Phase 5 was initiated in FY 2015. The Subsurface Microbiology Research Group, Groundwater Environment Research Group, and Sedimentary Rock Research Group each proposed several projects in field science and development of basic technology concerning biomethane generation in geological formations, as described below.
　The Subsurface Microbiology Research Group researched the microbial community structure of the diatomaceous mudstone formation of the JAEA’s Horonobe URL and created a cultivation model replicating in situ study of methanogenesis processes in geological formations. They began by expanding the microbial gene library and investigating the characteristics of the methanogenesis in the Koetoi and Wakkanai Formations. In each formation, it was discovered that acetic acid-producing microorganisms of the Acetobacterium genus could play an important role in methanogenesis. Moreover, the Group also investigated the survey methods used for the microbial community of coal formations in the Sarufutsu mining area of the Tempoku coalfields, improving water sampling methods for lignite-formation groundwater and devising and installing new water sampling equipment in anticipation of anaerobic water sampling. They improved cultivation models to reflect in situ temperatures, making it clear that the rate of methanogenesis was trending downward. They constructed an experimental system to analyze the intermediate metabolites of methane source materials, including monocyclic aromatic compounds, using gas chromatography/mass spectrometry (GC/MS). Using this improved culture model, they continued to isolate microorganisms and clarify their functions, obtaining acetic acid-producing microorganisms, humic acid degrading microorganisms, and methanogenic microbial communities that could grow at 10 °C, among others.
The Groundwater Environment Research Group conducted research in the Sarufutsu mining area of the Tempoku coalfields, using three observation wells to analyze interstitial water and methane, clarifying that methane associated with the Soya coal-bearing formation is of microbial origin, and formed as a result of the recharge of meteoric water and that high concentrations of iodine occur in interstitial water. In the Kushiro Coal Mine, the group analyzed the carbon and hydrogen isotopes of methane from the Cretaceous formations, determining that the methane found in closed pores originated from thermal decomposition and that methane found in open pores originated from microbes. Through collaborative research with the Underground Research Center of JAEA, the Group devised a method for quickly determining dissolved gas concentrations in groundwater. When this method was applied to the groundwater of the JAEA’s Horonobe URL, precise measurements of dissolved methane and carbon dioxide concentrations were obtained. As part of research into the development of basic technology, the Group continued its project from the previous fiscal year on testing the reactivity between lignite and low-concentration hydrogen peroxide solutions. Based on results of these tests, they estimated the activation energy of the hydrogen peroxide consumption reaction, and suggested the possibility of radicalization of the hydroquinone skeleton in lignite as a primary factor in the high reactivity between lignite and hydrogen peroxide.
　The Sedimentary Rock Research Group began to conduct a variety of physical and mechanical tests on lignite in order to explore changes in the mechanical properties of sedimentary rock during biomethane generation. In particular, the Group evaluated the physical and mechanical properties of lignite by measuring elastic wave velocity using the ultrasonic pulse method and uniaxial compression tests, carried out elemental analysis via non-destructive X-ray fluorescence, measured permeability using one-dimensional permeability tests, and imaged lignite structures using a micro-focus X-ray CT scanner, using samples of lignite from rocky outcrops and boring cores. The Group also examined changes in the physical and mechanical properties of lignite samples before and after decomposition of organic matter by hydrogen peroxide. To support the development of in situ tests of biomethane production technology in sedimentary rock formations, the Group enhanced the decomposition of organic matter in sedimentary rock through the use of hydrogen peroxide, assessing the chemical properties of solvents over time as well as changes in the physical properties of sedimentary rock. The Group also proposed evaluation tests of methane production volume from organic matter decomposition, and carried out environmental assessments of the liquid phase following decomposition of organic matter in lignite.