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Initial Results for a Hydrogeochemical Investigation of Coalbed Natural Gas Produced Water Application via Subsurface Drip Irrigation, Powder River Basin, Wyoming

Engle, Mark 1; Bern, Carleton 2; Sams, James 3; Zupancic, John 4; Schroeder, Karl 3
1 Eastern Energy Resources Team, U.S. Geological Survey, Reston, VA.
2 Crustal Imaging & Characterization Team, U.S. Geological Survey, Denver, CO.
3 National Energy Technology Lab, U.S. Dept. of Energy, Pittsburgh, PA.
4 BeneTerra, Sheridan, WY.

Several methods (e.g., infiltration, direct discharge, irrigation) are used to utilize and/or dispose of produced waters from coalbed natural gas (CBNG) development. Some CBNG produced waters in the Powder River Basin exhibit high conductivity and sodium adsorption ratios raising environmental concerns. Subsurface drip irrigation (SDI) is an emerging technology in the Basin to beneficially use pre-treated CBNG waters by emitting them into the root zone of agricultural land to aid in irrigation. The method is designed to minimize environmental impacts by storing salts in the vadose zone. The purpose of this research project is to investigate the transport and fate of the water and salts from the injected CBNG produced waters at an alluvial SDI site, adjacent to the Powder River, in Johnson County, Wyoming.

Geochemical data used in this study include pre- and post-SDI installation inorganic analyses of groundwater, soil water, surface water, and soil samples from sites within and adjacent to the SDI system. Local groundwater and the treated CBNG waters at the site exhibit relatively high sodium adsorption ratios indicating surface irrigation, which some non-SDI CBNG produced water use/disposal methods rely on, could negatively impact local soils. Substantial ranges in conductivity (2732-9830 μS/cm) and dominant cation chemistry (Ca-SO4 to Na-SO4) have been identified in initial groundwater samples from the site. Major ion chemistry and geochemical modeling using PHREEQC indicate that local groundwater composition is controlled by equilibrium with gypsum and ion-exchange reactions. Initial changes to groundwater, soil, and soil-water geochemistry resulting from initial SDI operation will also be presented.

Water flow and mass data collected at the SDI site include water level measurement and pressure transducer data (groundwater), soil moisture sensors (soil water), and USGS streamflow data (surface). Groundwater level and pressure transducer results exhibit transient responses to changes in infiltration, surface water-groundwater interaction, and evapotranspiration. Water balance responses to initiation of the SDI system will be presented.

Findings from this project will provide a critical understanding of the dynamics of water and salts in SDI systems during the application of CBNG produced water. The information obtained can be used to improve SDI and other CBNG produced water use/disposal technologies in order to minimize future potential impacts.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009