The evolution of methane vents that pierce the hydrate stability zone in the world's oceans

Abstract

We present a one-dimensional model that couples the thermodynamics of hydrate solidification with multiphase flow to illuminate how gas vents pierce the hydrate stability zone in the world's oceans. During the propagation phase, a free-gas/hydrate reaction front propagates toward the seafloor, elevating salinity and temperature to three-phase (gas, liquid, and hydrate) equilibrium. After the reaction front breaches the seafloor, the temperature gradient in the gas chimney dissipates to background values, and salinity increases to maintain three-phase equilibrium. Ultimately, a steady state is reached in which hydrate formation occurs just below the seabed at a rate necessary to replace salt loss. We show that at the Ursa vent in the Gulf of Mexico, the observed salinity and temperature gradients can be simulated as a steady state system with an upward flow of water equal to 9.5 mm yr À1 and a gas flux no less than 1.3 kg m À2 yr À1 . Many of the world's gas vents may record this steady state behavior, which is characterized by elevated temperatures and high salinities near the seafloor.

A joint electromagnetic and seismic study of an active pockmark within the hydrate stability field at the Vestnesa Ridge, West Svalbard margin

“…Similar fluid escape features have been observed in other gas hydrate provinces [ Bünz , ; Tréhu et al , ; Torres et al , ; Gay et al , ; Sultan et al , ] which has led to modeling studies, to understand why free gas and gas hydrate coexist within the GHSZ [ Liu and Flemings , , ; Smith et al , ]. These model studies also address how fluid escape features form and evolve over time to allow free gas to escape.…”

“…Free gas migration through the long‐term chimney is likely to occur through microfractures and faults [ Smith et al , ] which are not resolved by our seismic reflection data. However, the enhanced reflectors associated with the FGZ beneath the central chimney have a small vertical offset between the eastern and the western ridge flank, which is an argument for possible faulting within the central chimney.…”

“…Geochemical analysis of samples taken at the pockmark above the central chimney suggests episodic release of thermogenic gas through the chimney for over 20 ka [ Panieri et al , ; Smith et al , ]. The central chimney is therefore likely a long‐term feature with three‐phase equilibrium for gas, hydrate, and water extending throughout the GHSZ [ Liu and Flemings , ; Smith et al , ]. We therefore expect both gas and hydrate to coexist within the chimney.…”

Quantifying hydrate solidification front advancing using method of characteristics

“…However, our model includes the formation of a solid phase that has no mobility, includes the development of a three‐phase region, and incorporates the enormous volume changes associated with the transformation of gas to hydrate (e.g., methane or CO 2 ). The approach validates previous numerical approaches [e.g., Liu and Flemings , ; Smith et al ., ] and can be used to test other numerical models in the future. Perhaps, most important, the approach can be used to generalize the results and processes of hydrate solidification in gas‐rich systems.…”

Controlled-source electromagnetic and seismic delineation of subseafloor fluid flow structures in a gas hydrate province, offshore Norway

“…We infer that the saturation of hydrate is higher in this upper region than elsewhere along the pipe structure and at the BGHSZ. High hydrate saturation at that location is probably due to preferential gas advection and interaction of the gas hydrate stability field with the free gas (Flemings et al 2003;Liu & Flemings 2007;Smith et al 2014). While high resistivities can result from both free gas and gas hydrate, high seismic velocities can only be caused by gas hydrate.…”