Study of methane hydrate formation and distribution in Arctic regions : from pore scale to field scale
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We study hydrate formation and distribution in two scales. Pore-scale network modeling for drainage and imbibition and 1D field-scale sedimentological model are proposed for such purpose. The network modeling is applied in a novel way to obtain the possible hydrate and fluid saturations in the porous medium. The sedimentological model later uses these results to predict field-scale hydrate distribution. In the model proposed by (Behseresht et al., 2009a), gas charge in the reservoir firstly takes place when BGHSZ (Base of Gas Hydrate Stability Zone) is still above the reservoir. Methane gas migrates from deep source and is contained in the reservoir by the capillary barrier. The gas saturation distribution is determined by gas/water capillary pressure, and is modeled by network modeling of drainage. When gas charge is complete, the gas column in the reservoir is assumed to be disconnected from the deep source, and BGHSZ begins to descend. Hydrate formation is assumed to occur only at BGHSZ. At the microscopic scale it first occurs at the methane/water interface. A review of the possible modes of growth leads to the assumption that hydrate grows into the gaseous phase. It is assumed that the hydrate formation at the pore scale follows the path of imbibition process (displacement of gas phase by aqueous phase), and can be predicted by the network modeling of imbibition. Two scenarios, corresponding to slow and fast influx of water to the BGHSZ, are proposed to give the maximum and minimum hydrate saturations, respectively. The volume of hydrate is smaller than the total volume of gas and water that are converted at fixed temperature and pressure. Therefore, vacancy is created to draw free gas from below the BGHSZ and water into the BGHSZ. BGHSZ keeps descending and converting all the gas at BGHSZ into hydrate. The final hydrate profile has a characteristic pattern, in which a region of high hydrate saturation sits on top of a region with low hydrate saturation. This pattern agrees with the observation in Mount Elbert and Mallik sites. The low hydrate saturation in certain regions with good lithology shows that hydrate distribution is not only controlled by the quality of lithology, but also the gas redistribution during hydrate formation.