Vol.7 No.4 2015
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Research paper : Applicability of the technologies to the assessment of methane hydrate sediments (N. TENMA)−222−Synthesiology - English edition Vol.7 No.4 (2015) 2)Application for various production methods (depressurization method, thermal recovery method, and combined method) to extract methane gas from MH sediment.3)Treatment of MH dissociation/re-formation.4)Treatment of ice solidification/melting.So far, reproduction of the laboratory experiment results has been carried out to validate the simulator.[10][11] Various sensitivity analyses were conducted on a field scale using this simulator.[12] Figure 5 shows an example of predictive investigation conducted using the simulator on a field scale.[13] The model used is an axisymmetric model of the 2D cylindrical coordinate system, with the well as the central axis. The right-half region of the well is broken down into its elements. In this model, there is a MH layer sandwiched between mud layers. A simple model is assumed for the MH layer of alternating sand-mud layers, based on field surveys. Specifically, the thickness is set at 1 m each of alternating sand and mud. The calculation is done assuming that depressurization takes place in the section in which the well reaches the MH layer. The contour diagram of water pressure, MH saturation, and deformation of 1 day, 10 days, and 100 days after the start of the operation of depressurization are shown. As it can be seen from the water pressure change, the area with decreased water pressure in the vicinity of the depressurization zone spreads after the start of depressurization. The low-pressure area spreads by the depressurization method, and the MH dissociation area spreads in the sand layer corresponding to such areas. The effect of deformation by MH dissociation and consolidation can be seen in the sand layer. As the MH dissociation area spreads, the overall deformation of the alternating sand-mud layers progresses. Particularly, the effects caused by consolidation and MH dissociation coexist in the deformation, and the effect manifests in a greater area than the dissociation area. However, the calculations show that the subsidence gradually decreases and that the area in the vicinity of the well stabilizes after a certain degree of subsidence. Since the effects of MH dissociation can be seen, the deformation is thought to be greater than the consolidation deformation of the layer, but it appears that the deformation gradually subsides, since the structure within the sand layer is maintained to some degree, even after dissociation of the MH. Our current numerical model is constructed based on the knowledge of sensitivity analyses and the information from the first offshore production Fig. 7 Fig Title Arial Bold 12Q 15H-840-825-810100806040200DeformationMH saturationWater pressure1 day10 day100 day1 day10 day100 day1 day10 day100 day100806040200100806040200-840-825-8100.00.20.40.6345678Consolidation + MH dissociation = deformation(b) Contour diagram of calculation result (water pressure, MH saturation, deformation)MH bearing Sand layerUpper layerDepressurization zoneProduction well100 m30 m(a) Diagram of the field-scale model used in preliminary investigationFig. 5 Example of field-scale sensitivity analysis resultThis is a contour map that investigates the deformation during the operation of depressurization using the field-scale simplified model. The effects of deformation caused by MH dissociation and consolidation of the MH layer are shown.Deformation values of the contour map are expressed at 30 times the calculated values.

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