Abstract
The migration of methane through the gas hydrate stability zone (GHSZ) in the marine subsurface is characterized by highly dynamic reactive transport processes coupled to thermodynamic phase transitions between solid gas hydrates, free methane gas, and dissolved methane in the aqueous phase. The marine subsurface is essentially a water-saturated porous medium where the thermodynamic instability of the hydrate phase can cause free gas pockets to appear and disappear locally, causing the model to degenerate. This poses serious convergence issues for the general-purpose nonlinear solvers (e.g., standard Newton), and often leads to extremely small time-step sizes. The convergence problem is particularly severe when the rate of hydrate phase change is much lower than the rate of gas dissolution. In order to overcome this numerical challenge, we have developed an all-at-once Newton scheme tailored to our gas hydrate model, which can handle rate-based hydrate phase change coupled with equilibrium gas dissolution in a mathematically consistent and robust manner.
Highlights
Methane hydrates constitute a dominant organic carbon pool in the Earth system, and an important intermediate “capacitor” in the global methane budget
In order to overcome this numerical challenge, we have developed an all-at-once Newton scheme tailored to our gas hydrate model, which can handle rate-based hydrate phase change coupled with equilibrium gas dissolution in a mathematically consistent and robust manner
We present a robust implicit semi-smooth Newton scheme based on an NCP approach for handling the phase transitions in our methane hydrate model which includes rate-based hydrate phase change coupled with the vapor–liquid equilibrium across the gas–water interface
Summary
Methane hydrates constitute a dominant organic carbon pool in the Earth system, and an important intermediate “capacitor” in the global methane budget. Gas hydrates (gas hydrates and methane hydrates are used interchangeably, as in the settings of our interest gas hydrates are predominantly composed of methane; the contribution of other gases is negligible) are predominantly formed from biogenic methane generated by the microbial degradation of organic matter (methanogenesis) in the deep biosphere. This methane migrates upwards as free gas or methane-rich porewater by advection. Some methane gas by-passes the GHSZ and AOM zone if the upward flow is larger than the reaction rates
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
