Pump jack. Credit: Eric Kounce.
In studying the water and wastewater management practices of industries and manufacturers over the past few months, we have focused mostly on how water and wastewater get dealt with inside the fenceline -- at plants and factories. That's certainly an issue in the oil and gas industry, but what we're finding is that water concerns arise in the petroleum business throughout its products' lifecycles in unique and challenging ways.
Wastewater Treatment at the Refinery
As a concentrated industrial facility, a petroleum refinery will have its inside-the-fenceline wastewater treatment needs. As the environmental manager at one manufacturer told me, "in industrial wastewater treatment systems, each waste stream is different, depending on the actual chemicals used at the facility." Most industrial facilities will base their approaches on the standard wastewater treatment model
that employs primary treatment to remove solids, secondary treatment to remove organic matter, and tertiary treatment to provide final polishing before discharging effluent into the receiving environment.
According to IPIECA
Oil refinery in Washington State. Credit: Walter Siegmund, CC BY-SA 3.0
(originally the International Petroleum Industry Environmental Conservation Association), "Petroleum refineries are complex systems of multiple operations that depend on the type of crude refined and the desired products." For that reason, processes involving water, such as reuse, recycling, and treatment, can vary considerably from one facility to another. Refinery wastewater is particularly affected by its contact with hydrocarbons in the process units. For example, wash water is used in the removal of salts from crude oil, resulting in brine that has to be treated. Oil is often removed during the primary treatment stage employing gravity separation, but it can also be removed in a pretreatment process prior to discharge into the wastewater system.
The Problem of Produced Water
While significant, the treatment of wastewater at the refinery stage could be characterized as a relatively minor issue.
Oil and gas extraction results in "produced water," as oil and gas reservoirs frequently contain layers of water, which gets pumped to the surface along with the crude or gas. According to Argonne National Laboratory
, produced water is "by far the largest volume byproduct or waste stream associated with oil and gas production." In fact, the research firm Global Water Intelligence has gone so far as to characterize the oil industry
as "effectively a water industry which delivers oil as a byproduct."
This water presents a treatment challenge, as the many contaminants contained in produced water "can present a threat to aquatic life when they are discharged or to crops when the water is used for irrigation," says Argonne. U.S. oil wells reportedly produce an average of seven barrels (bbl) of water for each barrel of oil. During its lifetime, an oil well will produce an increasing ratio of water, growing as high as 98 percent near the end of the well's life.
Water is used to assist in oil extraction through the practice of water injection, which involves pumping water into an oil reservoir to increase pressure and bring more crude to the surface, improving production. The injection wells, or enhanced recovery wells, sometimes employ produced water, which helps somewhat with the wastewater treatment challenge of produced water. However, produced water still requires treatment with filters before it can be reused, as it typically contains hydrocarbons and solids that can damage equipment, and oxygen that can result in corrosion and bacterial growth.
Injection Wells Critical to Wastewater Disposal
Injection well head. Credit: Joshua Doubek, CC BY-SA 3.0
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.
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
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."