COALBED METHANE PRODUCT WATER CHEMISTRY IN THREE WYOMING WATERSHEDS¹

Abstract

The Powder River Basin in Wyoming has become one of the most active areas of coalbed methane (CBM) development in the western United States. Extraction of methane from coalbeds requires pumping of aquifer water, which is called product water. Two to ten extraction wells are manifolded into one discharge point and product water is released into nearby unlined holding ponds. The objective of this study was to evaluate the chemistry, salinity, and sodicity of CBM product water at discharge points and associated holding ponds as a function of watershed. The product water samples from the discharge points and associated holding ponds were collected from the Cheyenne River (CHR), Belle Fourche River (BFR), and Little Powder River (LPR) watersheds during the summers of 1999 and 2000. These samples were analyzed for pH, electrical conductivity (EC), total dissolved solids (TDS), alkalinity, sodium (Na), calcium (Ca), magnesium (Mg), potassium (K), sulfate (SO42‐), and chloride (C1‐). From the chemical data, practical sodium adsorption ratio (SARp) and true sodium adsorption ratio (SARt) were calculated for the CBM discharge water and pond water. The pH, EC, TDS, alkalinity, Na, Ca, Mg, K, SARp, and SARt of CBM discharge water increased significantly moving north from the CHR watershed to the LPR watershed. CBM discharge water in associated holding ponds showed significant increases in EC, TDS, alkalinity, Na, K, SARp, and SARt moving north from the CHR to the LPR watershed. Within watersheds, the only significant change was an increase in pH from 7.21 to 8.26 between discharge points and holding ponds in the LPR watershed. However, the LPR and BFR exhibited larger changes in mean chemistry values in pH, salinity (EC, TDS), and sodicity (SAR) between CBM product water discharges and associated holding ponds than the CHR watershed. For instance, the mean EC and TDS of CBM product water in LPR increased from 1.93 to 2.09 dS/m, and froml,232 to 1,336 mg/L, respectively, between discharge and pond waters. The CHR exhibited no change in EC, TDS, Na, or SAR between discharge water and pond water. Also, while not statistically significant, mean alkalinity of CBM product water in BFR and LPR watersheds decreased from 9.81 to 8.01 meq/L and from 19.87 to 18.14 meq/L, respectively, between discharge and pond waters. The results of this study suggest that release of CBM product water onto the rangelands of BFR and LPR watersheds may precipitate calcium carbonate (CaCO3) in soils, which in turn may decrease infiltration and increase runoff and erosion. Thus, use of CBM product water for irrigation in LPR and BFR watersheds may require careful planning based on water pH, EC, alkalinity, Na, and SAR, as well as local soil physical and chemical properties.

Tracking Salt and Sodium Build Up Due to Irrigation With Coalbed Natural Gas Product Water: Soil Solution Lysimeter and Soil Saturated Paste Extract Studies

“…The data show that EC and SAR values for water samples were well above desired levels of 0.75 dS m -1 and SAR of 10, recommended for irrigation water (U.S. Salinity Laboratory Staff, 1954). These results are in agreement with previous literature on CBNG water chemistry (Rice et al, 2002;McBeth et al, 2003). Soil Solution Chemistry Textures of irrigated soils ranged from sandy loam to clay, with CEC ranged from 5.2 cmol (+) kg -1 in 90-120 cm horizon of site 4 to 27 cmol (+) kg -1 in the 0-5 cm horizon of site 2 .…”

Research and Development Concerning Coalbed Natural Gas

“…These trace elements in sediment, with time, could migrate into shallow aquifers. Several studies were conducted (Jackson and Reddy 2007a, McBeth et al 2003a, McBeth et al 2003b, Jackson and Reddy 2007b in the PRB to examine the biogeochemical processes of CBNG produced water in disposal ponds. However, these studies did not investigate the accumulation and leaching potential of trace elements in CBNG disposal pond sediments.…”

“…The basin is bounded by the Yellowstone River to the north, the Laramie Mountains to the south, the Black Hills to the east, and Big Horn Mountains to the west. Elevation of the PRB ranges from 1,640 to 1,800m above sea level and its topography is characterized by high plains with rolling hills that are capped with clinker (BLM 2003, Jackson and Reddy 2007a, McBeth et al 2003a, McBeth et al 2003b). The predominate soils in the PRB consists of clay loam, fine loam, and loam (USGS 1986).…”

Subtask 1.18 - A Decision Tool for Watershed-Based Effluent Trading

“…The number of CBM-producing wells in the structural basin is expected to increase from the present 16,000 to perhaps 70,000 within the next 20-30 years (Montana Board of Oil and Gas Conservation, 2006), implying that approximately 5 to 50 million barrels (MMbbl) of produced water will be generated per day (McBeth et al, 2003;Rice et al, 2000).…”

“…The user should be reminded that these values may need to be further refined as additional data are available in the future. For additional information on water quality data collected within the Powder River Basin, please refer to Rice et al, 2000;McBeth et al, 2003;and Patz et al, 2004.…”