| Summary: | It has been widely reported that natural gas hydrate systems are often associated with the presence of cold seeps, seafloor vents or other manifestations of free gas co-existing within the thermodynamically controlled Gas Hydrate Stability Zone (GHSZ) and thus, being actively transported through hydrate-bearing sediments as an independent phase. Seismic data confirm the presence of gas-migrating and escaping structures situated directly within the GHSZ in many gas hydrate provinces (e.g. off-shore Svalbard, off-shore South Carolina, Arctic Ocean). In order to understand and simulate complex processes associated with gas hydrate and free gas co- existence and their phase dynamics, we have developed a numerical multiphase model which consists all major processes required for precise calculation of gas hydrate and free gas formation and dissolution rates under two distinct regimes: 1) local thermodynamic equilibrium and 2) kinetically controlled non-equilibrium state. Although it has been argued that gas hydrate crystallization occurs on time-scales much larger than any kinetically resolved process, the influence of presence, transport and, eventually, release of methane gas through the seafloor on gas hydrate formation potential is still discussable. Moreover, the new numerical model resolves for chemical reactions associated with in-situ POC degradation, anaerobic oxidation of methane (AOM), and sulfate reduction. To test our model, we have chosen the Blake Ridge Site, off-shore South Carolina where tens-of-meters thick discrepancy between thermodynamic GHSZ and observed BSRs depths has been reported, as well as seismically-imaged gas-escaping structures. The new numerical model allows us to simulate the entire depositional history of the investigated gas hydrate site by applying an instantaneous sedimentation and compaction with every time-step. The results of our model emphasize the importance of multi-phase transport resolving for free gas migration within the GHSZ, kinetically controlled rates of hydrate ...
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