Phase 6 examines the applicability of biomethane production by SCG method (Subsurface Cultivation and Gasification; biomethane deposit formation/production method) proposed by the institute in collaboration with the Mitsubishi Materials Corporations. We have been conducting in situ studies of the SCG method at the Tempoku Coalfield Koishi mining area of the Mitsubishi Materials Corporations to improve the current method. Since the in situ test was carried out jointly by the Subsurface Microbiology Research Group, Groundwater Environment Research Group, and Sedimentary Rock Research Group, this report first summarizes the outline of the in situ studies and the result obtained so far, and then describes the research results for each group.
The in situ studies of the lignite layer of the Tempoku Coalfield Koishi mining area was started in July 2019. After the unmanned measurement in the winter, deep sea water and hydrogen peroxide water injection test were conducted from May 8 to November 9, 2020 and water quality measurement during the winter season showed that unmanned measurement was successful for half a year. In the injection test, two tests were performed using the H30-6 borehole, injecting 1 m3 of deep-sea water and 1 m3 of 0.3% hydrogen peroxide solution. Both operations were completed safely, and injection to the lignite layer was successful. Changes in water pressure during injection and water quality in the hole after injection were measured and analyzed on the basis of the pH value, electrical conductivity, dissolved oxygen concentration, hydrogen peroxide concentration, organic acid concentration, and microbial density. The community structure was analyzed by regular water sampling. In these pioneering attempts, interesting findings have been obtained. In particular, it has been verified that organic acids are produced in the in situ coal bed by injecting hydrogen peroxide solution into the coal bed.

The research topics and results of each research group are as follows.
The Subsurface Microbiology Research Group monitored the microbial community in the borehole water over time at the Koishi mining area, confirming that the microbial density increased after the deep sea water injection and initially decreased after injection of hydrogen peroxide solution to increase later. Furthermore, microbial community structure analysis revealed that the proportion of methanogenic archaea living in the in situ environment decreased due to the effect of hydrogen peroxide solution injection. With regard to the methane production experiment conducted in our laboratory on the assumption of SCG method, methane production was confirmed under the condition that the incubation temperature was higher than the in situ temperature (about 8 °C). A new species of sulfate-reducing bacteria was successfully obtained from the groundwater sample collected during the deep borehole survey conducted by the Japan Atomic Energy Agency (JAEA). It is expected to be useful in elucidating methanogenic archaea and methane production mechanisms in the deep subsurface environment of Horonobe along with understanding microbial diversity in the future.

The Groundwater Environment Research Group first showed the results of water quality monitoring and analysis by sampling water at the Koishi mining area. In particular, they confirmed that the injected deep-sea water would be replaced with the in situ formation water in about one month. The change in hydrogen peroxide concentration after the injection of hydrogen peroxide, as well as the accompanying change in concentration of organic acids such as formic acid and acetic acid was elucidated. It was further highlighted that the methane in the coal and sedimentary rock strata of the Kushiro Coal Mine was microbial origin, and that calcite in the coal bed had precipitated in a methanogenic environment. Additionally, the origin of iodine in the hot spring water was elucidated by analyzing the water quality and microbial composition of the hot spring water at nine establishments in Hokkaido. Furthermore, using lignite from the Koishi mining area, a batch study and column study (8°C) of the hydrogen peroxide reaction was carried out in consideration of the in situ environment. Both studies confirmed that various organic acids were produced at a higher concentration, comparable to the test results at room temperature.

The Sedimentary Rock Research Group studied the initial permeability of the in situ lignite layer, as well as the changes in permeability caused by the injection of hydrogen peroxide. As a result of measuring the pressure change in the injection and the reference holes with the injection of deep-sea water, the hydraulic conductivity around the injection hole was evaluated to be 2.46 × 10-6 m/s, which was slightly higher than the hydraulic conductivity evaluated in the pumping test of the previous year. In addition, they found that change in water pressure in the injection hole due to hydrogen peroxide injection was significantly different from deep sea water injection and that the hydraulic conductivity apparently decreased due to the reaction between hydrogen peroxide and lignite around the injection hole. From the hydraulic behavior of the reference hole, it was interpreted that the injected hydrogen peroxide solution did not reach the reference hole. Paper disk and calorific value type groundwater flow velocity measurements were also carried out, confirming the flow velocity to be ≤0.01 cm/min implying the surrounding groundwater flow was not large.

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 source of vitality for research promotion.