Natalia Alekseeva

1. Natural gas in the shallow crust can exist in either free or hydrate (clathrate) form. Gas hydrates are ice-like, crystalline water-based solids that form under specific temperature and pressure...

Recent global warming trends are degrading Arctic permafrost, posing risks to the construction of facilities and leading to increased emission of greenhouse gases. Methane emitted from thawing permafrost may be released by dissociation of intrapermafrost gas hydrates. Northern West Siberian permafrost contains two hydrate-bearing layers: the present zone of gas hydrate stability below 200 m depth and a shallow metastable hydrate zone (150–200 m) preserved from past geological conditions due to the self-preservation effect. Original laboratory tests on the kinetics of phase transitions revealed considerable differences in thermal responses of hydrate-bearing sediments under different pressures and temperatures close to in situ permafrost conditions. According to the experimental results, stable pore gas hydrates are resistant to warming and dissociate only within the range of positive temperatures determined by the equilibrium gas pressure. On the contrary, metastable pore gas hydrates are highly sensitive to thermal effects and prone to rapid dissociation, as they dissociate at some critical temperatures ∼1.0°С–1.5°С below the thawing point for non-saline sand. In this respect, the geotechnical and environmental risks associated with gas production from high-latitude West Siberian reservoirs can be reduced by incorporating the empirical temperature of pore gas hydrate dissociation into the design, in addition to the permafrost thawing temperature.

In the Ross Sea region (Antarctica), evidence of warming, ice melt, and high levels of thermogenic gas hydrates indicate that deep-seated hydrocarbons contribute significantly to carbon cycles. The study investigates structural changes in the gas hydrate stability zone as methane moves from the hydrate-free zone to the seabed. A numerical fluid dynamics model is used to study the potential contribution of deep-seated carbon to the Ross Sea ice shelf, taking into account local thermodynamic equilibrium. The results of the calculation support the phenomenon of free gas from deep sediments that filtrates upward through the zone with methane hydrate stability conditions, partially converting into hydrate, while the remaining gas is capable of migrating higher into the water column. The process is accompanied by the formation of a three-phase (gas, hydrate, H ₂ O) hydrate equilibrium zone, replacing the initially two-phase (hydrate, H ₂O) gas hydrate stability zone. The modeling results explain the discrepancy between the radiocarbon dating of the core and the study of the history of the ice sheet in the region.

2D numerical modeling algorithms of multi-component, multi-phase filtration processes of mass transfer in frost-susceptible rocks using nonlinear partial differential equations are a valuable tool for problems of subsurface hydrodynamics considering the presence of free gas, free water, gas hydrates, ice formation and phase transitions. In this work, a previously developed one-dimensional numerical modeling approach is modified and 2D algorithms are formulated through means of the support-operators method (SOM) and presented for the entire area of the process extension. The SOM is used to generalize the method of finite difference for spatially irregular grids case. The approach is useful for objects where a lithological heterogeneity of rocks has a big influence on formation and accumulation of gas hydrates and therefore it allows to achieve a sufficiently good spatial approximation for numerical modeling of objects related to gas hydrates dissociation in porous media. The modeling approach presented here consistently applies the method of physical process splitting which allows to split the system into dissipative equation and hyperbolic unit. The governing variables were determined in flow areas of the hydrate equilibrium zone by applying the Gibbs phase rule. The problem of interaction of a vertical fault and horizontal formation containing gas hydrates was investigated and test calculations were done for understanding of influence of thermal effect of the fault on the formation fluid dynamic.