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COAL BED METHANE (CBM)

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

reservoirs.

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

Peat

Lignite

Sub-bituminous

Barrier

SandstoneOffshore Shale

LagoonalShale

Ocean

Offshore Shale

LagoonalShale

TidalInlet

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Bituminous

Semi-anthracite

Anthracite

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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

uncommon

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

Humic

coal

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

(charcoal)

Sapropeliccoal

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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BIOGENIC (EARLY)

Insitu (swamp gas)

BIOGENIC (MATURE OR SECOND STAGE)

Maceral & hydrocarbon (kerogen) types

Thermal maturity/burial

Hydrologic drive (cleat/fracture system)

THERMOGENIC (CATAGENIC)

Thermal maturity/burial history

MIGRATED THERMOGENIC

Thermal maturity/burial history

Hydrology

MIXED THERMOGENIC AND BIOGENIC

Thermal maturity/burial history

Hydrology

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

Temperature

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:

Structure

Coal seam thickness and continuity

Igneous intrusions

Burial history and reservoir (P & T)

Coal composition and rank

Hydrogeology and biogenic gas

Hydrocarbon

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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COALBED METHANE PRODUCIBILITY (From Scott, 1993)

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

Compaction

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

Cleat spacing

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Maceral:

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

Transport

Mechanism

Concentration gradient (FicksLaw) and Pressure Gradient

(Darcys law)

Pressure Gradient (Darcys law)

Permeability

Origin Butt cleat, face cleat, fractures Fractures, connectedness

Production

Performance

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

Mechanical

Properties

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

calculations

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.

However,

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

measurements

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Parameter Kualitas Batubara

Total Moisture

Proximate

Total Sulfur

Calorific Value

HGI

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

Sifat-sifatnya:

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

dianalisa.

Nilainya tergantung juga pada preparasi sample sebelum ADM dianalisa (Standar

preparasi)

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

batubara.

Sifatnya;

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.

Kegunaan:

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.

Sifat:

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

Kegunaan:

Volatile Matter digunakan sebagai parameter penentu dalam penentuan peringkat

batubara.

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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Sulfur

Organic sulfur, sulfat sulfur, pyritic sulfur

Sifat:

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.

Kegunaan:

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

limit.

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

Sifat:

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:

Drilling

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