Vol.7 No.4 2015

Research paper−218−Synthesiology - English edition Vol.7 No.4 pp.218-228 (Mar. 2015) while the gas is being produced from the MH layer, local deformation may occur between the production well and the sediment, and this may lead to barriers to production, such as flow path formation or gas leakage. Sediment deformation may also affect the stability of structures such as the well itself, making safe and long-term production difficult to maintain. To enable safe and long-term gas production from the MH layer, it is therefore necessary to develop evaluation technologies, such as a numerical simulator, that are able to predict sediment deformation behavior.However, the basic characteristics that must be handled in a numerical simulator such as “How do mechanical characteristics of the MH layer change during the dissociation process?” and “What role does MH play mechanically?” were not known at the start of the project, so we were unable to evaluate sediment deformation and consolidation. Therefore, “evaluation technologies for sedimentary characteristics” is being developed as a technology for safe and long-term gas production. As shown in Fig. 2, this technology is one of the developmental aims of gas production technology from the MH layer that will be explained below; MHRC conducts R&D as the main administrator of this area. The author is engaged in the development of evaluation technologies for sedimentary characteristics as the Deputy Director (also the Team Leader of Reservoir Simulator Team) of MHRC. The Reservoir Simulator Team is composed of a total of 14 people, including three researchers and technical staff, to develop evaluation technologies for sedimentary characteristics. The author is responsible for overall team management and the 1 IntroductionMethane hydrate (hereinafter MH) is a solid crystal in which the methane molecules are trapped in a basket of water molecules under high-pressure and low-temperature conditions. Environments where these high-pressure and low-temperature conditions exist include the permafrost zone on land and the sedimentary deposit layer at the continental margin in the sea (for example, 200–300 m below the seabed floor at ocean depths of 1,000 m or more). Under such conditions, MH exists in solid form. Lowering the pressure (depressurization) or raising the temperature causes the MH to break down into methane gas and water. Therefore, it has potential as a new natural gas resource. Surveys and research show MH to be present in abundance in the coast of Japan. The Methane Hydrate Resource Development Research Consortium (MH21) was set up in FY 2001 to promote MH resource exploration and development, and R&D is progressing.[1]The Methane Hydrate Research Center (MHRC) at AIST, which is in charge of development of production methods in MH21, has proposed a depressurization method as an efficient method of gas production.[2] In this method, the pressure is decreased by pumping up water in situ from the sediment layer, thereby dissociating the MH into methane gas and water, and then collecting the methane gas. This depressurization method is likely to lead to, the deformation and consolidation of the MH, layer due to its dissociation as shown in Fig. 1. For example, when the sediment deforms - Applicability of the technologies to the assessment of methane hydrate sediments-Methane hydrate (MH) is considered to be part of a new generation of energy resources. Depressurization has been proposed as a method of extracting methane gas from MH in marine sediments. During depressurization, sediment deformation may occur because of MH dissociation and increased effective stress. It is therefore important to develop long-term, safe methods for protecting equipment used on the sea floor against the impact of deformation. We have developed the “COTHMA” geo-mechanical simulator to predict sediment deformation during methane gas production from MH. We have also performed laboratory experiments (push-out tests) of well integrity to determine model parameters. Deformation and stress in the vicinity of a production well were evaluated to assess the integrity of the well. Our technologies for evaluating sedimentary characteristics consist of the development of the geo-mechanical simulator and the evaluation of well integrity and wide-area deformation. Based on this research, we are now preparing technologies for practical application. Development of evaluation technologies for sedimentary characteristicsKeywords : Methane hydrate (MH), MH21 Research Consortium, COTHMA, geo-mechanics[Translation from Synthesiology, Vol.7, No.4, p.228-237 (2014)]Norio TenmaMethane Hydrate Research Center, AIST Tsukuba West, 16-1 Onogawa, Tsukuba 305-8569, Japan *E-mail: Original manuscript received January 14, 2014, Revisions received August 25, 2014, Accepted August 28, 2014


page 22