In this study, a new in situ low-temperature oxidation (LTO) process was developed under the concept of effective utilization of heat generation, resulting from LTO of the injected organic substance (IOS), for the promotion of in situ dissociation of methane hydrate (MH) and the enhancement of gas recovery. When water containing the IOS component and air as an oxidant are injected into the MH reservoir, a high-temperature zone by heat generation is formed under the in situ condition. From this process, in addition to MH dissociation, a numerical model considering multicomponent flow in porous media with LTO reaction was constructed. From the calculation results, it was found that the high-temperature zone formed as a result of heat generation extended to the side of the production well, which promoted MH dissociation. In addition, gas recovery as high as 80% to 100% could be obtained through depressurization and in situ LTO process.
Methane hydrate (MH) existing in marine sediments near Japan is expected to be developed as a domestic energy resource in the future (Okuda, 1993; Sato, 2001; Sato and Aoki, 2001). As a gas recovery method for an MH reservoir, the depressurization process is regarded as the most effective process from the aspect of gas productivity and economic efficiency (Yamamoto, 2009). In March 2013 and May 2017, the methane hydrate offshore production test applying depressurization was conducted, and a continuous methane gas production was confirmed (MH21 Research Consortium, 2013; Ministry of Economy, Trade and Industry, 2017). However, because MH dissociation is an endothermic reaction, continuous dissociation by depressurization strongly depends on the sensible heat of the solid matrix and the heat conduction from the surrounding layers. As a result, if the reservoir temperature decreases as MH dissociation progresses, the stagnation of dissociation may result. The total gas recovery when depressurization is applied as a primary gas recovery process is generally estimated to be 40% to 50% (Kurihara et al., 2009). Therefore, it is particularly important to develop a secondary gas recovery process after the depressurization operation, by an additional heat supply into the reservoir in order to promote further MH dissociation.