Vol.5 No.2 2012

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研究論文:Development of methane hydrate production method(長尾)−93−Synthesiology Vol.5 No.2(2012) testFieldtestCoreExperiments on samples withalternating of sand and mudlayers To find higher gasproduction rate andrecovery rateTo observe impactimpeding production suchas sand productionComparative study withproduction simulator MH21-HYDRESEvaluation of stressdistribution around wellsduring gas productionEvaluation of sandy layersdeformation during gasproductionLarge-scale laboratory reactor formethane hydrate production testTo 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 are 1710 L and 9900 kg, respectively. This is four times larger than the large-scale production reactor LARS developed by the SUGAR Project in Germany[16]. The objective of the SUGAR Project is to clarify the characterization of CO2 geological storage and methane gas production using the reaction heat of CO2 hydrate generation in the methane hydrate reservoir. Our vessel can be loaded with core samples of sand with a diameter of 1000 mm and a length of 1000 mm. An inner plate is placed on top of the methane hydrate sedimentary sample to exert an overburden pressure of up to 16.5 MPa; this pressure is similar to that in a subsea environment. The overburden pressure is supplied by injecting water into the space between the upper chamber and the inner plate. A production well is simulated using a steel pipe with a diameter of 100 mm and a length of 1000 mm with 32 holes drilled along its length; the pipe is placed at the centre of a sandy sample layer. A sand screen can be placed over the holes to terminate sand production. A total of 50 holes in the sides and 19 holes in the bottom of the vessel are provided to allow the insertion of gas, water and temperature and pressure sensors. The position of sensors can be adjusted depending on the characteristics of the sand sample and the production conditions. To simulate conditions of a methane hydrate reservoir at the eastern Nankai Trough area, the vessel is placed in a large cabinet that can control Fig.4 Relationship between the experimental issues on large-scale laboratory reactor and the roles of research teams of the Research Group for Production Method and Modeling Evaluation of production behaviours such as (1) enhancement of production rate and recovery rate and (2) analysis of impact impeding production are the main experimental issues on a large-scale laboratory reactor. Also, various production conditions to obtain a higher gas production rate and recovery rate can be examined. The experimental results are compared with those from small scale core experiments and analyses of MH21-HYDRES, which is a numerical model of a large-scale laboratory reactor. Finally, the results will be compared to production results of real field tests which will be held in FY2012. However, research regarding geo-mechanical characterization has not been conducted. To achieve relatively uniform methane hydrate formation within the pore spaces of a sandy sample, the positions of the perforations cannot be adjusted for experiments on samples with alternating layers of sand and mud.

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