Discharge

Fact Sheet - Discharge

   
 
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In many parts of the world, discharge to surface water bodies and underground injection represent the principal options to dispose of produced water from oil and gas exploration and production operations. Subject to certain exceptions, the following trends can be observed:

  • Most U.S. onshore oil and gas operators inject their produced water for enhanced oil and gas recovery or final disposal. This reflects the prevailing regulatory situation of prohibiting discharges from most onshore wells.
  • Many U.S. CBM well operators prefer to discharge produced water to surface water bodies if authorized by the regulator. Tightening treatment requirements for CBM produced water under increasingly restrictive discharge standards may change the mix of management options in the future.
  • Most U.S. offshore operators discharge produced water to the ocean subject to all applicable regulatory requirements. Offshore produced water is also typically discharged in other parts of the world. Not surprisingly, different countries employ different discharge standards.

Injection of produced water for disposal is discussed in a separate fact sheet. This fact sheet describes the different types of discharge operations conducted by the industry.

  U.S. map with subcategories of EPA's standards for oil and gas discharges
The subcategories of EPA's standards for oil and gas discharges; Source: J. Veil, Argonne National Laboratory.

Where Can Produced Water Be Discharged? 
In the U.S., discharge activities are subject to all applicable regulatory controls required by the U.S. Environmental Protection Agency (EPA) and the state agencies. The EPA's national discharge standards - the effluent limitation guidelines (ELGs) for the oil and gas extraction point source category - include five subcategories. These are further described in the PWMIS Regulatory Module.

The "onshore" subcategory generally prohibits produced water discharge from onshore wells, subject to limited exceptions made available by two other subcategories for onshore wells.

  •   Stripper well.
    Stripper well; Source: J. Veil, Argonne National Laboratory.
     Under the "stripper" subcategory, states decide whether to authorize produced water discharges from very small oil wells. Since low oil production volumes do not contribute much income to stripper well operators, they are not able to undertake complicated or expensive treatment. Adewumi et al. (1992) describes a simple, low-cost system used for produced water treatment in Pennsylvania. It involves separation, pH adjustment, aeration, solids separation, and filtration. The U.S. Department of Energy (DOE) has funded a Stripper Well Consortium through the Pennsylvania State University. The initiative supports research to produce from stripper wells and manage the water most cost-effectively. For more project-specific information, consult the website of the Stripper Well Consortium, which is available at: http://www.energy.psu.edu/swc.
  • The "agricultural and wildlife" subcategory allows discharges of produced water that are clean enough (and with sufficiently low salinity). The discharge must meet a limit for oil and grease of 35 mg/L and must actually be put to a beneficial agricultural or wildlife reuse. Little information is available on the treatment methods used before discharging produced water from oil and conventional gas wells under this subcategory.
  Photo of discharge from offshore platform. 
Discharge from offshore platform; Source: J. Veil, Argonne National Laboratory.
Photo of platforms in Cook Inlet, Alaska.
Platforms in Cook Inlet, Alaska; Source: M. Puder, Argonne National Laboratory.

The "offshore" subcategory governs produced water discharges from as many as 4,000 U.S. platforms. Most offshore produced water is discharged to the ocean.

The primary pollutant of concern is oil and grease. Regulated by EPA's national ELGs for offshore activities, it is made part of all U.S. offshore discharge permits. The offshore subcategory requires an oil and grease limit of 29 mg/L monthly average and 42 mg/L daily maximum. In contrast to onshore operations, salinity does not pose a major concern for offshore discharges. In addition to the national oil and grease limit, the EPA's regional offices impose other discharge limitations, including restrictions on flow rate, toxicity testing, and monitoring for several toxic metals, organics, and naturally occurring radioactive material (NORM). Most of the treatment technology for offshore produced water is geared toward removing oil and grease.

Wells in the "coastal" subcategory are generally prohibited from discharging produced water, subject to an exception for wells located in Alaska's Cook Inlet. Discharges from these wells must meet all applicable offshore standards.

Produced Water Discharges from CBM Operations 
Because CBM was not widely produced at the time the EPA wrote its national discharge standards, the Agency never developed national ELGs for CBM water discharges. As of the end of 2006, the decision to allow CBM water discharges is made by state agencies, or by EPA regional offices, where states do not have permitting authority (Veil 2002). However, in December 2006, EPA announced that it would begin studying the CBM industry sector to determine if national ELGs were needed.

Some CBM produced water is clean enough to be discharged without treatment. Other produced water must be bubbled over rocks for aeration. This allows iron to precipitate out before the produced water is discharged to a stream. When more rigorous treatment is required to remove salinity and sodium, operators have sometimes used reverse osmosis, ion exchange, filtration, electrodialysis, capacitive deionization, or other technologies. These are described in other fact sheets.

  Photo of Produced water samples in analytical laboratory.Produced water samples in analytical laboratory; Source: J. Veil, Argonne National Laboratory.

What Is Oil and Grease? 
Oil and grease is a key constituent in produced water. It is therefore subject to regulation in nearly all permits authorizing produced water discharges. Oil and grease does not occur as a single chemical compound. Rather, as an "indicator pollutant" it is a measure of many different types of organic materials that respond to a particular analytical procedure. Different analytical methods will measure different organic fractions and compounds. Therefore, the specific analytical method used is important in determining the magnitude of an oil and grease measurement.

This is particularly important because of the phasing out of the use of Freon-113 as an extraction solvent (EPA Method 413.1) over the past decade. That longstanding standard-approved method (which was used to collect all the effluent data used in establishing the statistically derived discharge standards for oil and grease) has been replaced by EPA Method 1664, which uses n-hexane as the extraction solvent. Raia and Caudle (1999) describes a study sponsored by the American Petroleum Institute, comparing the results of the two methods. The standard deviations of the results occurred in the same order of magnitude or were larger than the means, thereby making it difficult to determine if the results are comparable. Most of the samples showed higher values when measured by the new method. This raises some concern over compliance. For example, if the new method measures 44 mg/L while the old method measures 40 mg/L for the same sample, this is the difference between compliance and noncompliance with the maximum discharge limit for oil and grease of 42 mg/L. It is also worth noting that the standard oil and grease measuring method approved for North Sea discharges differs from the two methods introduced above (Yang and Tulloch 2003).

A second point is that not all produced waters contain the same constituents even if they have the same oil and grease content. Oil and grease is made up of at least three forms: Free oil (this is in the form of large droplets that are readily removable by gravity separation methods), Dispersed oil (this is in the form of small droplets that are more difficult to remove), and Dissolved oil (these are hydrocarbons and other similar materials that are dissolved in the water stream; they are often challenging to remove).

For example, take two untreated produced water samples, both of which contain 100 mg/L of oil and grease. Produced water A has primarily free oil whereas produced water B has primarily dissolved oil. In order to meet the maximum discharge limit of 42 mg/L, the types of treatment processes and the cost of those processes would be vastly different. This is the challenge faced by offshore operators. McFarlane et al. (2002) describe a collaborative project between Oak Ridge National Laboratory and several major oil companies to better characterize and predict the types of water-soluble organics that are present in offshore produced water.

References
Adewumi, M.A., J.E. Erb, and R.W. Watson, 1992, "Initial Design Considerations for a Cost Effective Treatment of Stripper Oil Well Produced Water," in Produced Water, J.P. Ray and F.R. Engelhart (eds.), Plenum Press, New York.

McFarlane, J., D.T. Bostick, and H. Luo, 2002, "Characterization and Modeling of Produced Water," presented at the 2002 Ground Water Protection Council Produced Water Conference, Colorado Springs, CO, Oct. 16-17. (Available at: http://www.gwpc.org/meetings/special/PW%202002/Papers/Joanna_McFarlane_PWC2002.pdf[PDF external site].)

Raia, J.C., and D.D. Caudle, 1999, "Methods for the Analysis of Oil and Grease and Their Application to Produced Water from Oil and Gas Production Operations," presented at the 9th Produced Water Seminar, Houston, TX, Jan. 21-22.

Veil, J., "Regulatory Issues Affecting Management of Produced Water from Coal Bed Methane Wells," prepared for U.S. Department of Energy, Office of Fossil Energy (January 30, 2002). 14 pp. Available at: http://www.evs.anl.gov/pub/dsp_detail.cfm?PubID=1477 [external site].

Veil, J.A., M.G. Puder, D. Elcock, and R.J. Redweik, Jr., 2004, "A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane," prepared by Argonne National Laboratory for the U.S. Department of Energy, National Energy Technology Laboratory, January. Available at: http://www.evs.anl.gov/pub/dsp_detail.cfm?PubID=1715 [external site].

Yang, M., and S. Tulloch, 2003, "Oil-in-Water Monitoring - Where Are We Heading in the North Sea?" presented at the 13th Produced Water Seminar, Houston, TX, Jan. 15-17.

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