(454c) Performance Analysis of Texas Eagle Ford Shale Oil Hydro Fracturing Produced Water Treatment Process
Chris Skrivanos, Nico Dwarica, Cameron McKay, John Allen Floyd and Mahbub Uddin
Trinity University, San Antonio, Texas
With the rise of horizontal drilling, hydraulic fracturing has become increasingly common and it is estimated that 90 percent of new wells drilled in the United States are hydraulically fractured. Hydraulic fracturing involves the injection of a very large quantity of high pressure liquid to fracture the rock formation and pry open the fracture. This fracturing allows natural gas/oil to flow freely from the formation into the well for collection. The components used in the hydraulic fracturing process consist primarily of water, sand, salts, gel, diluted acid, surfactant corrosion inhibitor, scale inhibitor, and other additives to aid in the hydro fracturing process. During the hydro fracturing operation when the pressure used to inject the fracturing fluid into the well is released, some of the fluid returns to the surface; this fluid is called “flowback” water. Once gas production begins at the well, all wastewater emerging from the well is called “produced water”. Both types of wastewater – flowback and production phase water – contain potentially harmful constituents. These constituents can be broadly grouped into several principle categories: salts, organic hydrocarbons, metals, chemical additives and some naturally occurring radioactive material. Because of these constituents, shale oil/gas wastewater should not be released into the environment without adequate treatment. The purpose of this paper is to report the performance analysis of a water treatment process of Texas Eagle Ford Shale oil hydro fracturing produced. The treatment process consists of four primary unit operations: Adsorption column, Reverse osmosis, Ion-Exchange and Pump. The produced water is first pumped through the adsorption column, then a second high-pressure pump which then forces the water through the reverse osmosis membrane. The permeate of the membrane then flows through two ion exchange columns and received in a collection bin.
Adsorption is the adhesion of atoms, ions, or molecules from a fluid or dissolved solid to an insoluble surface and is used to remove suspended organics like oil and grease as well as dissolved organics like benzene and toluene. Our adsorption column consists of a cartridge and housing unit rated for operating pressures of 90 psi and temperatures no higher than 125 °F. The cartridge was 10” tall and accepted 10” x 4.5” cartridges. Reverse osmosis filters work on the principle of membrane divergence. An extremely high pressure (800 psi in our process) is applied at the housing entrance that forces the fluid through the fine membrane. Due to the pressure spike, particles that are small enough pass through the fine holes in the filter transfer, while the larger particles are rejected and thus removed from the membrane. In our application, the reverse osmosis membrane rejected the ions and other larger particles like metals, minerals, and organics, allowing only water and smaller particles to remain. The membrane we used is a FilmTec Sea Water Membrane designed to handle increased salinity (greater than 10,000 ppm). The reverse osmosis unit operation used had a flow ratio of permeate to retetante of 73 to 27, which was determined by a series of mass balances across the unit operation. We used a mixed ion exchange in order to remove any remaining positive or negative ions. Our process consists of two ion exchange columns in parallel, both of which were 20” vertical columns housed in separate units. The process uses two pumps. The first pump transferred the water from the tank to the adsorption column at a low pressure. The second pump primed the system to the operating pressure, 800 psi, to transfer the produced water through the reverse osmosis membrane and the ion exchange.
Each water sample analyzed was obtained from one of 4 major positions. Position 1: the untreated water, Position 2: after the activated carbon operation, Position 3: after the reverse osmosis operation and Position 4: after the ion exchange operation. Produced water processed by the system once is defined as first iteration treated water. Produced water processed by the system twice is defined as second iteration treated water. The first iteration sample collection began after flowing untreated produced water through the system for 60 seconds. The second iteration was run identically to the first iteration except produced water that had previously been treated by the system was used instead of untreated produced water. Once collected, samples were labeled according to the iteration, position and the trial number of extraction.
In order to assess the effectiveness of each unit operation and iteration in the wastewater treatment system, a series of experiments were designed to monitor the flow ratio, total dissolved solids present in solution, pH, and change in organic content. Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) was implemented as a means to measure the metallic and salt content present in the water at various purification steps. The initial metallic content was determined to be 184.45 ppm, and the initial salt concentration was determined to be 2262.26 ppm for the produced water. A simple pH test using a probe was conducted on samples to monitor the change in acidity of the water as it passed through each unit operation, and the initial pH of the produced water was determined to be 4.25. Gas chromatography measurements were implemented in order to monitor the changing levels of total organic compounds present in the water.
Analysis of Results
First iteration ICP-OES results showed that the initial salt content of the water was 2262.26 ppm and the heavy metal content was 184.45 ppm. The carbon adsorption column removed 99.01 percent of the initial salt content and 98.5 percent of the initial heavy metallic content. The reverse osmosis membrane removed an additional 0.14 percent and 0.16 percent of the initial salt and heavy metal content respectively. After the ion exchange column approximately 100 percent of the total salt content was removed and 99.84 percent of the total heavy metals were removed, leaving a heavy metal concentration of 0.29 ppm and the salt concentration of essentially 0 ppm. Analysis of the second iteration treatment indicated the carbon adsorption column added 0.37 ppm of heavy metal and 2.09 ppm of salt to the water. The reverse osmosis membrane removed 0.29 ppm of heavy metals and added 0.85 ppm of salt content. The ion exchange column removed 0.04 ppm of heavy metals and 1.55 ppm of salts, bringing the final concentration of the second iteration treated water to 0.32 ppm of heavy metals and 1.39 ppm of salts. The first iteration pH results showed that the produced water had an initial pH of 4.25. The carbon adsorption column increased the pH to 8.26, the reverse osmosis membrane lowered the pH to 7.24, and the ion exchange column lowered the pH to the final value of 5.31. Second iteration treated water pH results showed that the carbon adsorption column increased the pH from 5.31 to 8.27. The reverse osmosis membrane decreased the pH to 7.24. The ion exchange column brought the pH to a final value of 6.49. The produced water contained a plethora of unknown organic constituents and it would be unpractical to identify each individual constituent to generate a calibration curve. So instead of measuring the changing concentrations of each constituent, the GC measurements were used to observe the change in the overall organic content in the water, relative to position 1 of the first iteration which is representative of the initial organic content of the produced water. The first iteration GC results showed that the carbon adsorption column removed 48.4 percent of the initial organic content, the reverse osmosis membrane removed an additional 28.7 percent of the initial organic content, and the ion exchange column removed 6.4 percent of the total organic content; the system overall removed 83.5 percent of the initial organic content.
Conclusions and Recommendations
The produced water treated in our process in a single iteration met the EPA standards for acceptable levels of metallic and salt content for reclaimed water, however the pH is at the lower end of the standard. The first iteration treatment reduced the metallic content by 99.84 percent, reduced the salt content by essentially 100 percent, raised the pH from 4.25 to 5.31 and reduced organic content by 83.5 percent. With a second iteration, the treated water pH is within the acceptable range for reclaimed water. The second iteration was shown to be unnecessary since the metallic and salt concentration changed negligibly, and the organic content increased. The pH was raised to acceptable levels, but the addition of a controlled injection pH adjustment unit operation would allow the pH to be raised without the need for a second iteration, making the produced water treatment process more efficient. This change to the process would generate treated water that meets the EPA drinking water standard. Our analysis indicates that the wastewater treatment processes has applications in commercial and residential areas where hydraulic fracturing is common, to treat wastewater that meets EPA standards.