Injection well head. Credit: Joshua Doubek, CC BY-SA 3.0[/caption
The practice of underground injection is the preferred disposal method for produced water in the oil and gas industry; 98 percent of produced water from onshore wells gets re-injected.
Injection wells for oil and gas production are called Class II wells
; EPA, which regulates such wells, says there are 172,068 Class II wells in the U.S. The agency subdivides Class II wells as enhanced recovery wells (about 80 percent of Class II wells), disposal wells (about 20 percent of wells) and hydrocarbon storage wells (somewhat more than 100 wells). All of these types of wells have to be configured and managed so as to avoid contaminating surface water and groundwater. Disposal wells are used to inject brines and other fluids resulting from oil and gas production. The Groundwater Protection Council
(GWPC) says that over the past 30 years, 720 billion barrels of brine have been injected into the ground.
Class II wells. Credit: EPA.[/caption
Is underground injection really the best way to deal with produced water? As the world faces greater water stress, is it possible that all of that brine could somehow be treated and employed beneficially at the surface?
from the U.S. Department of the Interior Bureau of Reclamation estimates that in the 17 Western U.S. states total water use is 179,996 acre-feet per year (AFY). Produced water in the same region comes to 1,487 AFY; so the produced-water volume is relatively small, less that one percent of total needs. But even at that level, at some point the water's treatment and reuse could become economically feasible -- or even required, if new regulations were developed. Treated waters could be used for livestock watering, irrigation, stream-flow augmentation, or rangeland restoration. Industrial uses are certainly a possibility as well, such as reuse in oil and gas operations, dust suppression, fire protection or cooling towers. If produced water were treated up to sufficient quality, it could even be reused for municipal supplies.
Relatively little produced water can be safely reused without treatment. Produced water contain minerals dissolved through contact with underground formations. The most common contaminant in produced water is sodium, followed by calcium, magnesium, and potassium. Produced water contain bicarbonate, chloride, and sulfate, as well as trace elements such as boron, lithium, fluorine, bromine, and radium. Produced water contains dispersed oil and other organic matter, along with chemical additives.
Because produced water contains such a range and variety of contaminants, proposed treatment systems would have to involve multiple steps with multiple processes, principally organic and particulate removal, desalination
, and disinfection. Constructed wetlands
are now being employed as a tertiary treatment solution by some petroleum companies.
Improvements in Storage Tanks
Concerns over petroleum and its threat to clean water also extend further down the supply chain, where fuels are stored in tanks just prior to end use. GWPC reports that there are 640,000 federally regulated underground storage tanks (USTs) in the U.S., most of which contain petroleum products such as gasoline, diesel fuel, heating oil, kerosene, and jet fuel. On top of that, millions more USTs and above-ground storage tanks (ASTs) fall outside of federal regulations. GWPC says that 465,000 confirmed leaks as of September 2006 had occurred among the regulated petroleum storage tanks.
Petroleum storage tanks in Washington State. Credit: Walter Siegmund, CC BY-SA 3.0[/caption
Heightened awareness and recognition of the problems around storage tanks has reduced the frequency of leaks and have improved the cleanup response to such incidents. GWPC says that in gasoline the "main chemicals of concern are benzene, toluene, ethlybenzene, and xylenes, collectively called BTEX." Among these, benzene is the most hazardous and is a known carcinogen. The group says the nation's upgrade of its storage tanks is having a positive effect and is greatly reducing the water impacts of petroleum products. Most USTs now are "equipped with automatic tank gauges that monitor fuel levels and print out reports and sound alarms when a release is suspected." Steel tanks are now outfitted with "corrosion protection and/or reinforced-plastic jackets."