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Oil and Gas Wetlands Treatment Systems: Use of Constructed Wetlands to Treat Produced Fluids from Stripper Oil Wells - a Penn Soil Project

ABSTRACT

    Collectively, the oil wells in northwestern Pennsylvania constitute a significant economic benefit to the region. Individually, these wells provide limited economic return. On the average, the oil wells produce about 40 barrels of oil per year and brine at a ratio of 3:1 brine to oil. At current market prices, stripper oil well producers have limited resources and there are a limited number of economically feasible options to manage produced water. In 1993, the Department of Environmental Protection Oil and Gas Management Program, in cooperation with three independent oil operators, initiated a cooperative effort to examine the feasibility of using constructed wetlands to treat brine from stripper oil wells.

Constructed wetlands were selected for evaluation as a possible alternative for treatment of brine from oil wells because:

• They would be  low cost items to construct and operate, and could be constructed by the operator;
• Once constructed, daily operation and maintenance would be minimal;
• Chemicals, pumps and other equipment would not be needed;
• Wetland vegetation is common around land surface discharges of brine.

Constructed Wetland Treatment Systems

    In recent years, constructed wetlands have been used to treat or partially treat different types of wastewater discharges including discharges form coal mining and other mineral mining operations, storm water runoff, landfill leachate, sewage, agricultural runoff and others. Sometimes the constructed wetlands are the only means of treatment and other times they are a component of a larger treatment system. Constructed wetland treatment systems use natural processes to remove pollutants from wastewater. It is a technology that is still developing, especially as it applies to the treatment of brine from oil wells.

    A constructed wetland is a shallow basin or series of basins with substrate that is planted with vegetation.  The substrate may be soil or a mixture of soil and crushed stone which acts as the growing media for the vegetation. The vegetation must be tolerant of saturated soils and compatible with the wastewater being treated. The wastewater enters one end of the wetland and exits through an outlet structure that controls the depth of water in the wetland. As the wastewater travels through the basin, natural processes such as aeration, chemical precipitation, adsorption, settling, filtration, and biological degradation treat the wastewater.

   The primary advantage of a constructed wetland treatment system is the low operation and maintenance costs. The major limitation is suitable land area to place the system.

    In general terms, there are three types of constructed wetland: surface flow; subsurface flow; and a hybrid system (2). In a surface flow wetland, the water surface and flow path is above the substrate. The layer near the surface tends to be aerobic, while the layers near the substrate tend toward being anaerobic. In the subsurface flow wetland, the flow path is horizontally through the substrate to take advantage of the anaerobic conditions in the substrate. A hybrid system utilizes surface or subsurface flow systems in combination with other types of treatment.

Minimum Treatment Requirements

    Produced fluid or brine is salt water that is present in subsurface formations or is generated when fresh water is introduced into the formations. It is characterized as a saline solution high in total dissolved solids, sodium, calcium and chloride. Other common constituents are iron, barium, magnesium, and organic compounds such as phenol, benzene, toluene, and xylene

    Brine from oil wells varies in quality throughout the oilfields of northwest Pennsylvania. Brine associated with water floods is typically weak brine whereas brine associated with primary production is stronger brine. Other constituents in the brine also vary. For example, the iron concentration in the brine at the Rouse farm site in Warren County averaged 7 mg/L while the average iron concentration at the Drake site in Venango County was 22 mg/L.

The minimum treatment technology is:

    • Gravity separation and surface skimming or equivalent technology for oil and grease removal.
    • Flow equalization to ensure optimum treatment efficiency of the facilities.
    • Chemical addition for pH control and metals removal, if necessary.
    • Aeration or equivalent technology for reducing volatile petroleum hydrocarbons and oxidation for metals

           removal.         
    • Settling (retention) or filtration for removal of solids, including oxidized metals.

The Walton Site

    The Walton site is located in President Township, Venango County. This was the first facility constructed and placed into operation. The facility treats produced fluids (brine) generated from about 35 stripper oil wells on Drake Well's Walton lease (now North American Oil and Gas). Oil is produced from the Red Valley sand. The oil wells were typically pumped on a Monday, Wednesday, Friday schedule for about 45 minutes.  A central generator provides the electricity. The wells are piped to a 50 bbls central oil separator. Oil from the separator flows into the oil storage tank and the produced water (brine) is piped directly to the constructed wetland treatment system. The lease is operated year round except when winter conditions prevent operations.

    The passive wetland treatment system at this site was permitted to treat and discharge up to 1,000 gallons (23.8 bbls) per day. Treated brine can be discharged at a maximum rate of 0.69 gallons (2.6 L) per minute. The facility consists of three basins in series. Each basin was lined with a 20 mil polyethylene liner to prevent groundwater contamination.

    The first basin was designed as an equalization, oxidation and settling basin.  The second basin was constructed as an aerobic basin.  It was planted during the dormant season with common reed.  Finally the third basin was constructed as an anaerobic or successive alkalinity-producing system basin.

Treatment Results

    The Walton Facility was placed into operation in the fall of 1995 and for the first year of operation showed intermittent success with meeting the treatment objectives.  In 1997, the lease changed ownership and was inactive or being reworked.  In 1998, modifications were made to the system and are still being evaluated.  Since this was the first facility placed into operation, much experience was obtained on what not to do and what to do.

Conclusion

    This Project represents the first generation of constructed wetlands in northwestern Pa. to treat brine from stripper oil wells.  Design and sizing of a system is largely based on knowledge of the chemistry of the brine, understanding of the treatment process and judgment.  Additional work is needed to better understand the chemistry and biological process occurring in the wetland treatment systems and to develop design criteria (especially loading rates and detention times).  Vegetation plays an important role in the treatment process, and additional work is needed to establish vegetation and test other species, in particular, ones that can reduce pollutant concentrations by plant uptake of pollutants.  Use of clay liners also needs to be examined.

Reconstruction of the Walton site

   The next phase of this project includes combining and enlarging the existing ponds.  This project has been re-designed to utilize the local clay, for the lining of the pond and waterway surfaces.  The purpose of the low permeability clay is to protect the ground water from the waste water.  An inlet manifold will be constructed to disperse the incoming waste water along the rock pad.  The water will then flow through a bed of rocks into the first basin.  A fabric oil skimmer will then be installed.  This geotextile will be used for skimming off the excess oil that has come out of the separator.  Finally, wetland vegetation (salt-water cord grass) will be planted into a six-inch layer of organic soil, which is placed on top of the compacted clay liner.  The plants will then facilitate the cycle of growth and decay within the wetland soil, provide a medium for attached bacterial growth, absorb organic compounds, and provide structural integrity to the wetland bottom which will minimize erosion and prevent short circulating of flow into the basin.

Construction Complete   

    Final inspection was set on July 9, 2002.  On hand was Harvey Pinkerton (Penn Soil RC&D Coordinator), Doug Neely, Doug Parker, and Orest Kolodij (PA Dept. of Environmental Protection), and Clint Brown (Contractor).