The Basics of Gas Hydrate and the Importance of Methane Hydrate
Gas hydrate is an ice-like substance that forms when gas, at high concentrations, and water come into contact at high pressures and low temperatures (e.g., 60 bars, 4°C). Gas hydrate is composed of water molecules that bind together by hydrogen bonds to form a network of cages of various sizes. Small gas molecules such as methane, propane, and carbon dioxide (“guest” molecules) initiate cage formation and may become trapped in these cages (see opposite page). Other cages may remain vacant. Typically, large hydrate cages are more than 95 percent full of guests, while small cages are around 50 percent full of guests.
The most common, naturally occurring gas hydrate structure is known as structure l (“sl”; see opposite page), which contains methane “guest” molecules. Therefore, gas hydrate occurring naturally in permafrost and marine sediments (see images on third page of box) is often referred to as methane hydrate.a Microbial methanogenesis (the decay of organic matter at shallow depths and low temperatures) is commonly the source of the methane stored in these hydrates. The formation of other gas hydrate structures (e.g., sll and sH, which are not discussed further because these structures are less common in nature than sl) requires additional components of heavier hydrocarbon gases, which are minimally formed during methanogenic gas production. The existence of these heavier components may indicate a thermogenic gas source. Thermogenic processes occur at higher temperatures and greater depths within sedimentary rocks where buried organic material is thermally altered into liquid and gaseous hydrocarbons. Although most of these hydrocarbons may remain at depth as “conventional” oil and natural gas accumulations, some of the gases, including methane, may also migrate to shallow depths and form methane hydrate if appropriate pressure and temperature conditions and sufficient free water exist.
An important difference between methane hydrate deposits and those of “conventional” gas accumulations is the nature of the sedimentary rocks within which the gas is found: conventional natural gas fields trap gas in porous sedimentary beds, surrounded by impermeable rocks; methane hydrate deposits occur in relatively unconsolidated sediments where the ice-like hydrate structure itself serves as the trap for individual gas molecules. These characteristics add challenges to producing methane from methane hydrate—hence the description of methane hydrate as an “unconventional” gas resource.