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 [C. Ruppel, J. D. Kessler. Rev. Geophys. 55, 126–168 (2017)]. Methane hydrate (CH4⋅5.75H2O) is an ice-like solid that 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 gas-migration process. At a depth that is too shallow for hydrate formation, existing theories suggest that gas migration occurs via capillary invasion and/or initiation and propagation of fractures (Fig. 1). Within the HSZ, however, a theoretical mechanism that addresses the way in which hydrate formation participates in the gas-percolation process is missing. Here, we study, experimentally and computationally, the mechanics of gas percolation under hydrate-forming conditions. We uncover a phenomenon—crustal fingering—and demonstrate how it may control methane-gas migration in ocean sediments within the HSZ.

Hydrate Formation on Marine Seep Bubbles and the Implications for Water Column Methane Dissolution

“…When gas flow rate is sufficiently high, the rate of bubble surface creation and freshening outpaces the rate of hydrate shell growth (Fu et al., 2020). At the Barkley Canyon site, bubble release rates are high enough that no hydrate appears to form on the growing bubbles prior to seafloor release.…”

“…During bubble growth, internal gas circulation (Clift et al., 1978; Pinczewski, 1981) helps shift growing hydrate films away from the emerging meniscus toward the base of the bubble (Figure 9, upper panel). Such balance between gas flow rate and hydrate growth rate, however, is delicate and can be easily disrupted by changes in gas flux (Fu et al., 2020). As discussed in Section 4.3, when the growing bubble is cut off from the tube’s gas source, the bubble stops growing and the surface becomes fully hydrate‐coated in less than 0.3 ± 0.03 s (Figure 9, lower panel).…”

“…During bubble growth, internal gas circulation (Clift et al, 1978;Pinczewski, 1981) helps shift growing hydrate films away from the emerging meniscus toward the base of the bubble (Figure 9, upper panel). Such balance between gas flow rate and hydrate growth rate, however, is delicate and can be easily disrupted by changes in gas flux (Fu et al, 2020).…”

“…Figure 10 shows three stages of a chimney-growth bubble. This chimney-like bubble growth shape occurs because the hydrate growth directs bubble expansion toward the surface of least resistance (Fu et al, 2018(Fu et al, , 2020, which is generally the top of the bubble. The coupled effect of hydrate formation and bubble expansion also creates minor bends in the final bubble shape (Figure 10 here, and also Figure 4 in Fu et al, 2018).…”

A New Dynamic Modeling Approach to Predict Microbial Methane Generation and Consumption in Marine Sediments

“…Nevertheless, widespread seafloor methane venting reported in many regions of the world suggests the coexistence of free gas and hydrate in those places [25][26][27][28][29][30][31][32][33][34]. Thermal anomalies [35], salinity [36], capillarity [37], and crustal fingering [38] are some explanations for these field observations. Although many studies have addressed this subject from the thermodynamic point of view, few efforts have been devoted to understanding gas hydrate systems kinetics, which facilitates the free gas flow through GHSZ.…”