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|| Coalbed methane (CBM) is a source of natural gas that is recovered by dewatering coal beds and collecting volatile compounds, such as methane, that are desorbed from the coal. Coalbed methane currently accounts for approximately 7% of U.S. natural-gas production (U.S. Energy Information...
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Significant quantities of water are produced in the extraction of hydrocarbon energy resources (currently estimated at ~14 billion bbl/yr across the United States [1 bbl=42 gallons]). An additional volume of hydraulic fracturing (“fracing”) fluid, used to increase permeability, porosity, and hydrocarbon yield of reservoir rocks, is recovered at the end of the process (flowback fluids). Water is also generated from scrubbers in power plants, dewatering and extracting uranium resources, carbon sequestration, and development of unconventional energy sources. Although derived from a variety of different sources, these all represent sources of produced waters in that they are extracted in the process of trying to develop, extract, or dispose of energy-related products.
Photo: Sampling produced water from an
oil well in northern Louisiana.
Produced waters typically exhibit significant variations in salinity, sodicity, trace element composition, and organic geochemistry resulting from differences in environmental and geologic conditions. Some of these waters contain relatively high salinity values, sometimes greater than seawater, while others are potable. However, continued concerns over diminishing water resources and expanding needs for next generation energy sources have lead to the characterization of produced waters as possible resources.
Researchers in the U.S. Geological Survey (USGS) Energy Resources Program (ERP) and colleagues are actively engaged in examining several aspects related to characterization, use, and impact of produced waters. Currently research is focused in three areas:
- Assessing Impacts of Coalbed Methane Produced Waters – Coalbed methane is produced by de-watering coal beds, and has become an increasingly important source of energy in the United States. The USGS is studying the environmental impacts from use and disposal of related produced waters.
- Characterization and Sources of Appalachian Basin Produced Waters – Despite a long history of oil and gas development in the eastern United States, sparse compositional data exist for produced waters. This drive, along with renewed interest in Marcellus Shale gas accumulations, is sparking research on the source and chemistry of current and future produced waters from the Appalachian Basin.
- Water Balances for Energy Resource Production – USGS scientists are developing water budget methods for understanding inputs and outputs from regional oil and gas resources.
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Assessing Impacts of Coalbed
Methane Produced Waters
Assessing Impacts of Coalbed Methane Produced Waters
Coalbed methane (CBM), also called coalbed natural gas, currently contributes ~10% to U.S. natural gas production, but generates more water than traditional gas sources. CBM is generated by de-watering coal, and thus reducing pressure within the coal bed, allowing adsorbed volatile compounds, such as methane, to be transported out of the subsurface... [+]
and captured (fig. 1).
Figure 1. Simplified illustration of a coalbed
methane production well. (Modified from Rice and
Nuccio, 2000 by Eric A. Morrissey, USGS.)
Water generated from CBM production is typically re-injected into a different unit, treated, beneficially used and/or directly discharged into ponds or surface waters. The USGS and colleagues are involved in examining environmental impacts from several different disposal/beneficial use strategies for CBM Produced Waters.
One area of focus is the Powder River Basin (PRB) in Wyoming and Montana, the second largest producer of CBM in the United States. Coalbed methane activities within the Wyoming portion of the basin currently generate 570-680 million bbls of water per year (1bbl = 42 gallons), little of which is reinjected into the subsurface. The CBM produced waters from the PRB are Na-HCO3 type waters and contain relatively low concentrations of trace metals (Rice and others; 2000). However, some of the water samples collected from the basin exhibit low to moderate total dissolved solid concentrations (370-1,940 mg/L) and a relatively high sodium adsorption ratio (SAR=5.7-32). Direct discharge of these waters to the surface has the potential to damage soils and ecosystems. Additionally, infiltration of CBM waters have been shown to leach pre-existing salts from the unsaturated zone, and in some cases lead to high salinity plumes in shallow groundwater.
Figure 2. Cartoon showing hypothetical
distribution of water and salts in a working
SDI system employing CBM produced water,
Powder River Basin.
One method for using CBM produced water is the irrigation of crops via subsurface drip irrigation (fig. 2). However, few data exist which examine potential impacts from this technology. To better understand impacts of salt and water derived from CBM produced waters on a SDI system, the USGS and colleagues are investigating through application of geochemical and geophysical tools. Additionally, because CBM produced waters are derived from a coal, an organic-rich substance, they often contain elevated levels of organic compounds. Organic compounds are indicative of energy sources potentially available for biological organisms to potentially generate additional CBM, through biogenic processes. Alternately, some of these compounds are toxicants and may present environmental concerns. Therefore further work is being conducted to better understand the behavior and distribution of organic compounds in CBM produced waters.
Rice, C.A. and Nuccio, V., 2000, Water Produced with Coalbed Methane: U.S. Geological Survey Fact Sheet FS-156-00.
Rice, C.A., Ellis, M.S., and Bullock, J.H., Jr., 2000, Water co-produced with coalbed methane in the Powder River Basin, Wyoming: preliminary compositional data: U.S. Geological Survey Open-File Report 00-372, 20 p.
Characterization and Sources of Appalachian Basin Produced Waters
Considerable work has been done to characterize the inorganic, major element chemistry of produced waters from western basins of the United States, but equivalent data for the Appalachian Basin are scarce, despite significant oil and gas production. Across the Appalachian Basin, salinities of produced waters range from fresh to more than 350,000 mg/L TDS... [+]
Figure 1. Locations of wells in the Appalachian Basin
from which produced water samples were collected
and analyzed. Data from Breit (2002) and Osborn
and McIntosh (2010).
total dissolved solids; sea water is ~35,000 mg/L TDS). Initially, data from a variety of sources, including state and federal agencies, and possibly private sources, is being surveyed to compile basic information on the natural components of the deep groundwater. This information should be useful in advance planning for the disposal or possible recycling of produced formation water and flowback water. This information should be useful in advance planning for the disposal or possible recycling of produced formation water and flowback water.
A primary source of produced water geochemistry is a database compiled by Breit (2002) for localities throughout the United States, including 90 samples from the Appalachian Basin (fig. 1). The database reports the major cations and anions (i.e., Na, Ca, K, Mg, Cl, HCO3, and SO4) as well as mass and charge balance, pH, and total dissolved solids (TDS). The salinities of the Appalachian Basin produced waters compiled by Breit (2002) have a median of 246,000 mg/L TDS (fig. 2), markedly higher than for produced waters from almost all other oil and gas producing regions of the United States. The Rocky Mountain and Colorado Plateau regions, for example, have a median produced water salinity of 9000 mg/L TDS. The Appalachian Basin samples are predominantly Na-Cl-type waters; Ca is significant, but secondary to Na both in molality and equivalence. Concentrations of Mg, K, SO4 and HCO3, are minor to insignificant (fig. 3).
Figure 2. Boxplot comparing distribution of TDS
of water samples collected from the
Appalachian Basin and three western United
States basins. The upper and lower limits of
the box indicate the 1st and 3rd quartiles, the
median (2nd quartile) is denoted by the
horizontal line in the box. The 5th and 95th
percentiles are noted by the upper and lower
bars, respectively, on the whiskers.
Figure 3. Piper diagram showing range of
composition (percent equivalence) of water
samples collected from the Appalachian Basin.
The plot suggests that most produced waters
generated from the basin are Na-Cl
dominated waters, but that some variations
exist. Data from Breit (2002).
Figure 4. Plot of TDS versus reservoir age for water samples
collected from the Appalachian Basin. Data from Breit (2002).
The waters in a given reservoir are most commonly a mix of fluids from multiple horizons that have migrated varying distances through the basin over geologic time. In some cases however, reservoir fluid represent connate water, or the fluid originally trapped during deposition of the reservoir strata. The Appalachian Basin samples of Breit (2002) were obtained from reservoirs of Pennsylvanian through Cambrian age, with a majority from Silurian and Devonian reservoirs (fig. 4). Overall, these samples are not sufficiently evenly distributed throughout the stratigraphic section to permit conclusions as to reservoir age vs. salinity.
The USGS is investigating additional sources of data beyond the database of Breit (2002) in order to address the following long term objectives:
- Characterization of the major element chemistry of formation waters to define geographic trends in salinity, and/or trends relative to reservoir age. Possible new sample locations and types of analyses should be investigated.
- Identification of salinity sources (e.g., evaporite dissolution vs. connate water). This may help in predicting approximate salinity levels by reservoir age and location.
- Characterization of the sources and concentrations of NORM (naturally occurring radioactive material) and TENORM (technologically enhanced naturally occurring radioactive material) in formation waters and produced waters across the Appalachian Basin.
Water Balances for Energy
Water Balances for Energy Resource Production
New technologies have expanded domestic oil and gas production to include low-permeability formations once considered to be inaccessible, including the Bakken Formation in northern Montana and North Dakota, the Barnett Shale in Texas, and the Marcellus Shale in the Appalachian states (fig. 1). Hydrocarbon production from these formations requires considerable quantities of fresh water (surface and/or groundwater) to... [+]
Figure 1. This drill rig, outside of Parshall, North
Dakota, targets the Bakken Formation at a
depth of approximately 14,000 feet.
increase fluid conductivity of the reservoir unit through hydraulic fracturing (commonly called “fracing”). Fracing fluids must be removed prior to resource extraction. These returned fracing fluids, known as flowback water, generally contains salts and minerals from the formation in addition to the additives used to increase fracing efficiency. The large volumes of water involved in these practices (generally 1-5 million gallons per frac job) have already led to supply and disposal problems in some areas. To address such issues and to help stakeholders prepare appropriately, the USGS is developing water-budget methods for uses associated with oil and gas production. Established USGS energy assessments provide estimates of technically recoverable resources. These results will be extended to project the volume of water needed for hydrocarbon production.
Figure 2. Digital elevation model showing locations of
wells producing oil from the Bakken Formation (red dots).
The blue line shows the boundary of the Bakken
Formation, the red line shows the boundary of the
Williston Basin, and the green lines denotes the
approximate extent of the prairie potholes region. Data
source: IHS database.
The Williston Basin in Montana and North Dakota (fig. 2) is the area of our initial focus because of (1) the large quantity of oil present in the Bakken Formation, and (2) the potential for impacts on the Prairie Pothole wetlands that host large numbers of migrating waterfowl. Current, rapidly escalating production of Bakken Formation oil a million or more gallons of fresh water per well for fracing. Historic and ongoing conventional oil production from other Williston Basin formations has resulted in large volumes of highly saline produced waters; in the past, some of these waters escaped into shallow aquifers and have impacted wetlands. USGS scientists are estimating the quantities of water (fracing and produced) that would be involved in a range of future hydrocarbon production scenarios and comparing these quantities with estimated total water budgets for the region.
Why some Public-Supply Wells are More Vulnerable to Contamination Than Others
USGS Podcast (Episode 120)
See Podcast Transcript
Contaminants in 20 Percent of U.S. Private Wells
USGS Podcast (Episode 90)
See Podcast Transcript
Page Last Modified: Tuesday, February 26, 2013
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