Microbially Enhanced Coalbed Methane Research Articles

Biogenic methane clumped isotope signatures: Insights from microbially enhanced coal bed methane

Biogenic (microbial) methane is an imperative natural gas resource and plays an important role in the global carbon cycle. Determining isotope systematics in microbially enhanced coal bed methane (MECBM) informs methane biogeochemical processes and environmental impacts. Here, we conduct laboratory MECBM experiments using coal samples of differing maturities (from lignite to anthracite) and report analytical results of methane production, bulk and clumped isotopes (δ13C, δD, Δ13CH3D and Δ12CH2D2). We reproduce the differential reversibility in methanogenic reactions, which exhibits a correlation with kinetic clumped isotope fractionation (Δ13CH3D from +0.17 ‰ to +4.04 ‰). The exogenous combinatorial effects play a greater role in determining the negative Δ12CH2D2 (<–30 ‰). In the microcosm of MECBM, coal and amendments are suggested to significantly increase methane production (from 82.4 ± 17.1 to 344.6 ± 22.8 μmol CH4/g coal), with effects controlled by substrate coal bioavailability rather than coal thermal maturity. This suggests the possibility of MECBM attempts to utilize high thermal maturity coal. We perform machine learning models and improve the accuracy of biogenic 12CH2D2 predictions, which help understand the mechanism of methane formation, and further provide new insights into clumped isotope effects among different biogenic methane sources. Our incubation experimental and compiled dataset (n = 275) better constrain biological participation in the methane formation process and potentially be used for methane sources tracing.

The effect of organics transformation and migration on pore structure of bituminous coal and lignite during biomethane production.

Biomethane generation by coal degradation not only can increase coalbed methane (CBM) reserves, namely, microbially enhanced coalbed methane (MECBM), but also has a significant effect on the pore structure of coal which is the key factor in CBM extraction. The transformation and migration of organics in coal are essential to pore development under the action of microorganisms. Here, the biodegradation of bituminous coal and lignite to produce methane and the cultivation with inhibition of methanogenic activity by 2-bromoethanesulfonate (BES) were performed to analyze the effect of biodegradation on coal pore development by determining the changes of the pore structure and the organics in culture solution and coal. The results showed that the maximum methane productions from bituminous coal and lignite were 117.69μmol/g and 166.55μmol/g, respectively. Biodegradation mainly affected the development of micropore whose specific surface area (SSA) and pore volume (PV) decreased while the fractal dimension increased. After biodegradation, various organics were generated which were partly released into culture solution while a large number of them remained in residual coal. The content of newly generated heterocyclic organics and oxygen-containing aromatics in bituminous coal was 11.21% and 20.21%. And the content of heterocyclic organics in bituminous coal was negatively correlated with SSA and PV but positively correlated with the fractal dimension which suggested that the retention of organics contributed greatly to the decrease of pore development. But the retention effect on pore structure was relatively poor in lignite. Besides, microorganisms were observed around fissures in both coal samples after biodegradation which would not be conducive to the porosity of coal on the micron scale. These results revealed that the effect of biodegradation on pore development of coal was governed by the combined action of organics degradation to produce methane and organics retention in coal whose contributions were antagonistic and determined by coal rank and pore aperture. The better development of MECBM needs to enhance organics biodegradation and reduce organics retention in coal.

Multiple-experimental investigation on the physicochemical structures alternation during coal biogasification

The aim of microbially-enhanced coalbed methane (MECBM) generation is to replicate the natural process of microbial methane production in coal seams and to increase coalbed methane production. However, a systematic evaluation of the response of coal structure during biogasification is still lacking. Here, alternations of the pore and molecular structures were investigated during the bioconversion of coal to methane using proximate/ultimate analysis, scanning electron microscope, low-field nuclear magnetic resonance (LNMR), low-pressure nitrogen adsorption (LPNA), micro-Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The results showed that MECBM is a process of carbon enrichment, nitrogen/sulfur fixation and dehydrogenation/deoxygenation. Moreover, microbial modify coal pore structure by destroying aliphatic side chains, dissociating small molecular clusters and reducing the degree of aromaticity. These changes in the coal pore structure result in the “loose” of the coal reservoir spatial structure. We found that micropores (<10 nm) became transition pores (10–100 nm) and mesopores (100–1000 nm) after microbial degradation, which then led to the increase of the average pore width and the reduction of the specific surface area. This process benefits the desorption and transportation of coalbed methane. In addition, the surface fractal dimension decreased, and the volume fractal dimension increased according to the fitting date of LPNA. The fractal dimension of transition pores and mesopores increased of most coal after biotreatment based on NMR. These results led us to draw the hypothesis that the organic compounds formed by biodegradation may adhere to the pores, thus blocking the pores, making the structure of transition pores and mesopores complex, as demonstrated by the SEM images. This phenomenon is thought to be improved by continuous degradation or water movement.

Corrigendum to “Extending melengestrol acetate feeding from 14 to 18 days during an estrus synchronization and artificial insemination protocol for March-calving beef heifers” (Appl. Anim. Sci. 38:499–504)

The discovery that approximately 20% of natural gas is microbial in origin has elevated interest in microbially enhanced coalbed methane (MECoM). However, a rational approach to exploit this calls for the development of reservoir scale models that include the effect of microbial activity. To address this, we have developed a multiscale, multiphase, multicomponent reactive-transport model for the production of microbially enhanced coalbed methane (MECoM) that includes microbial kinetics. The model is used to evaluate field scale strategies for commercial MECoM production. Optimization studies are also conducted over a range of compositions of the injected nutrient and injector bottomhole pressures. In order to account for the effect of uncertainty in the model parameters, mean-variance robust optimization is performed, allowing a trade-off between performance and robustness. Proxy modeling is performed in a multivariate polynomial chaos expansion framework to evaluate the cost functions involved in the robust optimization and sparse expansions are constructed in order to deal with issues related to high dimensionality. The optimization strategy is tested for different trade-offs between robustness and performance. It is observed that for the given case, the location of robust optimal points does not vary unless only robustness is included in the objective function, and nominal performance is not.

Impact of Microbially Enhanced Coalbed Methane on the Pore Structure of Coal

Microbially enhanced coalbed methane (MECBM) has important theoretical and practical significance for reforming coal reservoir structure, alleviating greenhouse effects and energy crises and developing new sources of clean energy. In this study, No. 3 coal seams in Qinshui Basin were taken as research objects to analyze the pore structure characteristics after microbial treatment by means of low-temperature nitrogen adsorption (LTNA), mercury porosimetry (MP), and isothermal adsorption/desorption experiments. The results showed that after bioconversion, the specific surface area and pore volume increased from 1.79 m2/g and 0.0018 cm3/g to 4.01 m2/g and 0.0051 cm3/g respectively under liquid nitrogen testing; however, the specific surface area was reduced from 5.398 m2/g to 5.246 m2/g and the pore volume was increased from 0.053 cm3/g to 0.0626 cm3/g under MP. The fractal dimension based on the LTNA data indicated that the fractal dimension of micropores and minipores was increased from 2.73 to 2.60 to 2.89 and 2.81, however the fractal dimension of meso-macropores was decreased from 2.90 to 2.85. The volatile matter and fixed carbon were both reduced from 6.68% to 78.63%–5.09% to 75.63%, and the Langmuir volume and Langmuir pressure were increased from 34.84 cm3/g and 2.73 MPa to 36.34 cm3/g and 3.28 MPa, respectively. This result indicated that microorganism participated in the degradation of coal reservoir and promoted the production of methane gas, the meso-macropores were more obviously modified by microorganism, so that the pore diameter stabilized, the pores became smoother, the specific surface area decreased, and the pore volume increased. These are more beneficial to the adsorption and production of coalbed methane (CBM) after microbial treatment.

Feasibility study of enhanced biogenic coalbed methane production by super-critical CO2 extraction

Super-critical CO2 enhanced coalbed methane (Sc-CO2-ECBM) and microbially enhanced coalbed methane (MECBM) are environment-friendly technologies that can improve CBM recovery and generation. In this study, a new approach of MECBM based on Sc-CO2 extraction is presented. The Sc-CO2 pretreatment experiments of anthracite and bituminous coal were conducted to produce biomethane. The extracted organics and the changes of coal structure caused by Sc-CO2 were also analyzed to discuss the mechanism of methane stimulation. The results indicated that methane yields have been greatly improved after Sc-CO2 extraction by 734.85% and 148.15% for anthracite and bituminous coal, respectively. The extractions observed by GC-MS analysis is also favored to generate methane by microorganisms. The little increment of methane production from coal treated by subcritical CO2 indicated that the special characteristics of Sc-CO2 were critical for the stimulation of methane production. More functional groups and even new functional groups were formed to increase coal bioavailability after Sc-CO2 extraction. The specific surface area and total pore volume of bituminous coal increase after Sc-CO2 extraction that could provide more action sites for microorganisms and enzymes. These results strongly proved the feasibility of enhancing CBM by microbial degradation based on Sc-CO2 extraction.

Importance of specific substrate utilization by microbes in microbially enhanced coal-bed methane production: A modelling study

This study addresses a major gap in the understanding and control of microbially enhanced coal-bed methane (MECBM) production. A mathematical and conceptual model comprises a food-web that includes two types of bacteria and three types of archaea representing substrate-specific members of the community; the microbial community members are potentially interacting by competing for or being inhibited by substrates or products of other microbial community members. The model was calibrated using data sets from two different experimental setups. The calibrated model effectively predicted the methane concentrations within a 7% range of deviation from the experimental results. The results of additional batch experiments using varied conditions are also reproduced in an attempt to validate the model and to test the hypothesis of amendment-induced stimulation of microbial community members capable of converting coal into substrates available to methane producing microbes. This study significantly enhances the understanding of the complex interactions between microbial activity, substrate-specificity and bio-availability of coal for methane production, and provides the basis for including hydraulic flow and transport processes into future mathematical models important for the design and implementation of more sustainable methods of harvesting methane from un-mineable coalbeds.

Impact of bioconversion on matrix strain response of coal reservoirs: Part 1-Experimental insights

The technical feasibility of commercial gas production from microbially enhanced coalbed methane (MECBM) reservoirs can be divided into two broad categories: microbial treatment procedures and reservoir suitability. At this time, studies investigating the changes in reservoir-scale properties of coal as a result of bioconversion are lacking. This paper, first of a two-part series, presents the experimental work aimed at investigating the changes in sorption-induced strain response of coal to helium and methane flooding as a result of bioconversion. The strain and its variation, critical to producing methane after bioconversion, were established for coal treated for three periods, 15, 30 and 60 days. The results indicated a softer coal post-bioconversion which, in turn, resulted in inelastic strain with initial helium injection. However, past this initial compression, the bioconverted coal behavior was similar to untreated coal during continued helium injection/depletion. Methane flooding resulted in decreased sorption-induced shrinkage post-treatment, the decrease in Langmuir pressure and volume constants becoming larger with continued treatment. The observed results were used to understand the changes in coal structure for production of biogenic coalbed methane (BCBM) from depleted CBM reservoirs, recharged with methane using the bioconversion technology. The second part presents a detailed analysis of the experimental results to evaluate the behavior and performance of BCBM reservoirs.

Hydraulic fracturing for improved nutrient delivery in microbially-enhanced coalbed-methane (MECBM) production

Microbially enhanced coalbed methane (MECBM) recovery is a novel method to increase gas production by injecting nutrients, either with/without microorganisms, in depleted CBM wells. However, to be effective, methanogens require that the nutrient must be delivered efficiently by aqueous solution to a maximally large reservoir volume for microbial colonization. This study seeks to improve understanding of solute transport and microbial gas generation in naturally fractured reservoirs that are both pristine and hydraulically fractured. We complete a field-scale numerical simulation using an equivalent multi-continuum method to define the effectiveness of nutrient delivery. The complex pre-existing fracture pattern in the coalbed is represented by an overprinted discrete fracture network (DFN) to capture the natural heterogeneity and anisotropy of fracture permeability. A simplified PKN model is adopted to simulate hydraulic fracture propagation based on linear elastic fracture mechanics (LEFM). The hydraulically stimulated case is compared to the untreated control case, both without and with a network of natural fractures. Saturated cleat area, cumulative injection volume and prediction of methane yields are systematically modeled and analyzed for all three cases. We show that hydraulically stimulated fracture pathways, especially when connecting with a natural fracture network, optimally deliver nutrient remotely from the injection well, thereby increasing nutrient delivery and improving methane production and potential recovery. However, large magnitudes of proppant embedment and related permeability loss in the hydraulic fractures may reduce MECBM recovery. In the optimal production scenario, the methane production rate may reach 31 ft3/ton, an approximately 5-fold increase over that from the pristine unstimulated case.

An imaging and fractal approach towards understanding reservoir scale changes in coal due to bioconversion

Microbially enhanced coalbed methane (MECBM) aims to replicate the natural process of microbial methane generation in coal under in situ conditions. Considerable work has been reported over the years to optimize the associated microbial geo-chemistry. However, there is very little insight with regards to changes in the physical structure of coal due to bioconversion, and its impact on fluid flow properties. This paper presents the result of an image and fractal-based approach used to evaluate the changes in physical properties of coal. Samples of coal treated over 30, 60 and 120 days respectively were imaged before and after bioconversion. The results revealed that coal bioconversion resulted in swelling of the coal matrix. Cleats narrower than 5 µm reduced in its width post-bioconversion. A 77% drop in the permeability of coal is expected from the obtained results. Bioconversion also resulted in separation of thin layers of coal flakes from the coal surface, serving as a potential source of fines. Fractal analysis of the images revealed a decrease in the fractal dimension post bioconversion, which is in agreement with the previously reported studies, thus corroborating the variations in sorption-trends observed due to bioconversion. Additionally, the methodology to determine the fractal dimension via two-dimensional image processing was modified to improve its accuracy, and remove the dependency of the dimension to the scale of the obtained images. Bioconversion also resulted in formation of new pores/fractures. Shorter treatment duration resulted in nanometer-scale discontinuous pores, which do not contribute to Darcian flow. Longer treatment periods resulted in sub-micron wide continuous pores. Also, few larger fractures (>5 µm wide) saw an increase in its aperture post-treatment. This opens up new avenues, such as, utilizing artificially induced fracturing techniques to enhance biogenic methane production in future.

Modeling of microbial methane generation from coal and assessment of its impact on flow behavior

Enhanced microbial conversion of coal to methane has been a topic of research in the scientific community since its inception in the mid-nineties. Significant advances have since been made and the technique is now termed Microbially Enhanced Coalbed Methane (MECBM), with tremendous promise in the energy industry. However, analytical treatment of the bioconversion of coal using mathematical modeling is lacking. This paper is devoted to developing a fundamental outlook of the process of bioconversion of coal to methane using the concept of logistic population growth. The modeling exercise using laboratory results sheds light on the correlation between microbial growth rate, the ability of the environment to support the microbial population and environmental factors controlling the two. Furthermore, simulation of the impact of the technology on modifying the flow behavior of coal under in situ conditions is presented. The results demonstrate that the model can be used as a useful tool to evaluate the effect of any parameter, whether controllable for optimum gas production or an outcome of continued bioconversion, on actual gas production. The analysis of the results shows that the impact of environmental parameters, such as temperature, particle size and pH of the medium on bioconversion process can be optimized. Finally, it is shown that changes in flow behavior of coal are not very significant and can possibly even be negative.

Reaction of lignite with dilute hydrogen peroxide to produce substrates for methanogens at in situ subsurface temperatures

Since the beginning of the 21st century, various works have sought to enhance coalbed methane (CBM) production by stimulating indigenous microbes in coal seams. This concept of microbially enhanced coalbed methane (MECBM) generation relies greatly on the solubilization of the coal, which determines the rate of microbial conversion of the coal into biogenic methane. This study examined the potential of using dilute hydrogen peroxide (H2O2) (0.3%) to solubilize lignite at common subsurface temperatures (10–50°C) via batch experiments at a solid/liquid ratio of 1g/150mL. Dissolved organic carbon (maximum 450mgC/L) and organic acid (formic, acetic, oxalic, malonic, and succinic acid, maximum total of 145mgC/L) concentrations reached their maximum levels within periods of 1.2days (at 50°C) to 45days (at 10°C). H2O2 consumption was a first-order reaction with respect to its concentration, and the linear (r=0.998) Arrhenius plot for the rate constants yielded its activation energy to be 73kJ/mol. If the methanogens convert acetic and formic acids to methane, under the current experimental conditions they can produce about 6.0m3 of methane per ton of lignite. Dilute H2O2 would thus be useful for effective MECBM production at lignite seams. The aromatic-C content of the lignite markedly decreased by about 30% during the reaction, implying high reactivity of catechol structures in the lignite with H2O2.

Multiphase reactive-transport simulations for estimation and robust optimization of the field scale production of microbially enhanced coalbed methane

The discovery that approximately 20% of natural gas is microbial in origin has elevated interest in microbially enhanced coalbed methane (MECoM). However, a rational approach to exploit this calls for the development of reservoir scale models that include the effect of microbial activity. To address this, we have developed a multiscale, multiphase, multicomponent reactive-transport model for the production of microbially enhanced coalbed methane (MECoM) that includes microbial kinetics. The model is used to evaluate field scale strategies for commercial MECoM production. Optimization studies are also conducted over a range of compositions of the injected nutrient and injector bottomhole pressures. In order to account for the effect of uncertainty in the model parameters, mean-variance robust optimization is performed, allowing a trade-off between performance and robustness. Proxy modeling is performed in a multivariate polynomial chaos expansion framework to evaluate the cost functions involved in the robust optimization and sparse expansions are constructed in order to deal with issues related to high dimensionality. The optimization strategy is tested for different trade-offs between robustness and performance. It is observed that for the given case, the location of robust optimal points does not vary unless only robustness is included in the objective function, and nominal performance is not.

Microbial controls on the origin and evolution of coal seam gases and production waters of the Walloon Subgroup; Surat Basin, Australia

The Walloon Subgroup coal seam gas (CSG) play in the Surat Basin, Queensland, is Australia's pre-eminent onshore gas field. Concerted multi-disciplinary research is underway investigating the distribution, origin and composition of waters and gases in this dominantly microbial CSG reservoir, to guide both continued production and potential microbially enhanced coal bed methane (MECoM) applications. However, prior to the present research, a detailed study of co-produced waters and gases from across the Surat Basin was not available in the public domain. This study tested whether co-produced water compositional and stable isotopic data show relationships with production gas stable isotope compositions, to elucidate further evidence for microbial CO2 reduction and explore the down-dip geochemical evolution of Walloon coal bed waters and gases. A total of 41 wells were sampled with 50 water and 25 gas samples spanning the 3 major production areas of the Surat Basin. Detailed isotopic and hydrochemical analysis of these samples revealed distinct spatial trends between the different production locales. Water compositions were distinct for each of the production regions reflecting the different lithologies of adjacent recharge zones, differing fluid–rock interactions, likely different microbial consortia, and the extent of methanogenesis. On the western side of the basin near Roma, waters were the ‘freshest’ with the lowest median values for alkalinity (861mg/L), and Cl− (588mg/L) and a δ13CDIC of 14.2‰. On the eastern side of the basin, the Kogan Nose waters were the most saline with the highest median values for Na+ (1955mg/L), Cl− (2280mg/L) and δ13CDIC (20.0‰). Also in the east, in the present gas fairway, the Undulla Nose waters had the highest median alkalinity (1841mg/L) and were found to have a Na+ excess (median=1050mg/L) and a lower than expected median δ13CDIC (14.0‰). Co-mingled, produced methane carbon isotope values (δ13C −57.0‰ to −44.5‰) from both the upper (Juandah) and lower (Taroom) coal measures plot within the mixed ‘thermogenic/microbial’ genetic field. By contrast, deuterium isotopic difference [Δ2H(H2O–CH4)] values and cross-plots of δ2H–H2O and δ18O–H2O suggest that microbially mediated CO2 reduction is the dominant methane generation process in situ. At a given depth, the Undulla Nose waters in the east are more depleted in 2H and 18O than elsewhere in the Surat Basin, which may suggest these samples have been more heavily impacted by water–rock–microbial reactions. 14C values from the 3 production regions (0.115 to 1.769pmC; age: 32,400 to >50,000years before present (B.P.)) suggest that Walloon coals likely recharged in the last ~50,000years (limit of radiocarbon dating). Consistent with these dates, δ2H–H2O and δ18O–H2O values for the Surat Basin (δ2H −32‰ to −56‰, δ18O −5.9‰ to −9.0‰) echo the stable isotopic composition of meteoric waters during the initial part of the last glacial period in southeast Queensland. Based on a strong correlation between δ2H–CH4 and δ2H–H2O, we suggest that methane was generated since the Late Pleistocene. PCA analysis showed a degree of positive correlation between total alkalinity and both the δ13CDIC (median 14.2‰) and δ13C–CH4 (median −52.1‰) vectors that is consistent with finite reservoir effects. The results inform ongoing studies of gas distribution and origins and MECoM potential in the Surat Basin, and underpin a broader study examining aquifer interactions.

Conceptual exploration targeting for microbially enhanced coal bed methane (MECoM) in the Walloon Subgroup, eastern Surat Basin, Australia

The sustainable in situ regeneration of microbial (biogenic) methane or microbially enhanced coal bed methane (‘MECoM’) is an emerging concept being investigated globally. Promising results and recommendations of a preliminary culture study of Walloon Subgroup co-produced coal seam gas (CSG) waters from the Surat Basin, Queensland established the presence of viable methanogens and suggested that in situ methanogenesis could be stimulated using physical and chemical reservoir treatments. This paper represents the culmination of a stepwise basin analysis project ultimately aimed at siting potential in situ bioreactor locations in the eastern Surat Basin. The integration of stratigraphically located data on the molecular and isotopic composition of desorbed gases, host coal properties and spatially associated waters through the core production zone has allowed the spatial variability and relative influence of hydro-geological factors on methanogenesis to be evaluated in detail for the first time. Higher gas contents and systematically enriched CH4 and CO2 carbon isotopic compositions in the stratigraphically central coal seams (oldest-youngest: in the upper Taroom Coal Measures, Tangalooma Sandstone lower Juandah Coal Measures) are best explained by increased rates of microbial CO2 reduction and substrate depletion. There is a building case to trial MECoM in the central coal seams in a depleted/underperforming well in an area of high permeability. Integrated microbiological, chemical engineering, hydro-chemical and geological studies are ongoing to further enhance understanding of Walloon Subgroup CSG and the bioreactor potential of the Surat Basin.

Stable isotopic and molecular composition of desorbed coal seam gases from the Walloon Subgroup, eastern Surat Basin, Australia

This study used compositional and stable isotopic analysis to test hypotheses on the distribution and origins of Walloon Subgroup coal seam gas (CSG) in the eastern Surat Basin, Queensland, Australia. The Middle Jurassic Walloon Subgroup play differs from many other low-rank CSG plays—particularly in methane carbon isotopic signature, i.e., the CSG is not as ‘microbial’ as could be expected. The carbon isotope compositions of desorbed methane from three cored appraisal wells fall within the generally accepted range for thermogenic or mixed gas (δ13C −58.5‰ to −45.3‰). The δ13C–CH4 values from stratigraphically placed coal core samples increased (became more ‘thermogenic’) from the top of the upper (Juandah) coal measures to the base of the Tangalooma Sandstone. Below the Tangalooma Sandstone, in the lower (Taroom) coal measures, the δ13C–CH4 values decreased with increasing depth. These positively parabolic δ13C profiles tracked total measured gas content in two out of the three wells studied. The third well displayed lower variance of δ13C–CH4 and gas content increased uniformly with depth.A genetic classification based on methane stable carbon isotopes alone might interpret this pattern as a transition from microbially- to thermogenically-sourced methane in the central coal seams. However, a δ13C–CO2 profile for one well tracks total gas content and δ13C–CH4, and exhibits an inverse relationship with δD–CH4. These results, together with the mostly dry nature of the gas samples [(C1/(C2+C3)) ratios up to ~10,000] and relatively uniform δD–CH4 values (δD −238‰ to −202‰), suggest that microbial CO2 reduction is the primary source of Walloon Subgroup methane. As such, stratigraphic variations in gas content mainly reflect the extent of microbial methanogenesis. We suggest that peak microbial utilisation of H2–CO2 occurred at the Tangalooma Sandstone level, enriching the residual CO2 pool and derived methane in 13C. Carbon [Δ13C(CO2–CH4)] and deuterium isotopic differences [ΔD(H2O–CH4)], and cross-plots of δD–H2O and δ18O–H2O are also consistent with kinetic isotope fractionation during microbial-mediated carbonate reduction. The results are relevant for applying microbially enhanced coal bed methane (MECoM) in the Surat Basin.