Modeling fracture propagation and seafloor gas release during seafloor warming‐induced hydrate dissociation

Abstract

The stability of marine methane hydrates and the potential release of methane gas to the ocean and atmosphere have received considerable attention in the past decade. Sophisticated hydraulic‐thermodynamic models are increasingly being applied to investigate the dynamics of bottom water warming, hydrate dissociation, and gas escape from the seafloor. However, these models often lack geomechanical coupling and neglect how overpressure development and fracture propagation affect the timing, rate, and magnitude of methane escape. In this study we integrate a geomechanical coupling into the widely used TOUGH + Hydrate model. It is shown that such coupling is crucial in sediments with permeability ≤10−16 m2, as fracture formation dramatically affects rates of dissociation and seafloor gas release. The geomechanical coupling also results in highly nonlinear seafloor gas release, which presents an additional mechanism for explaining the widely observed episodic nature of gas flares from seafloor sediments in a variety of tectonic and oceanographic settings.

Can anaerobic oxidation of methane prevent seafloor gas escape in a warming climate?

“…As shown in Stranne et al (2017), the upward transport of CH4 within destabilized hydrate-bearing 10 sediments can be divided into three flow regimes. These flow regimes depend on the sediment permeability, and encompass the expected range of permeabilities for hemipelagic sediments composed predominantly of terrigenous silts and clays (Fig.…”

“…Buffett & Archer (2004) speculate that slow diffusive transport of CH4 likely results in AOM within the sediments with negligible effect on climate, while a more rapid liberation of CH4 (in response to anthropogenic climate warming) can lead to fractured pathways within the sediment that bypass the microbial filter and allow for a larger proportion of the CH4 to reach the ocean and 20 atmosphere. This idea is supported by Stranne et al (2017), who showed that warming-induced hydrate dissociation in moderate to low permeability sediments (clays and silty-clays) leads to formation of hydraulic fractures and rapid release of CH4 from the seafloor.…”

“…The T+H-GeoMech (Moridis, 2014;Stranne et al, 2017) is set up for mid-latitude conditions with an initial bottom water temperature of 5°C and a methane hydrate stability zone (MHSZ) extending down to 20 m below seafloor (mbsf). This represents the most sensitive "feather edge" of hydrate stability on 10 the upper continental slope.…”

“…One example of the latter is the TOUGH+Hydrate (T+H) model which predicts the evolution of 35 pressure, temperature, salinity, and the phase saturation distributions in hydrate-bearing systems (Moridis, 2014). Stranne et al (2017) to as T+H-GeoMech in the text) and showed that such coupling is critical since dissociation of methane increases pore pressure and leads to hydraulic fracturing. Hydraulic fractures increase the permeability of sediments, and dramatically affect rates of dissociation and seafloor gas release.…”

“…The present study aims at taking a step in this direction, through the addition of a simplistic but novel and fully coupled AOM module to the T+H-GeoMech code. As in Stranne et al (2017) we focus on the 35 feather edge of hydrate stability -the part of the marine hydrate reservoir most sensitive to ocean warming (Ruppel, 2011). We address the hypothesis of Buffett & Archer (2004)…”