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

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

The 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 Coal Methane in coal is: Adsorbed on the surfaces of the coal Stored as free gas in the cleats and open pores

Cleats in Coal Face Cleats Butt Cleats

Canadas estimated CBM Reserves; 530 to 620 Tcf

Highlights of CBM Locations in Canada Alberta ~450 Tcf B.C. ~80 Tcf Sask ~15 Tcf E. Coast ~22 Tcf

ALBERTAS CBM POTENTIAL Up to 500 TCF in Alberta

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

Composition of Coal Anthracite Bituminous Coal Sub-Bituminous Coal Brown Coal How do you characterize 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 Vitrinite Increasing Reflectance

Coal Ranking & Quality 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 & Quality From Diessel (1992)

Macerals Macerals are the smallest organic materials in the coal Macerals are the smallest organic materials in the coal They are analogous to the minerals in rock – for example a rock quartz, feldspar, clay minerals, calcite and dolomite They are analogous to the minerals in rock – for example a rock quartz, feldspar, clay minerals, calcite and dolomite Macerals separated into 3 main groups: vitrinite, inertinite and liptinite Macerals separated into 3 main groups: vitrinite, inertinite and liptinite

Vitrinites ! Wood, bark and roots ! Contain less hydrogen than the liptinites

Liptinites ! Hydrogen-rich hydrocarbons derived from spores, pollen, cuticles and resins in the original plant material Inertinites Oxidation (burnt?) products of other macerals and are thus higher in carbon content

Maceral Analysis Vitrinite Pseudovitrinite Exinite Resinite Semi-Fusinite Semi-Macrinite Fusinite Macrinite Micrinite Vitrinite Three main groups: Exinite (liptinite) Inertinite

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

Physical 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 Adsorption The degree of physical adsorption decreases with increasing Temperature. Not limited to a monolayer but a series of layers may pile up.

Chemisorption Chemisorption usually involves sharing or transfer of an electron.

Chemisorption 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 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 Pressure Adsorption (scf/ton) The Adsorption Capacity defines the Reservoir Capacity An adsorption Isotherm curve defines the holding capacity of gas as a function of pressure.

Adsorption Isotherm Curve Pressure Adsorption (scf/ton) Adsorption Capacity and Coal Ranking Anthracite Bituminous Sub-Bituminous

Increasing: Vitrinite Reflectance (Carbon Content) (Energy Content) (Rank) Adsorption Capacity and Coal Ranking

Langmuir Theory The rate of molecules arriving and adsorbing on the solid surface The rate of molecules leaving from the solid surface =

Langmuir Theory Number of Sites Occupied θ = Number of Sites Available Rate of Adsorption = dθ = K A P(1 – θ)(1) dt Rate of Desorption = dθ = -K D θ(2) dt where K A and K D are the constants of adsorption and desorption respectively.

Langmuir Theory All the surface has the same activity for adsorption. All the surface has the same activity for adsorption. No interaction between adsorbed molecules. No interaction between adsorbed molecules. The same mechanism of adsorption for all molecules. The same mechanism of adsorption for all molecules. Extent of adsorption is less than one complete monolayer. Extent of adsorption is less than one complete monolayer. Irving Langmuir

Langmuir Terminologies Linear relationship between P/V vs. P

Irving Langmuir Langmuir Terminologies Langmuir Volume (Saturated Monolayer Volume)

Irving Langmuir Langmuir Terminologies Langmuir Pressure (Pressure at ½ of Langmuir Volume) ½ of Langmuir Vol.

Desorption of Methane

Methane Desorption Curve Adsorption Isotherm Curve The desorption of the methane gas generally follow down the adsorption isotherm curve. Pressure Adsorption (scf/ton)

Comparison of CBM and Typical Dry Gas Reservoir Reservoir Pressure Depleted by 50% 17% of Gas Produced CBM Reservoir

Comparison of CBM and Typical Dry Gas Reservoir Reservoir Pressure Depleted by 50% 44% of Gas Produced Conventional Gas Reservoir

Comparison of CBM and Typical Dry Gas Reservoir Conventional Gas Reservoir Depletes by 56% To get 50% of Gas Out CBM Reservoir Depletes by 78%

Another challenge is the decline in K ABS as pore pressure decreases... As pore pressure decreases, the net overburden stress increases. Net Overburden Stress Effective Permeability

Cleat width P PORE P OVBN The state of insitu virgin conditions

Cleat width P PORE P OVBN * As pore pressure decreases, the net overburden pressure increases.

Cleat width P PORE P OVBN 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 K ABS at reduced pore pressure can be very significant ! Well deliverability at 750 psi may only be 30% of that at 2300 psi Medium Volatile Bituminous Coal Pore Pressure (psi) Methane Permeability (mD)

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. P PORE P OVBN

At low reservoir pressures, the coal shrinkage can offset the net overburden effects ! Medium Volatile Bituminous Coal Pore Pressure (psi) Methane Permeability (mD)

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

Production of CBM, What really happens?

The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production

There are 3 main flow regimes in a typical coal seam: TIME R1 – Saturated flow Only water – above desorption pressure. R2 – Un-saturated flow – subcritical gas R3 – Full 2-phase flow 3 Flow Regimes

Adsorbed Methane Coal Pressure is above desorption pressure – therefore only water flows. Regime 1: Saturated Flow Water

Starting reservoir 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 point where commercial gas production begins can vary Depending on how fast you can depressurize the reservoir. In some cases, it may take up to 2 years!

Coal Regime 2: Un-Saturated Flow Water Bubbles of gas starts to evolve out but does not form continuous flow streams.

The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production

Coal Regime 3: Full 2-Phase Flow A continuous gas stream is achieved and gas flow increases – full 2- phase flow.

The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production

Typical CBM Well in Production Gas Water

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

Comparison of CBM and Typical Dry Gas Reservoir Conventional Gas Reservoir Depletes by 56% To get 50% of Gas Out CBM Reservoir Depletes by 78%

Comparison of CBM and Typical Dry Gas Reservoir Deplete Reservoir by 75% Conventional Gas Reservoir CBM Reservoir CBM Reservoir still has over 50% of Gas left behind !

At low reservoir pressures, the coal shrinkage can offset the net overburden effects ! Medium Volatile Bituminous Coal Pore Pressure (psi) Methane Permeability (mD)

For CBM reservoirs, we need to deplete the reservoir pressure down low to get the gas out This has implications in the spacing of the wells.

Pressure drawdown profile of a single well

Water Flow Only Discontinuous Gas Flow Continuous Gas Flow

With much closer well spacing, we can achieve the low pressure required for gas depletion Drawdown Curve for all 3 wells pumping Drawdown Curve for an individual well

Horizontal wells and hydraulic fractures are often used to increase drawdown Region of Continuous Gas Flow

Some Elaborate Horizontal well systems

Outline of Presentation 1.Origin and Locations of CBM 2.Basic Geology & Fundamentals 3.Production Phases of a CBM Reservoir 4.Common Production Techniques 5.Future Challenges

Many Areas of CBM Research Completions Completions Drilling fluids Drilling fluids Horizontal-well technology Horizontal-well technology Hydraulic Fracturing Hydraulic Fracturing Reservoir characterization Reservoir characterization Production forecasting (complex models) Production forecasting (complex models) Enhanced gas recovery Enhanced gas recovery

What about Enhanced Gas Recovery ?!?

Affinity of CO 2 Adsorption for Coal CO 2 CH 4

Affinity of CO 2 Adsorption for Coal Methane CO 2

Affinity of CO 2 was 3-4 times that of Methane ! Methane CO 2 Methane

What about the Affinity of H 2 S Adsorption for Coal H2SH2S CH 4

Affinity of H 2 S Adsorption for Coal Methane CO 2 H2SH2S

Affinity of H 2 S was more than 10 times of Methane ! Methane CO 2 H2SH2S H2SH2S Methane

Pressure maintenance can provide better flow characteristics. Net Overburden Stress Effective Permeability P PORE P OVBN

What if we use CO2 for pressure maintenance? Methane CO 2

Adsorbed CO2 Coal CO2 will preferentially displace the methane Displaced Methane

There is potential for using CO2 and/or H2S for pressure maintenance to enhance the rate of recovery of the methane and potentially increase the ultimate recovery of methane. Methane CO 2

Using CO 2 for pressure maintenance can also reduce CO 2 emissions (sequestration). CO2 Injection Methane Production

Thank you for your Attention...

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