Natural Gas Hydrate and the Environment
Compared with conventional oil and gas production, gas production from natural gas hydrate (NGH) is inherently safer as controlled dissociation must be artificially initiated and maintained for the gas to be released from the stable NGH. In the event of an uncontrolled gas release, simply stopping the conversion will result in the gas reverting to NGH. As the dissociation of NGH is endothermic, NGH is remarkably “self-preserving”; both growth and dissociation are self-buffering. In addition, seawater is undersaturated with respect to methane, so that even in a “worst case” where small amounts of natural gas are vented from the seafloor, it is unlikely that any methane would reach the atmosphere.
Liquid hydrocarbons do not act as “guest” molecules that combine with water to form NGH. Thus, the presence of oil in NGH-bearing sands is extremely rare.
When NGH forms in permafrost and ocean environments, much of the natural gas that would otherwise pass into the sea or atmosphere is sequestered or retained in NGH. The formation of NGH sequesters natural gas at some times and releases it back into the environment at other times depending on natural swings of temperature and sea level. NGH is characterized by its very reversible chemical reaction that is very responsive to changes in pressure and temperature of changing climates as well as being a persistent reservoir of huge amounts of natural gas. For instance, during glacial episodes the gas hydrate stability zone (GHSZ) thickens and more NGH can be accommodated while at the onset of interglacial warm periods, the GHSZ thins, which causes some NGH to convert and allows some natural gas to reenter the global biogeochemical system. Artificial warming can also cause NGH to convert to its constituent gas and water.
Methane, which is the dominant gas in NGH, is a potent greenhouse gas, far more so than carbon dioxide, but normally it occurs in the atmosphere in very small quantities. Thus, the large volumes of methane tied up in NGH may have had significant effect on global climate intermittently through geologic time, where there may have been relatively large releases or less sequestration of natural gas over geologically short periods of time.
The sudden increase of large volumes of methane in the atmosphere appears to be related to warming that marks the cessation of Pleistocene ice ages. Might NGH play a role in future climate change? What could be the impact on NGH if seafloor temperatures increase by 1 or 2 degrees? Is NGH one of the major positive climate feedback mechanisms?
Oceanic NGH becomes more stable as sea level rises, but some may become unstable if warming seawater warms the seafloor. NGH in permafrost terrane becomes less stable as it warms but known deposits are known to be already held firmly in geological traps that will not naturally vent to the atmosphere. Methane venting into deepwater is unlikely to reach the atmosphere as current evidence is that it is dissolved in the ocean. Methane venting from very shallow water, such as the broad continental shelf of the shallow East Siberian Sea, is less likely to be dissolved in the sea; most of this will reach the atmosphere. Research on these questions is continuing with the involvement of HEI.
HEI has the expertise to advise and help companies and governments on climate issues related to NGH.