Abstract
Horizontal boreholes have been routinely applied to coal seams as a cost-effective way to maximize coal seam gas production. However, these wells can encounter severe instability issues during field development due to significant horizontal stress loss and change in deviatoric stresses acting on the borehole. In this work, a general dual-porosity dual-permeability model is established and assigned to a coupled gas flow and coal deformation numerical model to investigate permeability change and borehole break-out regarding different in-situ stress regimes around a horizontal borehole. Mohr-Coulomb failure criterion is used in this model.
The results show that drilling parallel to the maximum horizontal stress direction neither achieves the best stability of the borehole nor maximizes the permeability ratio. Drilling along the minimum horizontal stress direction would maximize the permeability ratio, but it has the worst stability. The optimal drilling direction window considering both permeability ratio and borehole stability is recommended to be between 45– 60°.
