Grain-Displacive Gas Migration in Fine-grained Sediments
bySun, Z., Santamarina, J.C.
Sun, Z.; Santamarina, J.C. (2019). Grain-Displacive Gas Migration in Fine-grained Sediments. Journal of Geophysical Research Solid Earth.
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 ΔPAE is greater than the effective stress σ′. 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 γ 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.