Coalbed natural gas is currently experiencing a second wave of large-scale commercial development in the U.S. Rocky Mountain region. A decade ago, the Fruitland Formation coal in the San Juan Basin was the only significant commercial coalbed natural gas play in this region. One key factor that initially discouraged exploitation of coalbed natural gas resources elsewhere in this region was the general gas industry perception that the large coal deposits in other Rocky Mountain basins simply did not possess the reservoir property characteristics needed for a commercially viable coalbed natural gas play. Today, new coalbed natural gas plays in the Rocky Mountain region's Raton, Uinta and Powder River basins are undergoing large-scale commercial development. The gas industry's success in unlocking these three new coalbed natural gas play areas required exploration persistence as well as the development and use of innovative, low-cost drilling, completion and production solutions tailored to the unique site-specific reservoir properties of each play area.
The geologic setting and reservoir properties of the commercial coalbed gas play in the Fort Union Formation of the Powder River Basin are particularly noteworthy since they completely defy conventional gas industry wisdom, based on experiences in the Fruitland Formation coal of San Juan Basin, regarding the type of geologic setting and reservoir properties required for a commercial coalbed natural gas play. In the Powder River Basin, the commercially productive reservoirs are shallow beds of low gas content, subbituminous rank coal. In 2000, natural gas production from these low gas content, subbituminous coalbed reservoirs totaled 147 Bcf from 4,200 wells.
The enormous commercial potential of the low gas content, subbituminous coalbed reservoirs in the Powder River Basin Fort Union Formation was simply not recognized by the early reservoir property evaluations conducted by industry in the late 1980s and early 1990s. An important implication of this is that the production potential of subbituminous coal deposits in other Rocky Mountain region basins also may have been significantly underestimated during prior industry evaluation processes. An example is the subbituminous coal in the Laramie Formation of the Denver Basin. Gas content data measured during the late 1970s and early 1980s indicated that the Laramie Formation subbituminous coal contains only trace amounts of natural gas. These low gas content values helped discouraged industry interest in the Denver Basin as a coalbed natural gas play prospect. By contrast, during the early 1900s numerous Laramie Formation underground coal mines in the Denver Basin reported gas explosions, a clear indication that the Laramie Formation subbituminous coal may contain a significant amount of natural gas.
One of the important lessons that the gas industry learned from its experiences in the Powder River Basin is that reliably assessing the natural gas resource, production and producible reserve potential of low gas content, subbituminous coalbed reservoirs requires the use of reservoir property analysis methods custom tailored to the unique properties of subbituminous coal. Custom tailored analysis methods are essential because the bulk organic matter comprising subbituminous coal is both chemically reactive and physically unstable. It readily undergoes very rapid aerial oxidation and desiccation, which can result in significant underestimation errors in such key reservoir properties as the in-situ gas content, moisture holding capacity, sorbed phase gas composition, and percent gas saturation. Results from case studies illustrating effective methods for avoiding or minimizing subbituminous coalbed reservoir property analysis errors due to aerial oxidation and desiccation are described in this paper.