Crustal fingering facilitates free-gas methane migration through the hydrate stability zone

Abstract:

Widespread seafloor methane venting has been reported in many regions of the world oceans in the past decade. Identifying and quantifying where and how much methane is being released into the ocean remains a major challenge, and a critical gap in assessing the global carbon budget and predicting future climate. Methane hydrate forms from methane-water mixture under elevated pressure and low temperature conditions typical of the deep marine settings (>600 m depth), often referred to as the hydrate stability zone (HSZ). Wide-ranging field evidence indicates that methane seepage often coexists with hydrate-bearing sediments within the HSZ, suggesting that hydrate formation may play an important role during the methane gas migration process. At a depth that is too shallow for hydrate formation, existing theories suggest gas migration occurs via capillary invasion and/or initiation and propagation of fractures. Within the HSZ, however, a theoretical mechanism that addresses the way in which hydrate formation participates in the gas percolation process is missing.

In this work, we study, experimentally and computationally, the fluid mechanics of gas percolation under hydrate-forming conditions. We uncover a new phenomenon --- crustal fingering --- and demonstrate how it may control methane gas migration in ocean sediments within the HSZ. We extrapolate the underlying physics of this phenomenon to understand its implications at the scale of multiple methane seeps. Our field-scale simulations suggest that the crustal fingering mechanism could help interpret intermittent temporal dynamics of deep methane seeps, trace the origin of hydrate fabrics in the subsurface environment, and advance our understanding of hydrate-seep interactions in the past and future climate.