Natural Gas Hydrates: Composition and Structure
Natural gas hydrate is a naturally occurring solid form of a mixture of water and gas, such as methane, ethane, propane, or carbon dioxide. Despite their solid form and appearance, these types of hydrates behave more like gas than a solid. Natural gas hydrates are primarily found in polar regions—such as the Arctic and Antarctic environments—as well as on continental shelves. In shallow (less than 300 m), high water content areas, hydrates occur in the subsurface where temperatures and pressures are higher and there is a sufficient amount of naturally occurring gas.
Compositionally, natural gas hydrates are primarily composed of natural gas molecules trapped within a lattice-like structure of water molecules. The water molecules in the lattice form a cage structure and the natural gas molecules form a ball-like core of hydrate molecules. These hydrates can form both thermodynamically and metastably.
At thermodynamically stable conditions, natural gas hydrates will form as a result of the following: (1) an increase in pressure and/or decrease in temperature, (2) an increase in the mole fraction of gas relative to water, and (3) an increase in the sediment load. Thermodynamically stable hydrates typically form in polar regions, on the continental shelves, and in other deep-water environments where temperatures, pressures, and gas compositions are conducive to formation. As pressure and temperature decrease, the gas molecules remain in the hydrate lattice, preventing the gas from being released from the lattice and allowing for continued precipitation of the hydrate.
Metastable hydrates can form with other physical and/or chemical changes in the environment. When a physical change occurs—such as a decrease in pressure or temperature—the hydrate lattice can become unstable, which causes the gas molecules to be released from the lattice. This releases the natural gas from the hydrate lattice and allows for the precipitation of the hydrate. Chemical changes can also cause the destabilization of the hydrate lattice, where fluids with different compositions such as salt can cause the hydrate lattice to become unstable, releasing the gas molecules and allowing for the precipitation of the hydrate.
Both thermodynamically stable and metastable hydrates form in equilibrium conditions, meaning that the pressure and temperature in the environment must be in equilibrium with the hydrate lattice. If the pressure or temperature increases, the hydrate lattice destabilizes, releasing the natural gas molecules and allowing for precipitation of the hydrate. If the pressure or temperature decreases, the hydrate lattice becomes more stable, trapping the natural gas molecules and preventing them from being released from the lattice, allowing for continued precipitation of the hydrate.
The structure of naturally occurring hydrate deposits will vary from one location to the next. In colder regions, where temperatures are below the equilibrium point for hydrate formation, the deposits will be relatively homogenous in size and structure. In warmer regions where temperatures are closer to or even above the equilibrium point for hydrate formation, the hydrate deposits may be heterogeneous in size and structure, with distinct bands of hydrate and empty space in between.
In addition to naturally occurring hydrate deposits, human activities, such as oil and gas exploration, can also cause the formation of hydrates. Human activities can create concentrations of natural gas, which can in turn lead to the formation of condensates or hydrates in the subsurface. This is typically caused by an increase in both pressure and temperature near the surface and an increase in the mole fraction of gas in the reservoir.
In conclusion, natural gas hydrates form in both thermodynamically stable and metastable conditions, primarily composed of natural gas molecules trapped within a lattice-like structure of water molecules. Natural gas hydrates can form in polar regions, on the continental shelves, and in other deep-water environments. The hydrate deposits may be homogenous or heterogeneous in size and structure, depending on the environmental temperature and pressure. Human activities can also cause the formation of hydrates, typically caused by an increase in both pressure and temperature near the surface and an increase in the mole fraction of gas in the reservoir.
