Seite - 78 - in Water, Energy, and Environment - A Primer

theirmoons.Onour homeplanetmost of the hydrates are filled withCH4, and they are abundant. Methane hydrates form as a solid similar to ice under the right conditions of CH4 and water availability, temperature (low) and pressure (high). They are fragile, easily destabilized (i.e., returned to separated water and CH4) by pressure and/or temperature changes, and are found most often within, and occasionally on top of, sediments on ocean floors. They are called ‘fire ice’because they canbe lit by amatch. The most common type of methane hydrate (.99%) has a density of 0.9gcm−1 or just slightly less than that of water, so it can float. One litre of the fully saturated solid would yield 120 grams of CH4 or 169 litres of gas at standard temperature and pressure. It forms in the presence of water and methane under conditions found in the oceans, deep lakes, and under ice caps that fallwithin a gas hydrate stability zone. The seafloors of most of the world’s oceans fall within the hydrate stability zone. Methane hydrates are also found in Arctic permafrost and continental deposits in sandstone and limestone inAlaska andSiberia. These depositsmay cover even larger reservoirs ofCH4at greater depths. There are two sources for thismethane: thermogenicmethane that isformeddeepintheearthbythesamethermal/high-pressure processes that convert organic matter to coal, oil and gas, and which leaks upward toward the ocean floor where it forms hydrates when it comes in contact with highly pressurized cold (0–2°C) water; and methane generated by microbes degrading organic matter (plankton) in low-oxygen environments in sediments. This latter process is the dominant source ofCH4 for methanehydrates. Methanehydrates are importantbecauseof estimates that such hydrates containmore carbon (and thereforemore potential fuel) than all other fossil fuels combined. The EIA reports that these hydrates could hold as much as 10,000–100,000 trillion cubic feet (Tcf) of CH4. To put these numbers into perspective, total Water,Energy, andEnvironment–APrimer78