First Step: Basic Separation

Fact Sheet - First Step: Basic Separation

 
 
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When a well has been completed in a productive formation, a mixture of reservoir fluids enters the well bore and is brought to the surface, either pushed by internal reservoir pressure or lifted with some type of pump. The mixture contains three types of fluids (crude oil, natural gas, and produced water) and often some solids.

The first step of the production process involves separating the oil, gas, and water into different streams where they can be managed appropriately. This is typically accomplished by gravity separation in a horizontal or vertical separator.

Three-phase separator
Three-phase separator; Source: U.S. Environmental Protection Agency
Horizontal oil/water separator
Horizontal oil/water separator; Source: J. Veil, Argonne National Laboratory
Free water knockout tank
Free water knockout tank; Source: J. Veil, Argonne National Laboratory

Gas rises to the top of the tank where it can be collected. The oil layer floats on top of the water layer. Strategically placed outflows can collect each fluid layer separately. A common type of separator is the "free-water knockout tank."

 

Gas pipeline
Gas pipeline; Source: J. Veil, Argonne National Laboratory

Historically, incidental gas produced in oil wells was often burned (flared) or, at many wells, it was provided to the landowner for domestic use. Today, where gathering lines are available, the gas is often collected and sent to market. The marketable gas is generally treated to meet pipeline-quality standards at a gas processing plant.

The separated oil stream may contain some water, and the water stream may contain additional dissolved hydrocarbons or emulsified oil. Emulsions are droplets of one liquid suspended in another liquid. Since in most cases they cannot be removed through basic gravity separation, emulsions typically require additional treatment. One common oil field treatment method used to break emulsions is the application of heat generated by burning gas and passing the hot exhaust gas through a pipe running through the middle of a tank known as a "heater-treater". This treatment helps to break emulsions, thereby allowing gravity separation to take place.

Other approaches used to break emulsions involve electrostatic precipitation and emulsion-breaking chemicals (demulsifiers). Chemicals are used in many different steps of produced water management. They are an important part of optimizing the performance of many of the technologies described in the other fact sheets. Proper chemical selection, the point of chemical application within the water handling system, and chemical dosage all contribute to the effectiveness of the water treatment. PWMIS does not include a separate fact sheet covering the use of chemicals.

Tank battery
Tank battery; Source: J. Veil, Argonne National Laboratory

At some point, the oil is either sent to a refinery or stored in tanks until it can be collected by truck. Likewise, the produced water is either sent directly for management or is stored in tanks. Many onshore fields contain a series of tanks to hold and store crude oil and produced water. These are the "tank batteries." Certain sites have one tank battery per well. More commonly, however, one tank battery serves several wells in the same area.

Any solids carried into the separators and storage tanks settle to the bottom and form a layer of sediment or sludge. The solids can be sand particles from the formation, solids left over from the well stimulation, scale or corrosion products, cement particles from well construction, or other materials. The solids - "tank bottoms" -- accumulate, and eventually must be removed, usually by a vacuum truck hauling the material offsite for recycling or disposal. Sediment and sludge from separators and tanks are considered to be exploration and production waste.

References
Hyne, N.J., 1995, "Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production," PennWell Publishing, Tulsa, OK, 536 pp.

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]

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