The second long-term plan starting in 2012 and spanning nine years with medium-term targets set for each phase (4–6) for three-year periods. The goal of phase 6 (2018–2020) is to elucidate biomethane formation in the sedimentary rocks strata and its method of control in the northern Hokkaido region, while examining the applicability of SCG method (Subsurface Cultivation and Gasification; biomethane deposit) proposed by the institute and extracting points of improvement. Therefore, in FY2019, in collaboration with Mitsubishi Materials Corporations, we conducted an in situ test of the SCG method in the Tempoku Coalfield Koishi mining area, where the company has mining rights. Since the in situ test was carried out jointly by the Subsurface Microbiology Research Group, Groundwater Environment Research Group, Sedimentary Rock Research Group, this report first summarizes the outline of the in situ tests and the results obtained so far, and then describes the issues and research results of each research group.

The in situ test started in July 2019 on the lignite layer of the Tempoku Coalfield Koishi mining area. At the test site, three boreholes (H30-4, 5, and 6) were drilled, rock cores and water were sampled at regular intervals. In addition, the properties of the in situ lignite layer and groundwater condition were obtained by pumping tests and water quality monitoring. In particular, the hydraulic conductivity was between 1.1 to 2.0 × 10-6 m/s, and it was found that the groundwater composition was Na-HCO3 type freshwater with a salt concentration about 50 to 100 times lower than that of seawater. Multipurpose probes were also developed for long-term monitoring of gas and water quality, and in situ microbial culture. An automatic measurement system for winter was also built. According to the results that had been obtained until November 2019, the quality of the groundwater in the borehole changed to an anaerobic environment within few months after excavation.

The research topics and results of each research group are as follows.
The Subsurface Microbiology Research Group installed water sampling devices in the two boreholes (H30-4 and H30-6) in the Koishi mining area, and in situ groundwater was sampled to understand the initial state of the microbial flora. In addition, using the boreholes, an in situ microbial culture test aimed to search for microorganisms utilizing lignite as a carbon source and an analysis of solid microbial flora adhering to lignite was performed. Simultaneously, a culture experiment with the application of the SCG method and a laboratory experiment, using the Koishi H30-4 and H30-6 groundwater as microbial inoculation source was conducted, to which various carbon sources were added. In addition, microorganisms were isolated from the microbial community Sal-4 in humus, and several promising strains could be obtained. In the future, it is expected that the use of isolated strains will facilitate culture experiments from humus to methane production. Furthermore, analysis on the properties of the new microorganisms isolated from the groundwater of JAEA Horonobe URL is being conducted.
The Groundwater Environment Research Group conducted research targeting borehole water and core samples obtained at the Koishi mining area test site, rock samples obtained from the Kushiro Coal Mine, groundwater and rock samples collected from boreholes in the Hamasato area. In the Koishi field test, the carbon isotope ratio of dissolved methane in borehole water and absorbed methane in lignite core samples were measured, indicating that these were of microbial origin. In addition, the carbon isotope ratio of calcite in the coal bed of the Kushiro Coal Mine showed that it precipitated in the habitat of methanogens. There was a correlation between the iodine concentration in the groundwater and rocks collected from the Hamasato borehole, but no correlation was found in the isotope ratio, suggesting the possibility of isotopic fractionation of iodine. Furthermore, an accelerated reaction test at a temperature of 50°C, using 0.3% hydrogen peroxide solution was performed on the lignite of the boring core sample of the Koishi mining area.
The Sedimentary Rock Research Group conducted a pumping test with three test holes in order to evaluate the permeability characteristics of the in situ bed rock. In the pumping test, water was pumped from the boreholes, the flow rate was measured with a large electronic balance, and the water level in the borehole was measured using a water level gauge that was pumped from the borehole. In addition, a laboratory experimental system that simulates the injection of hydrogen peroxide solution into the in situ lignite layer was also installed. A test device was also set up to perform hydrogen peroxide solution injection test under pressure using the collected rock core.

We would like to express our gratitude to the Japan Atomic Energy Agency Horonobe Underground Research Center for providing samples from underground facilities and for various opportunities in promoting the research. The in situ test was conducted as joint research with Mitsubishi Materials Corporations, and we are grateful to everyone in the company. Furthermore, we would like to express our gratitude to all concerned individuals for the cooperation at Kushiro Coal Mine Co., Ltd. for sampling and analysis. Finally, we would like to express our gratitude for the support extended by Horonobe town and other related parties, which was a great platform for research promotion.