Treating flowback with dissolved oxygen
The sun was rising in the morning sky as engineering consultant Richard Owens made his way out of Austin, Texas. His destination was Gonzales, Texas, which lies along the edge of the hotbed of hydrofracturing activity known as the Eagle Ford Shale.
He had been given clearance by Todd Whiting, vice president of operations for Purestream Technology, to inspect the installation of four FracCure aerators, which were mounted atop 500-barrel frac tanks as part of Purestream’s water treatment system for its Gonzales client.
Driven partly by environmental concerns and partly by economics, a growing number of frac operators are embracing the benefits of dissolved oxygen in treating flowback, which is a water-based solution that flows back to the surface during and after fracking. To date, most aerators have been placed in frac pits; however, the use of aerators in mobile frac tanks to treat flowback is gaining popularity. Mostly, the dissolved oxygen is used to pre-treat the produced or flowback before it enters more sophisticated onsite treatment systems. The goal is to use dissolved oxygen to increase the efficiency of these high-tech systems while lowering operating costs.
In Gonzales, Purestream is treating the flowback using its induced gas flotation (IGF)system. The IGF removes oil, grease and suspended solids from the flowback. Because chlorides also need to be removed, Purestream is employing its Advanced Vapor Recompression technology, referred to as AVARA. The technology removes chlorides and other dissolved solids through a thermal-based vapor recovery process.
Purestream, along with a number of service providers, looks at dissolved oxygen as a low-cost, eco-friendly means of normalizing flowback. Basically, dissolved oxygen acts as an oxidizing agent that works through two pathways to remove or diminish flowback contaminants. First, dissolved oxygen forms oxides with metals such as iron or manganese. It also oxidizes the sulfides into insoluble sulfates which, along with the newly formed metal oxides, drop out of suspension. A side benefit is the removal of carbon dioxide and volatile gases.
Purestream has found that aeration has a positive effect on solids separation as well as on the control of total suspended solids. Notably, aeration reduces the need for and cost of harmful oxidation chemicals such as bleach, peroxide and paracetic acid.
The second pathway by which dissolved oxygen reduces contaminants involves the ramp-up in the population of aerobic bacteria. Frac operators know that an anaerobic environment in the hydrocarbon formation is conducive to the growth of sulfate-reducing bacteria, leading to the production of hydrogen sulfide gas. If organics are not removed from flowback, they will decompose. In the process, they will use up oxygen and create byproducts such as ammonia, nitrogen and soluble phosphates. Aeration takes away the organic food from the anaerobes and prevents their growth and reproduction. Aeration provides a continuous, low-cost, non-chemical means for reducing high-hydrogen sulfid gas and other water problems.
At its Gonzales site, Purestream receives raw water in a 500-barrel tank where initial chemical treatment takes place. From there the water is fed into the IGF unit, where floatable solids, iron sulfide and total suspended solids are reduced through flotation (bubbles) and polymers (floc). The effluent then proceeds to the four 500-barrel frac tanks where the FracCure aerators are installed. Next, the oxygenated water is pulled into the AVARA unit for further treatment. Two effluent streams flow out of the AVARA: One is distilled water (TDS less than 100 ppm) and the other is concentrated brine (TDS greater than 150,000 ppm).
One of the concerns with the use of recycled water is that if it is not properly treated, it can interfere with the next round of treatment chemicals. When aeration is used in a frac pit, there usually is sufficient contact time between the dissolved oxygen and the aerobes. As the aerobes increase, they consume some of the residues of gels, friction reducers and hydrocarbons in the flowback. These contaminants are known to hold solids in suspension and can form emulsions. Aerobic remediation actually can be jump-started through the use of microbial blends, which are added to the water. In contrast, when water is pulled from the pit for treatment, it must be routed quickly through the onsite treatment system. This limits the contact time between the dissolved oxygen aerobes and oxidizable contaminants. Thus, Purestream used four frac tanks in series, each with a FracCure aerator, to overcome the reduced contact time.
One of the first visible signs that dissolved oxygen is working is the almost immediate clarification of the flowback. Numerous samples pulled from Purestream’s aerated frac tanks showed water that was very clear. Field tests conducted by FracCure showed that black flowback in a frac tank can be turned almost clear in seven hours or less via aeration.
Many state regulators no longer allow produced water to be piped in and around the frac field. Recognizing this, Purestream uses a range of treatment technologies to bring the water to a level acceptable to both regulators and producers. To minimize the need to pipe contaminated water, Purestream places its treatment systems as close to the frac pit as possible.
One of the goals of frac operators is to reduce the amount of fresh water used in fracking. The industry is realizing that in some cases the recycled water does not need to be as free of contaminants as previously thought. Yet, some clarification of the water almost always is needed. In this case, a combination of dissolved oxygen and a treatment system such as Purestream’s IGF could provide a solution.
As Owens drove away from Purestream’s Gonzales operation, he was left with a distinct, positive impression. The industry has embraced the need to be responsible corporate players in the world of hydrofracturing. From protective eye glasses to plastic liners everywhere, it is apparent that the industry is embracing the need for safety and environmental awareness. Coupled with breakthroughs in technology, these attributes make the future look bright for this industry.