Possible controls of hydrogeological and stress regimes on the producibility Of coalbed methane in Upper Cretaceous - Tertiary strata of the Alberta basin, Canada

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/mi(2) in the lower coal zones and 12 bcf/mi(2) 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.