Accurately determining the gas-in-place volume is a crucial element of coalbed gas resource and reservoir evaluation. Different reservoir property analysis methods can yield very different gas resource and gas-in-place values. This paper presents results from evaluations of the accuracy of commonly used reservoir property analysis methods. Many commonly used analysis methods were found to have inherent shortcomings that can result in large (50% or greater) underestimation error in the gas-in-place value. Six major sources of error are: (1) basing the reservoir volume on the assumption that the coalbed reservoir thickness and lateral continuity are invariant, (2) using 1.75 g/cm3 as the maximum log density value for determining the gross reservoir thickness, (3) using 1.32 to 1.36 g/cm3 for the in-situ density of coalbed reservoirs, (4) basing the in-situ gas content on gas desorption data obtained at ambient temperature conditions, (5) basing the in-situ gas content on gas desorption data obtained from drill cuttings, and (6) basing the in-situ gas content on the assumption that the residual gas volume is negligible. Because these six error prone analysis practices have been so widely used in the past there may be significant potential for large gas-in-place and reserve estimate gains in many existing coalbed gas fields.

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