Biogenic methane in Indonesian coal : assessing the potential using culture enrichment, molecular phylogenetic and isotopic analysis

Abstract

Biogenic coal bed methane (CBM) has gained considerable interest because of its potential to be regenerated. The research reported in this thesis evaluated the potential to develop Indonesian coals as CH4 bioreactors using culture enrichment, molecular phylogenetic and isotopic composition analysis methods. The results increase the current understanding on microbial methanogenesis in coal, with a specific focus on Indonesian coals. Indonesian coals range in rank from lignite to bituminous, with anthracites occurring in specific areas. The coals are low in ash yield and dominated by vitrinite group macerals, with abundant liptinite group macerals, which is thought to make them good targets for methanogenesis. Stable isotope analysis of gas from pilot production wells confirmed the hypothesis that Indonesian CBM is partly biogenic in origin. Culture enrichment studies of formation water from these CBM pilot wells also indicated the presence of active coal to CH4 consortia that has the capability for degrading native South Sumatra Basin (SSB) coals and foreign Surat Basin coal from Australia. Among the Indonesian coal basins, the SSB currently has the greatest CBM potential and good accessibility for gas, water, and coal sampling that is required to assess the biogenic CH4 potential of Indonesian coal. Therefore, SSB was targeted for the experiments conducted in this thesis. Detailed laboratory experiments assessing both acetoclastic and hydrogenotrophic pathways for formation waters from five SSB CBM wells confirmed the presence of microbial communities capable of methanogenesis in all. The representative bacterial sequences were dominated by Bacteroidetes, Firmicutes and Deltaproteobacteria while the archaea sequences by members of Methanobacteriales, Methanosarcinales and Methanomicrobiales. When grown on Muaraenim SSB coal, under ideal laboratory conditions, SSB cultures yielded a maximum net CH4 production of 130 Scf t–1, warranting further study to confirm mechanisms. Although the application of the laboratory results to field conditions remains a challenge, the results are still promising. Considering the large amount of deep subsurface unutilized SSB coal, the conversion of only a small fraction of that coal to CH4 could significantly increase the SSB CBM field reserves and reservoir lifetime. The investigation of the microbial methanogen community structure grown on different coal substrates showed temporal changes in community structure over time, and suggested some influence of coal substrate on the microbial community composition. The community structure exhibited greater similarity when grown on coal from the same seam but of different ranks (Mangus iii sub-bituminous (SB) Rv 0.5% and Mangus Anthracite (A) Rv 2.2%) than for coal of similar types (Mangus SB Rv 0.5% and Burung SB Rv 0.39%). The obligate acetoclastic Methanosaeta members favor Burung SB coal, while metabolically versatile Methanosarcina members favor the Mangus coals and the obligate hydrogenotrophic methanogens are significant in the control cultures without coal. Regardless of the community structure similarities across all coal cultures, more CH4 was generated from the lower rank sub-bituminous coal cultures relative to the one high-rank semi anthracite coal. These results suggest a potential relationship between coal type and rank, microbial community composition and CH4 production, which warrants further investigation. While CH4 measurements and molecular phylogenetic analysis confirmed the production of biogenic CH4 in the cultures, the gas sampled from the culture headspace had δ13C-CH4 values ( 52.2‰ to 22.6‰) that mostly fell outside the range currently considered to indicate a biogenic origin. In this study, the apparent carbon fractionation factor (αc=1.02±0.006) and isotope effect (ec= 20.1‰±15.3) were found to be more useful indicators of methanogenic pathways than the absolute δ13C-CH4 values. Both values agreed with the calculated contribution of CO2 reduction pathway (ƒmc=18%±24%) and molecular analysis results that showed the dominance of acetoclastic over hydrogenotrophic methanogens. In contrast, the δD of H2+CO2 and acetate derived CH4 were relatively similar, suggesting that in this study δD isotopic measurement may not provide a reliable basis for distinguishing methanogenic pathways. The high δ13C-CH4 values found in this study indicate that the methanogens are operating at low substrate concentration. Substrate depletion is thought to strongly affect the SSB cultures δ13C-CH4 composition and may relate to a decrease in the relative abundance of the key bacterial coal degrader with formation water inoculum storage time. Overall, the research output demonstrated that Indonesian coals in the SSB contain an active CH4-forming microbial community, which adds to the known basins where microbial communities can form CH4 from coal measures and their formation waters. The results also emphasize the potential of Indonesian coal to be developed into CH4 bioreactors that could contribute to Indonesia's future energy supply. From a scientific point of view, the results underline a possible positive extension of δ13C-CH4 values for biogenic origin beyond that currently available in the literature. The research results also highlight the need for further more detailed studies on coals of different type and rank, to assess the potential for microbes to form CH4 directly from the coal itself.