By Cory Lewis, B.S. civil engineering, and Feiyu Ma, P.E. MS. Ch.E.
With the drilling for oil and gas comes “produced” water, defined by the EPA as “water brought up from the hydrocarbon-bearing formation strata during the extraction of oil and gas, and can include formation water, injection water, and any chemicals added downhole or during the oil/water separation process.”
Thus, the two main produced-water sources are formation water and injected water.
Formation water is found naturally in oilfields. As is well known, an oilfield is formed over long periods of time and is often aided by the flow of water underground. During this process, the least dense hydrocarbons float to the top, forming a gas cap, while salt water sinks to the bottom due to its high density. This leaves the oil in the middle of the formation. When the oilfield is drilled, the formation water comes up with the oil.
Injection water, on the other hand, is part of drilling, employed as an oilfield matures and loses pressure over time. Water is injected to maintain well pressure, an important factor in pumping oil. Injection water can contain chemicals and substances from the drilling fluids.
The barrels of water produced as a byproduct of drilling can be anywhere from one to 50 times the number of barrels of oil produced, depending on the formation’s geographical location and maturity. Since produced water contains salt, oil, grease and chemical additives, it requires treatment before its disposal or discharge.
The need to treat
Efficient and economic produced-water treatment is integral to maintaining an oilfield’s lifespan and a key consideration in decision making. In the past, produced water was treated in evaporation ponds. However, new technologies have been developed in recent decades, including use of modified-fiber enhanced coalescence oil/water separation technology.
Separation is based on using modified fibers to extract hard-to-remove emulsified oils from produced water.
Crude oil is the most difficult constituent of produced water to remove in most cases. It includes four types of hydrocarbons: alkanes, cycloalkanes, aromatic hydrocarbons and asphaltenes. Asphaltenes in particular have a very viscous and sticky consistency proven difficult to remove from water efficiently. Many types of coalescent or filter media equipment are fouled by crude oil or need to be constantly replaced due to their absorbent nature.
The modified fibers extract hard-to-remove emulsified oils and crude oils from produced water because they have been imbued with both oleophobic and oleophilic properties. In other words, the fiber has been chemically altered to repel or attract oils. This is obviously important when considering separation of oil from water via filter cartridges or similar filter elements.
Photo 1 shows a filter cartridge that employs both oleophobic and oleophilic fibers. It also uses a stainless steel casing to ensure that it is not damaged during the backwash phases.
The system operates using two fibers, of opposite affinities, in tandem to remove otherwise hard-to-extract emulsified oils. As small oil particles are attracted to the oleophilic fibers, a film forms and they grow in diameter. Over time, as more droplets accumulate their diameter gets large enough to be repelled by the adjacent oleophobic fiber. The droplets are at that point large enough to float to the surface on their own to be removed.
This method removes oil more quickly than others, and does so very precisely. The concentration of oil in produced water can be reduced from levels as high as 1,000 ppm to below 1 ppm.
The technology has been applied using the oleophobic fiber in the form of modified-fiber balls. Many fiber balls, approximately 40 mm each, are placed within a tank and when compressed can offer precision up to 2-5µm removal of oils and suspended solids. The produced water is fed into the tanks and through the system only after the fiber balls have been compressed by vertical plungers, allowing for precision. Once the head in the system reaches its limit, the system is backwashed.
The advantage is that despite high loading of suspended solids and oil, the media will not become fouled or blocked since the fibers are continuously repelling the oil. In addition, a backwash cycle using these fiber balls is very efficient due to the fibers’ tendency to repel oil and oily suspended solids. The fiber-ball application is excellent for treating wastewater and produced water with high amounts of suspended solids.
In another application, fibers are used together in a filter cartridge to effectively act as a self-cleaning filter, achieved by winding the two fibers adjacent to one another. These filter cartridges are placed inside of pressurized tanks and water is pumped vertically through the filters. The advantage is that the filter cleans itself, requires no backwashing and can last years before needing to be replaced.
This modified fiber technology has been most used on offshore oil platforms.
When treating produced water on an offshore platform the major considerations are space and efficiency. Space on offshore platforms is both incredibly limited and expensive. For one offshore oil platform in the South China Sea, use of modified fibers reduced the space required for treating produced water by 20%. The influent contained 30 ppm of suspended solids and 20-500 ppm of oil for a flow of 1650 GPM. The system was able to deliver treated water with less than 10 ppm of oil and less than 5 ppm of suspended solids.
This allowed the platform to directly discharge the treated water into the ocean based on governing regulations.
In the future, application of these modified fibers can continue to aid in the treatment of produced water as well as in oil water separation. Produced water will continue to be a limiting factor for oil and gas production, but as modified fibers and other technologies are developed, the oil industry can continue to make the most out of the earth’s reserves.
Cory Lewis, with a B.S. degree in civil engineering, is a sales and applications engineer, General Technologies SPC, with a focus on oil-water separation and water-treatment technologies. Feiyu Ma, P.E. MS.Che.E., president, General Technologies SPC, has more than 17 years of experience in the water treatment industry.
General Technologies SPC is a leader in providing high quality products and services, with the latest technologies, to customers and clients in the water treatment, power generation and other industries. Customers are primarily in North America, as well as the Middle East, Latin America, Singapore, Hong Kong, Taiwan, South Korea, Japan, Australia and New Zealand. GTSPC says it not only provides high-quality products, but also stands behind them with strong technical support and full customer services. .