Vol.7 No.4 2015

Research paper : Applicability of the technologies to the assessment of methane hydrate sediments (N. TENMA)−223−Synthesiology - English edition Vol.7 No.4 (2015) test. Analysis and evaluation continues for the deformation behavior during gas production.Parameters such as elastic modulus and strength (peak value of stress) of the MH-bearing sand sediments had never been measured, and it was impossible to obtain a core sample of the MH layer. It was therefore necessary to prepare samples and develop a testing device specifically for MH to obtain these parameters. The ways to make the artificial MH specimens were investigated by collaborating with universities and private companies utilizing the MH21 framework. We established a way to make such specimens by injecting the gas into a frozen sand specimen and then melting it at a certain pressure and temperature. We succeeded in redesigning and developing a triaxial testing device, originally designed for use in conventional soil studies, to handle the artificial specimens. This enabled laboratory experiments to be carried out on MH. As part of the R&D of MH21, basic boring was conducted in the Tokai-oki to Kumano-nada area from late-January to mid-May 2004. Natural MH core samples were obtained successfully. The elastic modulus and strength values for the MH core were obtained by conducting laboratory experiments on this natural MH core and artificial specimen. It was found that the elastic modulus and strength increased as the MH saturation ratio (volume ratio of MH in the pore) increased.[14]-[18] The experimental equation was derived based on laboratory experiments results and incorporated into the geo-mechanical simulator. This enabled the analysis of the mechanical behavior of the MH layer.We recently succeeded in obtaining a pressure core (a core in which a high pressure condition is maintained to prevent MH dissociation) at the offshore production test site in the summer of 2012. These cores are managed and stored at AIST Hokkaido. Detailed data analysis of the cores from the MH layer is now being carried out. Moreover, to obtain accurate parameters for MH cores, a “transparent acrylic cell triaxial testing (TACTT) system” that allows laboratory experiments to be carried out while maintaining high pressure (Photo 1) was adopted for conducting the tests.[19] The chief characteristic of this device is that the natural core, collected with maintained high pressure, can be transferred to the device without loss of pressure; a sufficient confining pressure is applied through the rubber sleeve to conduct the triaxial test. The sample cell section is made by acrylic to enable visual observation of the triaxial test under high pressure. These techniques made it possible to gain an understanding of the local deformation of the core during the test. Visual observation could be made of the sheared section when stress was applied to the layer, and local deformation and strain could thus be quantified. This device contributes to improvement of the precision of the parameters used in the geo-mechanical simulator.3.2 Evaluation of well integrityWe are working on evaluation of well integrity using the geo-mechanical simulator that is under development. For example, it is becoming apparent that the stress distribution vicinity production well varies according to the difference of depressurization rate (which means the period of decreasing pressure from hydrostatic pressure to 3 MPa at the bottom of the borehole).[20] To further improve precision of analysis, it is necessary to match the well model closely to the condition at the actual site. Wells are composed of numerous and complex materials, including the casing, cement, and sedimentary layers, and it is particularly important to understand the strength of the contact surfaces of these materials. However, there has not been much research on the contact surfaces of wells in deep-water or large depth conditions as seen in the case of MH layers. We are therefore conducting push-out tests to obtain these parameters in order to establish a well model that matches the actual site. Photograph 2 shows a sample (a)Overview of the apparatus(c)Photographing the sample(b)Sample cell section of the apparatus(d)Example of image analysis01020304050708090010-10-20-30-4020400246810121416182060(%)30010-10-20-30-40204030Unit(mm)Photograph 1. Transparent acrylic cell triaxial testing (TACTT) system(a) Overview of the apparatus, (b) sample cell section of the apparatus, (c) photographing the sample, and (d) example of image analysis. As it can be seen from the apparatus overview, the image can be obtained from multiple directions at the sample cell section, and the changes in the sample surface can be observed using the image data.[19]


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