Overestimating climate warming‐induced methane gas escape from the seafloor by neglecting multiphase flow dynamics

Abstract

Continental margins host large quantities of methane stored partly as hydrates in sediments. Release of methane through hydrate dissociation is implicated as a possible feedback mechanism to climate change. Large‐scale estimates of future warming‐induced methane release are commonly based on a hydrate stability approach that omits dynamic processes. Here we use the multiphase flow model TOUGH + hydrate (T + H) to quantitatively investigate how dynamic processes affect dissociation rates and methane release. The simulations involve shallow, 20–100 m thick hydrate deposits, forced by a bottom water temperature increase of 0.03°C yr−1 over 100 years. We show that on a centennial time scale, the hydrate stability approach can overestimate gas escape quantities by orders of magnitude. Our results indicate a time lag of > 40 years between the onset of warming and gas escape, meaning that recent climate warming may soon be manifested as widespread gas seepages along the world's continental margins.

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

“…This builds upon previous work using T + H where Stranne et al [2016b] investigated seafloor warming-induced hydrate dissociation and found that with a seafloor warming of 3°C over 100 years, they could only accurately simulate hydrate-bearing marine sediments with permeabilities higher than 10 À15 m 2 . Hydraulic fracture dominated fluid flow appears in sediments with k ≤ 10 À16 m 2 .…”

“…This means that the pore pressure at the dissociation front remains significantly lower in the base case (compare Figures 2a-2f with Figures 2g-2l) which allows for faster dissociation (as discussed at length in Stranne et al [2016b]). In the base case, pore pressure rises to just above a critical pressure (corresponding to λ Ã T ) at which point a hydraulic fracture forms.…”

“…Accurate predictions of marine hydrate dissociation and subsequent seafloor gas release in low-permeability sediments as a consequence of climate warming require coupled thermodynamic-hydraulicgemomechanical model simulations [Stranne et al, 2016b]. In this study, such coupled model simulations have been performed, and the main results are as follows: (1) Accounting for overpressure development and fracture propagation is extremely important in low-permeability sediments where seafloor gas release becomes large (actually larger than in high-permeability sediments), while without the fracture module there would be none; (2) Fracture formation and propagation increases dissociation rates in low-permeability sediments (almost a factor of 2 larger in k = 10 À17 m 2 sediments) due to the significantly lower pore pressure conditions during hydrate dissociation; (3) The time from the onset of seafloor warming to seafloor gas escape is decreased when hydraulic fracturing is represented (for low-permeability conditions the time decreases from several centuries to just a few decades); (4) There appears to be an optimal sediment permeability for retaining gas at around 10 À15.5 m 2 , where higher permeability leads to generally larger gas mobility Geophysical Research Letters 10.1002/2017GL074349 through pore spaces, while lower permeability leads to pressure build up and fracture formation; (5) During warming-induced hydrate dissociation the first signs of seafloor gas release would be via hydraulic fractures that form in low-permeability facies; (6) Overpressure generation and fracture propagation results in highly nonlinear methane flux rates from the seafloor, 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.…”

“…Their stability is primarily determined by temperature and pressure conditions at and beneath the seafloor. As such, the stability of methane hydrates and potential quantity of methane gas that could be released to the World's Oceans is a subject that has received considerable attention in the past decade [Dickens, 2001;Archer et al, 2009;Westbrook et al, 2009;Skarke et al, 2014;Stranne et al, 2016b]. As such, the stability of methane hydrates and potential quantity of methane gas that could be released to the World's Oceans is a subject that has received considerable attention in the past decade [Dickens, 2001;Archer et al, 2009;Westbrook et al, 2009;Skarke et al, 2014;Stranne et al, 2016b].…”

“…The simplest approaches model methane dissociation and gas release as a sole function of temperature change. As a result, simple hydrate stability approaches can lead to severe over estimates in terms of both CH4 gas production from dissociating hydrates and CH4 gas release from the seafloor [Stranne et al, 2016b]. These estimates neglect many thermodynamic processes controlling hydrate stability and complexities associated with multiphase flow in porous media.…”