What is produced water?
Produced water refers to the fluid mixture that comes back up the wellbore after hydraulic fracturing is used to stimulate a well. As can be inferred from the name, the vast majority of the fluid is water mixed with trace amounts of other chemicals.
Produced water is made up of water originating from two main sources. The first of these is the water-based mixture that was originally pumped down the wellbore to fracture the rock formation. While some of this fluid inevitably remains in the ground, “stuck” in the newly created fractures, most of it flows back up the wellbore and returns to the surface.
The second source of produced water is the rock formation itself. When the rock fractures, space is created for any water trapped in the previously impermeable formation to flow out and up the wellbore along with oil and natural gas. This water is often referred to as “formation water”4.
Produced water concerns
High produced water volume
Hydraulic fracturing operations generate produced water on a massive scale. The United States alone currently generates over 21 billion barrels—or 882 billion gallons—of produced water annually4. Excluding the US, worldwide hydraulic fracturing generates annual produced water volumes on the order of 50 billion barrels—or 2,100 billion gallons4.
This number is also likely to grow as oil and gas fields mature. In the beginning of a well’s life, the reservoir is saturated with oil or gas. The pressures inside the reservoir are relatively high and fuel flows readily up the wellbore with minimal fracturing operations. However, as more and more oil or gas is removed, pressures drop and the supply in the reservoir begins to become exhausted. The desired fuel flux begins to drop and more fracturing treatments—involving more water—are needed to maintain profitable production. Therefore, as a well matures, it’s produced water to produced fuel ratio will continue to climb until fuel extraction is no longer profitable.
Produced water chemicals
Certain chemicals often present in produced water are also cause for concern. Chemicals in produced water originate from one of the two water sources—fracking fluid or formation water. The major components of concern are4:
- Salt content
- Oil and grease
- Various natural inorganic and organic compounds or chemical additives used in drilling and fracturing
- Naturally occurring radioactive material (NORM)
Of these categories of chemicals, only one is attributed to the original fracking fluid. Engineers often add small amounts of organic and inorganic compounds to the fluid used to fracture a well as tools to manipulate flow properties and interactions with drilling pipes. Common chemical “tools” added are an acid, an antibiotic, a corrosion inhibitor, a friction reducer, a gelling agent, a scale inhibitor, and an oxygen scavenger1. While these additives are present in relatively small concentrations, they may harm wildlife by altering water chemistry if they make their way into natural bodies of water.
The other three categories of concern generally originate from formation water, or the formation itself. Oil and grease refer to organic compounds in general—in other words, the oil or gas that is being extracted. When water is used to fracture a well, oil or gas present from the well flows back with it and must be separated for refining and sale. Again, presence of organic compounds in water can be harmful to both local wildlife and humans.
Salt content and naturally occurring radioactive material (NORM) are both results of compounds dissolving in the water from the rock formation while it is underground. Injected water may dissolve underground salt deposits during the fracturing process, making it un-potable and unsafe for wildlife upon its return to the surface. As apparent in its name, NORM is material containing naturally occurring isotopes, such as radium, that are involved in radioactive decay. While these compounds are relatively harmless while buried deep underground, they are soluble in water and are dangerous to living organisms when brought to the surface dissolved in produced water.
Produced water cost
Finally, a concern for industry is the cost of produced water that results from the previously described problems. Produced water is expensive. All the water used in hydraulic fracturing must be purchased—and the cost is high, especially in areas with limited water resources, where water is often auctioned off to the highest bidder. And after the original purchase cost, the water must be disposed of once used. This means costly treatment to remove pollutants or transport costs for disposal—or both. Depending on the origin, content, and treatment method of produced water, disposal can cost anywhere from $1 per barrel to $802.
In considering the implications for industry as well as for communities and environmentalists, it is clear that we all have the same basic question. What do we do with produced water?
Produced water disposal options
What can we do with produced water? Let’s look at some options.
An elegant solution is to avoid letting produced water reach the surface at all. In certain cases, this can be accomplished using injection of polymer gels that block water from flowing through fractures3. Downhole separators can also be used to separate fuel from water within the well and reinject formation water into the reservoir before it ever returns to the surface3. However, these possibilities are dependent on unique well and formation characteristics as well as availability of specialized equipment.
An extremely common disposal method is reinjection of produced water. That is, produced water is reinjected into the same reservoir after oil and gas production has stopped, or it is injected into a new underground rock formation. In this way, salinity or NORM picked up underground is returned there and is not exposed to surface life or environment. Contamination of potable water can also be avoided in this way. At sufficient depths, salinity is high enough that any water naturally present is already unfit to support plant, animal, or human life. If produced water is injected at these depths, any water reservoir it may affect is already not potable. Disadvantages to this method include transport and surface storage costs as well as the eventual loss of high volumes of water.
Produced water can also be discharged into natural bodies of water3. In order for this to be a possibility, the water must meet established standards and regulations for safety and pollution levels. In most cases, this requires water treatment prior to discharge.
Produced water can also be reused. This option has not been widely used historically, but is an area of current research and development. Although this avenue will almost always require water treatment processes, a myriad of possibilities are being explored. Potential applications include irrigational and agricultural use, use in turbines and generators, and even reuse in the same oil and gas well3.
1) Arthur, J. D., & et al, (2008, September). Hydraulic fracturing considerations fornaturalgas wells ofthe marcellus shale. The ground water protection council annual forum, Cincinnati, Ohio.
2) Duhon, H. (2012). Produced water treatment: yesterday, today, and tomorrow. Journal of Petroleum Technology,
3) Langhus, B. G., Patel, C., & Arthur, J. D. (2005). Technical summary of oil and gas produced water treatment technologies. US DOE National Energy Technology Laboratory,
4) US Department of Energy, National Energy Technology Laboratory. (2013). Produced water management information system: Introduction to produced water