Timescales and Processes of Methane Hydrate Formation and Breakdown, With Application to Geologic Systems

Abstract

Gas hydrate is an ice‐like form of water and low molecular weight gas stable at temperatures of roughly −10°C to 25°C and pressures of ~3 to 30 MPa in geologic systems. Natural gas hydrates sequester an estimated one sixth of Earth's methane and are found primarily in deepwater marine sediments on continental margins, but also in permafrost areas and under continental ice sheets. When gas hydrate is removed from its stability field, its breakdown has implications for the global carbon cycle, ocean chemistry, marine geohazards, and interactions between the geosphere and the ocean‐atmosphere system. Gas hydrate breakdown can also be artificially driven as a component of studies assessing the resource potential of these deposits. Furthermore, geologic processes and perturbations to the ocean‐atmosphere system (e.g., warming temperatures) can cause not only dissociation, but also more widespread dissolution of hydrate or even formation of new hydrate in reservoirs. Linkages between gas hydrate and disparate aspects of Earth's near‐surface physical, chemical, and biological systems render an assessment of the rates and processes affecting the persistence of gas hydrate an appropriate Centennial Grand Challenge. This paper reviews the thermodynamic controls on methane hydrate stability and then describes the relative importance of kinetic, mass transfer, and heat transfer processes in the formation and breakdown (dissociation and dissolution) of gas hydrate. Results from numerical modeling, laboratory, and some field studies are used to summarize the rates of hydrate formation and breakdown, followed by an extensive treatment of hydrate dynamics in marine and cryospheric gas hydrate systems.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

“…Site U1518 lies within the hydrate stability zone, and widespread but cm‐thick hydrate accumulations identified within sand‐ and silt‐rich layers led to the conclusion that the vertical flux of methane is largely controlled by diffusive migration (Cook et al., 2020). Gas hydrate formation is a slow and exothermic process that depends on temperature, pressure, methane concentration and solubility in existing pore‐water, and geometry of the pore‐space available (Collett et al., 2019; Ruppel & Waite, 2020; You et al., 2019). The diffusive transport of methane probably results in its accumulation in brittle, more porous structures within the damage zone of the Pāpaku fault from where it is more favorable to travel in a fault parallel direction.…”

Effect of thermal formation/dissociation cycles on the kinetics of formation and pore-scale distribution of methane hydrates in porous media: a magnetic resonance imaging study

“…The top boundary of hydrate occurrence zone is usually shallower than 200 m in gas hydrate-bearing subglacial and permafrost settings, expanding upward and downward simultaneously in response to temperature cycles in the surrounding environment. 13 In contrast with deep submarine gas hydrate reservoirs where liberated methane is usually oxidized aerobically once it reaches ocean waters, 14 for permafrost and subglacial hydrates there is a higher risk for reaching the released methane into the atmosphere as it will encounter less barrier in its pathways to the surface. 15 Indeed, some of the liberated natural gas from these reservoirs may have already reached the atmosphere.…”

“…16 Returning to the effect of the cyclic temperature changes on shallow hydrate reserves, these could also alter hydrate composition, saturation, and distribution which in turn could cause geomechanical instabilities. 17 A similar issue could occur during drilling gas hydrate reservoirs, production of natural gas from gas hydrate reserves 13 or storing CO 2 in these reservoirs. 18 Furthermore, recovery of deep warm uid either in the context of geothermal energy or conventional fossil fuels could trigger dissociation of hydrate where the warm uid well crosses hydrate stability zone.…”

Methane Hydrate in Marine Sands: Its Reservoir Properties, Gas Production Behaviors, and Enhanced Recovery Methods

“…Pore-filling hydrate occurs in coarse sands whereas veined or nodule-type hydrate is more common in fine-grained sediments 10) . Seafloor hydrate can exist in areas of active gas seepage 11) .…”