Grain-Displacive Gas Migration in Fine-Grained Sediments

Gas migration mechanisms control the release of gas from seafloor sediments. We study underlying phenomena using transparent sediments subjected to controlled effective stress; this experimental approach allows high-resolution real-time monitoring of gas migration through cohesionless granular materials under 3-D boundary conditions. Observed migration patterns depend on the effective stress at the time of injection and the stress history. Gas migration transitions from pore invasive to grain displacive when the capillary pressure for air entry P-AE is greater than the effective stress sigma. This study focuses on grain-displacive gas migration. The morphology of grain-displacive gas bodies changes with depth as the sediment stiffness G increases and the effect of surface tension vanishes: spheroidal gas bubbles form in the near-surface, faceted cavities further down, and eventually open-mode fractures develop at depth. The gas injection pressure is proportional to the effective stress in grain-displacive migration. Preloading and overconsolidation cause the rotation of principal stresses and gas-driven openings align with the new minimum principal stress direction. Cyclic loading promotes the upward migration of gas-filled openings, and there is mechanical memory of previous gas pathways in sediments.