Phase Field Theory Modeling of CH4 and CO2 Fluxes from Exposed Natural Gas Hydrate Reserviors
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Natural gas hydrates are widely distributed in sediments along continental margins, and harbor enormous amounts of energy. Gas hydrates are crystalline solids which occur when water molecules form a cage like structure around a non-polar or slightly polar (eg. CO2, H2S) molecule. These enclathrated molecules are called guest molecules and obviously have to fit into the cavities in terms of volume. Massive hydrates that outcrop the sea floor have been reported in the Gulf of Mexico (MacDonald, et al., 1994). Hydrate accumulations have also been found in the upper sediment layers of Hydrate ridge, off the coast of Oregon and a fishing trawler off Vancouver Island recently recovered a bulk of hydrate of approximately 1000kg (Rehder, et al., 2004). Håkon Mosby Mud Volcano of Bear Island in the Barents Sea with hydrates openly exposed at the sea bottom (Egorov, Crane, Vogt, Rozhkov, & Shirshov, 1999). In oil and gas industry the most common guest molecules are methane, ethane, propane, butane, carbon dioxide and hydrogen sulfide. But hydrocarbons with up to seven carbons can create hydrate. The worldwide energy contained in hydrates is huge. But at the same time many of the natural hydrate resources are not well trapped below clay and shale layers and dissociate through contact with under saturated water. Arctic hydrates may be covered by ordinary geological trapping mechanisms and ice layers of varying thickness. The integrity of the geological trappings in these areas are, to a large extent unknown and many potential scenarios can occur when the ice is shrinking in these areas. One of the largest environmental problems facing mankind in the 21st century is the impacts on global weather patterns due to greenhouse gases like methane, carbon dioxide and chlorofluorocarbons. It also effects the distribution of ecosystems and sea level change due to the impact of increased temperature on the melting of arctic ice and the shrinking of other permafrost ice like for instance glaciers. As a greenhouse gas CH4 is in the order of 25 times as aggressive as carbon dioxide. It is therefore an important global challenge to be able to make reasonable predictions of the dissociation flux of exposed hydrate reservoirs, and the associated CH4 that escapes to the atmosphere after biological consumption and conversion through inorganic and organic reactions. There are several possible methods for reduction and stabilization of the CO2 content in the atmosphere, ocean disposal and storage stands out as one solution. There are several options for this (different depths). The seafloor lake alternative, which implies disposal of CO2 at depths for which the density of CO2 is higher than that of seawater, might be the most promising. None of the ocean storage options for CO2 are permanent. But the presence of a CO2 hydrate film at the interface between water and CO2 in the seafloor lake will significantly reduce the dissolution of CO2 into the ocean water. The primary focus in this thesis is on the dissociation of methane and carbon dioxide hydrates due to thermodynamic instabilities through direct contact with under saturated water. For this purpose Phase Field Theory (PFT) was chosen as the scientific method. This is the first work on these types of systems with this level of theoretical methods and the scope have initially been limited to PFT without hydrodynamics. This puts an inherent limit on the types of...
PublisherThe University of Bergen
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