COALBED NATURAL GAS DEVELOPMENT IN THE POWDER RIVER BASIN, WY AND MT: IMPACTS ON LAND AND WATER IN A SEMIARID ENVIRONMENT

Coalbed natural gas (CBNG) production provides approximately 8 percent of the nation's supply, and this resource is becoming an increasingly an important source for future domestic natural gas. CBNG production requires infrastructure such as roads, pipelines, well pads, construction staging areas, and water management facilities (outfalls, pits, reservoirs, and treatment facilities). Development in the Powder River Basin (PRB), Wyoming and Montana, provides examples of impacts of CBNG development on a semiarid environment.

As many as 70,000 wells may be drilled in areas covering much of the approximately 22,000 mi²of the PRB. Each group of wells (5-10 in individual pods) has gas collecting and metering sites, water outlets, reservoirs, treatment facilities, pipelines, and access roads. Gas collected from several well pods is directed to central compressor stations and, ultimately, to larger gas-gathering sites. Satellite imagery and aerial photos comparing pre- and post-CBNG development document the type and amount of associated land disturbance. Likewise, ground-based surveys in areas of CBNG development have documented increases in the number and amounts of non-native plant species introduced following site preparation, as well as increases in electrical conductivities of the disturbed sites.

Water co-produced with CBNG in the PRB (currently about 96 barrels per day per well) is used beneficially in stock ponds and (or) for irrigation, but some is disposed of by surface discharge under National Pollutant Discharge Elimination System permits to drainage systems or reservoirs. The water is generally of the sodium-bicarbonate type, containing low total dissolved solids (TDS; <5,000 mg/L). However, in the semiarid PRB, where soils can contain abundant swelling clays and soluble salts (Ca, Mg, SO₄), presents challenges for environmentally sound water management strategies. Research at a CBNG water impoundment reservoir using monitoring wells, airborne geophysical surveys, and solid phase characterization indicates that water-rock interactions can cause increases in TDS and alter local hydrogeologic conditions. In addition, minor components such as ammonia and trace elements (iron and barium) can impact surface water.