An Introduction to Coalbed Methane Special Session 31: Presented by: Tony Ma, Hycal Energy Research Laboratories BACK to BASIC Series,

An Introduction to An Introduction to Coalbed MethaneCoalbed Methane

Special Session 31:Special Session 31:

Presented by: Tony Ma,Presented by: Tony Ma,Hycal Energy Research Hycal Energy Research LaboratoriesLaboratories

BACK to BASIC Series,BACK to BASIC Series,

Outline of Outline of PresentationPresentation

1.1. Origin and Locations of CBMOrigin and Locations of CBM

2.2. Basic Geology & FundamentalsBasic Geology & Fundamentals

3.3. Production Phases of a CBM Production Phases of a CBM

ReservoirReservoir

4.4. Common Production TechniquesCommon Production Techniques

5.5. Future ChallengesFuture Challenges

ReservoirReservoir

The origin of CBMThe origin of CBM

1. Biogenesis from bio mass – Coal beds are formed

from direct burial of organic materials as opposed to

conventional hydrocarbons which are believed to have

migrated into place.

2. The process of coal formation is known as

coalification.

Methane Storage in CoalMethane Storage in Coal

Methane in coal is:

• Adsorbed on the surfaces of the coal

• Stored as free gas in the cleats and open pores

Cleats in CoalCleats in Coal

Face Face CleatsCleats

Butt Butt CleatsCleats

Canada’s estimated CBM Reserves; 530 to 620 Tcf

Highlights of CBM Locations in Highlights of CBM Locations in CanadaCanada

Alberta ~450 Tcf

B.C.~80 Tcf

Sask~15 Tcf

E. Coast~22 Tcf

ALBERTA’S CBM POTENTIAL

500 TCF in Alberta

ReservoirReservoir

Composition of CoalComposition of Coal

Anthracite

Bituminous Coal

Sub-Bituminous Coal

Brown Coal

How do you characterize How do you characterize coals?coals?

Vitrinite Reflectance is usually used as an indicator of the rank of the coal.

Low Quality Coal: Lignite (Brown) Coal

High Quality Coal: Anthracite Coal

Vitrinit

Increasin

Reflec

Coal Ranking & Coal Ranking & QualityQuality

Lower quality coal: • low gas capacity• high volatile matter• high moisture content

High quality coal:

• high gas capacity

• high Vitrinite Reflectance

• high carbon content

Coal Ranking & QualityCoal Ranking & Quality

Coal Rank % CarbonSpecific Energy

(MJ/kg)Vitrinite Reflectance

(Max %)

Anthracite 95 35.2 up to 7.0

Semi-Anthracite 92 36 2.83

Low VolatileBituminous Coal

91 36.4 1.97

Medium VolatileBituminous Coal

90 36 1.58

High VolatileBituminous Coal

86 35.6 1.03

Sub-BituminousCoal

80 33.5 0.63

Brown Coal 71 23 0.42

From Diessel (1992)

MaceralsMacerals

Macerals are the smallest Macerals are the smallest organic materials in the coalorganic materials in the coal

They are analogous to the They are analogous to the minerals in rock – for example minerals in rock – for example a rock quartz, feldspar, clay a rock quartz, feldspar, clay minerals, calcite and dolomiteminerals, calcite and dolomite

Macerals separated into 3 Macerals separated into 3 main groups: vitrinite, main groups: vitrinite, inertinite and liptiniteinertinite and liptinite

VitrinitesVitrinites

Wood, bark and rootsWood, bark and roots Contain less hydrogen than the Contain less hydrogen than the

liptinitesliptinites

LiptinitesLiptinites

Hydrogen-rich hydrocarbons derived from Hydrogen-rich hydrocarbons derived from spores, pollen, cuticles and resins in the spores, pollen, cuticles and resins in the original plant materialoriginal plant material

InertinitesInertinites• Oxidation (burnt?) products of other macerals and are thus higher in carbon content

Maceral AnalysisMaceral Analysis

Vitrinite

Pseudovitrinite

Exinite

Resinite

Semi-Fusinite

Semi-Macrinite

Fusinite

Macrinite

Micrinite

VitriniteThree main groups:

Exinite (liptinite)

Inertinite

Methane Storage in CoalMethane Storage in Coal

Methane in coal is:

• Adsorbed on the surfaces of the coal

• Stored as free gas in the cleats and open pores

Adsorption of MethaneAdsorption of Methane

Two types of adsorption are believed to occur between the gaseous methane phase and the coal (solid phase). These two types of adsorption are:

1. Physical Adsorption

2. Chemical or chemisorption

Physical AdsorptionPhysical Adsorption

• Involves intermolecular forces (van der Waals forces) between the gas molecules and the coal (solid) molecules.

Physical AdsorptionPhysical Adsorption• Physical adsorption is nearly instantaneous and

equilibrium is quickly established.

• Usually reversible due to low energy requirements – energy of activation is usually very low.

Physical AdsorptionPhysical Adsorption

• The degree of physical adsorption decreases with increasing Temperature.

• Not limited to a “monolayer” but a series of layers may “pile up”.

ChemisorptionChemisorption

• Chemisorption usually involves sharing or transfer of an electron.

• The heat released from chemisorption is much higher then physical adsorption. Therefore, the chemisorbed molecules generally requires an activation energy for it to release.

• Chemisorption is limited to the formation of a monolayer of molecules, but physical adsorption may take place on top of a chemisorbed monolayer.

Adsorption Isotherm Curve

0 500 1000 1500 2000 2500 3000

Pressure

The Adsorption Capacity The Adsorption Capacity defines the Reservoir Capacitydefines the Reservoir Capacity

An adsorption Isotherm curve

defines the holding capacity of

gas as a function of pressure.

Adsorption Isotherm Curve

0 500 1000 1500 2000 2500 3000

Pressure

Adsorption Capacity and Coal RankingAdsorption Capacity and Coal Ranking

Anthracite

Bituminous

Sub-Bitumino

Increasing:

• Vitrinite Reflectance

• (Carbon Content)

• (Energy Content)

• (Rank)

Adsorption Capacity and Coal RankingAdsorption Capacity and Coal Ranking

Langmuir Langmuir TheoryTheory

The rate of molecules arriving and adsorbing on the solid surface

The rate of molecules leaving from the solid surface

Number of Sites Occupiedθ = Number of Sites Available

Rate of Adsorption = dθ = KAP(1 – θ) (1) dtRate of Desorption = dθ = -KDθ (2) dtwhere KA and KD are the constants of adsorption and desorption respectively.

All the surface has the same All the surface has the same activity for adsorption.activity for adsorption.

No interaction between No interaction between adsorbed molecules.adsorbed molecules.

The same mechanism of The same mechanism of adsorption for all molecules.adsorption for all molecules.

Extent of adsorption is less Extent of adsorption is less than one complete monolayer.than one complete monolayer.

Irving Irving LangmuirLangmuir

Langmuir Langmuir TerminologiesTerminologies

Reservoir Pressure Psi

Linear relationship between P/V vs. P

Langmuir Volume (Saturated MonolayerVolume)

Langmuir Pressure (Pressure at ½ of

Langmuir Volume)

½ of Langmuir Vol.

Desorption of MethaneDesorption of Methane

Methane Desorption CurveMethane Desorption CurveAdsorption Isotherm Curve

0 500 1000 1500 2000 2500 3000

The desorption of the

methane gas generally

follow down the adsorption

isotherm curve.

Pressure

Comparison of CBM and Typical Dry Gas Reservoir

0 500 1000 1500 2000 2500

Reservoir Pressure (psi)

Reservoir Pressure Depleted by 50%

17% of Gas Produced

CBM Reserv

0 500 1000 1500 2000 2500

Reservoir Pressure Depleted by 50%

44% of Gas Produced

Conventional G

Reserv

0 500 1000 1500 2000 2500

Conventional Gas Reservoir Depletes by

To get 50% of Gas Out

CBM Reservoir Depletes by 78%

Another challenge is the decline Another challenge is the decline in Kin KABS ABS as pore pressure as pore pressure

decreases . . .decreases . . .

As pore pressure As pore pressure decreases, the decreases, the net overburden net overburden

stress increases.stress increases.

Net Overburden Net Overburden StressStress

Cleat width

The state of insitu stresses @ virgin conditionsThe state of insitu stresses @ virgin conditions

Cleat width

* As pore pressure decreases, * As pore pressure decreases, the the netnet overburden pressure overburden pressure increases.increases.

Cleat width

Permeability W 3

A reduction in fracture/ cleat width of 10% translates to permeability reduction of (0.90 x 0.90 x 0.90 = 0.729) 27.1%.W ↓ 20% = K ↓ 48.8%; W ↓ 40% = K ↓ 78.4%

The decline in KThe decline in KABS ABS at reduced pore at reduced pore pressure can be very significant !pressure can be very significant !

05001000150020002500

Well deliverability at 750 psi may only be

30% of that at 2300 psi

Medium Volatile Bituminous Coal

Pore Pressure (psi)

Cleat width

A mitigating factor is that as the pore pressure decreases, the desorbed gas will effectively shrink the volume of the coal. This tends to intensify the cleating in situ.

At low reservoir pressures, the coal At low reservoir pressures, the coal shrinkage can offset the net overburden shrinkage can offset the net overburden

effects !effects !

05001000150020002500

Pore Pressure (psi)

ReservoirReservoir

Production of CBM,Production of CBM,What really happens?What really happens?

The Three stages of CBM Production

Water Gas

Stage 1,

De-watering

Stage 2,

Mid Life

Stage 3,

Decline production

There are 3 main flow regimes There are 3 main flow regimes in a typical coal seam:in a typical coal seam:

R1 – Saturated flow Only water – above desorption pressure.

R2 – Un-saturated flow – subcritical gas R3 – Full 2-phase flow

3 Flow Regimes

Adsorbed Methane

CoalPressure is above desorption pressure – therefore only water flows.

Regime 1: Saturated FlowRegime 1: Saturated Flow

WaterWater

Regime 1: Saturated FlowRegime 1: Saturated Flow

Conventional Isotherm

0 500 1000 1500 2000 2500 3000 3500

Pressure (Psia)

Starting reservoir condition @ 2200 psia

Conventional Isotherm

0 500 1000 1500 2000 2500 3000 3500

Pressure (Psia)

Starting reservoir condition @ 2200 psia

At the initial reservoir pressure of 2200 psi, the coal could adsorb about 1020 scf/ton but only has ~680 scf/ton. To start to desorb gas, we therefore need to depressurize to 950 psi.

The time it takes to De-water a coal seam to the The time it takes to De-water a coal seam to the point where commercial gas production begins point where commercial gas production begins can vary . . .can vary . . .

. . . Depending on how fast you can depressurize . . . Depending on how fast you can depressurize the reservoir. In some cases, it may take up to the reservoir. In some cases, it may take up to 2 years!2 years!

Regime 2: Un-Saturated FlowRegime 2: Un-Saturated Flow

WaterWater

Bubbles of gas starts to evolve out but does not form continuous flow streams.

Water Gas

Stage 1,

De-watering

Stage 2,

Mid Life

Stage 3,

Decline production

Regime 3: Full 2-Phase FlowRegime 3: Full 2-Phase Flow

A continuous gas stream is achieved and gas flow increases – full 2-phase flow.

Water Gas

Stage 1,

De-watering

Stage 2,

Mid Life

Stage 3,

Decline production

Typical CBM Well in Production

GasWate

ReservoirReservoir

0 500 1000 1500 2000 2500

Conventional Gas Reservoir Depletes by

To get 50% of Gas Out

CBM Reservoir Depletes by 78%

0 500 1000 1500 2000 2500

Deplete Reservoir by 75%

Conventional G

ReservoirCBM

Reservoir

CBM Reservoir still has over 50%

of Gas left behind !

At low reservoir pressures, the coal At low reservoir pressures, the coal shrinkage can offset the net overburden shrinkage can offset the net overburden

effects !effects !

05001000150020002500

Pore Pressure (psi)

For CBM reservoirs, we need For CBM reservoirs, we need to deplete the reservoir to deplete the reservoir pressure down low to get the pressure down low to get the gas out . . .gas out . . .

. . . This has implications . . . This has implications in the spacing of the in the spacing of the wells.wells.

Pressure drawdown profile of a single Pressure drawdown profile of a single well well

Water Flow

OnlyDiscontinuou

sGas Flow

Continuous

Gas Flow

With much closer well spacing, we can With much closer well spacing, we can achieve the low pressure required for gas achieve the low pressure required for gas

depletiondepletion

Drawdown Curve forall 3 wells pumping

Drawdown Curve for an individual

Horizontal wells and hydraulic fractures Horizontal wells and hydraulic fractures are often used to increase drawdown are often used to increase drawdown

Region of ContinuousGas Flow

Some Elaborate Horizontal well systems Some Elaborate Horizontal well systems

ReservoirReservoir

Many Areas of CBM ResearchMany Areas of CBM Research

►CompletionsCompletions►Drilling fluidsDrilling fluids►Horizontal-well technologyHorizontal-well technology►Hydraulic FracturingHydraulic Fracturing►Reservoir characterizationReservoir characterization►Production forecasting (complex Production forecasting (complex

models)models)►Enhanced gas recoveryEnhanced gas recovery

0 500 1000 1500 2000 2500

What about Enhanced Gas What about Enhanced Gas Recovery ?!?Recovery ?!?

Affinity of COAffinity of CO22 Adsorption for Adsorption for CoalCoal

Affinity of COAffinity of CO22 Adsorption for Coal Adsorption for Coal

Methane

Affinity of COAffinity of CO22 was 3-4 times that of Methane ! was 3-4 times that of Methane !

Methane

CO2 CO2

Methane

What about the Affinity What about the Affinity of Hof H22S Adsorption for S Adsorption for

CoalCoal

Affinity of HAffinity of H22S Adsorption for CoalS Adsorption for Coal

Methane

Affinity of HAffinity of H22S was more than 10 times of S was more than 10 times of Methane !Methane !

Methane

Pressure maintenance can provide Pressure maintenance can provide better flow characteristics.better flow characteristics.

Net Overburden StressNet Overburden Stress

What if we use CO2 for pressure What if we use CO2 for pressure maintenance?maintenance?

Methane

Adsorbed CO2

CO2 will preferentially displace the CO2 will preferentially displace the methanemethane

Displaced Methane

There is potential for using CO2 and/or H2S for There is potential for using CO2 and/or H2S for pressure maintenance to enhance the rate of pressure maintenance to enhance the rate of

recovery of the methane and potentially increase the recovery of the methane and potentially increase the ultimate recovery of methane. ultimate recovery of methane.

Methane

Using COUsing CO22 for pressure maintenance can also reduce for pressure maintenance can also reduce COCO22 emissions (sequestration). emissions (sequestration).

CO2 CO2 InjectionInjection

Methane Methane ProductionProduction

Thank you for your Thank you for your Attention . . .Attention . . .

If you would like a copy of this If you would like a copy of this presentation, please visit presentation, please visit www.hycal.comwww.hycal.com

or email: general@hycal.comor email: general@hycal.com

BACK to BASIC Series: BACK to BASIC Series: An Introduction to Coalbed An Introduction to Coalbed

MethaneMethane

An Introduction to Coalbed Methane Special Session 31: Presented by: Tony Ma, Hycal Energy Research Laboratories BACK to BASIC Series,

coal ranking slide

chemisorption slide

tcf slide

reflectance slide

coal methane

liptinite slide

coal macerals

alberta slide

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