Treatment methodologies must meet a variety of requirements in order to be truly effective in practice. First, the process must minimize waste volume. Second, the process must meet stringent discharge requirements by removing almost all, if not all, contaminants. Third, the process must reduce labor and supervision requirements. Fourth, the process must be flexible enough to handle fluids with a high variation in water quality and quantity. Finally, the best processes are modular with a small footprint to maximize effectiveness while minimizing environmental disturbance.
One of our experts highlights some of the technologies behind water treatment in the oil and gas industry.
Water Treatment in Hydrocarbon Production – David Yoxtheimer – Penn State
Hydrocarbon production typically generates between 5 and 15 barrels of brine for every barrel of oil or million cubic feet of natural gas produced. These brines, commonly known as produced fluids, consist of saltwater, along with dissolved chemicals, metals and radionuclides. Also, they flow with hydrocarbon throughout production. Therefore, they require environmentally sound and cost effective management strategies. While generating 5 to 15 barrels of produced fluids for one barrel of oil sounds inefficient, the fluids can be reused, so in practice, the waste water to hydrocarbon ratio is lower. The produced fluids can be recycled to fracture other wells. Some companies recycle all fluids and others may dispose of them.
In deciding what to do with produced fluids, operators must strike a balance between economics, regulations and the environment. To add to the water management challenge, energy companies also must consider what to do with flow-back from fracturing operations. These fluids are pumped underground to fracture formations and then return to the surface over the first days and weeks of production.
There are four common strategies for non hydrocarbon fluid management. In direct reuse, without treatment, operators blend fresh water with flow-back and reuse it onsite. In this case, the waste water doesn’t have to be transported or treated, so the process is simple and the costs are low. In onsite treatment and reuse, operators recondition flow-back, then reuse it in subsequent fracturing operations. This is more efficient in terms of water usage, but it requires onsite equipment to treat the fluids. In offsite treatment and reuse, the waste water is transported to a facility where it is treated so it can be reused. This makes sense for smaller operators that use lower volumes of water, or when treatment facilities are nearby. Finally, in offsite treatment and disposal, waste water is transported offsite for injection, or advanced treatment and discharge. If it is injected for permanent disposal, that water is essentially removed from the water cycle.
In practice, operators may employ more than one of these strategies. That’s because as fluids are reused, the concentration of dissolved solids and ions increases. That means it becomes more complex and expensive to condition the fluids for reuse purposes. Further, if the water is slated for return to the environment as effluent from a treatment facility, the water must meet stringent discharge requirements. A variety of water treatment technologies may be utilized by an operator, since fluid quality and use varies. For example, filter socks are a relatively simple technology and are exactly what they sound like. A sock-like device that filters out solids. Because these solids may be hazardous, some states have recently imposed strict regulations to ensure used filter socks are disposed of properly.
Chemical precipitation units use a chemical process to change a fluid’s composition so the contaminants can be separated from the liquid. The precipitated sludge can then be disposed of properly. Electrocoagulation uses electrical current to cause suspended solids to coagulate so they can be removed. Evaporation facilities distill the fluids and leave behind solids to be processed or disposed of. Filtration modules can use a variety of media to filter out suspended and dissolved constituents of interest. Other technologies include reverse osmosis, nano-filtration, or forward osmosis desalination modules. Several of these methods may be used in series. For example, metals can quickly form scale and evaporation units. If used in combination with chemical precipitation as a pre-treatment, the metals can be removed and the fluids can then be distilled more efficiently.
Treatment costs in 2016 range from about $3 to $10 per barrel, depending on methods used, which includes sludge disposal. The sludge produced by waste water treatment must also be handled carefully, as it is hyper-concentrated with dissolved solids from the treatment process. These solids must be disposed of in accordance with regulations. This often results in disposal of sludges in landfills in an environmentally safe manner. As technology progresses, it is likely that waste water treatment will become more effective and cost efficient, but it will always be a critical component of hydrocarbon production.
The technologies deployed in recycling include both simple and complex solutions. The list includes but is not limited to filter socks, chemical precipitation, electrocoagulation, evaporation, oxidation, filtration and desalination. Depending on how many of these technologies are combined in series, the costs range from $2 to more than $10 per barrel. Filter socks remove solid precipitants from wastewater and most of the naturally occurring radioactive material. Chemical precipitation units remove 99% of all cations cost effectively. Electrocoagulation units also remove cations but usually with between 70% and 90% efficiency. These units also remove oil and grease emulsions along with scalants. Electrocoagulation requires less chemical additives and generates less sludge. Scalant technologies including chemical precipitation and electrocoagulation cost between $3 and $6 per barrel.
Several desalination treatment technologies exist including thermal distillation and membrane systems. Thermal units have the ability to treat water with high levels of TDS with consistently high output. These units require more energy and are also more expensive. Membrane systems such as reverse osmosis and nanofiltration are much cheaper by comparison but require higher maintenance and more experienced operators. The capacity for membrane systems maxes out at around 40,000 TDS, which can be limiting in many shale plays. Desalination costs between $6 and $10 per barrel.
To maximize effectiveness, many technologies are connected in series. For example, chemical precipitation methods can be combined with thermal desalination technologies for removal of scalants prior to desalination, which would otherwise foul the desalination units. Therefore, the combination of technologies allows these processes to work more efficiently.
Images: “Opole city sewage treatment plant” by Mariusz Szczygiel via Shutterstock; “Graphic” by Top Energy Training