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

Mar 31, 2015

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An Introduction to Coalbed Methane Special Session 31: Presented by: Tony Ma, Hycal Energy Research Laboratories BACK to BASIC Series, Slide 2 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 Slide 3 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 Slide 4 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. Slide 5 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 Slide 6 Cleats in Coal Face Cleats Butt Cleats Slide 7 Canadas estimated CBM Reserves; 530 to 620 Tcf Slide 8 Highlights of CBM Locations in Canada Alberta ~450 Tcf B.C. ~80 Tcf Sask ~15 Tcf E. Coast ~22 Tcf Slide 9 ALBERTAS CBM POTENTIAL Up to 500 TCF in Alberta Slide 10 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 Slide 11 Composition of Coal Anthracite Bituminous Coal Sub-Bituminous Coal Brown Coal How do you characterize coals? Slide 12 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 Slide 13 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 Slide 14 Coal Ranking & Quality From Diessel (1992) Slide 15 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 Slide 16 Vitrinites ! Wood, bark and roots ! Contain less hydrogen than the liptinites Slide 17 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 Slide 18 Maceral Analysis Vitrinite Pseudovitrinite Exinite Resinite Semi-Fusinite Semi-Macrinite Fusinite Macrinite Micrinite Vitrinite Three main groups: Exinite (liptinite) Inertinite Slide 19 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 Slide 20 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 Slide 21 Physical Adsorption Involves intermolecular forces (van der Waals forces) between the gas molecules and the coal (solid) molecules. Slide 22 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. Slide 23 Physical Adsorption The degree of physical adsorption decreases with increasing Temperature. Not limited to a monolayer but a series of layers may pile up. Slide 24 Chemisorption Chemisorption usually involves sharing or transfer of an electron. Slide 25 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. Slide 26 Chemisorption Chemisorption is limited to the formation of a monolayer of molecules, but physical adsorption may take place on top of a chemisorbed monolayer. Slide 27 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. Slide 28 Adsorption Isotherm Curve Pressure Adsorption (scf/ton) Adsorption Capacity and Coal Ranking Anthracite Bituminous Sub-Bituminous Slide 29 Increasing: Vitrinite Reflectance (Carbon Content) (Energy Content) (Rank) Adsorption Capacity and Coal Ranking Slide 30 Langmuir Theory The rate of molecules arriving and adsorbing on the solid surface The rate of molecules leaving from the solid surface = Slide 31 Langmuir Theory Number of Sites Occupied = Number of Sites Available Rate of Adsorption = d = K A P(1 )(1) dt Rate of Desorption = d = -K D (2) dt where K A and K D are the constants of adsorption and desorption respectively. Slide 32 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 Slide 33 Langmuir Terminologies Linear relationship between P/V vs. P Slide 34 Irving Langmuir Langmuir Terminologies Langmuir Volume (Saturated Monolayer Volume) Slide 35 Irving Langmuir Langmuir Terminologies Langmuir Pressure (Pressure at of Langmuir Volume) of Langmuir Vol. Slide 36 Desorption of Methane Slide 37 Methane Desorption Curve Adsorption Isotherm Curve The desorption of the methane gas generally follow down the adsorption isotherm curve. Pressure Adsorption (scf/ton) Slide 38 Comparison of CBM and Typical Dry Gas Reservoir Reservoir Pressure Depleted by 50% 17% of Gas Produced CBM Reservoir Slide 39 Comparison of CBM and Typical Dry Gas Reservoir Reservoir Pressure Depleted by 50% 44% of Gas Produced Conventional Gas Reservoir Slide 40 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% Slide 41 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 Slide 42 Cleat width P PORE P OVBN The state of insitu stresses @ virgin conditions Slide 43 Cleat width P PORE P OVBN * As pore pressure decreases, the net overburden pressure increases. Slide 44 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% Slide 45 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) Slide 46 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 Slide 47 At low reservoir pressures, the coal shrinkage can offset the net overburden effects ! Medium Volatile Bituminous Coal Pore Pressure (psi) Methane Permeability (mD) Slide 48 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 Slide 49 Production of CBM, What really happens? Slide 50 The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production Slide 51 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 Slide 52 Adsorbed Methane Coal Pressure is above desorption pressure therefore only water flows. Regime 1: Saturated Flow Water Slide 53 Starting reservoir condition @ 2200 psia Slide 54 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. Slide 55 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! Slide 56 Coal Regime 2: Un-Saturated Flow Water Bubbles of gas starts to evolve out but does not form continuous flow streams. Slide 57 The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production Slide 58 Coal Regime 3: Full 2-Phase Flow A continuous gas stream is achieved and gas flow increases full 2- phase flow. Slide 59 The Three stages of CBM Production Time MCFD or BPD Water Gas Stage 1, De-watering Stage 2, Mid Life Stage 3, Decline production Slide 60 Typical CBM Well in Production Gas Water Slide 61 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 Slide 62 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% Slide 63 Comparison of CBM and Typical Dry Gas Reservoir Deplete Reservoir by 75% Conventional Gas Reservoir CBM Reservoir CBM Reservoir still has