In hydraulic fracturing (fracking) operations at oil and gas wells, water and oil both come to the surface and must be separated. Oil and gas can be filtered from the water fairly easily, but the remaining water — called “produced water” — is contaminated and must be treated before being released into the environment.
Fracking is becoming widely used in the oil and gas industry, and as it grows in popularity, treating produced water coming from wells is becoming a major challenge, especially because the cost of cleaning produced water is about 300 times that of cleaning municipal wastewater.
A similar problem exists at oil refineries, where incoming crude oil contains water that must be removed prior to refining. At refineries, crude may contain anywhere from 3 percent to 5 percent water.
The volume of water produced is dependent on a number of factors, including source of the oil, age of the well, type of separator, and location in the refining process. Volumes ranging from several hundred to millions of gallons a day are possible.
Whether the separation is done at the well site or at the refinery, one of the most critical parts of the process is measuring the level of the interface between oil and water in the separation vessel. A correct measurement ensures the maximum amount of oil is drawn off for refining purposes, and a minimum amount of oil and hydrocarbons are sent to the water treatment process.
|Figure 1. In a separation vessel, oil and water separate, with oil moving to the top and water to the bottom. Capacitance level switches are used to detect the oil/water interface.|
Water separation processes
Oil in production fields contains varying amounts of water depending on the specific oil reservoir, the age of the well, and the methods used in extracting the oil from the ground. Some separation of water from the oil is done in the field to reduce the amount of water that is transported to the refinery.
At a well site, especially one that uses fracking, the separated water must be treated on site or at a nearby water treatment facility before the water can be returned to the environment.
Refineries take in crude oil from production fields with varying levels of Basic Sediment & Water (BS&W) measurements. The higher the BS&W measurement, the more water is in the crude. The refineries separate the water and other impurities through various separation processes, including, but not limited to, gravity separation, desalter vessels, and skim tanks.
In all these processes, water separates from the oil and must be evacuated from the bottom of the vessel, while the hydrocarbons typically are removed on a continuous basis by flowing over a baffle at one end of the separation vessel (Figure 1).
Essentially, all the various separation processes involve a tank where oil and water coexist, with oil on top and water on the bottom. As the separation process continues, the water must be removed.
The process of removing water from the separation vessel is referred to as a “Water Dump.” The water is “dumped” to a treatment plant for processing, while the oil is removed from the top for further refining. Because of the high cost of treating the water, it is critical to know when the interface between the water and oil reaches a low point to prevent dumping oil to the water treatment facility.
The main purpose of water dump control is to prevent excess hydrocarbons from being sent to the water treatment facility. There are two reasons for this: First, in some cases there are fines for dumping hydrocarbons to the water treatment plant; second, the idea is to maximize the hydrocarbon throughput to the refining process, not dump it down the sewer.
A number of methods can be used to measure the interface between water and oil. These include everything from manual sampling to sophisticated continuous level measurement instruments such as guided wave radar and gamma radiation transmitters. But for water dump control, an on/off capacitance level switch inserted near the water outlet is almost always the preferred alternative, for reasons delineated later in this article.