In-situ stress and coal bed methane potential in Western Canada

Laboratory testing has shown that the permeability of elastic rocks is related to confining stress. The same relationships have been demonstrated for coal. Production rates of coal bed methane have been shown to be inversely proportional to stress magnitude in Australia and the United States. Preferred fluid flow axes aligned with S-Hmax, the larger horizontal principal stress, have been documented. Relevant previous studies are reviewed and discussed. The findings are then applied to central and southern Alberta and specifically to a horizon approximating the MacKay coals of the Upper Cretaceous Belly River Formation. Methods for determining the vertical stress, S-V, and the smaller horizontal stress, S-Hmin are described and a data base is assembled from published data concerning micro-fracture and mini-fracture instantaneous shut in pressures, fracture breakdown pressures and leak-off pressures. This data base is used to generate a series of maps which are interpreted using relationships established outside Western Canada to establish optimum locations for coal bed methane productivity at the selected stratigraphic horizon. The paper ends with a plea to acquire more local primary data relating in-situ stress to the permeability and productivity of subsurface coal seams.