Vol.7 No.4 2015
Research paper : Applicability of the technologies to the assessment of methane hydrate sediments (N. TENMA)−225−Synthesiology - English edition Vol.7 No.4 (2015) fault during the depressurization operation. In the future, we plan to systematize the effects of sediment deformation on each parameter and establish a way of selecting sites that are suitable for MH development.We also analyzed the sediment deformation and strength change during the period from start of MH development to well abandonment, and evaluated the long-term effects of MH development by comparing the situation before and after MH development. Specifically, using the artificial MH specimen made in the particle size distribution obtained from exploratory drillings of the MH layers, the triaxial test is conducted under the conditions assumed for the situation before and after gas production. During the application of the depressurization method (i.e., during production) and during the recovery of water pressure after the stop of the depressurization operation (i.e., after production), the liquid pressure in the layer changes, and therefore the deformation strength characteristics before and after gas production can be understood by conducting tests in which the liquid pressure in the pore of the artificial specimens is varied. In the future, we plan to further develop a constitutive equation using the deformation strength obtained from the laboratory experiments, and to embark on long-term sediment deformation analysis by incorporating such equations into the geo-mechanical simulator.4 On future R&DThe “Basic Plan on the Ocean Policy” which was approved by the Cabinet on April 26, 2013, contains the following statement: “The preparation of technology shall be conducted to achieve commercialization with FY 2018 as the goal, taking into account the results of the offshore production test, to make MH, which appears to be abundantly present in the coast of Japan, a future energy resource. In doing so, technological development will be promoted such that projects for commercialization led by private companies can commence in the latter half of 2018, taking into account the international situation.” Following the “Basic Plan on the Ocean Policy,” the “Plan for the Development of Marine Energy and Mineral Resources” is currently under revision. And it is thought that the long-term, stable production technology will be promoted. It is necessary to continue steady research for the evaluation technologies for sedimentary characteristics in the future.Currently, a 3D model has been constructed and is being updated based on the results of core analysis obtained in the field test site. Analysis and evaluation of the mechanical behavior of the MH layer are being carried out through the investigation of offshore production test. We plan to continue improving the geo-mechanical simulator through such investigations. The world’s largest laboratory experiment apparatus, the High-pressure Giant Unit for Methane Hydrate Analysis (HiGUMA) for the MH layer is set up at AIST Hokkaido. This is a device for evaluating the behavior of gas production and MH layer dissociation that cannot be understood by merely using core-scale laboratory experiments when the depressurization method is applied. Using this apparatus, we are attempting to measure the deformation behavior in the vicinity of wells during the depressurization operation. We hope to measure the deformation behavior when the depressurization method is applied, and to improve the precision of the geo-mechanical simulator through experimental verification.By continuing the simulator development combining functions such as the MH development selection method established by evaluations for wide-area deformation evaluation and for well vicinity as mentioned in subchapters 3.2 and 3.3, we ultimately wish to develop a tool that contributes to the MH development area selection, the optimum well design method, and the design of equipment used on the seabed floor for gas production from the MH layer.5 ConclusionIn this paper, we discussed the “evaluation technologies for sedimentary characteristics” as follows: (1) the development of a geo-mechanical simulator, (2) the evaluation of well integrity, (3) the evaluation of wide-area deformation, and outline, result, and future development policy of this theme. The proposal of a long-term, stable production technology for the future is important for the practical realization of MH development. The viewpoint of evaluation technology in terms of mechanical characteristics is the core technology. Our research so far, we believe, has achieved major success in the form of our numerical simulator that can help understand the mechanical characteristics of the MH layer and can handle the mechanical behavior of MH, through obtainment of the natural MH core that is the research subject, establishment of ways to make the artificial specimen, and comparison with the results of laboratory experiments using the natural MH core.Verification of the results of the first offshore production test was also conducted through industry-academia-government collaboration based on the MH21 framework. These research activities also assisted the training of human resources for MH development. The development of various experimental apparatuses has also been accomplished through this research. By gaining a further understanding of the mechanical characteristics of the MH layer and by the improvement of the geo-mechanical simulator, we hope to apply the “evaluation technologies for sedimentary characteristics” to actual sites of gas production from MH sediments.