What Is Produced Water?
Produced water is water found in the same formations as oil and gas. When the oil and gas flow to the surface, the produced water is brought to the surface with the hydrocarbons. Produced water contains some of the chemical characteristics of the formation from which it was produced and from the associated hydrocarbons.
Produced water may originate as natural water in the formations holding oil and gas or can be water that was previously injected into those formations through activities designed to increase oil production from the formations such as water flooding or steam flooding operations. In some situations additional water from other formations adjacent to the hydrocarbon-bearing layers may become part of the produced water that comes to the surface.
Most wells in unconventional oil and gas formations are stimulated using hydraulic fracturing, through which water is injected under pressure into the formation to create pathways allowing the oil or gas to be recovered in a cost-effective manner. Immediately following hydraulic fracturing in the well (a frac job), some of the injected water returns to the surface and is known as flowback water. Flowback water is often managed in a similar manner to produced water and some engineers in the industry consider it as part of the produced water flow stream.
How Much Produced Water Is Generated?
Nearly every oil and gas well generates water as part of the production process. The United States has almost 1 million producing oil and gas wells. During 2012 (the most recent year for which national volume data have been collected), 21.2 billion barrels (bbl = 42 gallons) of produced water were generated in the United States. By comparison, this is more than twice the amount of water that typically flows over the Niagara Falls each day. This volume can be expressed in other units:
- 58 million bbl/day
- 890 billion gallons/year or 2.44 billion gallons/day
- 3.57 billion m3/year or 9.2 million m3/day
Regardless of how the volume is expressed, it is a huge quantity of water. The rest of the world generates large volumes of produced water too, but insufficient data are available to make a precise estimate.
97% of the produced water comes from onshore wells located in 31 states. The remainder comes from offshore platforms in U.S. waters. More than one third of the U.S. produced water is generated in the state of Texas.
Based on 2012 data from 21 states, an average of 9.2 bbl of produced water is generated for each bbl of oil. However, data supporting that calculation were not available from Texas and Oklahoma, which have tens of thousands of old wells producing high percentages of water. If those states had been averaged in, it is likely that the actual ratio of water to oil would be higher than 10-to-1.
Is the Volume of Produced Water Increasing Over Time?
Many people assume that produced water totals will increase over time. In most conventional oil and gas wells, the volume of produced water from an individual well does increase over time. However, at some point the cost of managing the water becomes too high. At that point the well operator can work over the well to make it more efficient or can close the well.
From a national perspective, total produced water volume did not increase significantly between 2007 and 2012. During that five-year period, U.S. oil production increased by 29%, gas production increased by 22%, but water production increased by less than 1%. One explanation for these data is that many of the older conventional wells that generated a high lifetime volume of water were closed. Many of the newer wells that were drilled were unconventional wells that generated a lower lifetime total of produced water.
What Are the Characteristics of Produced Water?
The physical and chemical properties of produced water vary considerably depending on the geographic location of the field, the geologic formation from which the water was produced, and the type of hydrocarbon product being produced. The major constituents of concern are:
• Salt content (often expressed as salinity, conductivity, or total dissolved solids [TDS]). Although some produced water is nearly fresh (<3,000 mg/L TDS), most produced water is saltier than seawater (~35,000 mg/L), and can be >300,000 mg/L). Removing salt is not difficult, but it is usually costly.
• Oil and grease. This is not a single chemical compound; the analytical method for oil and grease measures various organic compounds associated with hydrocarbons in the formation). Oil and grease can be found in different physical forms:
- Free oil: large droplets - readily removable by gravity separation methods
- Dispersed oil: small droplets - somewhat difficult to remove; and
- Dissolved oil: hydrocarbons and other similar materials dissolved in the water stream - very challenging to eliminate.
• Inorganic and organic toxic compounds. The toxics may be introduced as chemical additives to improve drilling and production operations or they may leach into the produced water from the formation rock or the hydrocarbon.
• Naturally occurring radioactive material (NORM). Some hydrocarbon-bearing formations contain natural radiation that leaches into the produced water. The presence and concentration of NORM varies between formations.
How Is the Produced Water Managed?
The most recent set of U.S. data on produced water management was collected for the year 2012. Most U.S. produced water was injected. About 91% of the produced water was injected underground (this included water injected for enhanced recovery, water injected for disposal, and water sent to offsite commercial disposal). Slightly more than half of that was injected into producing formations for enhanced recovery. Slightly less than half of the injected produced water was injected to non-commercial and commercial disposal wells.
About 80% of the produced water from offshore wells was treated on the platform and discharged to the ocean. Only about 3% of onshore produced water was discharged. The percentage discharged from all wells (onshore and offshore combined) was about 5.6%.
Nearly 7% of produced water was managed by sending it to an offsite commercial facility, where the water was treated and disposed. These are third-party businesses that charge a fee to receive incoming produced water and other oil and gas wastes. Water was treated and processed in various ways. Most of these facilities managed water by injection into disposal wells.
About 3.6% of all produced water was evaporated. In some arid western states, produced water was managed through evaporation from onsite ponds and pits. Several commercial facilities managed water by evaporation from large ponds.
At least 0.6% of the produced water and flowback water in 2012 was put to a beneficial reuse – it is likely that a higher percentage was reused, but data were not available to quantify the amount. Much of the reuse was done by recycling flowback water and produced water to make drilling fluids and frac fluids for new wells in the same fields. Produced water may also have been used for irrigation (when the water has low salinity) or for dust and ice control on roads.
What Type of Processing or Treatment Must Be Done to Produced Water?
Produced water receives various types of treatment before it is disposed, reused, or otherwise managed. Many types of processes and technologies can be used to treat produced water depending on how clean the water must be before it moves on to its destination. For example, produced water must be treated to remove oil and grease and toxic chemicals before discharging it to the ocean from an offshore platform. Produced water that is discharged to onshore freshwater rivers must be further treated to reduce salt content. Water that is injected for either enhanced recovery or for disposal is treated in a different way from water that is discharged. The treatment processes used prior to injection are designed to remove free oil, solids, and bacteria. Chemicals are often used to enhance treatment processes and to protect underground formations and equipment.
As oil, gas, and water are produced from a well, the fluids need to be separated into separate streams. This is typically done using some type of gravity separation, such as API separators, free water knockout tanks, or gun barrel separators. In addition to separating the fluids, these devices allow for large solid particles to settle out. When the oil and water are emulsified, they can be separated by applying heat or appropriate chemical treatments.
It is not possible or practical to list or describe all types of treatment technologies. As a starter, here are some generic types of technologies that may be used. They are grouped by which constituents of produced they are primarily designed to treat. In most instances, these technologies are not used by themselves – they are stages in a pretreatment/treatment system.
Technologies Used to Remove Oil and Grease and Other Organics
- Gravity separation
- Physical separation
- Combined physical and chemical processes
Technologies Used to Remove Metals and Other Inorganics
- pH Adjustment and Clarification
- Membrane processes
- Reverse osmosis
- Ion exchange
Technologies Used for Removing Salinity
- Reverse osmosis
- Thermal distillation
Technologies Used to Control Microbial Organisms
- Chemical biocides
- Ultraviolet light
Much more information on these technologies can be found through Internet searches.
Other Produced Water Considerations
Produced water has several properties that are frequently not well understood and ignorance of them complicates the treatment. First, produced water contains chemical equilibrium systems that shift with changes in temperature and pressure and cause reactions to occur. These reactions may result in mineral scales being formed, solid hydrocarbon deposition (paraffin formation) and changes in pH. Second, produced water does not normally contain oxygen. Some of its components are in a chemically reduced state and will react with oxygen if the water is allowed to contact air. This can result in deposition of iron compounds and elemental sulfur.
Treatment of produced water is complicated by the changing concentrations of constituents, intermittent addition of process chemicals, and a need to treat consistently and dependably in oil field settings that often have extreme temperatures, and a lack of utilities and infrastructure.
Some potential problems associated with managing produced water include:
- Plugging of disposal wells by solid particles and suspended oil droplets,
- Plugging of lines, valves, and orifices due to deposition of inorganic scales,
- Corrosion due to acid gases and electrochemical reactions of the water with piping and vessel walls,
- Growth of bacteria that plug lines and valves or result in the formation of harmful products.