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  • 7/31/2019 Rangkuman CBM



    CBM is a natural gas containing virtually 100% methane (CH4) produced from coal seam


    Form of natural gas extracted from coal beds

    Gas is held on coal matrix by sorption

    Why is CBM?

    Provide a clean-burning fuel.

    Increase substantially the natural gas reserve base.

    Improve safety of coal mining.

    Decrease methane vented to the atmosphere from coal mines that might affect global warming.

    Provide a means to use an abundant coal resource that is often too deep to mine.Characteristics of Coal Suitable for CBM Production

    1. High gas content: 15m3 - 30m3 per tonne is typical.

    2. Good permeability: 30mD - 50mD is typical.

    3. Shallow: Coal seams < 1,000m in depth.

    The pressure at greater depths is often too high to allow gas flow even when the seam has

    been completely dewatered. This is because the high pressure causes the cleat structure to

    close, reducing permeability.

    4. Coal rank: Most CBM projects produce gas from Bituminous coals, but it can be possible to

    access gas in Anthracite.

    Depositional System:

    Narrow range sedimentary environment

    Buried quickly, high water table & isolated for oxidation process

    Lingkungan pembentuk batubara : Mires

    o Marine Connected, termed paralic : lagoon

    o Fresh water connected, termed limnic : lakes/abundant river channel

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    Coalification -> Coal Rank

    2 Stages:

    Biochemical: micro-organisms initiate & aid the

    chemical decomposition of vegetation into peatand brown coal

    Physicochemical: initiated & maintained by

    post-depositional subsidence & effects of rising

    temperature & pressure

    Coal Maturation=Coal Rank

    Gradual Process characterized by stage

    A measure of thermal maturity

    6 ranks commonly recognized





    SandstoneOffshore Shale



    Offshore Shale



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    Coal Type

    is the unique composition of a coal

    the proportion of different organic macerals & inorganic minerals

    Humic Coal

    the most abundanttype

    rooted vegetation deposited in-situ in mires & accumulated as peat

    finely bedded at macroscopic scale

    Sapropelic Coal


    sub aquatic deposition

    floating vegetation (incl. algae) & re-deposited organic matter not formed in-situ

    massive, black/brown, non-bedded

    (at macroscopic scale)

    Coal Type Analysis

    Lithotype Description

    macroscopic description of seam profile

    profile reflects depositional history

    aids correlation

    lithotypes reflect micro-composition

    Maceral Analysis

    determine proportions of macerals (cf minerals)

    reflected-light microscopy

    implications for depositional environment

    implications for utilisation

    can be distinctive for seams

    Coal type Lithotype Macroscopic characteristics



    Vitrain Bright, black, usually brittle, frequently

    with cleats

    Clarain (Duroclarain) Semi-bright, black, finely stratified

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    (Clarodurain) Durain Dull, black or grey-black, hard, roughsurface

    Fusain Silky lustre, black fibrous, soft friable



    Cannel coal Dull or slightly greasy lustre, blackhomogenous, unstratified, hard,

    conchoidal fracture, black streak

    Boghead coal Like cannel coal but brownish colourand brown streak

    Coal Lithotype (Australian System)

    Pengaruh dari tipe dan rank batubara untuk CBM:

    volume gas yang dihasilkan

    kapasitas batubara untuk mempertahankan gas

    pembentukan cleat yang merupakan permeability pathways

    physical properties dan response untuk prosedur rangsangan

    Bri ht Coal: 80-100% bri ht laminae

    Banded Bright Coal: 60-80% bright laminae

    Banded Coal: 4060% bright laminae

    Banded Dull Coal: 20-40% bright laminae

    Dull Coal: 0-20% bright laminae

    Fibrous Coal: very dull, earthy, very friable

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    Methane in Coal

    Coal is a sedimentary rock that had its origin as an accumulation of inorganic and

    organic debris.

    A readily combustible rock containing more than 50% by weight and more than 70% by

    volume of carbonaceous material formed from compaction and induration of variously

    altered plant remains similar to those in peaty depositsSchopf, 1956

    A black rock that burns!

    Coal acts as both source rock and reservoir rock for methane

    Methane is generated by microbial (biogenic) or thermal (thermogenic) processes

    Shortly after burial and throughout the diagenetic cycle (resulting from further burial) gas

    is generated and is physically sorbed on coal surfaces in areas with coal micro porosity

    Coal as source rock

    Biogenic can form early or late

    early at the beginning of coalification

    late by bacterial action in the reservoir Dominant gas generallyThermogenic

    volume of gas increases with coalification (coal maturity)

    very large volumes of gas generated during coalification

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    Insitu (swamp gas)


    Maceral & hydrocarbon (kerogen) types

    Thermal maturity/burial

    Hydrologic drive (cleat/fracture system)


    Thermal maturity/burial history


    Thermal maturity/burial history



    Thermal maturity/burial history


    Hunt, 1979

    Gas Storage

    Gas is retained in the coal due to hydrostatic andlithostatic pressure

    Gas in coal seams is stored in three basic ways:

    Chemical adsorbed to coal particles (macerals) and held

    by molecular attraction

    Within pore spaces, cleats and fractures of the coal

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    Dissolved in water contained within the coal

    Storage capacity

    The amount of adsorbed gas depends on

    Ash content

    Rank of coal

    Burial history

    Chemical make up of the coal


    And gas lost over geologic time

    Gas Saturation

    Determined by laboratory adsorptiontest

    Saturation is ratio of desorbed to adsorbed gas content

    Gas Composition

    Controls on coalbed gas compositon

    Maceral content (kerogens)

    Coal rank Reservoir dynamics

    Migrated gas

    Biogenic gas

    Controls on gas accumulation and distribution:


    Coal seam thickness and continuity

    Igneous intrusions

    Burial history and reservoir (P & T)

    Coal composition and rank

    Hydrogeology and biogenic gas


    Methane 70-98%

    Ethane 1-10%

    Propane trace-5%

    Butanes trace-2%

    Pentanes trace-1%

    Hexanes trace-1/2%

    Heptanes trace-1/2%

    Non Hydrocarbon

    Nitrogen trace-15%

    Carbon Dioxide trace-5%

    Hydrogen Sulfide trace-3%

    Helium up to 5%, usually trace or none

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    Effect of Igneous Intrusion

    High gas contents in coal around sills

    Little effect on gas quality

    Still 95% CH4

    Enhanced permeability & desorption

    Characteristic micropores & slits

    Sills may act as seals

    Gas contents higher below sills

    Sills important on regional scale for heat flow

    Hydrogeology Impact:

    Unsually high gas content

    Unsually low gas content

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    Coal Permeability

    Coal itself is a low permeability reservoir Almost all the permeability of a coal bed

    is usually considered to be due to fractures,

    which in coal are in the form ofcleats

    The face cleats are continuous and provide

    paths of higher permeability, while butt

    cleats are non-continuous and end at face cleats.

    Cleat develops in a coal from:

    Dehydration during coalification

    Devolatilization during coalification

    Tectonic force


    Why is cleat important?

    matrix of coal very porous but very low permeability

    gas desorbs slowly from matrix but rapidly through

    cleat network, i.e coal is essentially a fractured reservoir

    all permeability derives from the cleat network

    permeability the most common limiting factor to

    economic CBM production

    cleat development influenced by both coal rank & type

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    Influences on Permeability:

    all Perm derives from fracture system (cleat)

    cleat system influenced by:

    rank (+)

    ash (-)

    vitrinite content (brightness) (+)

    mineralisation on cleat surface (-)

    reservoir influence on perm

    depth (-)

    in-situ stress (-)

    Cleat spacing:

    Cleat spacing varies with rank

    cleat density highest in med vol. Bituminous to semi-anthracite

    Cleat spacing varies with coal type

    cleat density highest in brighter lithotypes

    at low rank, dull and banded dull coals might not be cleated

    cleat often terminates at lithotype boundaries

    generally, very low ash coal zones will be bright, with good cleat.

    rarely, very low ash zones will be dry dull coals with poor/no cleat


    Cleat spacing

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    Coals are composed of macerals (microscopic organic particles in the coal)

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    CBM vs Conventional Gas Reservoir Characteristics

    Characteristic CBM Conventional

    Gas generationGas is generated and trapped

    within the coal

    Gas is generated in the source rock

    and then migrates into the reservoir

    Structure Uniformly-spaced cleats Randomly-spaced fractures

    Gas storage

    MechanismAdsorption Compression



    Concentration gradient (FicksLaw) and Pressure Gradient

    (Darcys law)

    Pressure Gradient (Darcys law)


    Origin Butt cleat, face cleat, fractures Fractures, connectedness



    Gas rate increases with time then

    declines. Initially the production

    is mainly water.

    GWR increases with time

    Gas rate starts high then decline.

    Little or no water initially.

    GWR decrease with time

    Gas contentprediction

    Cores Wire-line logs



    Young modules ~105

    Pore compressibility~ 10-4Young modules ~10Pore compressibility~ 10-6

    Advantages of CBM

    vs Conventional

    Disadvantages of CBM

    vs Conventional

    Often located in major markets

    Increasing production initially

    Long life

    Onshore shallow wells

    Potential for carbon sequestration

    Potentially large footprint

    Dewatering during gas production

    Low pressure requires many wells

    Higher well completion costs

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    Thickness (h)

    Coal shallower than 150m and deeper than 1200m is usually excluded from volumetric


    Too low gas content at shallow depths and occluded permeability at deeper depths

    SPEE/COGEH uses rules of thumb on requirements for clustering seams fordevelopment of:

    > 0.3m in thickness

    < 30.0 m vertical separation

    Another major consideration is the maximum completed interval thickness

    Ash and Moisture (%)

    Essentially these are the equivalents to the net-to-gross ratio of the individual coal bedCoal formed of non-net ash yield (other lithologies) and moisture content

    Analysis is performed in the laboratory and should have a relatively low range of measureduncertainty.


    Coals can vary significantly in ash yield vertically between seamsRelates to the grade and depositional environment of the coal

    Moisture (irreducible water) contents are usually low(< 3-5 %)

    SPEE/COGEH suggest a cut-off of 50% for ash yield to qualify the bed as a suitably useablecoal

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    Gas Content

    Errors arise when estimating the gas content of the coal during sampling, relating to:

    Q1- lost gas (lost during the retrieval)

    Q2- recovered gas (contained during the sampling)

    Q3- residual gas (released by crushing the coal)

    Large uncertainties can exist on estimation of Q1

    Majority of uncertainty relates to:

    Spatial distribution of the gas content

    Can vary considerably both laterally and vertically

    Requires detailed mapping and high density sampling

    Units- either As-received or Dry Ash Free (daf)

    Traditionally gas-content values are obtained by desorbing core samples in the laboratory and

    then correcting these values for lost and residual gas.

    Coal Density

    Coal has a density ranging between approximately 1.1 - 2.5 g/cm3

    Density used to convert gas content (scf/ton) to volume (scf/ft3)

    Dependent on ash/moisture content and rank of coal

    Rank is the degree of metamorphism of the coal

    Highest Gas Contents found in Sub-Bituminous to Bituminous coals

    Errors often occur relating to the density value not being used on the same basis as the gascontent value

    i.e. both must be As-received or daf

    Often a conservative density limit of 1.75 g/cm3 is applied

    The average insitu coal density can be estimated from from a density log or from core


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    Parameter Kualitas Batubara

    Total Moisture


    Total Sulfur

    Calorific Value


    Ultimate Analysis

    Ash Fusion Temperature

    Ash Analysis

    Total Moisture

    Moisture dalam batubara : Inherent moisture -> EQM ; MHC

    Extraneous moisture -> surface moisture

    TM = EQM + SM

    Tinggi Rendahnya Total Moisture akan

    tergantung pada :

    Peringkat Batubara

    Size Distribusi

    Kondisi Pada saat Sampling

    Semakin tinggi peringkat suatu batubara -> semakin kecil porositas batubara tersebut

    atau semakin padat batubara tersebut.-> Dengan demikian akan semakin kecil juga moisture

    yang dapat diserap atau ditampung dalam pori batubara tersebut.

    Hal ini menyebabkan semakin kecil kandungan moisturenya khususnya inherent moisturenya.

    Semakin kecil ukuran partikel batubara -> semakin besar luas permukaanya. Hal ini

    menyebabkan akan semakin tinggi surface moisturenya. Pada nilai inherentmoisture tetap,

    maka TM-nya akan naik yang dikarenakan naiknya surface moisture.

    Total Moisture dapat dipengaruhi oleh kondisi pada saat batubara tersebut di Sampling.

    Yang termasuk dalam kondisi sampling adalah :

    Kondisi batubara pada saat disampling

    Size distribusi sample batubara yang diambil terlalu besar atau terlalu kecil.

    Cuaca pada saat pengambilan sample

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    Penentuan Total Moisture:

    Penentuan Total Moisture biasanya dibagai menjadi dua tahap penentuan yaitu :

    Penentuan Free Moistrue atau air dry loss

    Penentuan Residual moisture

    TM = FM + RM(1-FM/100)

    Air Dried Moisture

    Adalah moisture yang terkandung dalam batubara setelah batubara tersebut dikering udarakan

    Moisture In the analysis samples

    Inherent Moisture


    Besar kecilnya nilai ADM dipengaruhi oleh peringkat batubara. Semakin tinggi peringkat

    batubara, semakin rendah kandungan ADM nya.

    Nilainya tergantung pada humuditas dan temperature ruangan dimana moisture tersebut


    Nilainya tergantung juga pada preparasi sample sebelum ADM dianalisa (Standar


    TM = ADL + RM (1-ADL/100)

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    Ash Content

    Batubara sebenarnya tidak mengandung abu, melainkan mengandung mineral matter. Namun

    sebagian mineral matter dianalisa dan dinyatakan sebagai kadar Abu atau Ash Content.

    Mineral Matter atau ash dalam batubara terdiri dari inherent dan extarneous.

    Inherent Ash ada dalam batubara sejak pada masa pembentukan batubara dan keberadaan

    dalam batubara terikat secara kimia dalam struktur molekul batubara

    Sedangkan Extraneous Ash, berasal dari dilusi atau sumber abu lainnya yang berasal dari luar



    Kadar abu dalam batubara tergantung pada banyaknya dan jenis mineral matter yang

    dikandung oleh batubara baik yang berasal dari inherent atau dari extraneous.

    Kadar abu relatif lebih stabil pada batubara yang sama. Oleh karena itu Ash sering

    dijadikan parameter penentu dalam beberpa kalibrasi alat preparasi maupun alat sampling.

    Semakin tinggi kadar abu pada jenis batubara yang sama, semakin rendah nilai kalorinya.

    Kadar abu juga sering mempengaruhi nilai HGI batubara.


    Kadar abu didalam penambangan batubara dapat dijadikan penentu apakah penambangan

    tersebut bersih atau tidak, yaitu dengan membandingkan kadar abu dari data geology atau

    planning, dengan kadar abu dari batubara produksi.

    Kadar abu dalam komersial sering dijadikan sebagai garansi spesifikasi atau bahkan

    sebagai rejection limit.

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    Volatile Matter

    Volatile matter/ zat terbang, adalah bagian organik batubara yang menguap ketika dipanaskan

    pada temperature tertentu.

    Volatile matter biasanya berasal dari gugus hidrokarbon dengan rantai alifatik atau rantai

    lurus. Yang mudah putus dengan pemanasan tanpa udara menjadi hidrokarbon yang lebih

    sederhana seperti methana atau ethane.


    Kadar Volatile Matter dalam batubara ditentukan oleh peringkat batubara.

    Semakin tinggi peringkat suatu batubara akan semakin rendah kadar volatile matternya.

    Volatile matter memiliki korelasi dengan vitrinite reflectance, semakin rendah volatile

    matter, semakin tinggi vitrinite reflectancenya


    Volatile Matter digunakan sebagai parameter penentu dalam penentuan peringkat


    Volatile matter dalam batubara dapat dijadikan sebagai indikasi reaktifitas batubara pada

    saat dibakar.

    Semakin tinggi peringkat suatu batubara akan semakin rendah kadar volatile matternya.


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    Organic sulfur, sulfat sulfur, pyritic sulfur


    Kandungan sulfur dalam batubara sangat bervariasi dan pada umumnya bersifat

    heterogen sekalipun dalam satu seam batubara yang sama. Baik heterogen secara vertikal

    maupun secara lateral.

    Namun demikian ditemukan juga beberapa seam yang sama memiliki kandungan sulfur

    yang relatif homogen.


    Sulfur dalam batubara thermal maupun metalurgi tidak diinginkan, karena Sulfur dapat

    mempengaruhi sifat-sifat pembakaran yang dapat menyebabkan slagging maupun

    mempengaruhi kualitas product dari besi baja. Selain itu dapat berpengaruh terhadap

    lingkungan karena emisi sulfur dapat menyebabkan hujan asam. Oleh karena itu dalam

    komersial, Sulfur dijadikan batasan garansi kualitas, bahkan dijadikan sebagai rejection


    Namun demikian dalam beberapa utilisasi batubara, Sulfur tidak menyebabkan masalah

    bahkan sulfur membantu performance dari utilisasi tersebut. Utilisasi tersebut misalnya

    pada proses pengolahan Nikel seperti di PT. INCO. Dan juga pada proses Coal

    Liquefaction (Pencairan Batubara).

    Calorific Value

    Specific Energy

    Higher heating Value

    Adalah nilai energi yang dapat dihasilkan dari pembakaran batubara.

    Nilai kalori batubara dapat dinyatakan dalam satuan: MJ/Kg , Kcal/kg, BTU/lb

    Nilai kalori tersebut dapat dinyatakan dalam Gross dan Net.

    Nilai Kalori dapat dinyatakan dalam satuan yang berbeda :

    Calorific Value (CV)(kcal/kg)

    Specific Energy (SE) .(Mj/kg)

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    Higher Heating Value (HHV) = Gross CV

    Lower Heating Value (LHV)= Net CV

    British Thermal Unit = Btu/lb


    Nilai Kalori batubara bergantung pada peringkat batubara. Semakin tinggi peringkat

    batubara, semakin tinggi nilai kalorinya.

    Pada batubara yang sama Nilai kalori dapat dipengaruhi oleh moisture dan juga Abu.

    Semakin tinggi moisture atau abu, semakin kecil nilai kalorinya.

    CBM Potential in Indonesia:

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    CBM exploration strategy

    1. Initial basin assesment

    Initial Geology and Geophysical study and data analysis

    2. Basin wide eksploration

    Drilling of core and stratigraphic wells

    CSG data analysis and technical evaluation

    3. Appraisal wells

    pilot test program

    Gas water flow testing

    Completion tests

    Commerciality analysis

    Good CBM Prospect (not definite)

    Parameter :

    Seam thickness : Best coal seam >8m thickness

    Depth : coal seam between 3001200m in depth

    Seam Properties

    Rank (mostly bituminous but possible also sub bituminous)

    Composition (preferably high in vitrinite content because generally generate good cleat )Ash content (low ash-high carbon content)

    Permeability (best 20mD-50mD, but >3mD can be economic with right stimulation and

    completion strategies)

    High gas content : 10m3-25m3 /tone (sometimes low gas content in thick and high

    permeability coal seams could be a good prospect as well)

    Structural trapping

    Access to market

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    CBM Technology:


    Hydraulic Fracturing

    Hydraulic fracturing (more commonly known as fracing) is the technique used to increase

    the surface area of the coal. The fluid systems and additives used in conventional wells are

    generally not suitable for CBM wells. This is because coal seam reservoirs have uniqueproperties and therefore specially developed materials need to be used.

    Well Completion