The second method is thermal stimulation, in which a source of heat provided directly in the form of injected steam or hot water or another heated liquid, or indirectly via electric or sonic means, is applied to the hydrate stability zone to raise its temperature, causing the hydrate to decompose.
In the thermal stimulation process, heat energy can be released into the methane hydrate strata to dissociate the gas. This process has a favorable net energy balance, as the heat energy required for dissociation is about 6% of the energy contained in the liberated gas. In simple terms, steam or hot water can be pumped down a drill hole to dissociate the hydrate and release methane. The methane released can then be pumped to the surface of the seafloor through another drill hole (Desa 2001).
The direct approach could be accomplished in either of two modes: a frontal sweep similar to the steam floods that are routinely used to produce heavy oil, or by pumping hot liquid through a vertical fracture between an injection well and a production well. A major disadvantage of the thermal stimulation method is that a considerable portion of the applied energy (up to 75%) could be lost to nonhydrate-bearing strata (thief zones). A second major disadvantage is that the producing horizon must have good porosity, on the order of 15% or more, for the heat flooding to be effective. These drawbacks make the thermal stimulation method quite expensive. Figure 5.2 shows the gas production by thermal stimulation (heat injection) process.
Laboratory studies have been conducted on pure hydrate specimens (Circone et al. 2004, 2005; Stern et al. 2001, 2003) and in sediments with synthetic hydrate (Handa and Stupin 1992; Ogasawara et al. 2005; Sakamoto et al. 2005; Sung et al. 2002; Uchida et al. 2004; Yousif et al. 1991). Heating has been implemented by either injecting hot water or heating the chamber (Kamata et al. 2005; Kamath and Holder 1987; Ogasawara et al. 2005; Sakamoto et al. 2004, 2005; Ullerich et al. 1987).
In Japan, methane hydrate has considerable potential as a new energy resource. As a method for production of natural gas from the methane-hydrate-bearing layer, depressurization or depressurization with a well-wall heating process seems to be economically effective. The depressurization process decreases the system
Fig. 5.2 Gas production by the thermal stimulation (heat injection) process
pressure below the pressure of hydrate formation at a specified temperature. The depressurization with a well-wall heating process is a combination of the thermal simulation method and the depressurization process. This process only heats the well wall at the hydrate-bearing layer; it is considered that the initial cost and the running cost is low. Therefore, a number of dissociation data are necessary for the assessment of the efficiency and to elucidate the decomposition process.