U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
Romeo M. Flores, Gary D. Stricker, Joseph
F. Meyer, Thomas E. Doll,
Digital products by Scott Kinney, Heather Mitchell,
and Steve Dunn
Open-File Report 01-126
Production of coalbed methane usually involves withdrawing water from the coal bed to lower the reservoir pressure and allow the methane to desorb from the coal. A major impact related to dewatering the coal beds is groundwater drawdown resulting from lowering of the water table. A computer simulation of groundwater drawdown modeled by Bureau of Land Management (1999) is shown on Figure 18. This simulated drawdown attributed to CBM development for the Wyodak-Anderson coal zone during the 15-year period to 2015, shows maximum drawdown of as much as 550 ft in the Gillette South assessment area and about 300 to 375 ft in the southern and northern parts (north of Gillette) of the EIS area, respectively.
Another major concern of CBM development is the effect of surface disposal of co-produced water and the effect of this additional water on watersheds within the EIS project area. Figure 19 is a map of the major watershed areas and related drainages of the Belle Fourche, Cheyenne, Powder, and Tongue River basins as well as Caballo Creek. Co-produced water from CBM development is presently discharged either directly into exiting surface waters or to drainages. The Wyoming Pollutant Discharge Elimination System (WPDES) permits surface discharge because the water is generally potable (freshwater), mainly low total dissolved solids (TDS), and of good quality. Total dissolved solid, pH, chloride, and sulfate concentrations in the produced water are near or lower than the levels recommended for drinking water standards (Rice and others, 2000; Table 1). It is expected that surface disposal of co-produced water may result in erosion or drowning of drainage draws and associated vegetation within the project (BLM/EIS) area (see Fig. 1 and Fig. 2). Several companies have been experimenting with reinjecting the produced water into sandstones and coal beds in the Wasatch and Fort Union Formations. Pennaco Energy is presently reinjecting water into an aquifer used by the city of Gillette (Tollefson, 2000).
Groundwater withdrawal from aquifers is a particularly sensitive issue to landowners who “beneficially use” groundwater for their livestock and for irrigation. Generally, gas operators have cooperated with landowners by diverting co-produced water from CBM wells into stock tanks or other holding areas for their livestock.
Another important issue related to co-produced water is the effect of the water composition on soils. The sodium absorption ratio or SAR is a calculated parameter comparing the amount of sodium in the water versus the amounts of calcium and magnesium and is frequently used in agricultural applications to determine the compatibility of irrigation water and soils. The SAR is a measure of the tendency of the sodium to replace other cations in clays in the soil, potentially reducing soil permeability, with higher SAR values favoring replacement. Higher water SAR values may limit surface disposal of co-produced CBM waters in some areas depending on where the water is being discharged and the soil characteristics in that area. Initial results of co-produced water analyses indicate the SAR values increase from southeast to northwest in the basin (Fig. 20; Rice and others, 2000).
CBM development impacts the mining of the same coal beds
along the eastern margin of the Powder River Basin. There are 18
surface coal mines (Fig. 21) along the eastern part of the Campbell County
and the northernmost part of Converse County (Fort Union Coal Assessment
Team, 1999, Flores and others, 1999). Last year, these coal mines
produced about 300 million short tons from the Wyodak-Anderson coal zone,
which is being explored and developed for CBM by about 80 gas operators
basinwide. The produced coal from these mines made up about 30 percent
of the total U.S. coal production in 2000 and was shipped to more than
140 electric-power generating plants in the western, midwestern, southern,
and southeastern U.S. Minor amounts of this coal is shipped to foreign
countries (Canada, France, and Spain). The major impact of CBM development
on coal mining is groundwater withdrawal from the coal. Although
this does not affect the amount of coal that is produced, it reduces the
available water for mining operations. In addition, the conflicts
between coal mines and CBM operators arise in regard to lost gas due to
mining. That is, because water is withdrawn during surface mining,
reservoir pressures can be reduced resulting in the liberation of stored
gas in the coal, which may escape along mine highwalls. The joint
USGS-BLM CBM cooperative project is currently involved in the resolution
of this conflict by estimating the gas content of the coal beds within
the area of coal mine and CBM leases. Results of this study are published
and listed on the joint USGS-BLM website (www.wy.blm.gov/minerals/og/res.mgt.html)
maintained by the Wyoming BLM office in Cheyenne.
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