Vol.5 No.2 2012

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Research paper : Development of methane hydrate production method (J. Nagao)−92−Synthesiology - English edition Vol.5 No.2 (2012) 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 the temperature of the high-pressure vessel from −5 to 20 °C. Holes in the sides and bottom of the vessel for the insertion of gas and water are connected to a CH4 gas supplier and pumps that supply pure water into the sandy sample layers, respectively. The production well is connected to a gas and water separator. Real-time observations of the rate of the production of gas and water as well as the amount of fine sand particles can be performed under various temperature and pressure conditions.Pure water is injected into the high-pressure vessel via the holes in the sides of the vessel and the centre pipe. Once the designated amount of pure water has been filled in the vessel, sand particles are added to the pure water, and vibration is applied to ensure homogeneous accumulation of sand particles. After the vessel is filled with wet sand particles, the inner plate is positioned above the sand sample layer, and the top chamber is closed. Pure water is injected into the interior of the top chamber to apply overburden pressure to the sandy sample layer by pressurizing the inner plate. To adjust the water content, water in the sandy sample layer can pass through the holes in the bottom of the vessel.For the formation of methane hydrate in the sandy sample layer and control of the confinement pressure, the flow rate of CH4 is adjusted. CH4 is continuously supplied via holes in the sides of the vessel. The temperature of the cabinet is decreased below the equilibrium temperature of methane hydrate formation. By calculating the injected volume of methane gas and the initial water content, the end of the methane hydrate formation can be estimated. After methane hydrate formation, pure water is injected into the pore spaces of the sandy sample layer because natural gas hydrate reservoirs are usually saturated with water.The top of the centre pipe is connected to a backpressure regulator. To examine the depressurization method, the pressure value of the regulator is adjusted to a designated pressure. After adjustment, gas and water flow out through the centre pipe, which may contain fine sand components. The centre pipe is connected to the gas-water separator, and each line tube is connected to a fluid flow metre that measures water and gas volumes during the experiment. To evaluate the sand production phenomenon, a water flow line is connected to the accumulation chamber to collect the fine sand particles.Figure 6 shows the predictions of water and gas production by the MH21-HYDRES production simulator using the results of depressurization experiments conducted in the large-scale laboratory reactor. The results show the water and gas production behaviours when pressure is decreased from 10 to 3 MPa. The parameters for the numerical simulation were temperature of 10 °C, pressure of 10 MPa, permeability of sandy sample layer of 1000 mD,[17] initial effective permeability of 26 mD,Term1 hydrate saturation of 60 % and 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 testFig.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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