Coalbed Methane：A Review

Review articleCoalbed methane:A reviewTim A.Moore ⁎Cipher Coal Consulting Ltd.,173Clifton Terrace,Sumner,Christchurch,8081,New Zealand Department of Geological Sciences,University of Canterbury,Christchurch,New Zealanda b s t r a c ta r t i c l e i n f o Article history:Received 29December 2011Received in revised form 19May 2012Accepted 20May 2012Available online 14July 2012Keywords:Coalbed methane Coal seam gas Reservoir BiogenicThermogenic Gas saturation Production Exploration Diffusion Permeability Desorption Adsorption Microbiology Methanogens PoresOrganic compositionThe commercial extraction of methane from coal beds is now well established in a number of countries throughout the world,including the USA,Australia,China,India and Canada.Because coal is almost pure carbon,its reservoir character is fundamentally different to conventional gas plays.Coalbed methane (CBM)forms as either biogenically-or thermogenically-derived gas.The former occurs in ‘under mature ’(b 0.5%vitrinite re flectance)coals and is the result of bacterial conversion of coal into CO 2or acetate,which is then transformed by archaea into CH 4.Thermogenic gas is formed as part of the coali fication process and is purely a chemical devolatilization that releases CH 4.Methane is primarily stored in coal through adsorption onto the coal surface;thus it is pore surface area that determines the maximum gas holding potential of a reservoir (as opposed to pore volume in a conventional reservoir).Although macro-,meso-,and micropores are present in the coal matrix,it is thought that the micropores are where most methane adsorption occurs.In many of the micropores,the methane molecule may actually stretch,minutely,the pore and thus with de-gassing of the reservoir,could result in matrix shrinkage,allowing opening of the fracture (cleat)system in the coal and thus enhancing permeability.The organic composition of the coal is paramount in determining porosity and perme-ability character and thus maximum gas holding capacity.In general,the higher the vitrinite content the higher the gas holding potential (and ultimately the amount of desorbed gas)and permeability (all other factors being the same).There are other organic component/gas property relationships but these seem to be speci fic to individual basins,or even seams.Characterising a CBM reservoir during an exploration programme is a challenge but the two most vital measures to determine are permeability and %gas saturation.Permeability will largely determine gas (and water)flow rate,dictating how commercial a prospect might be.Gas saturation,determined from desorption and adsorption measurements,also in fluences gas rate and the ultimate recoverability of gas from a reservoir.Modelling of gas flow from the reservoir is highly dependent on knowledge of these parameters.Designing a successful pilot well programme and ultimately production wells will rely mostly on the permeability and %gas saturation character.Certi fication of resources and reserves,which is also very important to CBM companies as they explore and develop their permits,depends heavily on accurate estimates of reservoir character;primarily seam continuity,%gas saturation and permeability.©2012Elsevier B.V.All rights reserved.Contents 1.Introduction ...............................................................371.1.Previous compilations .......................................................382.Producing fields of the world .......................................................383.Gas type,quality and measurement ....................................................393.1.Biogenic gas and the microbes that create it .............................................403.2.Generation of thermogenic gas ...................................................413.3.Gas quality ............................................................423.4.Desorption ............................................................443.5.Desorption accuracy and use ....................................................473.6.Adsorption isotherms ......... (47)International Journal of Coal Geology 101(2012)36–81⁎Tel.:+64212466641.E-mail address:tmoore@.0166-5162/$–see front matter ©2012Elsevier B.V.All rights reserved.doi:10.1016/j.coal.2012.05.011Contents lists available at SciVerse ScienceDirectInternational Journal of Coal Geologyj o u r na l h o me p a g e :ww w.e l s e v i e r.c o m /l o c a t e /i j c o a l g e o4.Geological framework (49)4.1.General rank effects (50)4.2.Impact of original depositional environment (54)4.3.Role of structure and stress (54)4.4.Influence of external effects (54)5.Reservoir characterisation (55)5.1.Pores (55)5.2.Sorption and diffusion (55)5.3.Matrix shrinkage and swelling (56)5.4.Permeability (57)5.4.1.Matrix (57)5.4.2.Fracture/cleat (57)5.4.3.Induced fracturing (59)5.5.Gas saturation (60)6.Coal composition–gas property relationships (61)6.1.Pores and permeability (61)6.2.Maximum gas holding capacity (63)6.3.Desorbed gas (64)7.Production,modelling and water (65)7.1.Production profiles and modelling (65)7.2.Co-produced water (69)8.Key reservoir measurements during exploration (70)8.1.The importance of sampling protocol and mitigation of uncertainty (71)9.Enhanced production (71)10.The impact of CBM (72)10.1.Project viability (72)10.2.From resource to reserve certification (73)11.Summary (74)eful terms and conversions (75)Acknowledgements (75)References (76)1.IntroductionThe approach taken in this review aims to satisfy an apparent need for a current and brief summary of coalbed methane for both students and new colleagues in the industry.Coalbed methane(CBM)is also known as coal seam gas(CSG),coal seam methane(CSM),and coal seam natural gas(CSNG)—all of which are identical for the purposes of this paper.It is recognised,however,that any gas coming from coal is not just pure methane.But the term‘coalbed methane’will be used in this paper because it conveys the two principal words of utmost interest for this forum:coal and methane.This paper is slanted towards the practical and is intended to be broad rather than deep. The tone will be less formal than pure research papers in order to reach a wider than normal audience.However,it is intended that the scientific foundations of gas in coal are clearly laid out and referenced so that the reader can dip deeper where individual interest may alight.The paper will be organised with two major bounding milestones: exploration and production—but in the opposite order.By looking at the production of some CBM basinsfirst we can gain a perspective of the scale and potential that CBM can manifest itself in the world's markets.Towards the end of the paper,critical reservoir properties used in the assessment of an exploration programme will be reviewed. The middle part of the paper will explore how a coal reservoir works and what controls gas occurrence.Finally,two brief reviews will be given,one on enhanced CBM and the other on the impacts of CBM.It is worthwhile to note here that CBM is considered an‘uncon-ventional’gas resource.Though somewhat of an arbitrary designation it is unconventional only because it isn't classically how oil and gas are extracted from clastic and limestone reservoirs.It is true,however, that gas in coal is held in a fundamentally different way.Since gas is held in such a different way in coal,than in conventional reser-voirs,this influences everything from what the significant reservoir properties are to what are the most important measurements to take during exploration,to how to complete production wells,and even how to produce the gas and certify the reserves.Just how different CBM is from conventional gas resources should not be underestimated.So there is no confusion,let me say what this paper will not address.The more theoretical aspects of methane generation from coal will not be covered in depth and nor will methane in coal mining. Methane has long plagued the coal industry(Raymond,1986).This aspect of methane from coal has been covered in other places,including Bibler et al.(1998),Boyer et al.(1990),Karacan et al.(2011),Sloss (2005)and United Nations(2010).Various methods are used to identify and extract methane in order to allow mining with the least cost to human life.Although canaries were once used to detect methane and have earned their place in pop culture(e.g.Sumner,1980),technologi-cal sophistication has progressed substantially in the detection and drainage of gas from coal mines.Extraction and potential monetization of methane from current or abandoned coal mines(commonly termed ‘coal mine methane’[CMM])is,as yet,unquantified but surely has significant potential,both commercially and in mitigation of green-house gases(GHG).However,concepts on methane in coal as explained here are readily transferable from‘virgin’seams to those being or having been mined.The paper will also not deal in any kind of depth of the engineering aspects of vertical or horizontal well completions although the reader is referred to articles by Gentzis(2009),Gentzis et al.(2009a,2009b),Han et al.(2009),Nie et al.(2012)and references within.Any review on a subject as deeply complicated as methane in coal is doomed to disappoint someone.To grasp the entirety of coalbed methane would be impossible within the constraints of a journal article.Thus it is best to view this review as a narrative,and as in any narrative it is limited to the point of view and experience of its author.It is necessarily partial and incomplete.Its author will uniquely determine its path;another author would take a different path.37T.A.Moore/International Journal of Coal Geology101(2012)36–81Finally,a note on the units of measure that are used throughout this paper.The petroleum industry and people in the USA tend to use imperial measurements whereas others tend to use metric.This paper adopts the metric system for most units(for example gas charge is expressed in m3/ton)though for gasflow and resource/reserve es-timates the imperial system is used(for example MCF[thousand cubic feet]).There are various reasons for this but mostly because those units are generally accepted and known throughout the CBM in-dustry.The author has found that most people in the CBM industry can use either metric or imperial when it comes to gas contents but gen-erally always refer toflow rates and reserves exclusively in imperial.In any case,the last section of this paper(Section12)gives some useful conversions.1.1.Previous compilationsIt would be remiss not to identify previous CBM compilations and reviews.I will admit that most of this review will be English language-centric in its references and I apologise for the limitation. Also,there is no doubt that other regionally produced proceedings addressing CBM are not listed here,but this in no way reduces their significance.The oil crisis of the1970s in the USA spurred a focused interest in potential gas resources derived from coal beds.Rightmire et al.(1984) give a good summary of reports that were commissioned by the U.S. Department of Energy on thirteen basins in the USA.Rightmire et al. (1984)make the case that these reports,republished together in 1984as part of an AAPG Special Publication prompted a deepening interest in development of CBM in the USA.One of thefirst major compilations of research on gas in coal,how-ever,was published almost a decade later(Law and Rice,1993)and covered virtually all aspects of hydrocarbon generation in coal.Most papers in the volume delve into the detailed foundation,origins and influences on CBM.Notably,the paper by Levine(1993)outlines the aspects and processes of coalification of organic material and the pathways to formation of CBM.Similarly,the paper by Rice(1993) clarifies the origins of coal gas and its formation,as understood at the time.Flores(1993)also outlines the depositional setting of gas producing sequences,notably presaging the advent of significant pro-duction from the Powder River Basin.Other papers in the compilation deal with natural fractures in coal(Close,1993),the mechanisms of gas sorption(Yee et al.,1993)and methane in underground mining (Diamond,1993)to name just a few.If I were to recommend one set of papers to read to a colleague new to CBM,it would be this volume.Another compendium of papers was published by the Geological Society(Gayer and Harris,1996)and covers various aspects of methane resources(e.g.the calculation of resources),mainly in Europe but it also contains brief discussions of plays in the USA.In addition,there are several papers that report on the coal character,such as mineralisation, in relation to reservoir properties.In almost all cases these papers focus on high-rank coals as CBM producers.A geographic summary of CBM characteristics for Russia,Germany,Ukraine and the UK characterises the coal reservoir in terms of such things as resource size and the poten-tial for development.The paper by Levine(1996)is an early work on coal shrinkage in relation to gas production and permeability changes; an aspect of a CBM reservoir which is still not fully understood.Later,an evaluation of CBM as a danger and a resource was collated by Flores(1998).These papers addressed aspects of underground coal mine outbursts and emissions,as well as sorption characteristics,per-meability,stratigraphy and resource assessment.The composition of coal derived methane gas was studied(Clayton,1998)—a subject returned to a decade later,in a slightly different way,in a volume of special papers by Flores(2008).A broad reaching book edited by Mastalerz et al.(1999)covered many aspects of CBM including regulatory regimes,resource assess-ments,controls on CBM occurrence,reservoir evaluation,methane emissions and modelling,among other subjects.This was thefirst stand-alone book that took such an encompassing view of CBM and, importantly,discussed microbial enhancement of CBM(Scott,1999), probability analysis of CBM properties(Zuber,1999),gas saturation in coal reservoirs(Khavari-Khoransani and Michelsen,1999)and pore space in coals specifically applied to CBM reservoirs(Radlinski and Radlinska,1999).By the mid1990s CBM production from high-rank coals in the USA (i.e.the Black Warrior and San Juan basins)began to level off(see Section2)which accelerated exploration and development in low-rank coals such as those in the Powder River basin of Wyoming and Montana.These developments in turn catalysed research into mechanisms of gas formation in low-rank coals.The special volume edited by Flores(2008)was thefirst internationally available set of papers dealing exclusively with CBM in low-rank coal and thus addressing some of the issues of the genesis and implications of bio-genic gas.The major difference between thefirst set of papers edited by Flores(1998)and this later set was the rise in CBM production from what would be termed‘non-mature’coals.Many of the papers in this volume address the methanogenic consortium in coals and describe laboratory experiments to determine their biokinetics.Though much work is still to be done,this set of papers heralds an important step in defining and understanding a type of methane generation that a decade ago was barely recognised in the petroleum industry.An importantfinding revealed in this set of papers was that methane can be generated in stages and is not simply biogenic or thermogenic(see Section3.1)(Flores et al.,2008).Other compilations of papers exist and the reader is directed to four other special volumes,thefirst of which was edited by Lyons(1998) and focuses on the reserves and geological controls of Appalachian CBM.The second is the volume by Collett and Barker(2003),which discusses coalbed methane properties from the Ferron coals of Utah. The third compilation,by Karacan et al.(2009),discusses aspects of CO2sequestration and methane recovery in coal beds and the last,by Golding et al.(2010),has papers covering topics from Appalachian basin resources to controls on coal cleating.In addition to the above compilations,two institutions in North America have made significant contributions to research into CBM. The Gas Technology Institute in Illinois(formally known as the Gas Research Institute[GRI])has produced countless reports and presenta-tions on all aspects of gas in coal and the reader is encouraged to visit their web site and browse their publications(www.gastechnology. org).Similarly,the University of Alabama has run the International Coalbed Methane Symposium for decades and much new research and understanding has come from those proceedings(www.coalbed. ).There are also some non-aligned,independent,broad-interest and review CBM papers and the reader is directed to Ayers(2002),Bell (2006),Busch and Gensterblum(2011),Bustin and Clarkson(1998), Liu et al.(2011),Pashin(1998),and Scott(2002).Finally,although the paper by Laxminarayana and Crosdale(2002)is primarily about sorption character of selected Indian coals,it does discuss and present some fundamental,and certainly quite broadly applicable,gas–coal relationships.2.Producingfields of the worldWherever there are coal basins,it is inevitable that there will be at least some methane.But in many cases,monetising that gas is prob-lematic because of unfavourable reservoir properties,barriers to market,or a total lack of markets(think basins in remote jungles,or high latitude arctic or Antarctic regions).There are,however,some stellar examples where the right geology,geography and economics have combined to produce a vibrant industry.Even when there is abundant production,it is not easy to get accurate data on gas volumes.As most of us realise,hydrocarbon reserves directly38T.A.Moore/International Journal of Coal Geology101(2012)36–81in fluence a country's economic standing.Thus,there is always potential for ‘gaming ’of the numbers.With this caveat in mind,and in order to give an indication of the scale and importance of coal-derived methane on world economics,some examples are given below.Four countries will be discussed:the USA,Australia,China and India.These countries are currently producing and selling CBM as pipeline gas,or utilising for electrical generation directly on or near site.In all cases,the gas is used domestically.Canada is also a fairly major producer of CBM although will not be discussed here because the USA and Australia already give ample examples of major fields;China and India are brie fly mentioned to highlight emerging CBM markets.Other countries on the verge of commercial CBM production,are Indonesia and Russia but will not be discussed in this paper.The USA currently has the largest CBM production in the world with over 1.91TCF of gas sold in 2009(Fig.1;see also Pashin,2011).The majority of CBM production has come out of three basins:The Black Warrior (Alabama),San Juan (New Mexico,Utah,Colorado)and the Powder River (primarily Wyoming)(Fig.2).The fourth basin,the Raton,has also produced CBM but is smaller and more restricted than the other USA basins thus will not be discussed.In the USA commercial production began in the Black Warrior Basin in the early 1980s but by 1989the San Juan Basin was the major producer (Fig.3).Since the mid 1990s,annual production in the Black Warrior Basin has stabilised around 115BCF.CBM in the San Juan Basin peaked in 1997with an annual production of 597BCF,but has since declined and,as of 2009,was approximately 432BCF per annum.Although most of the signi ficant production in the San Juan Basin began in the late 1980s,there was an early and spectacular well that began in the mid 1950s —the San Juan 32–7well reportedly produced methane for over 40years,though generally at rates around 100MCF/day (Murray,1996).Although both the San Juan and Black Warrior basins are in long term decline at present,they are expected to produce signi ficant methane for a decade or two to come.Production through the 1980s in the USA saw double digit annual percentage increases which fell to less than 5%by 1999(because of the decline in production in the Black Warrior and San Juan basins).However,by 2000the Powder River basin ramp-up (Fig.3)resulted in signi ficant annual increases in the overall USA CBM production.Although initial production in the Powder River basin started as early as 1984,it took over 15years before the right combination of technology and economics resulted in previously unheard of development rates.Over seven years,the number of wells drilled in the Powder River basin went from a few dozen to over 500per annum (Montgomery,1999)and in 2001up to 3655wells were drilled (Ayers,2002).Although the annual CBM production in the Powder River basin has dropped in the last two years,the field is still expected to grow and produce signif-icant methane for a decade or two.The basin has already produced 4.3TCF of gas since 1984(Wyoming Oil and Gas Commission,2011).A de fining attribute of the Powder River basin CBM production is the amount of water produced.Over 678million barrels of water was produced in 2006alone and since 1984a total of 6.5billion barrels of water have been produced (Fig.4).Much of this water is treated and discharged (see Section 7.2).With its huge high-rank coal reserves and numerous underground mines,gas has always been a mining hazard in the coal fields of Australia.A number of early trials at development of CBM by major oil companies failed but by 1996production began with signi ficant ramp up occurring in 2001(Fig.5).Practically all of the gas comes from either the Bowen or the Surat basins in Queensland,however some development and pro-duction occur in the Sydney Basin in New South Wales (Fig.6).Although a signi ficant amount of Australia's production comes from coal of high-rank,the Surat basin,which is mostly subbituminous in rank,does con-tribute signi ficantly to overall gas production.Finally,there have been exploration activities in the brown coals of Victoria,but no production has occurred to date.Reliable production data for CBM in China is elusive,therefore values reported here should only be taken as indicative at best.Simi-larly,gaining information on the number and types of wells is virtually impossible,but the general feeling in the industry is that thousands of development wells have been drilled,but commercial utilisation of the gas is localised as compared to the large integrated network of CBM pipelines seen in the USA,Australia and Canada.What is known is that practically all of the production either comes from the high-rank coals in the Ordos or the Qinshui basins (Fig.7)although some CBM production can be found almost anywhere in China where there is coal and people,which covers much of the country.Data reported in the general press has CBM production in 2006at just over 1BCF and by 2010at 51BCF (Fig.8).Although most certainly inaccurate and underestimating total production,these values probably do re flect the rate of increase in large CBM production projects in China over the last five years.The other certainty is that CBM in China,because of both the size of its coal resources (estimated to be in the trillions of tons)and its voracious energy needs,will likely increase to rival other producing countries in the world.Like China,India has a huge population and growing economy and is looking for ways to maximise its energy resources.Therefore,India has had an aggressive initiative for the last few years to explore and develop its CBM resources through international tendering mercial production has started and is believed to have been just over 2BCF in 2009(Fig.9).Again,the data most likely under represents the actual CBM being produced and sold.A final note on CBM production;over the last few years there has been a race to feed LNG plants with methane from CBM prospects.This is mainly occurring in Australia where it is likely that the first pur-pose built,relatively large scale,plant will take coal seam-derived gas in the next 5years.A similar race is also ensuing within Indonesia.The carrot for all the companies is the relatively high export price paid for LNG;but in order for this to work economically,a large resource has to be able to be developed and sustained over a large number of years.3.Gas type,quality and measurementCBM is primarily of interest because of its energy potential;therefore this review considers the gas itself up front.Reviews of origins of gas in coal and the composition or ‘quality ’of the gas have been conducted by Clayton (1998),Hunt (1979),Rice (1993),Rice and Claypool (1981)and Whiticar et al.(1986).The reader is directed to these papers,and the references within,for an additional perspective and an in-depth exam-ination of gas formation and gas composition in coal beds.To start out with,however,we can consider that there are two primary origins of CBM:biogenic and thermogenic (Fig.10).The gas in the coal seam may contain varying proportions of methane relative to other gases,referred to as gas quality (see Section 3.3for further discussion on gasquality).Fig.1.Coalbed methane (CBM)production in the USA from 1989to 2009.Source:U.S.Energy Information Administration,Washington,DC;/dnav/ng/hist/rngr52nus_1a.htm .39T.A.Moore /International Journal of Coal Geology 101(2012)36–81From a practical perspective,purely biogenic gas plays are almost always lower in gas content than thermogenically-derived CBM.Gas contents are rarely above 4to 6m 3/t in biogenically derived resources though this is not a hard and fast rule.High-rank coals can be in excess (though rarely)of 20m 3/t.Remember too that the transi-tion from biogenic to thermogenic is not instantaneous and there will no doubt be mixing of biogenic and thermogenic gases (see next Section below).And finally,within a single basin,the shallow coal beds may be biogenic plays whereas the deeper coals may be thermogenic (e.g.Hackley et al.,2009).3.1.Biogenic gas and the microbes that create itIt is now recognised that life below the surface of the earth is vast (Gold,1992)and could account for half of Earth's biomass (/contents/dark_life.shtml ).Mostly,life below our lawns consists of microscopic organisms (microbes)(Fig.11)which play a huge role not only in decay of plant material (Rogoff et al.,1962;Waksman and Stevens,1929)but also in the generation of methane inpeat and low-rank coal (Str ąpo ćet al.,2008a;Wang et al.,1996).methane is derived from microbes,it is said to be biogenically It is interesting to read Rogoff et al.(1962),which is an early of the microbiology of coal.The questions and doubts raised in paper are now just beginning to be addressed and answered (seepapers in Flores,2008as well as Midgley et al.,2010,Penner et al.,2010and Str ąpo ćet al.,2008a ).A number of recent studies have categorised microbial taxa found in coal-derived water and coal matrix that have been deeply (500to 1000m)buried (e.g.Green et al.,2008;Hendry et al.,2007;Li et al.,2008;Midgley et al.,2010;Penner et al.,2010;Shimizu et al.,2007;Str ąpo ćet al.,2008a ).Usually the microbial assemblage found in coal will consist of hundreds of taxa but it is generally believed that there is only a smaller subgroup,aptly termed methanogens,which In terms of the taxa present,creation of biogenic methane from coal requires the cooperation of organisms from two out of the three Domains of Life —bacteria and archaea (note that the three Domains [Superkingdoms]of Life are:Eukarya,Bacteria and Archaea (Woese et al.,1990).Plants and animals (humans among them)lie within the Eukarya branch of life).Collectively,the organisms involved are referred to as methanogenic consortia because there are literally hundreds of species of both bacteria and archaea that have their own special role in making methane (Green et al.,2008).Bacteria begin the process and archaea complete it (Fig.12).There is a developing body of research on biogenic gas formation;however it is recognised that there are many more details still to be understood.The following discussion is gleaned from a few papers (Green et al.,2008;Midgley et al.,2010;Shimizu et al.,2007;Str ąpo ćet al.,2008a;Ulrich and Bower,Fig.2.Map showing locations of USA basins discussed in this paper.Fig.3.Coalbed methane production pro file for the Black Warrior basin,USA from 1980to 2009,San Juan basin,USA from 1989to 2009and the Powder River Basin,USA 1985to 2010.Source:Energy Information Association /dnav/ng/hist/rngr52sal_1a.htm ,and Wyoming Oil and Gas Conservation Commission:/.Fig.4.Co-produced water from the Powder River basin from 1985to 2010.Source:Wyoming Oil and Gas Conservation Commission:/.40T.A.Moore /International Journal of Coal Geology 101(2012)36–81。