Geology

introduction to the geology of coalbed methane

Course Description

In the U.S, coalbed methane (CBM) accounts for approximately 10% of both natural gas production and reserves. Internationally, however, the CBM plays a much less important role in meeting energy demands, even in countries where coal and CBM are abundant. This course reviews (1) CBM origins, (2) controls on CBM occurrence and production, (3) methods of assessing CBM resources and producibility in various geologic settings, (4) controls on coalbed gas composition, (5) influences on permeability and production rates, (6) hydrologic issues and water production, and (7) methods of drilling, completing, and stimulating CBM wells.

Examples from several basins are used to clarify controls on CBM occurrence and producibility and to demonstrate the origins of highly productive CBM fairways or sweetspots. The properties of contrasting coalbed gas systems (i.e., thermogenic vs. biogenic system and water-productive and water-free systems) are described in terms of their origins, gas resources, and CBM play characteristics. Drilling, completion and stimulation of CBM wells are considerations in relation to geologic controls on best engineering practices. Understanding these factors may facilitate development of CBM resources from diverse physical settings.

Audience

This course is designed for reservoir and production engineers and geoscientists and for managers responsible for making exploration and development decisions and optimizing recovery from coalbed methane reservoirs.

Prerequisites

Basic knowledge of petroleum geology and engineering principles.

Course Schedule

1

1. INTRODUCTION

  • History and importance of coalbed gas
  • USA coalbed gas basins
  • What is coalbed gas and how do coalbed reservoirs differ from conventional reservoirs
    • Self-sourcing reservoirs
    • Storage mechanism
    • Decline curves
    • Water issues

2. COAL DEPOSITIONAL SYSTEMS

  • Coal depositional environments
    • Controls on reservoir extent and continuity
    • Controls on coal quality
    • Effects on well completion and stimulation

3. RESOURCE DELINEATION QUESTIONS

  • Regional structural setting
  • Depth of the coal reservoirs
  • Total coal thickness
  • Maximum (thickest) individual coal bed
  • Coal occurrence – one coal bed; a few, grouped beds, or many, thin beds distributed over an immense interval
  • Coal continuity

4. STRUCTURAL SETTING

  • Site-specific structural complexity
  • Can compartments accommodate enough wells to support infrastructure?
  • Fault blocks isolated from hydrologic recharge
  • Sealing vs. transmissive
  • Evidence for structural traps
  • Indications that coal is degassed below unconformities

5. MACERAL TYPES AND RESERVOIR FLUID COMPOSITION

  • Maceral (kerogen) types
  • Liquid-  vs. gas-prone kerogen
  • Effects of maceral type on gas sorption and diffusion
  • Have macerals generated hydrocarbons?
  • Coal moisture content?
2

6. COALBED GAS ORIGINS and COAL THERMAL MATURITY

  • Relationship between maturation level and present burial depth
  • Coal rank (thermal maturity) and gas generation
  • Thermogenic vs. biogenic gas
  • Mixed gas and migrated gas
  • Coalbed gas composition and heating value
    • Hydrocarbon composition
    • Gas composition maps
    • Carbon dioxide or other noncombustible gases that it may be necessary to remove

7. COALBED PERMEABILITY

  • Coal permeability
    • Typical permeability values for various coalbed systems
  • Face and butt cleat
    • Origins
    • Orientations
    • Regional and local variations
    • Spacing
    • Mineralization
    • Permeability anisotropy
  • In-situ stress
    • Overburden stress and permeability
    • Regional tectonic stress regimes
    • Magnitude of minimum horizontal stress relative to overburden stress (complex induced fracture geometries)
    • Orientation of maximum horizontal stress
      • Orientation of induced fractures
      • Orientation of induced fractures relative to face cleat
    • Effects of in-situ stress magnitude on fractures permeability
    • Matrix shrinkage and swelling
      • Causes of matrix shrinkage
      • Effect on coalbed permeability
      • CO2 injection and matrix swelling – permeability reduction
      • CO2 sequestration and enhanced coalbed gas production
        • Critical issues
        • Studies and Field tests (San Juan basin, W. Canada, Tx Gulf Coast)

8. HYDROLOGIC ANALYSIS

  • Produced water composition
  • Hydrologic maps as indicators of reservoir compartmentalization and permeability, and pressure regime
    • Potentiometric surface maps
    • Pressure gradient maps
    • Hydrochemical maps
    • Relations to gas content and composition - biogenic gas
  •  
    • Re-pressured and re-saturated coal - biogenic gas?

9. ESTIMATING COAL TONNAGE AND COALBED GAS VOLUMES

  • Gas content of coal
    • Methods of determinations
    • General GIP equation
    • Pitfalls
      • Measured gas content
      • Inferred gas content

10. COALBED GAS DRILLING, COMPLETIONS AND PRODUCTION METHODS

  • Surface drilling and completion methods
    • Vertical wells
      • Fracture stimulation
      • Open hole
      • Open hole cavity
      • Topset and under-ream
      • Vertical wells in advance of mining (GOB wells)
      • Slant and horizontal wells from surface
        • Horizontal
        • Pinnate
  • Subsurface drilling methods
    • In-seam boreholes
    • Cross-seam boreholes
  • Some typical coalbed gas well costs
3

11. COALBED GAS DRILLING, STIMULATION AND COMPLETION ADVISOR

  • Sunil Ramaswamy M.S. research
  • Software based on literature reviews and interviews with industry experts
  • Guides engineers to best engineering practices in 2007

12. WATER LIFT METHODS FOR COALBED GAS WELLS

  • Water production methods
    • Electric submersible pumps (ESP)
    • Sucker rod pumps (beam pumps)
    • Progressing cavity pumps
    • Gas lift

13. WATER DISPOSAL METHODS

  • Stream discharge; e.g., BW basin (TDS< 230 mg/L; iron <6 mg/L)
  • Land application
  • Deep well injection (may cost 15X > surface discharge)
  • Surface evaporation
  • Reuse in hydraulic stimulation
  • Off-site commercial disposal
  • Reverse osmosis
  • Artificial wetlands

14. COALBED GAS EXPLORATION MODELS

  • Thermogenic gas (e.g., Pottsville fm., Black Warrior Basin)
  • Biogenic gas (e.g., Ft. Union fm., Powder River Basin)
  • Mixed thermogenic and biogenic gas systems (e.g., Fruitland fm., northern San Juan Basin)
  • Dry coal systems (e.g., Horseshoe Canyon fm, W. Canada Sedimentary Basin)

15. EXERCISES

  • Hypothetical cases
  • Historical cases

Instructor

Dr. Walter Ayers

Instructors may vary based on location and schedule.

Classes

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