Hydraulic Fracturing Waste Streams—An Overview
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Hydraulic fracturing for natural gas uses millions of gallons of water for each well and during the life of the well, a large percentage of that water returns to the surface and becomes a waste stream. In the vernacular of hydraulic fracturing, there are two types of water resulting from the process: Flowback and production water.
Flowback is a water-based solution that returns to the surface during and after the completion of hydraulic fracturing. This flowback water contains the original fluid used to fracture the formation, as well as formation constituents like clays, chemical additives, dissolved metal ions, hydrocarbons, and total dissolved solids (TDS).
According to the U.S. Department of Energy (DOE), most of the flowback occurs in the first 7 to10 days while the rest can occur over the next 3 to 4 weeks. The volume of flowback water is generally between 20 percent and 40 percent of the volume that was initially injected into the well. The rest of the fluid remains absorbed in the formation. The water can have a murky appearance from the high levels of suspended particles. The following table provides general flow profiles, however, the DOE cautions “there could be wide variation across geographical locations and due to operator bias …”
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|
Time Flowback Rate |
Flowback Recovery |
% Frac Fluid |
|
1–5 days |
100–150 bbl*/hr |
10–25% |
|
5–15 days |
20–60 bbl/hr |
8–12% |
|
15–30 days |
5–10 bbl/hr |
1–5% |
|
30–90 days |
10 bbl/day |
1–2% |
*1 barrel (bbl) = 42 U.S. gallons
Although the composition of flowback can be generalized, the DOE notes that “Flowback water is not a uniform ‘raw material’ from a process development perspective. The physical and chemical properties of flowback water vary considerably depending on the geographic location of the play, the geological formation, and the chemicals introduced during the drilling and fracturing operations.” Also important is the fact that both flowback volume and water properties can vary throughout the lifetime of the well.
As a result, the DOE contends that “As oil fields mature and water/oil ratios rise, and as increased levels of hydraulic fracturing in both oil and gas-bearing shales lead to increased volumes of flowback water, the challenges of cleaning and/or disposing of oilfield waste will only increase.”
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But flowback is just one part of the hydraulic fracturing waste stream and the second is produced water. Produced water is created from water that is trapped in underground formations and brought to the surface along with oil or gas. Held underground for millennia in hydrocarbon-bearing formations, this water contains chemical characteristics of the formation and the hydrocarbon itself as well as constituents of injected hydraulic fracturing fluids such as chemicals added during the production and treatment processes.
According to the DOE, produced water is also called “brine” and “formation water.” The major constituents of concern in produced water are:
- Salt content measured as (salinity, total dissolved solids, electrical conductivity),
- Oil and grease (this is a measure of organic chemical content),
- Various natural inorganic and organic compounds or chemical additives used in drilling and operating the well, and
- Naturally occurring radioactive material (NORM).
Similar to flowback, produced water is not a single commodity but rather has varying physical and chemical properties depending on the geographic location of the field, the geological host formation, and the type of hydrocarbon product being produced. Produced water properties and volume can also vary throughout the lifetime of a reservoir.
By volume, produced water far exceeds that of flowback as a waste stream from oil and gas exploration and production. According to the DOE, each year in the United States nearly a million wells generate approximately 21 billion bbl of produced water, or about 2.4 billion gallons per day. Due to these extremely high volumes of wastewater, research is ongoing to find ways to manage these wastes sustainably. Tomorrow we look at one way to clean up wastes that has a profitable side as well.