Hydrates of Natural Gases and Small Molecules: Structures, Properties, and Exploitation Perspectives

Abstract

Starting from the discovery, in the mid-1930s, that petroleum pipelines in the colder regions of the Northern hemisphere contained crusts of some crystals, and were often blocked by them, a short history of the development of research on the structures, properties, and possible exploitation of the class of inclusion compounds known as gas hydrates is given. The state of the assessment of the natural reservoirs and their perspectives for exploitation are presented, together with an analysis of the hypotheses on the origins of the hydrates. Finally, the phase diagrams are shown in relation to environmental problems arising from the instability of the hydrate fields due to global warming or geological activity.

The Role of Methane Emissions on Ancient and Present Climatic Changes

“…The global warming indeed caused in the last years an abnormous increase in the summer mean temperature (about 4°C) of Siberian rivers and coastal seas. Other phenomena were observed around the Svalbard Islands An alternative hypothesis concerning the decomposition of the methane hydrates seems more convincing (Barone & Chianese, 2009) in this case. Moreover other dramatic episodes were registered some time ago in very different environments, as the abrupt emission of a great amount of natural gases near the Namibia coasts, were an animal slaughter occurred (Ward, 2007).…”

“…It must be noted that methane is about 25 times more active as greenhouse gas than the CO 2 (O'Connor et al, 2010;Barone & Chianese, 2009; United States Environmental Protection Agency [U.S. EPA], 2010a). The longer perturbation lifetime of CH 4 is primarily a result of feedbacks between CH 4 , OH, and its by-product CO which is also removed by reactions with OH.…”

“…Structure of a geological trap of natural gas hydrates, originated both by migration from petroleum reservoirs, or by local biological processes. (Adapted and redrawn from Barone & Chianese, 2009;Kvenvolden, 1983 andKoh, 2008). The filled spots in the upper layers represent gas bubbles eventually seeping trough fractures in the clays or overhanging rocks.…”

“…However permafrost thawing implies the formation of marshes or can be associated with thermokarst erosion, which can form (or drain) lakes: in the first two cases the anoxic fermentation of organic sediments can occur giving methane emissions (Walter at al., 2007). Moreover, if gas hydrates are associated with permafrost, then permafrost thawing promotes destabilization of these hydrates, with methane release, potentially in large quantities (Barone & Chianese, 2009). Another consideration is changes in wetland vegetation and moisture status associated with permafrost thawing; this would be an issue for some, but not all, permafrost landscapes.…”

“…Oceanic and onshore continental reserves are believed to be stable at present, which means that they are not currently emitting CH 4 . Offshore continental shelf reserves are currently unstable, however, and are believed to emit CH 4 (Shakhova, 2010;Barone & Chianese, 2009). Estimates assume that the CH 4 being liberated from the gas hydrate form is released into the atmosphere.…”