Environmental Issues Related to Coalbed Methane Production
Maria Mastalerz, Indiana Geological Survey, Indiana University, Bloomington, IN 47405
Many coal basins worldwide contain economic quantities of coalbed methane (CBM), a natural gas generated and trapped in coal. In the United States, CBM has been an important energy fuel for more than 20 years (Figure 1). Many U.S. basins contain vast resources of CBM (Figure 2), with proved reserves estimated at 18,000 Bcf (billions of cubic feet). CBM currently accounts for approximately 8% of total U.S. natural gas production and 7% of proved natural gas reserves (EIA, 2010).
CBM can be produced from vertical or horizontal wells. Horizontal wells are drilled along and within the methane-bearing coal layer (called a “seam”). They are especially successful if they trend across a strong set of natural fractures in coal seams.
CBM production poses environmental challenges, one of which is the disposal of the water co-produced with the gas. This water can be re-injected into geologic formations, used for irrigation, or stored in evaporation ponds. The water can cause damage to the environment if it is disposed of improperly; therefore, knowing the quality of the water is essential for planning water disposal and treatment options. CBM producers can apply for a “permit” to dispose of the water, if its quality meets the required standards. If the water does not meet the requirements (for example, if the salinity is too high), it must to be treated before it is disposed.
Another potential environmental issue related to CBM production is the application of fracture stimulation techniques, so-called “fracking”. Fracking involves pumping large volumes of fluids, usually with sand and some chemicals, into the targeted coal seam (EPA 2004). The purpose of fracking is to create or reactivate fractures that allow gas to flow more easily towards the production well. Fractures formed this way commonly extend beyond the coal seam and may serve as conduits between the coal seam and groundwater aquifers. In these cases groundwater contamination by methane, fracking fluids, or disposed CBM water may result. To prevent such contamination, detailed knowledge of coal seam properties (porosity, fluid conductivity, seam thickness, etc.) is required before the decision about the location for a CBM production well is made. Efforts to prevent water contamination are important parts of CBM operations because the remediation of contaminated groundwater is a complex and costly process.
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Data from U.S. DOE, EIA
Figure 1. Annual coalbed methane production in the United States, rising from 91 Bcf in 1989 to 1966 Bcf in 2009 (data from U.S. Department of Energy, Energy Information Administration, accessed September 19, 2011).
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U.S. DOE, EIA
Figure 2. Basins producing coalbed methane in the lower 48 states in the U.S. (from U.S. Department of Energy, Energy Information Administration, 2009).
Coal-bed methane: Potential and Concerns
Coalbed methane, CBM – World Coal Association