Biogenic Methane Generation Research Articles

Experimental research on biogenic methane generation pathway and microflora constraints based on in-situ gas-water-microbe

Biogenic coalbed methane (CBM), generated through the biodegradation process of coal organic matter, is a relatively clean and environment-friendly unconventional natural gas resource. To investigate the differences in microbial communities (bacteria and archaea) as well as their environmental influencing factors between the Permian Shanxi Formation (P1s) and Taiyuan Formation (P1t), we systematically collected in-situ CBM samples along with coalbed water and microbial samples from a representative biogenic CBM field – the Baode block locked in the Ordos basin in China. The relative content of biogenic CBM was quantitatively calculated while analyzing gas composition, stable isotope, ion content, and high-throughput sequencing analysis to discuss the generation pathway for biogenic CBM and environmental control factors affecting microbial community structure. The results indicate that P1s and P1t are mixed with early thermogenic gas and secondary biogenic gas genetic types within their respective CBMs. The average proportion for biogenic gas within current coal seams is approximately 62.24 % for P1s, while it reaches around 64.37 % for P1t. Water quality type primarily consists of Na-HCO3 type with total dissolved solids (TDS) measuring at approximately 1015.86 mg/L for P1s, whereas P1t exhibits Na-Ca-HCO3-Cl water type with TDS content reaching about 2314.85 mg/L. Combining dominant Methanobacterium species, along with Δ13C(CO2-CH4) ratios and δD(CH4) versus δD(H2O) indexes, represents the biogenic CBM primarily generated through the hydrogenotrophic methanogenesis pathway. The in-situ bacterial communities of P1s are mainly influenced by Ni, pH, and TDS. The archaeal communities exhibit higher sensitivity to environmental factors (such as Co, pH, Ni, and dissolved organic carbon) than bacteria. These findings hold significant implications for understanding the generation process of biogenic CBM in middle and low-rank coal areas.

Laboratory investigation and core flood demonstration of enhanced biogenic methane generation from lignite.

Over the last several decades, coalbed methane (CBM) has emerged as an important energy source in developing nations like India as well as worldwide and is expected to play a significant role in the energy portfolio of the future. The current scenario of rapid exhaustion of fossil fuels is leading to the need to explore alternative and efficient fuel resources. The present study demonstrates enhanced methane production per gram of lignite (lowest-rank coal). Optimization of the bioconversion of lignite to methane revealed 55°C temperature and 1.5g/L NaCl concentration as ambient conditions for the process. A scale-up study in the optimized condition showed 2,800mM methane production per 25g of lignite in anaerobic conditions. Further, Fourier transform Infrared (FTIR) and Gas Chromatography Mass Spectrometry (GCMS) analysis showed bioconversion of lignite into simpler intermediate substrates required for methane production. The results highlighted that the bacterial action first converts lignite into volatile fatty acids, which subsequently get converted into methane. Further, the exploration of indigenous microbial consortia in Tharad well (THAA) mainly comprises the order Methanosarcinales and Methanomicrobiales. The pathogenicity of the microbial consortium THAA was declared safe for use in mice via the oral route by The Energy and Resources Institute (TERI), India. The study demonstrated the development of indigenous consortia (TERI THAA), which can potentially enhance methane production from the lowest coal grade under extreme conditions in Indian coal beds.

Generation of secondary microbial methane of high-rank coals: insights from the microbial community and carbon isotope.

Secondary microbial methane could provide a valuable energy source if it were better understood. Although coal seam is an ideal environment for investigating secondary microbial methane, there are few studies to trace the secondary microbial methane of high-rank coals. Here, we collected co-produced water samples from coalbeds in the Qinshui Basin (China) and analyzed the microbial community structure by 16S ribosomal RNA (16S rRNA) amplicon sequencing analysis. 16S rRNA sequencing demonstrated abundant methanogens in coalbeds including 6 orders (Methanobacteriales, Methanococcales, Methanofastidiosales, Methanomassiliicoccale, Methanomicrobiales, and Methanosarciniales) and 22 genera of methanogens. Superheavy DIC (δ13CDIC ranging from -4.2‰ to 34.8‰) and abundance of methanogenic microbes in co-produced water revealed the generation of secondary biogenic methane in high-rank coal seams in the Qingshui Basin. Hydrogenotrophic methanogenesis is the main pathway for secondary biogenic methane production. In deeply buried coal seams, biogenic methane is dominated by CO2 and H2 reduction methanogenesis, and in shallow buried coal seams, it may be produced synergistically by hydrocarbon degradation and hydrogenotrophic methanogenic microbes. The study discussed here is important for a better understanding of the generation of secondary microbial methane in high-rank coal.

Biogenic methane generation from lignite coal at different temperatures

Microbial gasification of coal resources in situ is a novel method for mining legacy and deep coal resources. The complexity of the in-situ coal environment and the singularity of strains limit the gas production efficiency. To improve the gas production efficiency in terms of strains, multiple strains with high species density were enriched from anaerobic sludge. Anaerobic fermentation experiments were conducted using lignite at 15 °C, 35 °C, and 55 °C. To obtain complete gas production curves and to reduce measurement errors, gas production volumes were collected using an online continuous micro-gas measurement device, with an error rate less than 1%. The results showed that the gas production delay periods of multiple strains were very short; additionally, there was a stagnation period in the middle of the gas production at 35 °C and 55 °C. Moreover, these strains produced the most gas at 35 °C with the lowest total organic carbon (TOC) in solution. The maximum methane ratio reached 39.94%, the maximum gas production rate was 0.1 mL/g/d, and the maximum yield was 1.22 mL/g. When gas production stopped, the potential of hydrogen in the fermentation solution decreased and the redox potential increased. A logistic model was fitted when the temperature reached 15 °C and 55 °C. A gas production curve was produced using a segmented S-shaped curve to determine the best-fit model. The results of this study provide insights into the use of multiple strains of exogenous bacteria for gasification of lignite under different coal seam temperature conditions and also provide data to support the estimation of parameters of the numerical model for the microbial gasification of lignite.

Microscopic Characteristics and Formation Mechanism of Effective Reservoirs in the Xihu Depression, China: The Important Role of the Poikilotopic Calcite Cements in Tide-Dominated Delta Systems

The Xihu depression is an offshore sag located on the East China Sea Shelf Basin, which is currently one of the major oil and gas basins along the coast of China. In this study, an integrated approach using thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), cathodoluminescence (CL), high-resolution 3D CT core scanning and stable isotope analysis was applied to examine the diagenetic evolution and investigate the microscopic characteristics and formation mechanisms associated with effective reservoirs. Four types were distinguished: upper conventional reservoirs (UC reservoirs), lower conventional reservoirs (LC reservoirs), “bottom calcium” low-permeability reservoirs (“bottom calcium” reservoirs) and “MI clay” low-permeability reservoirs (“MI clay” reservoirs). Poikilotopic calcite cements play an important role in the diagenetic alterations and reservoir quality evolution, precipitating during early eogenesis, provided a framework that retards the adverse impacts of UC reservoirs by compaction. Conversely, in LC reservoirs, with limited poikilotopic calcite, secondary porosity is mostly due to the dissolution of feldspar or unstable rock fragments. UC reservoirs normally develop in the middle of tidal channels and in subaqueous distributary channels, with the base of the sand-body being extensively cemented by carbonate cements, such as late calcite, Fe-calcite and dolomite, which formed the “bottom calcium” reservoir. Combined evidence from petrographic and geochemical analyses suggests that calcite precipitates from diagenetic fluids of mixed marine and meteoric waters, with additional external sources from calcareous siltstones and bioclasts. The carbon sources of calcite mostly originate from the dissolution of carbonates clacts or bioclasts within sandstone beds or adjacent silty mudstones, while dolomite cements have an isotopic composition that is more comparable to the generation of biogenic methane. This study demonstrates how poikilotopic calcite, developed in tide-dominated delta systems, affects the vertical heterogeneity. The results can be used to improve the reservoir evolution model of tide-dominated delta systems and provide a basic understanding for researchers conducting reservoir studies of similar sedimentary systems. Our results can act as a geological basis for further oil and gas exploration.

An operative laboratory investigation of bioconversion route from waste coal to natural energy

PurposeIn the present research, the potential of reactivated consortium for the methane production consuming waste coal as a carbon source (1% w/v) in the modified media at mesophilic temperature (37 °C) was determined.MethodsMedia modification was conducted for the enhancement of methane production by selecting three different components from the two media, i.e., Methanosprillium sp. producing media (MSP) and methane-producing bacteria media (MPB). From MSP medium, C2H2NaO2 (sodium acetate), KH2PO4 (potassium dihydrogen the phosphate), and NaHCO3 (sodium bicarbonate) whereas from MPB medium; yeast extract, peptone, and NH4Cl (ammonium chloride) were selected in the range of 0.5–2.5 (g/l). Analytical assay, i.e., Fourier transform infrared spectroscopy (FTIR), gas chromatography mass spectrophotometry (GCMS), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) were conducted. Further, compatibility study and pathogenicity was performed.ResultsIn the present study, reactivated consortia was used therefore key components of the media were modified. In case of MPB medium, 2 g/l of yeast extract, 2 g/l peptone, and 1 g/l NH4Cl showed the promising results; whereas for MSP medium, 1 g/l of KH2PO4, 0.5 g/l of NaHCO3, and 1.5 g/l of C2H2NaO2 were noted to be the suitable range for methane production. Analytical studies confirmed the presences of -OH and aliphatic groups which majorly belongs to alkane, alkene, and phenol derivative compounds whereas SEM and EDX studies delineated the active interaction of bacteria with coal particles and presences of carbon (C) as a major peak in untreated coal and absence of C peak in microbial treated coal. In addition, a compatibility study was performed and their successful results aid in the future approach of field implementation. Further, pathogenicity data indicated the non-virulent and non-toxic nature of the consortia.ConclusionsThe production of waste coal is one of the most problematic and common activities of the mining industry. They release toxic substances into the environment (water, air, and soil) and damage the local biodiversity. Therefore, the generation of biogenic methane from waste coal is an environmentally friendly approach to overcome this problem.

Investigation on future perspectives of ex-situ biogenic methane generation from solid waste coal and coal washery rejects

Biogenic methane production from the coal faces difficulties in the conventional combustion process. Understanding the coal rank, structure, and the biogenic pathway is essential to introduce the innovations in the exploitation of coal reserves. Furthermore, in-depth investigation of the microbial diversity present in the coal beds is necessary to develop cost-effective and environment-friendly techniques for energy production. In the present review, we provide an overview of the availability, properties, and processing of coal for the production of biogenic methane. The survey on the microbial communities present in various coal seams and laboratory-scale conversion of ex-situ coal to methane is also provided. We have elaborated the pathway of methanogenesis and the source of microbes for the coalification process. Finally we discussed the enhancement strategies to improve the bioavailability of coal towards microbes. Recent researchers showed that a maximum of 22.9 mM/g of coal, 0.162 mM/g coal, and 0.08 mM/g coal methane has been produced from various consortiums. The bioaugment and biositmulation process has lead to 25% increase in methane production which proves the possibility of transiting technology from laboratory to large scale.

The characteristics of coalbed water and coal in a coal seam situated in the Red River Basin, Vietnam

An in-depth understanding of the hydrogeochemical characteristics of coal mines is helpful in establishing an effective and successful exploration program of coalbed methane (CBM). This study provides a comprehensive analysis of hydrogeological characteristics, characteristics of coalbed water, and characteristics of the coal sample from a coal seam located in the Red River Basin (RRB). These physicochemical characteristics along with the microbial composition of coalbed water were critically analyzed. A high concentration of chloride and sodium was found in the coalbed water, presumably due to the coal mine's stratigraphic association with marine or marine-transitional beds. A correlation between the occurrence of microbes and the chemical components in the coalbed water was established. The characteristics of the coal were systematically analyzed, including proximate, ultimate, and petrographic analyses. Based on the coal macerals, coal rank is classified as low-rank (sub-bituminous) with a vitrinite reflectance (Ro, max) of 0.36%, suggesting that this type of low-rank coal is favorable for biogenic methane generation. Pore structures and pore types were characterized using different methods, including low-temperature nitrogen adsorption/desorption (LTNA), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). Coal from the study area has microporous and macroporous features. Pore types of the coal were also characterized using SEM. The primary genetic pore types of the Red River coal include plant tissue holes and blowholes.

Biogenic methane generation from Vietnamese coal after pretreatment with hydrogen peroxide

The application of coal as a methanogenic substrate is limited due to its recalcitrant nature, thus, requiring pretreatment for increasing bioavailability of coal. This paper describes the pretreatment of Vietnamese coal from the Red River Basin. The coal sample was treated with 3% (vol/vol) hydrogen peroxide, which resulted in 45.4% (weight %) solubilization of coal and release of various organics into the liquid phase. This dissolved fraction was analyzed for total dissolved organic carbon, Fourier transform infrared spectroscopy, high-performance size exclusion chromatography, and high-performance liquid chromatography. The detailed analytical investigations revealed the presence of aliphatics and aromatics hydrocarbons in the liquid phase. Short-chain organic acids, including oxalic, citric, malic, malonic, formic, and acetic acids were also observed. The dissolved fraction was subjected to methanogenesis. Methane generation achieved the peak at 218.5 μmol/g coal after 500 days of incubation. The effect of the pretreatment process on inorganic matter of the residual coal samples was also assessed using inductively coupled plasma mass spectrometry. The pretreatment caused a reduction in metal content of the residual coal sample. The findings of the paper can be prospected as a feasibility study of methane production from Vietnamese coal after its pretreatment using hydrogen peroxide.

First investigation of microbial diversity and biogenic methane potential in coal mines located in the Red River Basin, Vietnam

The generation of biogenic methane from coal mines is considered a sustainable approach for enhancing the recovery of natural gas from coalbeds. This study aims to provide the first quantitative data about the characteristics of coal samples, microbial community, and biogenic methane potential in coal mines located in the Red River Basin (RRB), Vietnam. Coal samples from the RRB were characterized and classified into Lignite B and Subbituminous A. Formation water (FW) and coal samples from the investigated boreholes were also analyzed and characterized for microbial communities. Bacterial communities were found to be more dominant (65.8%) than archaea (34.2%) in the FW sample. Four bacterial phyla, including Proteobacteria, Firmicute, Actinobacteria, and Bacteroidetes, were found in the FW sample, in which Proteobacteria were the most abundant phylum, accounting for 56% of total identified bacteria. In the Proteobacteria phylum, only Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria were found in the FW sample; and Gammaproteobacteria were the most abundant class (50.5%). At the genus level, more than 30 microbial genera were detected in the FW sample with the relative abundance >0.1%, in which Shewanella was the most abundant genus, followed by uncultured Rhodobacteraceae, Paracoccus, and Pseudomonas. The microbial population in the coal sample was less diverse than that in the FW sample. The final methane yields after 21 days of incubation with coal samples varied from 2.51 to 5.21 mL of methane per gram coal, exhibiting the great potential for the biotransformation of coal to methane. The results of this study have shown that the presence of indigenous microbial consortia along with available substrates can play an important role in the generation of biogenic methane in coal mines.

Thermogenic gas controls high saturation gas hydrate distribution in the Pearl River Mouth Basin: Evidence from numerical modeling and seismic anomalies

We integrate three-dimensional (3D) seismic data, well log and two-dimensional (2D) modeling to understand the accumulation and saturation of gas hydrates and their relationships with biogenic and thermogenic gases in the Pearl River Mouth Basin, South China Sea (Sites SH2, SH5, W17, and W19). The model based on physical, geochemical and geological parameters from shallow gas hydrate sites, deep oil sites and seismic interpretation, and it is tied to well-log and seismic quantitative interpretation, to predict the main controlling factors for gas hydrate distribution. The modeling results show that the biogenic methane generation is distributed within the strata shallower than 1500 mbsf (meters below the seafloor), whereas thermogenic gas generation ranges from 2300 mbsf to 6000 mbsf. The erosion surfaces of migrating canyons represent important pathways for upward migration of biogenic gas resulting in average hydrate saturation <20% below the ridges of canyons. The total organic carbon content is an important factor related to biogenically-sourced gas hydrate distribution. Higher gas saturation (>40%) at the base of gas hydrate stability zone (BGHSZ) is related to the focused migration of thermogenic gas along faults and throughout permeable carrier beds. Although gas chimneys are widely identified from the coherence attribute extracted from 3D seismic data, their impact on the obviously higher (locally up to 60%) gas hydrate saturations in the fine-grained sediments appears site-dependent and associated with fault activity. By comparing well log-derived gas hydrate saturations, our modeling shows that about 80% of the methane-forming hydrate is thermogenically-sourced at Sites W19 and W17. The integrated modeling effort presented in this work, based on provides a better understanding of the different contributions from different gas sources for gas hydrate accumulation along the northern slope of the South China Sea.

A record of seafloor methane seepage across the last 150 million years

Seafloor methane seepage is a significant source of carbon in the marine environment. The processes and temporal patterns of seafloor methane seepage over multi-million-year time scales are still poorly understood. The microbial oxidation of methane can store carbon in sediments through precipitation of carbonate minerals, thus providing a record of past methane emission. In this study, we compiled data on methane-derived carbonates to build a proxy time series of methane emission over the last 150 My and statistically compared it with the main hypothesised geological controllers of methane emission. We quantitatively demonstrate that variations in sea level and organic carbon burial are the dominant controls on methane leakage since the Early Cretaceous. Sea level controls methane seepage variations by imposing smooth trends on timescales in the order of tens of My. Organic carbon burial is affected by the same cyclicities, and instantaneously controls methane release because of the geologically rapid generation of biogenic methane. Both the identified fundamental (26–27 My) and higher (12 My) cyclicities relate to global phenomena. Temporal correlation analysis supports the evidence that modern expansion of hypoxic areas and its effect on organic carbon burial may lead to higher seawater methane concentrations over the coming centuries.

Enhancing biogenic methane generation in coalbed methane reservoirs – Core flooding experiments on coals at in-situ conditions

Stimulation of microbial methanogenesis on intact coal under in-situ conditions is demonstrated in a series of 18 anaerobic core flooding experiments. The experiments used core samples from producing coalbed methane fields in the Surat Basin and Bowen Basin, with formation water containing methanogenic microbial consortia sourced from within the matching basin. The formation water was amended with nutrients containing phosphorus and nitrogen before being used in core flooding experiments of between 6 and 20 weeks duration. Since the core floods used degassed coal core at reservoir pressure, gas generated during the core flood could be adsorbed and retained by the coal. Therefore the quantity of methane generated was determined at the end of the experiment by decreasing the pore pressure and measurement of the volume of gas desorbed from the core sample. The amount of methane generated, averaged on a per week basis, varied considerably across the different core floods but was up to 7.75 μmol methane per gram of coal per week, comparable to previous studies where biostimulation experiments were conducted on crushed coal particles. During the core floods, the nutrient concentration was measured periodically in inflow and outflow water samples and the rates of nutrient consumption determined. The concentration of methane dissolved in the water samples was also determined and used as an indicator during a core flood of the relative rate of methanogenesis. Plateaus in gas generation were observed in the majority of core floods despite constant nutrient supply, indicating nutrient availability is not the single limiting factor to sustained methanogenesis. Higher gas generating core floods displayed higher rates of utilisation of phosphorus, underlining its importance in stimulation of microbial activity, while nutrient utilisation and gas generation measured in the absence of continuous nutrient delivery suggests the possibility of injection and production from a single CBM well. Results demonstrate the ability to stimulate gas generation on intact coal under reservoir conditions, an important step in development of technology for increasing gas content in depleted or undersaturated CBM fields.

Basin-scale estimates on petroleum components generation in the Western Black Sea basin based on 3-D numerical modelling

A new numerical model reconstructing the depositional history (98–0 Ma) of the Western Black Sea sub-basin is presented. The model accounts for changing boundary conditions (i.e. water depth, bottom water temperature, heat flow evolution over time) and estimates the rates and total amounts of the in-situ biogenic methane generation and thermally-driven organic matter maturation in the source rocks. The overall thermogenic and biogenic gas generation predicted by the model is estimated at ~1560 Gt and ~3100 Gt, respectively.

Evaluating indigenous diversity and its potential for microbial methane generation from thermogenic coal bed methane reservoir

Microbial enhanced coal bed methane process has emerged as cheaper, feasible and environment friendly approach for enhancing coal bed methane (CBM) generation worldwide.Despite the fact that India has tremendous potential for CBM, there are many challenges towards microbial translation of CBM. Therefore, to explore the possibility of microbial enhanced CBM generation from the deep thermogenic CBM reservoir, diversity of the microbial community was investigated using the 16S rRNA gene amplification methods. Additionally, a culture dependent analysis from the formation water of the reservoir was performed to access the ability of the indigenous microbes to produce methane from coal when enriched with suitable nutrient supplements. Culture independent data revealed the nature of communities i.e. bacterial and archaeal domain, majorly from thermophilic profile. The bacterial community was dominated by proteobacterial and Firmicutes taxa, while archaeal community was dominated by Euryarchaeota taxa (acetoclastic methanogens). The enrichment cultures derived from the formation water were able to produce methane (1002 µmol/g coal) at 45 °C. The 16S rRNA gene analysis depicted that the enriched microbial culture was dominated with anaerobic thermophilic fermentative bacteria of Clostridiales family and acetoclastic Methanosarcinales methanogens. The present study depicts the biogenic methane generation in the thermogenic-gas containing CBM reservoir could be enhanced by stimulating the indigenous microorganisms through providing suitable nutrient supplements. Present investigation is likewise the prime response to deliberate about the capability of thermogenic CBM reservoir (temperature > 40 °C) in India for biogenic methane generation through culture independent and culture dependent examination.

Effect of chemical structure of lignite and high-volatile bituminous coal on the generation of biogenic coalbed methane

Biogenic coalbed methane has been attracted much attention due to massive reserves and green performance, but the current research is relatively weak. To further investigate its formation mechanism, the experimental simulation of biogas generation was performed by using lignite, high-volatile bituminous coals and their treated coals as substrates for 90 days. The results show that the lignite has a greater biogas potential than high-volatile bituminous coal in this study. Total gas production in lignite groups (L-BZ-1 and L-LJ-1) is 12.6% more than the bituminous coal groups in the experimental simulation. This is because the lignite has a low degree of thermal evolution and rich in soluble organic matter: chloroform asphalt “A” (CAA), which plays an important role in microbial metabolism to produce biogas. The CAA in coal has a conversion rate of 3.08 mL/g into biogas in this experimental simulation. The saturated hydrocarbons and nonhydrocarbons are more biodegradable components in the CAA. In saturated hydrocarbons, the n-alkanes with shorter chains or an odd carbon number are preferentially degraded. In the experimental simulation, biogas generation results in the consumption of a large number of organic matters with heteroatomic groups such as hydroxyl, carboxyl, pyridine and nitrogen oxides. Aliphatics show a stronger bioactivity than aromatics. The aromatic structure in coal is a few biodegraded in the later stage.

Potential and Constraints of Biogenic Methane Generation from Coals and Mudstones from Huaibei Coalfield, Eastern China

Coalbed methane (CBM) resources formed biogenically and thermogenically have been discovered in the Permian coalbeds of the Huaibei coalfield. Four coals, three mudstones, and five coalbed-produced water samples collected from the Linhuan, Luling, and Haizi coal mines of the Huaibei coalfield were characterized geochemically and biologically to gain an understanding of the biogenic methane generation potential and the microbial communities involved in situ and in coalbed-produced water-enriched samples. The 16S rRNA gene high-throughput sequencing results showed that the archaeal communities from in situ and enriched cultures were dominated by Methanolobus and Methanobacterium species. The organic material of coals and mudstones could be biodegraded under an anaerobic incubation. The maximum biogenic methane generation potentials of coal and mudstones were 98.5 and 72.5 μmol/g within 123 days, respectively. Volatile matter and total organic carbon (TOC) content were the most important internal factors aff...

The role of meteoric water recharge in stimulating biogenic methane generation: A case study from the Tempoku Coal Field, Japan

While meteoric water recharge is known to stimulate biogenic methane formation in shale and coal seams, the underlying mechanisms are currently unresolved. To this end, we conducted fieldwork in the Tempoku Coal Field, Japan. Pore water in core samples and well water in boreholes were analyzed for dissolved components and isotopic compositions. The hydraulic gradient was determined using values of borehole hydraulic head. Cl− concentrations (38 mg L−1 to 16,600 mg L−1), δ18O(H2O) values (−10.5‰ to −2.4‰), and δD(H2O) values (−71.6‰ to −17.8‰) increased with depth (<200 m), indicating meteoric water recharge. The positive correlation between δ13C(CH4) values (−74.9‰ to −42.7‰) and δ13C(CO2) values (−27.5‰ to +13.3‰), as well as between δD(H2O) and δD(CH4) values (−264‰ to −200‰), in the core samples and groundwater, indicted in situ methanogenesis in the zone of mixing and migration of meteoric water and saline groundwater. Some of the pore water samples contained biogenic acetate, propionate, and succinate at remarkably high concentrations (~200 mg L−1), implying: (i) the thermodynamic inhibition of fermentation at fermentation sites, and (ii) spatial separation between the fermentation sites and methanogenesis sites. Planar fracture modeling indicates that at distances greater than a millimeter between fermentation and methanogenesis sites, the advective transport of fermentation products dominates rather than diffusive transport of those. Hence, meteoric water recharge would stimulate biogenic methane formation by inducing advective transport of the fermentation products, thus (i) relaxing the thermodynamic inhibition of fermentation at the site of the fermentation, and (ii) enhancing the rate of transport of the fermentation products to the site of methanogenesis.

Long-term succession in a coal seam microbiome during in situ biostimulation of coalbed-methane generation

Despite the significance of biogenic methane generation in coal beds, there has never been a systematic long-term evaluation of the ecological response to biostimulation for enhanced methanogenesis in situ. Biostimulation tests in a gas-free coal seam were analysed over 1.5 years encompassing methane production, cell abundance, planktonic and surface associated community composition and chemical parameters of the coal formation water. Evidence is presented that sulfate reducing bacteria are energy limited whilst methanogenic archaea are nutrient limited. Methane production was highest in a nutrient amended well after an oxic preincubation phase to enhance coal biofragmentation (calcium peroxide amendment). Compound-specific isotope analyses indicated the predominance of acetoclastic methanogenesis. Acetoclastic methanogenic archaea of the Methanosaeta and Methanosarcina genera increased with methane concentration. Acetate was the main precursor for methanogenesis, however more acetate was consumed than methane produced in an acetate amended well. DNA stable isotope probing showed incorporation of 13C-labelled acetate into methanogenic archaea, Geobacter species and sulfate reducing bacteria. Community characterisation of coal surfaces confirmed that methanogenic archaea make up a substantial proportion of coal associated biofilm communities. Ultimately, methane production from a gas-free subbituminous coal seam was stimulated despite high concentrations of sulfate and sulfate-reducing bacteria in the coal formation water. These findings provide a new conceptual framework for understanding the coal reservoir biosphere.

Biogenic methane generation using solutions from column reactions of lignite with hydrogen peroxide

A microbial consortium associated with coal from the coal-bearing Soya Formation in the Tempoku Coalfield (northern Hokkaido, Japan) was cultivated with reaction solutions of lignite and hydrogen peroxide (H2O2) (i.e., chemically solubilized lignite) to evaluate in situ biogenic methane generation. Column experiments with H2O2 flowing through pulverized lignite achieved maximum concentrations of dissolved organic carbon, acetic acid, and formic acid of 6330, 612, and 1810 mg/L, respectively. Cultivation experiments using the above reaction solution as a substrate for methanogens produced nearly 6 cm3 CH4 per g lignite with a maximum rate of 0.14 cm3 per g per day. Pyrosequencing analysis of the microbial consortium after cultivation confirmed the presence of methanogens, such as Methanoregulaceae and Methanosarcinaceae, as the dominant archaea in the culture. The observed production of biogenic methane from a reaction solution of lignite and H2O2 under the simulated conditions indicates the potential for successful in situ microbially enhanced coalbed methane generation using H2O2.