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AAPG Bulletin, V.
Possible controls of hydrogeological and stress regimes on the producibility of coalbed methane in Upper CretaceousTertiary strata of the Alberta basin, Canada
1Alberta Geological Survey, 4999-98 Avenue, Edmonton, Alberta, Canada T6B 2X3; email: [email protected]
2Alberta Geological Survey, 4999-98 Avenue, Edmonton, Alberta, Canada T6B 2X3; email: [email protected]
Stefan holds engineering and M.Sc. degrees and a Ph.D in water resources, hydrogeology, and transport processes. Currently, he is senior geoscience advisor in the Alberta Geological Survey, Alberta Energy and Utilities Board. Over the years, Stefan has been involved in various research activities: numerical modeling of deep waste injection, reservoir analysis and characterization, geothermics and hydrogeology of sedimentary basins, and recently, sequestration of carbon dioxide in geological media. Stefan has published more than 90 papers in scientific journals and conference proceedings and has made more than 100 presentations at professional conferences and symposia.
Karsten Michael received his degree of Diplom Geologe (an M.Sc. degree equivalent, 1997) from the Technische Universitaet Berlin (Germany) and his Ph.D. (2000) from the University of Alberta (Canada). He joined the Alberta Geological Survey of the Alberta Energy of Utilities Board as a hydrogeologist in 2000. He currently works on hydrogeological issues related to coalbed methane, CO2 geological sequestration, and oil sands development.
The authors would like to gratefully acknowledge the help with coal quality information from Andrew Beaton and their colleague at Alberta Geological Survey and financial support from the Alberta Science and Research Authority.
The coalbed methane potential and producibility of any coal-bearing strata are strongly affected by the hydrogeological regime of formation waters and by coal permeability, which in turn depends on the effective stress regime of the coals. Peat that accumulated in the Alberta basin during the Late Cretaceous and early Tertiary led to the formation of coal deposits that may contain significant coalbed methane resources. The flow of formation waters plays an important role in the maintenance and producibility of this resource. The present-day flow is driven by gravity (topography) and erosional rebound and is controlled by rock permeability and the presence of gas-saturated sandstones. The estimated gas in place in the Tertiary–Upper Cretaceous coals decreases significantly with stratigraphic age, ranging between less than 2 bcf/mi2 in the lower coal zones and 12 bcf/mi2 in the uppermost coals. The gas content, especially of the deeper coals, is lower than would be expected for the corresponding coal rank and burial depth, most likely because the underpressuring has caused the release of gas from the coals and accumulation in adjacent sands. The shallow coals, although of low rank, may contain important amounts of late-stage biogenic methane. The salinity of formation water in shallow coal seams, where the flow is driven by topography, is low, generally less than 1500 mg/L, although in places, it reaches 3000–5000 mg/L. The salinity of formation water in the deeper, underpressured strata in the west-central part of the basin is significantly higher, reaching 18,000 mg/L. This affects treatment and/or disposal strategies with regard to the water produced concurrent to coalbed methane.
The producibility of this resource depends on coal permeability, which decreases west-southwestward with increasing burial depth, from the order of several darcys in the shallow zones to millidarcys in the deep zones. The minimum effective stress, which affects coal permeability by closing fractures, increases west-southwestward from zero at the erosional edge of these strata to approximately 20 MPa near the Rocky Mountain deformation front. Fractures, including those in coal seams, will generally be vertical and will propagate on a southwest-northeast axis along the direction of the maximum horizontal stress, in a direction generally perpendicular to the Rocky Mountain deformation front.
Considering the hydrogeological and stress regimes in conjunction with estimations of the gas content in coals, the region with probably good coalbed methane potential and producibility are the Ardley coal zone in the Scollard Formation and maybe, to a lesser extent, the coal zones of the stratigraphically deeper Edmonton and Belly River groups along their respective subcrop in central and southern Alberta. The deep Edmonton and Belly River strata in western and central Alberta have most likely a reduced coalbed methane potential as a result of lower gas content and of low permeability. These regional considerations need to be applied against local studies of coal thickness, rank, permeability, and gas content to identify the best targets for coalbed methane exploration and production.
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