Vol.5 No.2 2012

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Research paper : Development of methane hydrate production method (J. Nagao)−89−Synthesiology - English edition Vol.5 No.2 (2012) below the equilibrium pressure of methane hydrate formation at the reservoir temperature. This method appears to be a cost-effective solution for producing natural gas from methane-hydrate-bearing layers.[6] On the basis of numerical simulations of gas productivity, this method is considered to be predictable and effective for producing gas from the reservoirs consisting of alternating layers of sand and mud. However, hydrate dissociation is a very complex process of coupling heat and mass transfers with the kinetics of hydrate dissociation. Therefore, to understand the dissociation process of methane hydrate existing within the pore spaces of sandy sediments, dissociation experiments on methane-hydrate-bearing cores in a laboratory would be useful.[7]-[10]The performance of gas production strongly depends on the size and permeability of the samples. Heat transfer is a predominant factor in dissociation experiments on methane-hydrate-bearing cores performed in a laboratory (of the order of a few centimetres), whereas mass transfer dominates the dissociation process in an actual reservoir field (of the order of a few 100 m). This difference in the dominant factors between core-scale experiments and field-scale production is responsible for the difference in gas production behaviours. To overcome this problem and to establish gas production conditions at a reservoir field, it is necessary to conduct methane hydrate sedimentary core production experiments on a larger scale. Thus, AIST recently developed and introduced a large-scale apparatus for methane hydrate laboratory production tests, which can conduct gas production experiments under conditions similar to those at actual natural methane hydrate reservoir fields. In this paper, I first present an overview of the Methane Hydrate Research and Development Program.[11] Then I describe the problems in conducting research issues such as methane hydrate production experiments at a laboratory scale, actual field production tests and numerical prediction of productivity, and finally, I report the advantage and certification of a large-scale reactor developed recently to overcome such problems.2 Overview of Japan’s Methane Hydrate Research and Development ProgramThe Methane Hydrate Research and Development Program has a three-phase approach.[11] At the starting period of this program, since AIST had high potential in gas hydrate chemistry, the Methane Hydrate Research Laboratory (now Methane Hydrate Research Center: MHRC) joined as a conducting member of research on the production method and modeling. In phase 1, from FY 2001 to 2008, the MHRC performed laboratory experiments on methane-hydrate-bearing cores taken from the eastern Nankai Trough, where the methane hydrate reservoir consists of alternating layers of sand and mud. The experiments showed that methane hydrate existed within the pore spaces of sand layers. Details of physical properties such as absolute permeability, porosity, methane hydrate saturation, thermal conductivity and sedimentary strength were also obtained. To evaluate gas production performances from methane hydrate reservoirs, a numerical production simulator called MH21-HYDRES was developed. Through laboratory experiments and numerical simulations using MH21-HYDRES performed by the MHRC, MH21 Research Consortium revealed that the depressurization method was determined to be the optimal production method for use in a methane-hydrate-bearing layer, which is the main sedimentary structure in the eastern Nankai Trough. For the first time, the validity of the depressurization method was verified by means of an onshore gas hydrate production field test conducted in March 2008 in a permafrost zone in Canada. In phase 2, from FY 2009 to 2015, the use of methane hydrate extracted off the shores of Japan will be evaluated as a highly reliable energy resource. In addition, although it has been known that the depressurization is a useful method for gas production from methane hydrate reservoirs by laboratory characterization of core samples, the technical difficulties of commercializing gas production from methane hydrate reservoirs will be studied, where the commercialization will be inducted by public and private sectors in phase 3 from FY 2016. The MH21 Research Consortium has set up four Fig. 2 Methane hydrate distribution off the shores of Japan calculated by observing bottom simulating reflectors Red: MH concentrated zones are confirmed partially by detailed surveys (5,000 km2), Blue: Characteristics of MH concentration are suggested in some areas (61,000 km2), Green: Characteristics of MH concentration are not recognized (20,000 km2) and, light blue: Surveys are insufficient for the evaluation of MH (36,000km2). Total BSR area is approximately 122,000 km2. (Copyright@MH21 Research Consortium)

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