Aquifer Controls on Coalbed Methane Development in the Powder River Basin, Wyoming

Abstract

Abstract Development of coalbed methane (CBM) in the Wyodak-Anderson seams of the Powder River Basin, Wyoming, is one of the most active plays in the world. Compared to other CBM producing basins, the Powder River Basin subbituminous coal has relatively low gas content (20–50 scf/ton), but high permeability (100s of millidarcy to 1+ darcy). Because of the shallow depth, Powder River Basin CBM development is normally on 40- or 80-acre well spacing. Another significant difference between the Powder River Basin and other producing CBM basins is the fact that the coal seams of the Powder River Basin are overlain and underlain by large, thick aquifers that are essentially infinite-acting in nature. The degree of hydraulic connection between these aquifers and the coal is a critical factor in the economics of gas production from this play. Where the aquifers are well connected to the coal, water influx or leakance is high, leading to a nearly steady-state pressure environment that will deter efforts to dewater the coal and produce methane. Using reservoir simulation analyses and actual field data on gas and water production, this paper examines the effect of aquifer leakance on the producibility of the coal. Optimum well spacing in the Powder River Basin is shown to depend on having sufficient wells per section to lift the water that leaks into the coal from the aquifers. In fact, in some parts of the basin, aquifer leakance will maintain reservoir pressure high enough that it will curtail methane desorption and restrict ultimate gas recovery. Further testing and analysis to better characterize the aquifers and their connection to the coal is necessary.