Application of Geophysical Well-Logs in Coal Bed Methane

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Application of Geophysical Well Logs in Coal bed Methane Prospect Evaluation____________________________________________________________________________________________________________

Application of Geophysical Well-Logs in Coal bed MethaneProspect Evaluation

Dr.V.Gopala ao and Mr..!.Cha"raborty#L$ Asia Ltd.

AbstractGeophysical Well logs are important information source for CBM prospect evaluation !t identifiescoal layers "ithout any ambiguity and offers a permanent record# "hich facilitates spatialcorrelation# and "or$ out geological history of the area by "or$ers of different discipline at thesame time or at different point of time Geophysical "ell log characteristics of coal beds bears astrong semblance to coal ran$ing# recogni%es cleat potential & fracture %ones and providesinferences on thermal history# pressure regime and about stress field of the area that are vital forprospect evaluation Well logs are most reliable data source in various map preparation neededfor reserve estimation# forecast production potential and plan for optimal recovery of methanegas Well log data being a permanent record# offers a strong foundation for generating transforms

bet"een laboratories derived data vis-'-vis log data for continuous evaluation and applycorrective reality measures to incorporate subtle local variations caused due to geologicenvironmental changes (hese transforms have found "ider acceptance throughout the globe forvarious assessment# and handy# to ma$e forecast on production potential Among the array oflogs used in CBM prospect evaluation# density log occupies a special position (his is because ofits "ider use eg in various assessments# framing transforms# and Gas !n Place )G!P* calculationthrough volumetric method

Introduction

CBM is an energy source and areheld in coal layers in a li+uid li$estate Coals can accommodate si, orseven times more gas volume thanconventional sand reservoirs Coalshave high micro-porosity and largeinternal surface area that providesthe re+uisite accommodation spaceto host methane gas# "hich can berecovered before# during and aftermining# and even from the debrisafter abandonment of mining activity#called gob gas After e,traction of

CBM# coal layers can act as a goodhost for C.se+uestration becauseC. has high degree of affinity tocoal layers CBM e,ploitation istherefore advantageous from thestandpoint of protecting planet Earthfrom potent green house effectleading to high global "arming

Methane has ./ times more "armingcapability than C.

Coals are of follo"ing types0 Peat#Lignite# 1ub-Bituminous# Bituminous#and Anthracite Ma2or elements incoal are0 carbon# hydrogen# o,ygen#sulfur and nitrogen Minor +uantitiesof metals eg iron# sodium# calcium#mercury etc and many organicmolecules are also present (heirorganic structure varies dependingon content and grade rganicstructures in coal generally are toocomplicated as can be vie"ed from

the e,ample presented here in )3ig/* !t is in these molecular sieves#methane molecule is stored (oe,ploit this gas economically# it isnecessary to assess its potentialfirst (o this effect# "ell logs throughout the "orld have been found veryeffective and potent data source

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Coalification, Methane Gene-ration and Entrapment

Coal is a carbon rich roc$ derivedfrom plant material in s"amps Peat

is the first stage of transformation !tis created under specific conditionseg "ater logging# lac$ of o,ygen ornutrients# high acidity and lo"temperature !ts subse+uent burialby ongoing geologic processes#"hich led to high temperature andpressure "ith increasing depth ofburial# affect the plant materialundergo coalification# releasingvolatile matters )"ater# C.# light

hydrocarbons including methane4C56* With on going coalificationprocess# coal becomes progressivelyrich in carbon content and continuesto e,pel volatile matter

CBM origin can be biogenic orthermogenic (he re+uirements forbiogenic gas generation are0 ano,icenvironment# lo" sulfate concen-tration# lo" temperature# abundant

organic matter# high p5 values#ade+uate pore space and rapidsedimentation Biogenic methanegas is generated over a period oftens of thousands of years

Biogenic gas can be generated int"o stages0 ne# in the early stage#from the peat# favored by rapidsedimentation )!t is believed thatmost of the biogenic gases "ere

probably generated during this earlystage* and t"o# in the late stage# inrecent geological time and isassociated "ith active ground "atersystem# "hich provides favorableenvironment for bacterial activity#including methane generation (hislate stage biogenic gas generation

can ta$e place in coal beds of anyran$

(hermogenic origin ta$es place inhigher coal ran$s )vitrinite reflec-

tance# 78 value 9 8: ;*< tempe-rature plays a big role in gas yieldGas generation is ma,imum in

Anthracite and too lo" in Bituminouscoal

Gas is trapped in the coal seam inpart by "ater pressure and in part by"ea$ co-valent bonding# $no"n as=an de)r* Walls forces Largeamount of gas can be stored in the

molecular cage at lo" pressure andis adsorbed into the internal structureof the coal

CBM Prospect Evaluation

CBM Prospect evaluation needs0$no"ledge of thermal history#pressure regime# hydro-dynamicsand regional stress (hese areinferred parameters !n addition to

these# the coal characteristics# coalran$ing# gas adsorption capacity#initial gas phase concentration andcritical saturation are importantlaboratory derived parametersneeded to predict gas generationand production potential (o "or$ outthese parameters# one relies onfollo"ing information sources0 Coresand cuttings# Well logs# !nput fromMines# utcrop Analogue# Geology

and 1eismic data (he approach toevaluation# therefore# is on inte-gration and co-vie"ing of multi-disciplinary data

n thermal history front# vitrinitereflectance )78* and presence ofclays are important contributors#

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particularly $aolinite# "hich indicatesthat the temperature involved incoalification must have been lessthan .88> C At and above thistemperature# the common clay

minerals are metamorphosed

3or specific development# rela-tionship bet"een gas content andgas holding capacity determinedfrom sorption isotherm is important1orption isotherm is the relationshipbet"een gas storage capacity andpressure at constant temperature !tis usually referred as Langmuirsorption isotherm )3ig.* (he

relationship is e,ponential and is"or$ed out on actual coal sample inthe laboratory Characteristics ofLangmuir sorption isotherm aredependent on inorganic content#organic properties and ontemperature

3or estimating production potential#$no"ledge on follo"ing three factorsis critical0 ?esorption of gas from the

coal matri, )follo"s sorptionisotherm*# diffusion of gas in thecleat system and flo" throughfractures )hydrodynamics* 3romproduction engineering standpoint#these can be put under t"o heads0Permeability and Gas content

Most mature coals "ere originallyfully saturated# but coal looses gasas reservoir pressure is reduced# due

to0 uplift# faulting or unconformities)3ig@* Coals can store more gas atlo"er temperatures after a basin isuplifted

Coals can become fully saturated"hen more gas is added due tosecondary biogenic gas generation

By combining gas content data "ithan adsorption isotherm# it is possibleto determine the gas saturationcondition )3ig6*

CBM Production

CBM production is a function ofLangmuir isotherm When the effectsof adsorption and the t"o-phase flo"are combined# the characteristiccurves assume distinct shape 3or"ater# "e e,pect to see significantinitial production follo"ed by adecline# and for gas# "e e,pect tosee no initial gas production follo"ed

by an increasing gas rate# and finallya decline )3ig*

A field may ta$e years before theonset of actual gas production (histogether "ith environment compli-ance "ater disposal system deter-mines the commercial viability ofCBM pro2ect

Contribution of Geophysical

Well o!s

Well logs are important informationsource for identifying coal layers)3ig:* and infer their characteristicseg cleats# ash and moisturecontent !t provides several importantparameters eg net pay thic$ness#reservoir pressure and host of inputsfor map preparation necessary forreserve estimation# and forecasts

spatial e,tent# gas potential andrecovery factor Core-log transformshave been found very helpful to thiseffect

?etermination of Ash content fromdensity log is routine feature )3ig*?irect relationship e,ists bet"een

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density and ash content Bye,trapolation of this relationship# ashdensity )75a* & pure# ash free#organic coal density )75o* can becalculated (his input is important

because gas content decreases "ithincrease of inorganic content3urther# gas-in-place formula)=olumetric e+uation* contains bul$density parameter as direct input#)3ig* ma$ing density log a vitaldata source

With the onset of production#pressure measurements areroutinely made# often near the

producing layer)s* through "ire-linegauges to have reliable first handinformation on pressure regime

Dno"ledge about stress field isdeduced from full "ave sonic logs

(hus host of information aregathered from "ire-line services#"hich in con2unction "ith thesupplementary sources# reduce

dependency on actual sample mea-surements and enable continuousevaluation instead of discreteevaluation# and incorporate subtlechanges observed in coalcharacteristics "hich other"ise canbe missed

E"amples

Well log response against coal layers

yields net pay thic$ness andcharacteri%es coal layers# "hich areeventually lin$ed to coal ran$ing

Micro log have been found useful inestablishing cleat potential anddelineating fractures )3ig*

?ensity log is useful for ash content#gas content and other miscellaneousevaluation )3igs/8# //# /.# &/@*

Feutron log at times is useful formoisture content estimation

3ull "ave sonic logs help inestimating mechanical properties ofcoal layers and in establishing thestress field direction (hese inputsare used in designing hydrofracturing and or cavitations 2ob toincrease productivity

Pressure measurements at regularinterval facilitates in estimatingoriginal gas in place )G!P* throughmaterial balance and# from thedecline curve analysis# the ultimaterecovery of methane gas

3uture e,ploration# evaluation ande,ploitation practices shall rely moreon seismic attribute analysis and "elllog integration for reliable spatialcoal characteri%ation (his "illenable engineers dra" successfule,ploitation plans by incorporatingdynamic properties eg reservoir andproduction engineering data to coalcharacteri%ation !n the entireprocess of integration# "ell logsdirectly or indirectly )throughtransforms* provide the re+uisite lin$of multi-faced disciplines

Conclusions

Well logs have been found useful inall stages of CBM pro2ectmanagementH venture e,ploration#evaluation and e,ploitation

Ability of "ell logs in delineating coallayers "ithout any ambiguity and

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their geophysical characteristics lin$to coal ran$ing have made them anaccepted data source in the industry

Well logs provide the re+uisite lin$ in

integrating multi-disciplinary sub2ectsnecessary for providing globalsolution

3uture "ill see more intenseintegration "ith seismic attributes forbetter and more reliable spatialcharacteri%ation (his "ill provide afirm base for reservoir andproduction engineers to dra"successful "or$ plan for optimal

e,ploitation

?ensity log finds a special place inCBM property analysis becausedensity value is directly used involumetric method for Gas in Place)G!P* assessment# +uantitative ashand )at least* +ualitative gas contentdetermination !t is indirectly relatedto moisture content as "ell (herelationship bet"een volume of

moisture and ash content is linear

Micro-log response against coalbeds gives good indication aboutcleat potential and presence of li$elyfractures# providing operator usefulinformation about future drillingtechni+ues and completion design

1hear "ave is generally notsupported in coals (herecommended practice is to estimatethis value from 1tonley "ave velocityor from a $no"n value of PoissionIsratio 3rom inferred shear "avearrival time# mechanical properties ofcoal beds are estimated and arestretched further for determiningstress direction (his useful

information is used in designinghydro-fracturing and cavitations 2ob

Application of imagery tool andnuclear magnetic tool hold

tremendous potential in permeabilityassessment "hich needs to bee,plored

Ac#no$led!ements

Authors "ould li$e to e,press theirac$no"ledgements to the manage-ment of 5L1 Asia Ltd for thepermission and resources to "ritethis paper

%eferences

Mullen# MJ# //# KCoalbedMethane 7esource Evaluation fromWireline Logs in Fortheastern 1anJuan Basin0 A Case 1tudy# 1PE/6:# PP /:/-/.

Mullen# MJ# //# KCleat ?etectionin Coalbeds using the Micro Log#7oc$y Mountain Association ofGeologists# PP /@-/6

Mullen# MJ /# KLog Evaluation inWells ?rilled for Coal-bed Methane#7oc$y Mountain Association ofGeologists# PP //@-/.6

5alliburton ?ocument on CBMLogging (echni+ues and Evaluation

1ircar and Anirbid# August# .888# KA7evie" of Coalbed MethaneE,ploration and E,ploitation#Current 1cience# =ol # F 6

Fational Energy (echnologyLaboratory K3uture 1upply and

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Emerging 7esources4CoalbedFatural Gas document

K?etermining Gas ProductionCharacteristics of Coal 1eams#

"""sigracomaupprcsgdethtml

KCoal Bed Methane Play andProspect Evaluation NsingGeoGraphi, 1oft"are# ?ocument

3ig/ rganic 1tructure of Coal

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http://www.sigra.com.au/ppr_csgdet.htmlhttp://www.sigra.com.au/ppr_csgdet.html


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3ig. Langmuir !sotherm

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Thermogenic gas moved up-dip

Biogenic gas migrated up -dip

3ig@ Gas Migration

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3ig6 1aturated 7eservoir converting to Nnder-saturated 7eservoir

3ig 3irst Water Production then Gas Production

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3ig: Log characters against coal and carbonaceous layers

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Dry Ash Content

Density

Ash Content vs. Density

.. ..

RHOa

RHOo

3ig Ash content determination from ?ensity

Gas in place ormula

G!" # $%&'.( )A* )h* ) Rho B* )Gc* +

),%&/ )A* )h* ) "or* )$- 01* )Bg*2here3

A # Areah # thic4ness

Gc =Gas content

Bg # Gas compressi5ility actor

6irst part is or adsor5ed gas second part is or ree gasstored in cleat system )insigniicant 7 89*

3ig ?ensity value is a direct input to Gas in Place formula

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:on-clea ted

6ractured or poorlycleated

;o1 to air permea5ility

2ellcleated

Good p ermea5ility

Thin laminated coal

"ro5a5ly som e racturing

< ost li4ely lo1 permea5ility

< icro log Response

3ig Microlog 7esponse in cleated and fractured coal

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ccm*

Gas Content vs. Bul4 Density

3ig/8 Gas Content 7elated to ?ensity

"u5lished

;ocal

/8/ ,/

? Ash )9*

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?moisture


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? Ash )9*

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i=e d c a r 5

o n

;ocal

"u5lished

Ash vs 6i=ed car5on

3ig/. Carbon Content and Ash Content 7elationship

/ 8/ ,/

? Ash )9*

8/

,/

./

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i=e d c a r5 o nd ry a

sh r

e e

;ocal

"u5li

shed

Ash vs 6i=ed car5on dry ash ree

3ig/@ Carbon Content and ?ry Ash Content 7elationship

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Application of Geophysical Well-Logs in Coal Bed Methane

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