Thereafter, a free gas pathway is formed below the pockmarks and the reservoir gas drains quickly. The pockmark depth is a function of thickness of free gas column beneath the hydrate and depth of the hydrate seal (bottom of hydrate layer). Interestingly, pockmark depth does not depend on sediment permeability. Pockmark depth implies the resource amount of free gas beneath hydrate layer.The model shows that a 22-m-thick free gas layer at least is needed to form a 4-m-deep pockmark on the rise of Blake Ridge.