Volumetric Strain Associated with Methane Desorption and Its Impact on Coalbed Gas Production from Deep Coal Seams

R. Marc Bustin and Xiaojun Cui
University of British Columbia, Vancouver, BC

The permeability of deep (>1000 m) coal seams is commonly low. For deep coal seams, significant reservoir pressure drawdown is required to promote gas desorption because of the shape of the Langmuir-type isotherm that typifies coals. Hence further permeability decline may occur due to pressure drawdown and resulting increase in effective stress, depending on coal properties and stress field during production. However, the permeability decline can be potentially offset by the enhanced permeability due to matrix shrinkage associated with methane desorption. The predictability of varying permeability is critical for coalbed gas exploration and production well management. We have investigated quantitatively the effects of reservoir pressure and sorption-induced volumetric strain on coal-seam permeability with constrains from the adsorption isotherms and associated volumetric strain measured on a Mesaverde Group coal (Piceance Basin) and a derived a stress-dependent permeability model. Our results suggest that the favorable coal properties that can result in less permeability reduction during earlier production and an earlier strong permeability rebound (increase in permeability due to coal shrinkage) with methane desorption include: 1) large bulk or Young's modulus; 2) large adsorption or Langmuir volume; 3) relatively large Langmuir pressure; 4) high initial permeability and dense cleat spacing; and 5) low initial reservoir pressure. Permeability variation with gas production is further dependent on orientation of the coal seam, the reservoir stress field, and the cleat structure. Injection of N₂ and displacement of CH₄ only results in short term enhanced permeability due to matrix shrinkage associated with nitrogen adsorption and methane desorption.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005