Deep Coal Seams: Potential for Long-Term Natural Gas Supplies

Abstract

Considerable attention has been focused on the long-term supplies and potential of natural gas. Studies have included resources that range from conventional gas in stratigraphic traps to unconventional gas locked in hydrates. These studies do not include potentially productive gas stored in deep coal seams, which is considerable worldwide. For example, in the U.S., the bulk of the 84 Tcf of coalbed methane contained in the Piceance basin is in deep coals at depths >5,000 ft. Even larger amounts of gas exist in deep coal seams in Alberta, Canada. Other deep coal basins around the world, including those in Australia, China, Europe, Kazakhstan, and Russia, contain many hundreds of trillions of cubic feet. Currently, the conventional wisdom is that coals at >5,000 ft will be extremely low in permeability and thus unproductive. This viewpoint stems from the early laboratory-based tests of confining pressure vs. permeability and from the initial field tests in the deeper portions of the Piceance basin. The most damaging evidence came from laboratory-based permeability measurements made on coal seams in the Piceance, San Juan, and Warrior basins. These studies implied that coal-seam permeability dropped to <1 md at depths >3,000 ft, falling to <0.001 md at 7,000 ft. On the basis of this and other information, it is not surprising that coalbed methane developers have been reluctant to pursue coals at depths >3,000 ft. Yet, significant methane recovery, on the order >10 Tcf, is being obtained from deeper 3,500-ft coals in the San Juan basin. Here, actual gas- and water-flow rates show that the permeabilities are one to two orders of magnitude higher than those presented in the previously published log-log depth vs. permeability plots. It would appear that either the initial data and postulated theory are in error or that alternative geologic regions exist that have not been captured by the initial data. A review of the theory and assumptions used in the initial studies on depth vs. permeability indicate the conclusions may be unduly pessimistic for several reasons. As Kuuskraa and Wyman point out, the calculation of the effective stress may be higher when an assumed lithostatic (vertical) stress is used instead of the lower horizontal stress that more likely controls permeability of vertical fractures. Because stress (not depth) is the controlling variable, a coal seam could be buried much deeper and still retain the same permeability as a shallower coal with a high-stress gradient. Studies of two basins (Bowen and Warrior) have shown that alternative stress regions rather than a single high-stress region exist within each basin. The resulting data indicate three broad stress regions: low, moderate, and high. We used these stress regions to construct alternative-stress (depth) vs. permeability curves for these two basins. Fig. 1 shows the resulting curves for the Bowen basin. These results, combined with reservoir simulation studies, encourage the theory that basin areas of relaxed stress could provide favorable production at even greater depths. Completions between 5,000 and 10,000 ft have been made in the Piceance basin of Colorado, the Uinta basin of Utah, and the Deep basin of Alberta. Gas rates varied from <100 to >1 million ft3/D. Of particular interest was a completion attempt in a coal seam at 10,000 ft in the Elmworth area of Alberta. Gas was produced from this seam at stabilized rates, but mechanical difficulties prevented final cleanout and completion of this interval. The potential of significant production from deep coal seams remains, but further experience is needed to establish its economic feasibility in various basins. References Wyman, R.E.: "Natural Gas," 1992 Yearbook of Science and Technology, McGraw-Hill Publishing Co. Inc., New York City (1992) 281-84. McKee, C.R., Bumb, A.C., and Koenig, R.A.: "Stress Dependent Permeability and Porosity of Coal," Geology and Coalbed Methane Resources of the Northern San Juan Basin, Colorado and New Mexico, Rocky Mountain Assn. of Geologists, Denver (1988) 143-153. Kuuskraa, V.A. and Wyman, R.E.: "Deep Coal Seams: An Overlooked Source for Long-Term Natural Gas Supplies," paper SPE 26196 presented at the 1993 SPE Gas Technology Symposium, Calgary, Alta., June 28-30. P. 774