Microbial methane from carbon dioxide in coal beds - usea.org methane... · Coal mine methane (CH 4) has posed a danger in working coal mines and is a stronger greenhouse gas than

Microbial methane from carbondioxide in coal beds

Irene M Smith

CCC/174

August 2010

Copyright IEA Clean Coal Centre

ISBN 978-92-9029-494-8

Abstract

Microbial CH4 chemistry and its formation in coal are summarised. The results of recent research on microbial CH4 formation arereviewed from Australia, China, Germany, Japan, and the USA. Two fields of interest are considered in this report. Enhancedmicrobial CH4 production is under investigation to improve the yield of CBM. A further development is to inject CO2 in coalseams, using microbes to convert it to CH4 for recovery.

There is reliable evidence for the production of CH4 through recent microbial activity in coal beds. Microbial CH4 may beenhanced artificially and there is an incentive to turn CBM, where appropriate, to a continuously renewing system. Theintroduction of large quantities of CO2 from carbon capture systems could have a favourable effect on microbial CH4 formation.However, there are uncertainties about the supply of H2 which is essential to form CH4. Field and laboratory studies are inprogress to address gaps of knowledge in the geology, microbiology and engineering.

BGR Federal Institute for Geosciences and Natural Resources, GermanyBMBF Federal Ministry of Education and Research, GermanyCBM coal bed methaneDMF dimethylformamideIEA International Energy AgencyPRB Powder River BasinUQ University of Queensland, Australia

2 IEA CLEAN COAL CENTRE

Acronyms and abbreviations

Acronyms and abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2 Methane production from coal beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1 Microbial methane chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 Microbial methane formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Sources of H2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2 Studies in different countries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Enhanced microbial methane production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.1 Enhanced CBM production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143.2 CO2 sequestration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

4 Ongoing and planned research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3Microbial methane from carbon dioxide in coal beds

Contents

IEA CLEAN COAL CENTRE4

Coal mine methane (CH4) has posed a danger in working coalmines and is a stronger greenhouse gas than CO2, but isrecognised as a valuable source of energy. Coal bed methane(CBM) from unmined coal production in the USA accountsfor more than 70% of global CBM production, which exceeds59 Gm3. The remainder is produced in descending order ofproduction by Australia, India, Canada, China, the UK,Colombia, Russia, Ukraine, and Austria (Flores, 2008). Powerplants fired with coal mine CH4 have been operating inAustralia, China, France, Germany, Poland, Russia, theUkraine, and the USA. The most favourable outlook forfurther power projects was identified for Australia, China, andGermany. Many countries in Eastern Europe, Africa andSouth America are interested in such projects but most wouldrequire international expertise and funding to ensure theirsuccess (Sloss, 2005, 2006).

Biological processing of coal was demonstrated in the early1980s, with bacteria solubilising part of the organic phase ofhard coal in Germany and wood-rot fungi solubilising lowerrank coal in the USA. Early findings showed that low rankcoals were more susceptible to biosolubilisation. Additionally,this susceptibility decreased progressively with increasing rankand increased with pre-oxidation (Catcheside and Ralph, 1999).In Germany, CH4 from active and abandoned mining areas isbeing used increasingly for heat and power production,especially after the introduction of the renewable energy lawin 2000, which gives fiscal benefits to renewable energysources. Hence there is interest in the discovery that part of theCH4 has been produced recently by microbes (Krger andothers, 2008). Long-term (14 months) observations in the RuhrBasin, Germany, showed that the carbon source for microbialCH4 might be coal, but could also be other fossil or recentbiomass. Further, between 38% and 90% of the recentlysampled CH4 was of microbial origin. The proof of livingmethanogenic microbes in mine water made it possible that atleast a portion of this microbial CH4 had been producedrecently by CO2 reduction (Thielemann and others, 2004a).

This raises the question as to whether CO2 stored in coal beds,after capture from power plants, could be used to enhancemicrobial CH4 production as a source of renewable CH4. Ifso, this CO2 sequestration would be a means of completelyutilising the coal, recycling CO2, and would complementunderground coal gasification (see the detailed review byCouch, 2009). These processes would also increase the globalresource of useable coal for gas production.

Enhanced microbial CH4 production concerns firstlyenhanced CBM production and secondly the effects of CO2sequestration on microbial activity. There are many gaps inthe knowledge of various aspects of this topic. These arebeing addressed by ongoing and planned research in severalcountries. More information on technologies that can reducegreenhouse gas emissions derived from the use of fossil fuelsis provided at www.ieagreen.org.uk by the IEA GreenhouseGas R&D Programme, which is also an implementingagreement of the International Energy Agency (IEA).


This review focusses on biogenic CH4 production from coalmines. There are also thermogenic sources of CH4 which arenot discussed here. It is necessary to distinguish between theorganic setting which is mainly coal and the CH4 associatedwith inorganic, sedimentary rocks. The formation ofmicrobial CH4 and the associated chemistry is summarisedfrom results of recent studies in Australia, China, Germany,Japan, and the USA.

1 Introduction

www.ieagreen.org.uk

Biogenic CH4 production, or methanogenesis can occurnaturally in coal beds under specific geochemical conditionsin the presence of methanogenic organisms. This CH4 maycontribute to undesirable CH4 emissions or provide a usefulenergy source as CBM. The biogenic CH4 production ratemay be affected by coal rank, the availability of carbon (C),the presence of microbes, nutrient limitations, temperatureand pressure, as well as coal surface area. In general, biogenicgas production occurs on the order of 100s to 1000s of years,but sometimes the rates are too low to be detected (Budwill,2008a,b).

In addition to the background production of microbial CH4in coal bed reservoirs, there is a great potential to enhanceworldwide CH4 reserves through the chemical conversion ofCO2 to CH4, during CO2 sequestration. This is very much stillin the research stage (Massarotto, 2009).

2.1 Microbial methane chemistry

The biochemistry of methanogenesis has been detailed inearlier reports, for example, by Whiticar (1999) and Thauer(1998). The formation of microbial CH4 is complex. Methaneis produced from the reduction of CO2 by over five reactionpaths, involving eight enzymes, mostly at pH 6.87.2. Thesimplified reaction is:

4H2(g) + CO2(aq) r CH4(g) + 2H2O (liq) 1

The reaction is isothermal with a release of energy upto -140 kJ/mol, at hydrogen (H2) and CH4 partial pressures of1 bar (0.1 MPa) and for CO2(aq) and H2O(liq) of 1 mol/L. AsH2 is only present naturally in low concentrations, the energyrelease through this reaction is usually only 0 to -20 kJ/mol(Thielemann and others, 2004a,b).

The conversion of complex substrates such as coal to CH4requires fermentative and acetogenic bacteria in addition tomethanogens. First fermentative bacteria hydrolyse and thenferment complex substrates to produce acetate, longer chainfatty acids, CO2, H2, NH+4, and HS. Acetogenic bacteriaconsume H2 and CO2 to produce more acetate. In addition,they can demethoxylate low-molecular weight, ligneousmaterials and ferment some hydroxylated aromaticcompounds to produce acetate. Acetogens which produce H2,convert fatty acids, alcohols and some aromatic and aminoacid to the H2, CO2, and acetate needed by the methanogens.Low rank coals are especially enriched in lower molecularweight, leachable organic compounds which may beamenable to microbial degradation (Green and others, 2008).

The autotrophic pathway (equation 1) is distinct fromheterotrophic metabolism. The autotrophic pathway dependson bacteria synthesising their own food by taking upinorganic compounds. Heterotrophic metabolism obtainsready made organic food from the environment, for example,the acetotrophic or methylotrophic pathway. The simplified


overall equation for the acetotrophic pathway, which releasesenergy, is (Hoth and others, 2005):

CH3COOH r CH4 + CO2 2

Fermentation reactions that produce H2 are described by:

(CH2O)n + nH2O r nCO2 + 2nH2 3

The reduction of CO2 would occur only with incompletefermentation. The hydrogenic pathway of fermentation maystimulate the autotrophic methanogens. It may hinder acetateconsuming methanogens. In anoxic marine sediments,acetogenic fermentation has been described by:

4H2 + 2CO2 r CH3COOH + 2H2O 4

A combination of acetogenic fermentation (equation 4) andacetate consuming methanogenesis (equation 2) yields asecond pathway of net CO2 transformation to CH4 (with thenet equation 1). Biogenic coal bed CH4 generation throughtransformation of CO2 to CH4 requires large reaction areas,long time scales and the avoidance of leakage to theatmosphere. All these demands can be met in the deepsubsurface. Hence this favours the use of subsurface oversurface technologies (Hoth and others, 2005).

The biogenesis of CH4 is a process that concentrates H2 in thefinal reaction products. The hydrocarbon substrates partiallyconsumed by this process have varying ratios of H2 to C. Coalhas H2:C ratios of less than one whereas oil has H2:C ratioscloser to two. Methane has a ratio of 4:1 and clearlyrepresents the most H2-enriched and reduced form ofhydrocarbon. Hydrogen availability is the limiting factor inestimating the resource potential of a particular substrate(DeBruyn and others, 2004).

2.2 Microbial methane formation

Methane production from a coal substrate is strictly ananaerobic process. Aerobic degradation prior to anaerobicdegradation may be helpful in improving coal bioavailability,but it is not required (Gilcrease, 2010). Anaerobic organismswork in a consortium (collection of different microbialspecies), providing methanogens with acetate or H2 and CO2that then produce CH4. The methanogen electron-transportchain and specific enzymes formed in the process use CO2 asthe final electron acceptor and reduce it to CH4. Salts such assulphates promote growth of sulphate reducers, thatout-compete the methanogens for H2 and acetate, inhibitingmethanogen growth. Other inhibitors include oxygen (O2),nitrates and sulphur. In simple terms, a methanogenic coalbed consortium works as a step-wise process by which someprimary fermenting and hydrolytic microbes might breakdown coals directly to high molecular-weight, H2-richcompounds. These can be consumed by other groups ofmicrobe that convert these products to organic acids. Another

2 Methane production from coal beds

pathway may involve oxidation of organic matter, hosted bycoal, that partially or wholly solubilises the coal into humicacids. These organic acids can be utilised by another group ofbacteria to produce the acetate or CO2 and H2 that themethanogens would need for metabolism. It has beendemonstrated that low rank coals are particularly susceptibleto these initial oxidation reactions. Alternatively,wood-degrading fungi have been shown to be capable ofdepolymerising coal humates into low molecular-weightorganic compounds, like alkanes. These could also beconsumed by the consortia, producing acetate, H2 and CO2(Barker and Dallegge, 2006).

The conclusion from research on methanogenic consortia isthat a sensitive chemical and biological balance appears toexist. This may explain why some coals generate CH4 atpresent and other coals do not, although similar in bulkcharacteristics. For example, in nature, formation waterassociated with CBM appears to have a characteristicchemical composition with alkaline pH that contains sodiumand bicarbonate, but typically lacks sulphate, calcium, andmagnesium. This may be due to the reduction of sulphate bysulphate reducing bacteria and isolation of calcium andmagnesium by chelating agents (Barker and Dallegge, 2006).

Temperature exerts a strong control on the rate ofmethanogenesis. Methanogenesis rates are lower when theambient temperature is below 15C. The rates and overallactivity generally increase at temperatures of about 100C.The optimal temperature for many methanogeniccommunities is about 2040C (Barker and Dallegge, 2006).The temperature range for methanogenic activity iscommonly reported as between 4100C but thermophilicmethanogens may generate CH4 at temperatures >100C (Faizand Hendry, 2006). Other environmental factors that limitmicrobial conversion of coal to CH4 are: pH, salinity,nutrients, trace metals, and coal surface area/bioavailability(Gilcrease, 2008).

Groundwater flow along a coal bed aquifer flushes CH4 andCO2 downstream differentially because of their differentsolubilities and adsorption characteristics; CO2 is about20 times more soluble than CH4 in water at low temperaturesand has a stronger adsorption in most coals. This can lead to aseparation between CO2 and CH4 along the flow path in thecoal bed. The differential transport of coal bed gases bygroundwater flow along coal bed aquifers can essentiallyseparate CO2 from CH4, accumulating gas rich in CO2 atdownstream or near discharge zones if gas traps exist, andretaining gas with less CO2 content in recharge regions. Thismay explain the CO2 and CH4 distribution in the San Juan andPowder River Basins in the USA. Active groundwater flowmay facilitate the late-stage coal bed gas generation bysupporting microbial activities in coal seams. This is due tonutrients, such as alcohols, organic acids, and phenols, carriedin meteoric water along with gas-producing bacteria whichflush microbial communities in coal seams and generatevariable amounts of CO2 and CH4. In addition, O2 carried inmeteoric water may enhance microbial oxidation anddecarboxylation reactions to produce CO2 in coal seams closeto recharge regions. The generated CO2 may be furthertransported to a more reducing environment in coal seams,

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Methane production from coal beds

Microbial methane from carbon dioxide in coal beds

enriched in methanogens, where it is reduced to CH4. Theseactivities result in coal bed gas with distinct chemical andisotopic signatures (Cui and others, 2004).

In some unique basins that host coal bed CH4 such as theVelenje lignite basin, Slovenia, CH4 migrates faster than CO2and accumulates at the subsurface. This accumulation occurswithin the water-saturated layers of the Velenje basin as a freegas phase. The CO2 remains mostly adsorbed in the coal bedstructure or preferentially dissolved in water (Kanduc andPezdic, 2005).

Significant quantities of CO2 are generated from coals duringthermal maturation. Assuming that this CO2 remainsassociated with a sourcing coal bed as uplift or erosionprovide conditions conducive for microbial methanogenesis,the resulting quantities of CH4 generated by complete CO2reduction can exceed the quantities of thermogenic CH4generated by factors of 25 (Kotarba and Lewan, 2004).

Isotopic fractionation of the carbon and hydrogen isotopicsignatures of the CH4 is one of the most useful diagnostictools in the determination of the CH4 source. It is used todistinguish CH4 generated by microbial CO2 reduction fromthermogenic CH4 formation. Biogenic CH4 is generallyisotopically light, with 13C values less than about -55.However, it is affected by other factors such as the isotopiccomposition of the original substrate, the temperature, partialpressure of H2, methanogenic pathways and the species ofmethanogens involved. More definitive insights into the originof these gases were obtained when 13C values werecombined with H2 isotope values of deuterium (D) for CH4and gas dryness indices (see Faiz and Hendry, 2006).

Since both microbial CO2 reduction and methyl fermentation(aceticlastic) reactions can occur during microbial activity incoals, both C and H2 isotopes are necessary to distinguishbetween these two methanogenic pathways. Microbial CO2reduction derives H2 from formation water, whereas microbialmethyl fermentation uses H2 primarily from methyl groups oforganic matter and secondarily from formation water.Methane has a wide range of C and H2 isotope ratios varyingfrom -50 to -110 for 13C and from -150 to -400 forD. Methane resulting from CO2 reduction may bedifferentiated from that derived from microbial methylfermentation by its isotopic characteristics. A summary ofprevious studies (Flores and others, 2008) showed the ranges:

Microbial CH4 13C, D,

CO2 reduction -55 to -110 -150 to -250

Methyl fermentation -40 to -70 -250 to -400

The most comprehensive and fundamental work on this isconsidered to be by Whiticar (1999) and deals with C and H2isotope systematics of bacterial formation and oxidation ofCH4. Also, the biochemistry of methanogenesis issummarised by Thauer (1998).

Carbon and H2 isotopic studies of CH4 from gasesaccompanying bituminous coals and anthracites in coal basins

of Germany, China, the former Soviet Union, TheNetherlands, Australia and Poland revealed high variability ofboth 13C for CH4 from -80 to -12 and D for CH4of -333 to -117. Such high isotopic variations may resultfrom various primary (generation) and secondary (migration)processes. For example, the Upper Silesian and Lublin coalbasins in Poland contain coal bed gases including CH4 andCO2 which had been generated during thermogenic andprobably microbial processes, followed by migration andmixing. After coalification the uplifted coal beds were subjectto erosion, denudation and intensive degassing (Kotarba,2001).

2.2.1 Sources of H2

The supply of H2 may be the most critical factor for theformation of CH4 from CO2 in coal reservoirs and thereforethe methanogenic conversion of sequestered CO2 (Gilcrease,2009). This applies to both heterotrophic and autotrophicpathways (see Section 2.1). The production of H2 gas byanaerobic bacteria is a widely known and well-documentedphenomenon, occurring for example in coal samples in thepresence of water (Faraj and others, 2004). Furtherinformation on microbial production and oxidation of CH4 inthe deep subsurface is provided by Kotelnikova (2002).Unfortunately, no information was found on H2 supplies incoal seams but only on minerals in sandstones and basalts

Although H2 concentrations in the deep subsurface far exceedthat in surface aquatic environments, the H2 content insubsurface fluids and the supply from juvenile geogas is smallcompared to the demand for autotrophic transformation ofsequestered CO2. The H2 concentrations in the deep fluidsrepresent only an intermediate state since H2 is rapidlytransformed. Hence the relatively low H2 concentrations donot imply low H2 supply rates. Earlier data cited showed thatthe H2 content for Fe(III) zones varied in the range0.20.8 nmol/L for SO4 at terminal electron acceptingprocesses from 1 to 4 nmol/L. By contrast, the H2 content inmethanogenic areas may even reach 530 nmol/L (see studiesreviewed by Hoth and others, 2005).

In sandstones, iron rich clay minerals of cements areconsidered to be the most reactive minerals with respect to H2formation. This is because water reduction is commonlyrelated to oxidation of mineral bound ferrous iron. Themineral cements from milled rock samples from the Westfal Csandstone formation at 2800 m depth were analysed todetermine their composition and ability to generate H2.Dissolved H2 concentrations in the tests were 1016 mol/L,exceeding the reported environmental H2 contents of, forexample, sulphate reducing or methanogenic aquifers and ofbasaltic ground waters. The concentrations of H2 formed arecomparable to the calculated H2 fluid concentrations of themid-ocean ridge basalt (Kassahun and others, 2007b). Thus ithas been shown that H2 is generated on iron silicates. Furtherexperiments within the RECOBIO research project (describedin Chapter 4) emphasised the generation of H2 on ironchlorites and its fast consumption, coupled to sulphatereduction (Hoth and others, 2008).

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IEA CLEAN COAL CENTRE

Anaerobic incubation tests of milled rock material frompotential CO2 sequestration sites in sandstone-hosted oil andgas fields in Germany revealed the generation of up to500 nmol H2 per gram rock sample. Dissolved H2concentrations of 20450 mol/L exceeded that of sulphatereducing or methanogenic aquifers at 130 nmol/L. The rocksamples contained 1 wt% chlorite and up to 35 wt% of layersilicate. The results indicated microbial consumption of theH2 for CO2 transformation to CH4 (Kassahun and others,2007a). Organic matter on silicate mineral surfaces was alsolikely to be an important factor causing microbial colonisationand subsequent biofilm formation through the formation ofmolecular H2 (Kassahun and others, 2008).

The deep subsurface methanogenesis of sequestered CO2 wasdiscussed by Koide and Yamazaki (2001) for Japanese sites.Sources of H2 could include microbial decomposition oforganic matter, geochemical water-rock interaction, and deepgeothermal activities. They note that biogenic methanogenesisis active even in deep aquifers in basaltic lava layers, despitethe fact that these lack organic matter. In deep basalticaquifers, methanogens might produce CH4 from CO2 ofinorganic origin and H2 derived from reduction of thermalwater by Fe(II) in basaltic glass. CO2 injection into deepbasaltic lava layers would make the aquifer environmentsimilar to an ancient reducing environment wheremethanogens were dominant under conditions of high CO2pressure, high temperature, and low concentrations of organicmatter. Artificial stimulation of the natural deepbiogeochemical carbon cycle by CO2 injection may facilitategreenhouse gas control with the added benefit of renewableCH4 supplies. Perhaps in the future the biogeochemicalcarbon cycle in basaltic layers in the oceanic crust couldprovide a virtually limitless CO2 sink and a source of CH4(Koide, 2001).

2.2.2 Studies in different countries

The formation of microbial CH4 has been investigatedrecently in Australia, China, Germany, Japan, and the USA.The results are described in the following sections.

AustraliaCommercial CH4 production from coal has become a rapidlygrowing industry along the eastern seaboard. Two of the mainbasins where deep underground mining is widespread are theSydney Basin in New South Wales and the Bowen Basin inQueensland. Extensive reservoir testing is being conductedfor other eastern Australian basins such as the Surat,Genedah, Gloucester and Otway Basins (Faiz and Hendry,2006).

The Bowen Basin, Queensland, is a major source of CBMfrom bituminous coal seams. Water and gas production havebeen variable across the field. Stable isotope analysis andaccessory water quality tests were conducted on CBMproduction gas and water samples collected from two of theCBM producing coal seams. Time slice mapping over a fouryear period led to the conclusion that the domains of higherand lower gas production could be related tocompartmentalisation of the reservoir due to tectonic

activity that led to folding and faulting. The samples ofwater and gas were geochemically analysed to determinewhether there were significant differences between tectonicregions of different production behaviour (for geological,hydrological, and experimental details see Kinnon andothers, 2009).

The gas isotope analysis showed that production gases hadboth biogenic and thermogenic origins and that secondarybiogenic gas generated through CO2 reduction comprised asignificant portion of the CBM produced from this field. It isgenerally accepted that CH4 from Australian coal seam gaseswith 13C compositions less than -60 have biogenic originsand 13C compositions greater than -50 are thermogenic.Dry gases (those dominated by CH4 rather than C2+hydrocarbons) are characteristic of biogenesis. Watersassociated with CBM are typically highly sodic. Tests showedthat water sodicity increased with depth. The highest gasproducing wells were distinguished by producing a mixture ofbiogenic and thermogenic CH4, a well depth of between200 m and 300 m, and more positive CO2-CH4 carbonisotopic fractionations between 60.8 and 62.9. Waterrecharge (washing) may introduce bacteria and increasebiogenesis. Areas of recharge had less positive CO2-CH4carbon isotopic fractionations ranging from 48.6 to 57.6;but the residual CH4 is either enriched or depleted in 13Cdepending upon whether aquifer flow rate is fast or slowrespectively (Kinnon and others, 2009).

The results of other studies which were carried out in theGunndah and Bowen Basin, eastern Australia, show that theinteraction of fluids with the coal seams along with thethermal degradation of organic matter has lead to formation ofCO2 and CH4. The CO2 generated from these processes hasbeen incorporated into the Ca-Mg-Fe carbonates. The bulk ofthe CO2 currently stored in the coal seams is of magmaticorigin and variously associated with tectonism and majorepisodes of igneous activity. Biogenic and thermogenicalteration of organic matter, both produce CO2 and CH4 butthe source of these products can be determined throughgeochemical analysis. Gas stable isotopes confirm generationof secondary biogenic CH4 in CO2-rich coal seams byreduction of CO2. The relative proportions of different gasesgenerated by thermal alteration depend on temperature andthe type of organic matter. Significant CO2 is producedthrough the thermal alteration of humic or coaly (type IIIkerogen) source rocks, whereas only minor CO2 is generatedfrom sapropelic (type I/IIkerogen) source rocks. The studiessuggest that methanogenesis may provide an additionalsequestration mechanism for CO2 in coal seams. These coalseams with high CO2 content are natural analogues of theprocesses likely to occur as a result of CO2 injection andstorage in coal systems (Golding and others, 2009a,b).

Li and others (2008) surveyed the microbial populations insome Australian CBM reservoirs (Surat, Sydney, and PortPhillip Basins). The results of polymerase chain reactionstudies suggested that members of the domain Archaea (aunique group of microorganisms classified as archaeobacteriabut genetically and metabolically different from all otherknown bacteria) were relatively rare in the coal samples andabsent from the water samples. The dominant archaeal

9



species belonged to the genus Archaeologlobus, an anaerobewith very weak CH4 generating capacity. This result wasunexpected as true methanogens had been detected in both theSydney and Surat Basins. This suggested that there waspotential to enhance, by artificial means, the biogenicproduction of CH4 from these coal samples. Further coal andformation water samples collected from the CBM productioncoal basins in eastern Australia were being studied in detail todetermine the mechanisms of secondary biogenic gasgeneration.

A review of microbial activity in Australian CBM reservoirs(see Fais and Hendry, 2006) summarises the processesinvolved in CBM formation. Geochemical data for gases andcoal indicated extensive microbial activity, especially in coalseams shallower than about 600 m. Evidence suggested thatCO2 reduction was the main pathway of secondary biogenicCBM generation. There was no evidence for CBM formationfrom aceticlastic reactions, which differed from the results ofstudies of Powder River Basin (PRB) gases in the USA. Thereason for the difference was not clear but the PRB coals areof lower rank and more permeable than the Australian coals.The variations in the water chemistry may also be anotherfactor that could cause these differences.

ChinaThe formation of secondary biological coal bed gas in theXinji area, Anhui, China, was investigated by Tao and others(2007). The biogenic component of the coal bed gas wasestimated at 60.1% to 68.5% of the total CH4 generated. Thecoal bed gas was super-dry, which is the characteristic ofsecondary biogenic gas. By contrast, the thermogenic gas waswet from the thermal evolution of the coal. Intense tectonicuplift, faults and erosion in the studied area created favourableconditions for the infiltration of surface water, the abundanceof microbes and hence the formation of secondary biogenicCH4. The excess of biogenic over thermogenic CH4 isattributed to the uplift of the coal beds or erosion of theiroverburdens with the presence of CO2. The concentration ofCO2 in the coal bed gas samples from the Xinji area was only0.51% to 1.93%, less than the 5% found in the Sydney andBowen Basins in Australia.

Isotopic tests indicated that the CO2 was of organic origin andderived from the coal bed. The content of N2 in the coal bedgas was 2.86% to 42.4% and it had a strongly negative linearcorrelation with CH4. This indicated that the N2 was mainlyfrom the atmosphere and was supported by the isotopiccomposition of N2 and by helium data. As N2 is a majorcomponent of the atmosphere, there is abundant N2 dissolvedin the surface water. Nitrogen can infiltrate into undergroundcoal beds with the permeation of the surface water. Theexistence of atmospheric N2 in the coal bed gas indicated aneffective infiltration of surface water and favourableconditions in the coal bed for the bloom of microbes andformation of secondary biogenic gas, which can also beattributed to the rapid tectonic uplift and erosion in the area.The Xinji area coal bed gas was comprised of secondarybiogenic gas, thermogenic gas and some atmospheric N2. Theproportion of secondary biogenic CH4 was estimated at60.1% to 68.5% and that of thermogenic CH4 at 31.5% to39.9% (Tao and others, 2007).

GermanyThe formational history of coal seam gas in the Ruhr Basinwas investigated using methods detailed by Thielemann andothers (2004a,b). Isotope and gas wetness analyses indicatedthat the coal seam gas was a mixture of thermal and microbialCH4. The latter was formed through CO2 reduction andcomprised 3890% of the total coal seam and coal mine CH4.The occurrence of coal seam gas varied over the three regionsof the Ruhr Basin. In the centre, hard coal had been mined for800 years, and intensively since 1850. This area showed ahigh proportion of microbial CH4. In another area, coal hadbeen mined only since 1900 and the CH4 production showedonly a small microbial signature. A third area had not beenaffected by mining. The microbial CH4 from this area was

The isotopic ratios of the Yubari CBM implied an abiotic,thermogenic origin. However, the presence of methanogens inthe coal seam was suggested by methanogenic enrichmentcultures and gene sequences, most of which were closelyrelated to methanogenic Archaea and acetogenic/H2generating bacteria in association with methanogens. Thiswas consistent with the idea that most of the CBM wasformed under high temperature and high pressure and thatmethanogenic and associated microbial populationsdeveloped with the decrease of in situ temperature due to theuplift of coal seams and/or increased flow of groundwater(Shimizu and others, 2007).

USAThe Powder River and San Juan Basins, USA, are two of themost productive CBM reserves in the world. Geochemical andisotopic indicators establish that the gas in these basinscontains microbial CH4. The microbial biodegradation of thecoal beds is a significant component of the total gas resourcesin the San Juan Basin and the sole source for the shallowcoals of the PRB. Localised hydrological conditions andsubsurface geology are likely to play important roles incontrolling the extent of biodegradation and ofmethanogenesis. Biodegradation of hydrocarbons coupledwith methanogenesis may develop regardless of the organicmatter source across a range of inherited thermal maturities inthe coal. The highest degrees of biodegradation were confinedto particular coal seams, suggesting that zones ofbiodegradation were ultimately controlled by stratigraphicvariation in the subsurface. These variations includedfractures and cleat systems in coals, allowing localisedgroundwater flow through particular strata, and stimulatingmicrobial biodegradation of hydrocarbons (Formolo andothers, 2008).

The biodegradation signatures preserved in the coals did notspecifically follow observed trends in petroleum, crude oil, orin shale-hosted gas reservoirs. They showed elevatedbiodegradation of aromatic compounds prior to complete orextensive removal of more aliphatic compounds in boththermally mature and thermally immature coal seams. Moreresearch is necessary to link the signatures of organic matterbiodegradation to active microbial methanogenesis (Formoloand others, 2008).

Flores and others (2008) investigated the composition andisotopes of gases and co-produced water from samplescollected basin wide from varied CBM coal reservoirs in thePRB, Montana and Wyoming. They linked the analytical datato the basin wide coal geology and stratigraphy. Theirconclusions, relating to recent CH4 formation, were that: gas generated from CO2 reduction accumulated and was

dominant in the centre of the basin; thermogenic gas migrated into the deep, central part of

the basin and mixed with microbial gas, forming gaswith a transitional isotopic signature;

groundwater recharge events continued to the present; groundwater recharge events, also previously interpreted

to have played an important role in the generation ofmicrobial gas from CO2 reduction along the easternbasin margin, played an important role in the generation

11



and accumulation of younger gas derived from methylfermentation in the northwestern part of the basin;

several generations of methanogenesis produced youngergases from methyl fermentation along basin margins thatoverprinted older gas derived from mixed CO2 reductionand methyl fermentation.

Geological factors play an important role in the origin of thecoal bed gases. These include the direction of groundwaterrecharge, depth of burial, thermal and maturation history,lateral and vertical continuity of stratigraphic units, degree offaulting and fracturing, coalification processes, and thenatural burning of the coal beds. All these factors influencethe extent, methanogenesis, gas composition, gas generation,accumulation, and preservation of these resources. Thus,ongoing and future microbiological, biogeochemical, andhydrological studies must correlate their data and results tothe coal geology, structural geology, and stratigraphy of thebasin in order to be meaningful (Flores and others, 2008).

The effect of coal beds of differing thermal maturity on theorigin of coal bed gases was investigated by Strpo andothers (2007) in the southeastern Illinois Basin, Indiana,USA. The gradient of thermal maturity in the Illinois Basingives rise to biogenic and/or thermogenic coal bed gasesdepending on coal properties and geologic setting. The gasgeneration pathways in Illinois Basin coals at lower maturityin Indiana were compared with those with higher maturity inwestern Kentucky. Biogenic CH4 from CO2 reduction wasprevalent in the less mature Indiana coals, whereas moremature coals from western Kentucky produced predominantlythermogenic hydrocarbons through cracking of coal organicmatter. The two different types of coal bed gas were distinctin gas and isotopic composition. The fast fluid exchange withoxygenated surface waters in Indiana coal beds makes itunlikely that hydrocarbon gases have been accumulating sincethe deposition of Pennsylvanian coal swamps. Instead, thepresence of viable methanogens in coal waters with abundantCBM, suggests that biogenic CH4 in Indiana coal beds iscontinuously forming as a quasi-renewable form of fossilenergy, although the rate of in situ CH4 generation remainsunknown.

The isotopic results of tests on the coal bed gas from Indianaindicated microbial methanogenesis by CO2 reduction. Aplausible scenario for decomposition of coal organic matterby microbial consortia leading to methanogenesis throughCO2 reduction could be that (Strpo and others, 2007): methanogens require elemental H2 and CO2 which can

be produced by other microbes; H2 and CO2 are probably provided by cooperating

species that decompose organic matter; such asfermenting anaerobes which generate CO2, acetogensproducing H2, proton reducers and others.

Predominant macerals of Indiana coals, and especiallyvitrinite, are suitable substrates to support microbialgeneration of H2 and CO2 by fermenters and acetogens, thusproviding the raw materials for methanogenesis as a terminalstep of organic matter decomposition in coal. Alternativesupplies of H2 could result from thermal maturation ofunderlying marine shales, rich in organic matter, or may be

generated by deep metamorphic processes. If that were thecase, methanogenic Archaea could generate CH4 in thesubsurface independently from bacterial consortiadecomposing organic molecules to CO2 and H2 (Strpo andothers, 2007).

Molecular and geochemical studies were performed onmicrobial communities and CBM throughout the CBMformations in the eastern Illinois Basin. The results suggestthat organic matter is biodegraded to simple molecules, suchas H2 and CO2 which fuel methanogenesis, principally byMethanocorpusculum, generating large CBM reserves. Therate limiting step of coal biodegradation is the initialfragmentation of the macromolecular, polycyclic,lignin-derived macromolecules which tend to be relativelyresistant to degradation. Lignin degradation can be achievedby extracellular enzymes used by fungi and some microbes. Ithas also been shown that up to 40% of the weight of somecoals can be dissolved using extracted microbial enzymes.Enrichments have been developed capable of anaerobicdegradation of methylated and ethylated aromatic compoundsor even polycyclic aromatic hydrocarbons. In the study of theeastern Illinois Basin, enrichments from coal waters showedhigh rates of CH4 generation and the presence of abundant

12



methanogens. Anoxia, low salinity, and moderatetemperatures (about 17C here) are common characteristics ofall described Methanocorpusculum niches. However, thespecies is tolerant of temperatures as low as 1C, although itgrows primarily in the range 2535C. Proposed mechanismsfor stepwise biodegradation of organic matter in coal areshown in Figure 1. The data from the study suggested that thecontribution of acetoclastic methanogenesis to bulk CH4generated was minor (Strpo and others, 2008).

The presence of minerals in coal cleats may have greatsignificance for CH4 generation and extraction from coalbeds. This is because mineral fillings affect fluid flow andpermeability, and the isotopic composition of authigenic(secondary) calcite is diagnostic for the onset of microbialmethanogenesis relative to calcite mineralisation. Althoughcoal bed gas CO2 in the Illinois Basin, Indiana, features strong13C-enrichment in the presence of microbial CO2 reduction,the available 13Ccalcite values are not in thermodynamicequilibrium with modern coal bed CO2. Relatively negative18Ocalcite values indicate that calcites in Indiana coal cleatscrystallised at a time when CO2 in coal beds was far more13C-depleted than it is today. Apparently microbialfractionation of CO2 through CO2 reductive methanogenesis

1spirochaeta, 2sporomusa, 3cytophaga, 4acidoaminococcus, 5flavobacterium, 6methanocorpusculum, 7rhodobacter(?)

CO2-reduction methanogenesis6 CH4

acetoclastic methanogenesis6

geomacromolecules1,2,3,7

demethylation2 andring cleavage3

fragmentation of coals

SH

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Figure 1 Proposed mechanisms for biodegradation of organic matter in coal (Strpo and others, 2008)

did not occur at the time of calcite crystallisation, possiblyowing to prior sterilisation of coal during deep burial andheating. This suggests that inoculation of uplifted coals withmethanogenic microbial consortia and the onset of microbialmethanogenesis were relatively late events. This furthersupports the work of Strpo and others (2007), suggestingthe previously heat-sterilised sedimentary environment hadnot yet been inoculated with methanogenic microbialconsortia and that microbial methanogenesis may be arelatively recent development. Inoculation with CO2-reducingmethanogens must have occurred when microbes weremigrating with fluids from cooler strata above the coal seams.Prior, discontinuous mineralisation along cleats may havepreserved the cleat framework, kept pathways open for fluidmigration, and thus facilitated access of methanogenicbacteria into coals. On the other hand, cleat mineralisationcould also inhibit the migration of fluids, including that ofcoal bed gases by sealing the faults (Solano-Acosta andothers, 2008).

Laboratory experiments on subbituminous, humic coals fromAlaska resulted in additional, CH4-rich gas generation afteradding nutrients to the coal cuttings and canister water andculturing the microbial consortia under anaerobic conditions.The canister water contained common, fluorescent, rod-likemicrobes comparable to Methanobacterium species and rod,cocci and spherical forms of microbes attached to the coalsurface. These microbes apparently represented at least aportion of the microbial consortia needed to depolymerisecoal, as well as to generate the observed secondary CH4emission (Barker and Dallegge, 2006).

13



The benefits of enhancing biogenic CH4 production rates areto reverse declining production in older wells, improve it incoal seams with low gas content and possibly increasepermeabilities. A further development is to inject CO2 forstorage in coal seams, using microbes to convert it to CH4 forenhanced recovery. Field trials are necessary to demonstratethe technology (Budwill, 2008a,b).

3.1 Enhanced CBM production

Investigations aimed at enhancing CBM reserves, but not yetrecycling CO2, are also pertinent to the subject of this review.Extensive laboratory-based studies have confirmed that theintroduction of a suitable nutrient stimulates the growth andactivity of indigenous methanogens, leading to the real-timegeneration of new CH4 from coal. Research is therefore inprogress to develop a technology which would involve theinjection of nutrients into deep, unmineable coal beds(see Figure 2) or other unconventional reservoirs for cleanenergy generation (Budwill, 2008a).

The nutrient needs to be introduced into the target reservoirseam to ensure that it comes into contact with the nativepopulation of bacteria and methanogens as much as possiblein order to achieve a successful field demonstration of themethanogenesis technology. However, microbial activitytypically concentrates close to the nutrient injection point.This produces a limited amount of CH4 from easilysolubilised and metabolised geological matrix components.Microbial activity is also limited to areas where water is


retained within the reservoir pore-fracture network. Theseconditions may mean that a large portion of availablereservoir surface may not be utilised within the formationbecause it is beyond the zone of nutrient and microbialinfluence. This limits new CH4 generation (Budwill, 2008a).

A patent entitled Biogenic fuel gas generation in geologichydrocarbon deposits (US 12/136,728) by Luca TechnologiesInc (LUCA, 2009a,b) covers methods of cultivating CH4production from geologic deposits such as coal, oil or shaleby: collecting a methanogenic bacterial consortium from

water extracted from an underground hydrocarbonformation, under anaerobic conditions;

using the anaerobic microbial concentrate to inoculateanother geologic hydrocarbon deposit; and

harvesting the CH4 that the consortium produces overtime from anerobic water within the deposit or from thewell head space.

An analogous patent, covering the same aspects ofmicrobially enhanced CH4 production methods has beenissued in New Zealand (Patent No. 5638868). Initial successwith these methods in cultivating CH4 production under fieldconditions in the Wyoming coal beds suggests this technologyoffers the potential for long-term, large-scale sustainableenergy generation (LUCA, 2009b).

Traditional extraction techniques for CBM production entailpumping groundwater out of the coal beds. A damaging andunforeseen consequence of this pumping is that it can allow

3 Enhanced microbial methane production

60

Time, d

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CH

4/to

nne

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50403020100 70

proprietary nutrient

tryptoneyeast extractsoytonebrain-heart infusion broth

20

10

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Figure 2 Enhanced CH4 production in coal-based methanogenic cultures grown with crushed coaland amended with nutrients (Budwill, 2008a)

the influx of air, containing abundant free O2 which is toxic tothe CH4 producing microorganisms. Removal of water alsodamages the microorganisms. LUCA has established a libraryof anaerobic core samples from known and suspectedgeobioreactors for study and experimentation, using genomicsand modern biotechnology. Developing a better understandingof the biology and ecology of these organisms shouldhopefully lead to the ability to create functionalgeobioreactors from currently barren or non-CH4 producinghydrocarbon reservoirs. It is believed by LUCA that wellmanaged, functional geobioreactors not only have thepotential to meet the need for CH4 but that it may also bepossible to engineer CH4-producing organisms genetically toproduce free H2 instead as their final product (LUCA, 2009a).

The low rank, high permeability, and high water content ofPRB coal suggest that seam conditions are still favourable forCH4 biogenesis. A fundamental understanding of theprocesses of biogenesis could lead to enhanced CH4production (Green and others, 2008).

Earlier experiments with coal cores from the PRB ofnortheastern Wyoming were described by LUCA (2004).Biogenic CH4 production could be increased rapidly andsubstantially by the addition of specific nutrients and otheramendments in laboratory experiments. In addition,laboratory results suggested that formation water had an asyet unidentified role in stimulating biogenesis perhapsacting as a conduit of nutrients to entrenched microbes withinthe coal. A better understanding of how this microbialcommunity interacts with its environment and how itswell-being could be managed through prudent productionpractices was considered paramount in efforts to produceclean energy from the methanogenic bioreactor. Laboratoryevidence for recent biogenic CH4 formation in oil is describedby DeBruyn and others (2004).

The patented Arctech Process converts coal into CH4 andhumic substances. Natural microorganisms are adapted todigest coal under anaerobic conditions, resulting in a mixtureof CH4 and humic substances. The microbes have beenengineered from the digestive tract of termites. Thehumic-rich carbon by-product may be used to improve soilfertility, replenish water, and neutralise munitions, convertingthem into organic fertiliser. A wholly owned subsidiary,Humaxx, has been formed by Arctech, Inc to market cleanCH4 and humic-rich products. Further information is availableat www.arctech.com and www.humaxx.com (Chopra, 2009).

Bioconversion is accomplished by a three-step process, usingappropriate nutrient components (Humaxx, 2009):1 microbes convert the coal into volatile organic liquids,

by a hydrolytic and fermentation process;2 the liquids, along with gases produced, are contacted

with CH4 producing microbes (methanogens) thathydrogenate the acetate and CO2 into CH4;

3 the CH4 is separated and unconverted residual coalresidue is converted into humic acid for formulatingagricultural and environmental products.

Coal-culturing experiments reported by Barker and Dallegge(2006) indicated that it may be possible to use simple

15

Enhanced microbial methane production


nutrients, such as yeast extract, to stimulate the indigenousmethanogenic consortia as an alternative to injecting alienmethanogen cultures. They calculated that if they couldachieve a one percent conversion of PRB coals by nutrientstimulation, they could increase the recoverable CBM byaround 1330%.

Studies have shown that the overall biodegradation rates fororganic solids can be limited by the solid-liquid mass transferrate when the microbial concentration is high and/or the solidsurface area is low. Hence the rate and quantity of CH4generated from coal may depend on the exposed surface area,and the insolubility and impermeability of coal representmajor constraints. Although the diameter of coal pores rangesfrom 0.04 to 30 m many of these pores are too small topermit microbial entry (

concentrations in the formation waters. After depletion of thesulphate, CH4 formation can start. Generally the processes arelinked to carbonate formation through a shift within the pHvalues and the supply of cations. These carbonate phases canresult in an important increase of the sequestration capacity(Hoth and others, 2007a,b).

There is insufficient knowledge of the impact of sequestratedCO2 on the deep biosphere. The occurrence ofmicroorganisms in gas field fluids was therefore studied byEhinger and others (2009) to obtain more fundamentalknowledge of the microbial processes in potential CO2sequestration sites. Active microbial cells were absent in gasfield fluids with high salinity (239 g/L Cl-) and low pH (4.9)but present in those with lower salinity and higher pH values.A consortium of fermenting and sulphate-reducing bacteriatogether with the methanogenic Archaea were involved in thecomplex degradation processes in the gas reservoir fluids.Only two methanogenic genera dominated, Methanlobus andMethanoculleus, in the Schneeren gas field. This combination

16



had also been reported for a deep coal seam aquifer at Yubariin northern Japan (Shimizu and others, 2007).

The abundance of various sulphate reducing, fermenting andmethanogenic microorganisms, coexisting in the deep saline,sulphate-rich gas field fluids, indicate that the gas reservoirSchneeren harbours microbial activity despite the extremephysicochemical conditions. Variability in the composition ofthe methanogenic community shows their adaptation toaltered environmental conditions and the dynamic withinthese methanogenic processes. Sequestration of CO2 in suchdeep gas reservoirs will change physical and chemicalparameters. For example, a cooling effect is predicted in thestorage formation due to the expansion and evaporation of theCO2 after injection. These cooling effects can be significant infissures and cavities if CO2 is migrating upwards. Coolingmay decrease microbial activity (see Section 3.1) althoughsmall temperature changes could enhance microbial activityin some regions of the reservoir. There is some evidence forrelease of minerals and dissolved metals, as well as organic

0.20

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Figure 3 Effect of temperature, pH, particle size, and enhancing solvents on microbial CH4 production(Green and others, 2008)

material which would affect microbial activity. It is notcurrently possible to predict the effect of the injection of largeamounts of CO2 on the biogeochemistry in the subsurfacebecause investigations are at an early stage (Ehinger andothers, 2009).

Natural CO2 occurrences have three principal sources: mantleor mantle-derived igneous rocks, metamorphism ordissolution of marine carbonate-bearing sedimentary rocks,and modification of organic material. Coal seams containinghigh CO2 contents occur in many coal basins worldwide, forexample, in the Gunnedah and Bowen Basin, eastern Australia(see Section 2.2). High purity CO2 deposits (>90%) occur inseveral fields in the US (Colorado), Hungary (PannonianBasin), France (Massif Central) and Turkey (West and EastAnatolia). Also, in central Italy, CO2 dissolved in cold springwaters is derived largely from deep, mantle related sources.Nearly pure high concentrations of CO2 occur in Permian gasfields of Germany, connected to Late Tertiary volcanism.These basins provide natural analogues of the processes likelyto occur as a result of CO2 injection and storage in coalsystems (Golding and others, 2008, 2009a,b).

The CO2 is stored predominantly as adsorbed molecules onmicropore surfaces (adsorption trapping). These allow higherdensities and greater volumes at shallower depths than insandstone and carbonate reservoirs where CO2 is storedinitially as a free phase (structural/stratigraphic trapping). Inthe long term, CO2 dissolves in formation water and reactswith minerals in the host formation (solution/ionic trapping)and may be precipitated as carbonate minerals (mineraltrapping). Natural analogue studies in the Bowen andGunnedah Basins in Australia indicate that magmatic CO2 hasbeen stored in coal and sandstone formations since theMesozoic through a combination of adsorption and mineralcarbonation reactions. Gas stable isotopes confirm generationof secondary biogenic CH4 in CO2-rich coal seams byreduction of CO2, suggesting that methanogenesis mayprovide an additional sequestration mechanism for CO2 incoal seams (see Section 2.2). These observations comparewell with those seen previously in the Sydney Basin (Goldingand others, 2008, 2009a,b).

The mineralogy and geochemistry of high CO2 coal seams inthe Bowen and Gunnedah Basins were compared withadjacent low CO2 coal seams of the same formation.Experimental details are described by Golding and others(2009b). The study aimed to determine the impact of highCO2 contents on the coals and the mechanisms that kept theCO2 naturally sequestered. Hydrogen isotope compositions ofCH4 were used to distinguish between microbial CO2reduction and methyl fermentation. The CH4 D valuesranged from -213 to -223 in the low CO2 hole and-214 to -221 in the high CO2 hole. The more negative13C values of CH4 from the high CO2 hole (mainly -60) suggested thatCH4 in the high CO2 environment formed by microbialreduction of CO2, when considered with the D values. Thismay indicate that CH4 generated by coalification has beendisplaced by CO2 of inorganic origin with the current CH4 inplace largely the result of subsequent microbial CO2reduction.

17



The experiments carried out at the Yubari site, northern Japan(see Section 2.2) used N2 injection at one well and subsequentproduction of N2 at the other as a substitute for CO2. Thisaffected the genetic diversity of the methanogenic community,as well as the pH and Eh in the groundwater of the productionwell. This evidence suggested that N2 injection into the coalseam might affect the cycling of organic matter bymethanogens in situ. Similarly, the planned large-scale CO2injection (and breakthrough) would affect groundwaterproperties and methanogenic community structure. Thedegree and extent of the possible influence of CO2sequestration should be assessed and the recoverability ofgeochemical and microbial properties after CO2 sequestrationshould also be estimated (Shimizu and others, 2007).

Research is in progress and planned in several countries toaddress critical gaps in our current knowledge of the effects ofCO2 injection on microbial CH4 formation. These include: potential fracturing of the coal seams; dissolution of constituents in coal and their

bioavailability; destruction of permeable networks in the coal structure; potential increase in H2 availability to counteract the H2

limiting factor in coal; H2 from water.

AustraliaA research project is in progress at the University ofQueensland (UQ) on underground conversion of CO2 to CH4,using coal as a substrate. The scope is (Massarotto, 2009):1 active promotion of microbial CH4 technology as a

research project to local CBM companies;2 collection and analysis of formation waters from coal

fields, which has confirmed the presence ofmethanogenic bacteria;

3 the coal and water samples were provided to Dr PatrickGilcrease, South Dakota School of Mines, USA, and hehas confirmed good quantities of biogenic CH4generation in preliminary tests. This collaboration mayextend to a one-year sabbatical at UQ in 2010;

Details of a project using coals as CH4 bioreactors, initiatedwith Gilcrease, are summarised by Golding (2009). The gas tosupply the proposed Gladstone liquefied natural gas facilitieswill require an enormous area to be drained of water and thenCH4 an area that will need to be continually expanded assections of the coal measures are exhausted of their gasinventory. Preliminary research results suggest that CH4 canbe regenerated in situ within exhausted areas. This essentiallycreates a sustainable supply of coal seam CH4 and permits thereuse of wells and associated infrastructure. Moreover theregeneration occurs in a remarkably short period of time,within weeks in a controlled laboratory environment. It usesmicroorganisms which occur naturally and are already presentwithin the coal seams. The aim is to expand the preliminaryresults and develop them to methods suitable for fielddevelopment. A successful outcome would significantlyincrease coal seam CH4 production and reserves, and reducethe cost of their recovery.

The approach proposed for this project is to confirm andcharacterise the processes involved in microbial CH4generation in the Walloon coal measures in the Surat Basin,Queensland, as a basis to understand and develop theconcepts, preceding work on other coal types, ranks andmeasures (not part of this project). Field studies andlaboratory experiments will address the following geology,microbiology, and engineering gaps (Golding, 2009): evaluate the distribution of microbial CH4 resources, the

role of geology and formation water chemistry inmicrobial CH4 generation, and the geochemicalsignatures of associated gas and formation waters;

define the main biogeochemical processes active in the


Walloon coal measures and the relative importance of theacetate fermentation and CO2-reduction pathways ingenerating in situ microbial CH4;

undertake coal seam and formation water metagenomicsin conjunction with sequencing of isolates to improveunderstanding of biogeochemical processes andecosystem functioning in medium-depth coal seams;

determine the reaction rates, coal to CH4 conversionfactors and yields, and the nutrient and environmentalrequirements and limits (including temperature andpressure), of living microbial consortia from the coalseams to elucidate the potential for microbially enhancedcoal seam gas; and

evaluate potential physical and chemical reservoirtreatments to stimulate the in situ activity ofmethanogenic consortia in coal seams, determine theireffect on CO2 permeability/injectivity and CH4permeability/productivity at wells, and on the overallfluid transport process in the reservoir.

CanadaThe Alberta Research Council (now part of AlbertaInnovateas Technology Futures)has been investigating anddeveloping technology to enhance biogenic CH4 production,or methanogenesis, in deep unmineable coal beds and otherunconventional reservoirs for clean energy generation. Furtherresearch is planned to (Budwill, 2008a): enhance nutrient delivery and dispersion; maximise microbe-coal interaction; utilise robust geochemical monitoring tools to monitor

the process.

A continuous flow-through column that can simulate reservoirconditions of elevated pressure and groundwater movementwill be used to conduct the experiments involving nutrientintroduction. The end result will be a methodology for in situmethanogenesis which will be transferable to differentgeological conditions and realise the full potential of CH4production (Budwill, 2008a).

It is important to understand the microbial ecology andmethanogenesis processes so that in situ conditions can bemodified to enhance microbial CH4 production toeconomically viable amounts. Proposed research thereforeincludes (Budwill, 2008a, 2010): identification of the native microbial species in

unconventional reservoirs using DNA sequencingtechniques;

linking the assemblage of microbial species with thereservoir geochemistry;

participation in a metagenomics project that will look atcommunity structure and function of coal beds and otherenergy-related environments such as oil sands and oilreservoirs (see www.hydrocarbonmetagenomics.com).

The outcomes of this research will allow for the prospectingof unconventional reservoirs for the stimulation ofmethanogenesis. This may be in the form of ensuring that

4 Ongoing and planned research

active methanogenic communities are present in the targetcoal seam; or identifying the dominant bacterial and/orArchaeal species and customising nutrient amendments tostimulate growth and metabolic activity of the species. Theresearch will also allow for the development of strategies forin situ monitoring, without which analysis of the complexinteractions between changes in nutrient availability, naturallyoccurring microbial communities, and geological matrixproperties cannot be quantified during a field trial, much lessunderstood (Budwill, 2008a).

The injection of CO2 for storage requires H2 for enhancedproduction of CH4 (see Figure 4). Hence coal containing CO2and H2 produced CH4 more quickly than coal with CO2 andnutrients (Budwill, 2008b).

Future research is required to (Budwill, 2008b): determine the optimum nutrient injection procedure to

provide greater nutrient distribution and to maximise theCH4 production potential;

assess water saturation characteristics; understand the effect of coal surface chemistry on

methanogenic activity, including which components arebeing degraded and limits to biofilms;

model the geochemistry of biogenic coal bedenvironments;

determine the microbial ecology of coal bedenvironments.

GermanyThe Federal Ministry of Education and Research (BMBF) hasbeen funding the RECOBIO project which is linked to anational R&D programme called GEOTECHNOLOGIEN(see http://www.geotechnologien.de). Research withinRECOBIO 1 was carried out from 2005 to 2008 by the

19

Ongoing and planned research


Technical University of Freiberg, Dresden GroundwaterResearch Centre and the GEOS Ingenieurgesellschaft mbH,Freiberg. The recycling of sequestered CO2 by microbialtransformation in the deep subsurface was studied.Sub-projects included a laboratory investigation of thebiogeochemical CO2 transformation with molecular-geneticmethods and the impact of mineral reactions on the microbialand biogeochemical transformation of sequestered CO2. Thelong-term biogeochemical reactions are important to interpretthe resulting pressure reductions in CO2 which mightotherwise be interpreted as leakage in the storage reservoir(Ehinger, 2009; Hoth, 2009; Hoth and others, 2005).

The research is continuing within RECOBIO 2 which focuseson the biogeochemical transformation of injected CO2 in thedeep subsurface, including induced carbonate phase formationand the importance of accompanied impurities in the storedCO2. The Technical University of Freiberg and the FederalInstitute for Geosciences and Natural Resources (BGR) areinvestigating the effects of elevated CO2 partial pressures onthe efficiency of CH4 production in different coal and gasreservoirs (GEOTECHNOLOGIEN, 2009; Hoth, 2009;Krger, 2009).

The influence of microbial processes, as well as fluid/CO2interactions and reservoir topology, determine the storageefficiencies and long-term safety for the sequestration of CO2in underground reservoirs. The consequence of changes inthese parameters under elevated CO2 concentrations are beingstudied at the BGR within the frame of the EU-Network ofExcellence CO2GeoNet (Krger, ND).

JapanThe research and development programme on undergroundmicrobial CO2 sequestration and CH4 factory

65Time, d

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Figure 4 The importance of H2 in methanogenesis (Budwill, 2008b)

http://www.geotechnologien.de

(see Section 2.2) was scheduled for the period 2002-04(Koide and others, 2003). Shimizu and others (2007) note thatgas production at the Yubari site has been monitored since2004. No further information has been found on anysubsequent research plans.

USACollaboration is in progress between the University ofQueensland, Australia, and the South Dakota School ofMines, USA (see above and Massarotto, 2009). Gilcrease(2009) speculates that when coal is used as a source of C andH2 for microbial CH4, the system is H2 limited rather thanCO2 limited, and that adding CO2 does not enhance CH4yields. He is currently in the process of evaluating thishypothesis using thermodynamics, stoichiometry, andlaboratory experiments. However, if a geological source ofreduced hydrogen (other than coal) is available at a site, thenthe addition of CO2 could stimulate additional methaneproduction. No further information on research plans for thistopic has been obtained from US researchers contacted.

20

Ongoing and planned research


There is reliable evidence for the production of CBM throughrecent microbial activity in coal beds. However, theconditions to initiate, support, and enhance this production arenot fully understood. Nevertheless, microbial CH4 may beenhanced artificially and techniques have been patented in theUSA and New Zealand. The usual CBM production methodsof removing water prevent further microbial CH4 formation.There is an incentive to turn CBM, where appropriate, to acontinuously renewing system by changing the productionmethods and by stimulating production.

The introduction of large quantities of CO2 from carboncapture systems could have a favourable effect on microbialCH4 formation. However, there are uncertainties about thesupply of H2 which is essential to form CH4. Research is inprogress to determine the effect of CO2 storage on microbialCH4 formation. The coal seams in Australia with high CO2content, and containing recent microbial CH4, provide naturalanalogues of the processes likely to occur as a result of CO2injection and storage in coal systems.

Methane production from coal bedsThe microbial formation of CH4 in coal seams is widelyaccepted. There is great potential for increasing CH4 reservesthrough the chemical conversion of CO2 to CH4, by enhancedmethanogenesis. However, this is still in the research stage. Theformation of microbial CH4 from complex substrates such ascoal requires several steps. First fermentative bacteriahydrolyse and then ferment complex substrates to produceacetate, longer chain fatty acids, CO2, H2, NH+4, and HS.Acetogenic bacteria consume H2 and CO2 to produce moreacetate. In addition, they can demethoxylate low-molecularweight, ligneous materials and ferment some hydroxylatedaromatic compounds to produce acetate. Acetogens whichproduce H2, convert fatty acids, alcohols and some aromaticand amino acid to the H2, CO2, and acetate needed by themethanogens to produce CH4. In terrestrial, coal-bearingbasins, the transformation of CO2 to CH4 requires largereaction areas, appropriate microbial communities andformation water chemistry, long time-scales, and a reservoirthat restricts the escape of CH4 to the atmosphere. Thesedemands can be met in the deep subsurface. The biogenesis ofCH4 is a process that concentrates H2 in the resultinghydrocarbon products. Coal has H2:C ratios of less than onewhereas oil has H2:C ratios closer to two. Methane has a ratioof 4:1 and represents the most H2-enriched form ofhydrocarbon molecule. Hydrogen availability is the limitingfactor in estimating CH4 resource potential.

Formation water associated with CBM in nature appears tohave a characteristic chemical composition. It has alkalinepH, containing sodium and bicarbonate, but typically lackssulphate, calcium, and magnesium. The temperature range formethanogenic activity is 4100C. The optimal temperaturefor many methanogens is between 20C and 40C.Thermophilic methanogens may generate CH4 at temperatures>100C. Other environmental factors that limit microbialconversion of coal to CH4 are: pH, salinity, nutrients, trace


metals, and coal surface area/bioavailability

Isotopic fractionation and gas wetness or dryness arecommonly used to distinguish CH4 generated by CO2reduction from thermogenic CH4 sources. Biogenic CH4 isgenerally isotopically light, with 13C values less thanabout -55. However, it is affected by other factors such asthe isotopic composition of the original substrate,temperature, partial pressure of H2, methanogenic pathways,and the species of methanogens involved. The origin of thesegases may be determined in conjunction with H2 isotopevalues of D for CH4 and gas dryness indices. Thisdifferentiates between microbial CO2 reduction, deriving H2from formation water, and microbial methyl fermentationusing H2 primarily from methyl groups of organic matter andsecondarily from formation water.

Hydrogen formation from water reduction is commonlyrelated to oxidation of mineral bound ferrous iron and may bea process occurring in several environments. This process hasbeen observed in reactions involving cements of sandstones inpotential CO2 sequestration sites in Germany. It has also beenshown that organic matter on silicate mineral surfaces is likelyto favour microbial colonisation through the formation ofmolecular H2. The deep basaltic aquifers in Japan lackorganic matter but show active biogenic methane formation.The H2 may be derived from reduction of water by iron inbasaltic glass.

Secondary biogenic CH4 formed through CO2 reductioncomprises a significant portion of the CBM produced ineastern Australian coal basins. There is no evidence for CBMformation through the acetate pathway. The coal seams withhigh CO2 content are natural analogues of the processes likelyto occur as a result of CO2 injection and storage in coalsystems. Interaction of fluids with the coal seams and thermaldegradation has lead to formation of CO2 and CH4 with theCO2 incorporated into the Ca-Mg-Fe carbonates in long-termstorage. Evidence suggests that there is potential forartificially enhancing the biogenic production of CH4 fromCO2 injection into coals.

The recent microbial CH4 formation in the Powder River andSan Juan basins in the USA did show a minor contributionfrom the acetate pathway for CH4 formation although the CO2pathway dominated (see Figure 1). Differences from theAustralian results may be due to the lower rank and greaterpermeability of the US coals, and possibly variations in waterchemistry. In the US microbial bio-degradation of the coal(providing H2 and CO2) was stimulated by localisedgroundwater flow through fractures and cleat systems.

Isotopic tests of coal bed gas in the Xinji area, China,indicated that the CO2 was derived from organic matter in thecoal but there was less CO2 than in the Australian coal basins.Uplift of the coal beds or erosion of their overburdens withCO2 created favourable conditions for infiltration of surfacewater and microbes for CH4 formation. In Hokkaido, Japan,

5 Conclusions

isotopic ratios of the CBM suggested that it had mostlyformed under high temperature and pressure but thatmethanogenic and associated microbial populationsproliferated at lower temperatures following the uplift of coalseams and/or increased flow of groundwater.

The occurrence of recently formed coal seam gas varies overthree regions of the Ruhr Basin in Germany. It is consideredthat this variability may be partly due to differences in themining activities. The microbial contribution of CH4 seems tobe more pronounced at sites of active and especiallyabandoned coal mining than at unmined places. Anabandoned coal mine where the mine water contained bothmine timber and pieces of coal showed evidence for CH4production through both H2 and acetate. The best explanationwas that residual O2 after mining activities initiatedweathering of the coal and timber, facilitating a subsequentmicrobial degradation under anoxic conditions. The CO2reducing methanogens may compete with or benefit fromacetate consuming methanogens, sulphate reducing and ironreducing bacteria, or fermentation processes. The processes ofCH4 formation and other microbial processes can generally belinked to carbonate formation which can substantiallyincrease the CO2 sequestration capacity.

Enhanced microbial methane productionNew CH4 may be generated from coal by introducing asuitable nutrient such as yeast extract to stimulate indigenousmicrobial activity. Research is in progress in Canada to injectnutrients into deep, unmineable coal beds to stimulate CH4formation (see Figure 2). There are limitations caused bymicrobial activity concentrating close to the nutrient injectionpoint, and large areas of available reservoir surface may beunavailable due to lack of water.

Enhanced CBM production processes have been patented inthe USA, by Luca Technologies Inc and Arctech Inc and inNew Zealand. Successful CH4 production under fieldconditions in the Wyoming coal beds indicates that the Lucatechnology offers the potential for long term, large scalesustainable energy generation. Laboratory studies suggest thatformation water has an as yet unidentified role in stimulatingbiogenesis perhaps acting as a conduit of nutrients tomicrobes in the coal. However, traditional extractiontechniques for CBM production entail pumping groundwaterout of the coal beds. The removal of water damages the CH4producing microorganisms and can allow the influx of airwhich is toxic to them. LUCA considers that well managed,geobioreactors have the potential not only to meet the needfor CH4 but that it may also be possible to engineer CH4producing organisms genetically to produce free H2 instead astheir final product. The Arctech Process converts coal intoCH4 and humic substances, using microorganisms. In additionto the CH4, the humic substances may be used to improve soilfertility, replenish water, and neutralise munitions, convertingthem into organic fertiliser.

The rate and quantity of CH4 generated from coal may dependon the exposed surface area. The insolubility andimpermeability of coal are major constraints. Many coal poresare too small to permit microbial entry (

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