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 I seals have the property that they fall 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