Coupled flow and geomechanical processes during gas production from coal seams

Permeability in coal seams is sensitive to stress and pore pressure changes. An additional influence on this behaviour is the nature of the coal matrix to shrink with gas desorption and swell with adsorption. Concise relationships for permeability have been developed which incorporate these mechanisms, some of which are derived from a geomechanical basis such as the Shi–Durucan model and the Palmer–Mansoori model. While these expressions are attractive approaches for defining coal permeability during gas migration, the geomechanical behaviour has been simplified by assuming uni-axial strain and constant vertical stress. In this paper a coupled numerical model is developed and used to investigate the applicability of these geomechanical assumptions for gas drainage from coal seams. The modelling approach involved coupling the existing coal seam gas reservoir simulator, SIMED II, with the geomechanical simulator, FLAC3D. While SIMED II was used to simulate gas migration in a hypothetical coal seam and a series of production scenarios, FLAC3D simulated the geomechanical response of the coal and the adjacent non-coal geological formations to fluid pressure and gas content changes imported from SIMED II. The simulations, which considered a range of property values relevant to the San Juan basin, found that while the assumption of uni-axial strain introduced negligible discrepancy, the assumption of constant vertical stress leads to significant differences between the Shi–Durucan permeability estimate and that calculated from the SIMED–FLAC3D simulation, especially at early times during production. These differences were a result of the pressure and sorption strain changes induced by production leading to arching of stresses in the vicinity of the production well. The mechanism was shown to produce significant differences in the calculated gas rate particularly at early times during production.