Vol.5 No.2 2012

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Research paper : Development of methane hydrate production method (J. Nagao)−91−Synthesiology - English edition Vol.5 No.2 (2012) these parameters are obtained from methane-hydrate-bearing core analyses, and the obtained results will be evaluated and fine-tuned through comparisons with results from the real field production tests. A dissociation experiment on methane-hydrate-bearing cores in a laboratory is useful for determining chemical and structural properties and understanding dissociation behaviour of methane hydrate distributed within pore spaces. However, because of the size of methane-hydrate-bearing cores (of the order of a few centimetres), heat transfer becomes a predominant factor. As mass transfer dominates the dissociation process in an actual reservoir field, the difference in dominant factors between core-scale experiments and field-scale production would result in a difference in gas production behaviours. As mentioned above, these R&D concepts have advantages and disadvantages and are closely related to each other, as shown in Figure 3.To overcome the above problems, AIST developed a large-scale laboratory reactor for methane hydrate production tests. Especially, to design this reactor, we have focused on solving the problem of predominant factors on hydrate dissociation, and a numerical analysis by MH21-HYDRES has been performed.[15] From this analysis, we cleared that mass transfer dominates the dissociation process for sandy sample having over 1m-size. Furthermore, taking into account the research activities of the Research Group for Production Method and Modeling, the reactor was designed by considering the technical issues, as presented in Figure 4. As stated above, three main research activities need to be conducted by the Research Group for Production Method and Modeling. Although it has been determined that the depressurization method is economically suitable for gas production from methane hydrate reservoirs off the shores of Japan, detailed conditions and procedures for depressurization remain unknown. Thus, AIST designed the large-scale laboratory reactor to aid the development of technologies for advanced production methods and to analyze the impact of sand production, skin formation, and flow obstructions. To achieve these goals, in the reactor, highly sensitive temperature and pressure sensors with a wide range and fluid flow metres are arrayed to examine a range of production conditions so that a higher gas production rate and a higher recovery rate can be obtained. To evaluate the sand production phenomenon, a sand screen is fitted to a well tube. The overall volumes of the high-pressure vessel and line tubes are estimated to reduce data error enabling comparison of the results with those of numerical predictions obtained by MH21-HYDRES. Thus, evaluation of mechanical properties can be avoided. To verify the deformation of sandy samples during gas production, it is necessary to position mechanical sensors at many locations for measuring changes in stress and confinement pressure. For this purpose, holes need to be configured in the sides and bottom of the vessel, which is a complex task.A schematic diagram of the large-scale laboratory reactor is shown in Figure 5. The steel high-pressure vessel has an inner diameter of 1000 mm and a height of 1500 mm. The vessel consists of three chambers, and its volume and weight Fig. 3 Large-scale laboratory reactor for resolving disadvantages of production tests, core analyses and production simulations These issues are the main research concepts for establishing gas production methods and evaluating conditions in methane-hydrate-bearing layers.

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