This paper presents results from evaluations of geochemical, petrographic and reservoir properties of Powder River Basin, Paleocene Fort Union Formation Canyon seam subbituminous coal at the Triton Federal #21C-2623 coalbed methane well in Campbell County, WY. The core analysis data reveal that at this location the Canyon seam coalbed is a multi-layered reservoir with each layer having different bulk compositional and fluid mobility properties. A very unique reservoir property of this subbituminous coal is its extremely fast gas desorption rate, which, in turn, facilitates a high early time gas production rate from the coalbed reservoir. The core analysis data indicate that the gas desorption rate is dependent on the maceral composition of the bulk coal organic matter.

It was found that several commonly used coal and coalbed reservoir property analysis methods required modification to prevent or minimize errors due to subbituminous coal's propensity to undergo aerial oxidation and desiccation. The occurrence of these processes during sample handling, storage or testing causes significant (15% or greater) underestimation errors in key reservoir properties such as the in-situ gas content, moisture holding capacity, sorbed gas composition, and percent gas saturation. Effective methods for avoiding or minimizing subbituminous coal property analysis errors due to oxidation and desiccation are described in this paper. Because these error prone analysis practices have been commonly used in the past, there may be significant potential for large coalbed methane resource and reserve estimate gains in this and other Rocky Mountain region basins.

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