Sequestration in Coal Seams: An Overview

Grant S. Bromhal

Carbon Sequestration Leadership Forum

National Energy Technology LaboratoryPittsburgh, PA

May 11, 2007

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Coalbeds are a major source of natural gas.*

• 3000 to 9000 Tm3

estimated global resource

• 8 to 17 Tm3 estimated U.S. resource

• Estimates of between 155 and 185 Billion tonnes of CO2 storage capacity in North American coals

*C.M. White, D.H. Smith, K.L. Jones, et al,”Sequestration of Carbon Dioxide in Coal with Enhanced Coalbed Methane Recovery--A Review,” Energy & Fuels 19, 659-724 (2005).

Data from Energy Information Administration. U.S. Crude Oil, Natural Gas, and Natural Gas Liquids Reserves 2003 Annual Report, Table 12.

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Production from coal can be a complex process.

• Flow through coal is defined in large part by the cleat network.

• Methane sorbed on the coal is released by decreasing pressure.

• Water needs to be moved out of cleats before gas can flow.

• CO2 injection causes reverse process (adsorption instead of desorption.

SORPTION DIFFUSION ADVECTION

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Often, gas production is inhibited by initial water production.

• Relative permeability in the cleats affects flow of gas.

• Most of the water around the injection area should be removed before CO2 is injected.

Meek and Levine, 2006

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However, sometimes water has short or little effect.

Meek and Levine, 2006

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CO2 sequestration in coal will occur with ECBM.

• Five-spot pattern for thick coals (San Juan Basin)

• For thinner seams, horizontal wells may be used.

<915 m(<3000 ft)

Lateral #11

Well A

Well C

Well B

Lateral #6

Lateral #5

Lateral #7

Lateral #12 Lateral #10

Lateral #9

Lateral #8

AAPilot holes forLaterals #9 and #10

Pilot holes forLaterals #11 and #12

305 m(1000 ft)

<915 m(<3000 ft)

305 m(1000 ft)

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How much CO2 can be stored (and methane removed)?

• Sorption is typically considered to follow a Langmuir isotherm.

• The gas pressure in the coal seam determines how much is sorbed to the coal.

• CO2 is preferentially sorbed to coal, compared to methane.

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Original Coal

Coal shrinkage and swelling can have significant impact of cleat permeability.

• Based on the Palmer-Mansoori model

• Most important effect is on permeability

• Most important coal properties for shrinkage and swelling:− Porosity (φ)− Young’s modulus (E)− Poisson’s ratio (ν)

Coal matrix

Cleat space

Coal ShrinkageCoal Swelling

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Swelling may significantly reduce cleat permeability and therefore injectivity.

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Concluding Statements

• We have a couple of decades of experience in CBM production.

• CO2-ECBM is still in its infancy and much is being learned.

• A lot of capacity may be available.• Swelling could prove to be a big issue.

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We found optimum engineering designs for many different values of the coal properties.

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We found which physical parameters are most important for sequestration economics.

• Vertical wells.• DOE horizontal well project:

− Shorter injectors better for enhanced gas production, but not for sequestration.

− Natural gas price important for profitability, but for the cases studied, had little effect on optimal design.

− CO2 price/credit determines optimized design.

• Initiated discussions for possible addition to IECM.

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Young’s Modulus x (106) kPa

6 PoissonRatio

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$)

For the Technology Opportunity “modeling studies of swelling,” we predicted which petrophysical

parameters are economically important.*

• Young’s modulus generally more important than Poisson’s ratio.

• Preferred cleat porosity depends on Young’s modulus.

*Sequestration Technology Roadmap

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Our simulations and experiments are mutually supportive.

• CT imaging of coal cores: before, during, after injection

• Petrography

• Gas sorption and uptake rates, at lithostatic pressures

• Young’s modulus, etc.

• DEMONSTRATED THAT SORPTION ISOTHERMS ARE NEEDED AT LITHOSTATIC PRESSURES.

E x c e s s a d s o r p t i o n i s o t h e r m s f o r C a r b o n D i o x i d e o n a s r e c e i v e d B a i l e y M i n e C o a l

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P o w d e r T = 2 9 9 . 6 5 K

T = 3 0 3 K

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