Gary D. Stricker1, Romeo M. Flores1, Ronald W. Stanton2, Cyndi A. Rice1, Margaret S. Ellis1, Allan M. Ochs3, Dwain E. McGarry4, Fredrick J. Crockett4, Dean P. Stilwell4
(1) U.S. Geological Survey, MS 939, Denver, CO
(2) U.S. Geological Survey, MS 956, Reston, VA
(3) Independent, Denver, CO
(4) U.S. Bureau of Land Management, Wyoming Reservoir Management Group, Casper, WY
ABSTRACT: Challenges Confronting Coalbed Methane (CBM) Development in the Powder River Basin, Wyoming and Montana
Coalbed methane developers in the Powder River Basin in Wyoming and Montana are frequently challenged with assessing multiple coal-bed reservoirs in the Paleocene Fort Union Formation. These heterogeneous and laterally discontinuous coal beds present characteristics and problems that are different from conventional reservoirs. This paper presents preliminary results of a two-year investigation in the basin by the USGS and BLM in cooperation with gas operators on variations of the coal stratigraphy, gas content, storage capacity, gas composition, coal petrology, and co-produced water.
Fort Union coal beds are often incorrectly correlated and identified leading to confusion in the nomenclature of reservoir beds. Thus, the vertical and lateral variability of the coal reservoirs has often led to misinterpretations of the connectivity among these reservoirs, which in turn affect gas drainage flow. Gas content as measured by direct-method desorption tests and gas storage capacity indicated by isotherms generally increase with depth; however, these parameters also vary between coal beds and with overburden thickness. For example, coal-bed gas content and storage capacity increase away from coal mine highwalls depending on the groundwater table levels and overburden. Gas composition, which mainly indicates a biogenic origin, also varies with depth and among coal beds. Facies of the coal beds suggest that gas content is controlled by maceral composition and bedding structure. Finally, the chemical composition of co-produced water varies among coal reservoirs.
In order to meet production and recoverability targets, which project as many as 70,000 wells for completion and 25 tcf recoverable gas (Goolsby, 2000), a rigorous characterization of coal reservoirs must be performed.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado