Experimental Study on Consolidation Behavior and Permeability Characteristics during Dissociation of Methane Hydrate by Depressurization Process

Methane hydrate (MH) is one of the potential resources of natural gas in the near future, because large amount of MH exists in marine sediments or in permafrost regions worldwide. Depressurization process is regarded as the most effective process for gas recovery from the viewpoints of gas productivity and economical efficiency, compared with the other in-situ dissociation processes of MH. However, increase of effective stress during depressurization causes consolidation of MH sediments and permeability reduction. As a result, decrease of gas productivity is also supposed. Therefore, it is very important to understand the behavior in MH reservoir, especially in developing the extraction system for MH, and when considering the environmental impacts due to the development. In this study, we conducted an experimental study on consolidation and gas production behavior during MH dissociation by depressurization, using the special type apparatus. To reproduce the real flow condition of gas and water, we used disc shape samples as simulated MH sediment. Horizontal radial flow in porous media during MH dissociation was constructed whereas vertical load system was used to simulate rock pressure conditions in real MH sediment. From an experimental observation, it was found that dissociation and gas production during depressurization consisted of the following three stages depending on temperature and pressure conditions, such as 1) expansion of methane gas in pore space by depressurization, 2) dissociation due to the latent heat of sand grain and each phases and 3) dissociation due to thermal conduction from outer temperature boundary. In addition, we confirmed that increase of effective stress at the initial stage of depressurization was dominant factor on compaction behavior, and deformation after constant effective stress condition was primarily dependent on the creep effect rather than MH dissociation. Then, dissociation pressure, sand grain size, MH saturation and initial temperature was changed as experimental parameters, we discussed the effect of these parameters on MH dissociation, consolidation, and dissociated gas production.