Alaska Coalbed Methane

http://www.blm.gov/ak/ak940/fluids/coalbed.html

The advent of coal bed methane (CBM) production in Alaska has brought the potential for rural Alaskan communities to become self-sufficient through the production of a local, cleaner, low-cost energy source. The estimated CBM resource in Alaska is around 1,000 trillion cubic feet and there are more than 35 rural villages that are situated on, or are immediately adjacent to a coal source, but traditional drilling methods are too expensive and are not well suited for exploration in remote Alaska. This summer in Fort Yukon a consortium comprised of the U.S. Bureau of Land Management, U.S. Department of Energy Arctic Energy Office, Alaska Division of Geological & Geophysical Surveys, U.S. Geological Survey, and University of Alaska Fairbanks tested a light weight/low cost drilling technology. A slim hole was drilled using a helicopter-portable Christianson CS 1000 drill rig that can drill to approximately 3000 feet. The project is intended to demonstrate the use of this light weight drill rig for exploration and development of natural gas from coal, and to assess the economic feasibility of extracting natural gas from coal in remote Alaskan villages. Drilling began on August 21, 2004; coring of the first coal began on August 26 at a depth of approximately 1,280 feet. Coal cores were collected for analysis of gas content and composition; water from the coal seam was sampled for chemical analysis. A second coal seam was successfully cored on September 1 at a depth of approximately 1,905 feet with the final hole depth of 2,287 feet reached on September 3.Over the next few months core samples will be analyzed for methane content, formation transmissivity and water quality, and economic modeling will be conducted. Results will be released in the second quarter of 2005. If the information from this summers work is favorable, additional testing and analysis as it relates to slim hole production will be initiated in the Summer of 2005. (BLM Alaska Coalbed Methane: http://www.blm.gov/ak/ak940/fluids/coalbed.html)

Assessment of Coalbed Gas Resources in Cretaceous and Tertiary Rocks on the North Slope, Alaska, 2006

The North Slope of Alaska is a vast area of land north of the Brooks Range, extending from the Chukchi Sea eastward to the Canadian border (fig. 1). This Arctic region is known to contain extensive coal deposits; hypothetical coal resource estimates indicate that nearly 4 trillion short tons of coal are in Cretaceous and Tertiary rocks (Flores and others, 2004). Because of the large volume of coal, other studies have indicated that this region might also have potential for significant coalbed gas resources (for example, Smith, 1995; Tyler and others, 1998).

The present study represents the first detailed assessment of undiscovered coalbed gas resources beneath the North Slope by the USGS. The assessment is based on the total petroleum system (TPS) concept. Geologic elements within a TPS relate to hydrocarbon source rocks (maturity, hydrocarbon generation, migration), the characteristics of reservoir rocks, and trap and seal formation. In the case of coalbed gas, the coal beds serve as both source rock and reservoir. The Brookian Coalbed Gas Com­posite TPS (fig. 1) includes coal-bearing rocks in Cretaceous and Tertiary strata underlying the North Slope and adjacent Alaska State waters. Assessment units (AUs) within the TPS (from oldest to youngest) include the Nanushuk Formation Coalbed Gas AU, the Prince Creek and Tuluvak Formations Coalbed Gas AU, and the Sagavanirktok Formation Coalbed Gas AU.

Resource Summary

USGS assessments provide estimates of the volumes of un­discovered petroleum resources (oil, gas, and natural gas liquids) that are technically recoverable and have the potential to be added to known reserves. Because there is currently no production of coalbed gas on the North Slope, all assessment units within the Brookian Coalbed Gas Composite TPS are considered hypothetical. Geologic factors that influenced the resource estimates include (1) the distribution and cumulative thickness of coal beds in each AU, (2) depth of permafrost relative to coal-bearing strata, (3) thermal maturity of the coal, and (4) key structural elements (folds, faults) that might affect gas migration and entrapment. This assessment also utilized production data from wells in coalbed gas fields in the Wasatch Plateau (Utah) and the Powder River Basin (Wyoming) to gain insight into production potential that might be applied to North Slope coalbed gas wells. The Utah and Wyoming fields are consid­ered viable analogs for assessing North Slope resources based on similarities in such criteria as coal thickness, thermal maturity, and overall geologic setting. In addition, this assessment only consid­ered coals within 6,000 ft of the ground surface because of the potential decrease in coal permeability with increasing depth.

Coal beds in the Nanushuk Formation Coalbed Gas AU are distributed over the largest area; cumulative total coal thicknesses exceed 200 ft and individual coal bed thicknesses are as much as 30 ft. The Sagavanirktok Formation Coalbed Gas AU has cumulative coal thicknesses of more than 180 ft with individual bed thicknesses as much as 35 ft. Coal in the Prince Creek and Tuluvak Formations Coalbed Gas AU is more limited in areal extent. Cumulative coal thickness can exceed 70 ft, and individual beds are as thick as 15 ft. Of the mean total estimated undiscovered coalbed gas re­sources of 18 TCF in the Brookian Coalbed Gas Composite TPS, about 84 percent or 15 TCF is estimated to be in the Nanushuk Formation. About 12 percent (2.2 TCF) is in the Sagavanirktok Formation and about 4 percent (0.8 TCF) is in the Prince Creek and Tuluvak Formations. (USGS Fact Sheet 2006-3105,September 2006 http://pubs.usgs.gov/fs/2006/3105/pdf/FS06-3105_508.pdf)