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Unconventional oil and gas (UOG) extraction is a multistep process that involves horizontal drilling, hydraulic fracturing, and massive infrastructure to handle fossil fuel resource recovery and associated wastewater generation.
Unconventional oil and gas (UOG) extraction is a multistep process that involves horizontal drilling (1), hydraulic fracturing, and massive infrastructure to handle fossil fuel resource recovery and associated wastewater generation. Such processes are required to extract oil and gas from low-permeability shale formations. Because of the success of this industry, the United States has become the leading producer of petroleum in the world. However, because UOG is a massive industrial process, there are significant concerns for inefficiencies that could lead to deleterious environmental impacts (2–6). Opportunities exist to increase the efficiency of UOG, especially in terms of wastewater handling and treatment; analytical chemistry is needed to guide choice and optimization of available technologies for wastewater treatment.
Hydraulic fracturing involves pumping large volumes of fluids thousands of feet underground and applying pressure to induce the formation of fissures in the shale and release sequestered hydrocarbon resources (1). When the hydrocarbon resources return to the surface, they are initially accompanied by significant volumes of the hydraulic fracturing fluids (termed flowback water) (7). Within about a week, the flowback water is replaced by waters predominantly originating from the shale itself (termed produced water) and this wastewater will continue to be produced as long as the UOG well is operational. In the Eagle Ford Shale in south Texas, approximately 1 barrel (bbl) of produced water is obtained for every barrel of oil recovered. In the Permian Basin in west Texas, it is not uncommon for 6 bbl of wastewater to be produced for every barrel of oil (8).
Currently, over 3.5 million bbl/day of oil is produced from the Permian Basin, and this value has been increasing by about 50,000 bbl/day each month for the past couple of years. As such, the volumes of wastewater that must be handled are astronomical. Currently, 98% of this wastewater is reinjected underground into disposal wells. This practice has led to induced seismicity and earthquakes in regions not historically known for such activity (9,10). As a consequence, regulations have been tightened regarding the volumes of waste that can be disposed in given locations, and a reduced number of disposal well permits are being issued.
Reduced availability of disposal wells has both positive and negative consequences. Wastewater reinjected underground is essentially lost forever, so a reduction in this process potentially keeps more water in the water cycle. However, something has to be done with the large volumes of wastewater produced. Increasingly, small and large companies are entering the oilfield wastewater treatment business with a variety of treatment technologies. The goal is to treat the wastewater to a sufficient extent so that it can be reused in subsequent hydraulic fracturing operations or recycled for other purposes, such as agricultural irrigation or surface water discharge.
An underlying challenge in the treatment of oilfield wastewater is its inherently complex nature. It has been referred to as possibly one of the most complex aqueous sample matrices to analyze (11). Total dissolved solids (essentially, salts and minerals) can range from a few thousand up to 400,000 mg/L. The salinity of Permian Basin produced water ranges from 8 to 12%, or 2.5–3 times the salinity of seawater (12). Mixtures are laden with a wide variety of metals and ions, organics, and bacteria (13–16). In some areas, the levels of naturally occurring radioactive matter (NORM) in produced water is a significant concern.
Only recently has some guidance been given on the level of water quality needed for the reuse of wastewater in hydraulic fracturing. It is imperative that levels of certain species be below various thresholds to limit damage to infrastructure (such as scaling), interferences with hydraulic stimulation chemistries (such as crosslinker efficiency), and contamination of extracted resources (such as oil souring by microbes) (17). Different operators can have different thresholds, which is not surprising given that chemistry needs can vary with different geological formations; additionally, operators may work with different third-party suppliers of hydraulic fracturing chemicals, so their susceptibility to certain components from wastewater may vary. Further guidance for water quality in the application of treated waters for agriculture or surface discharge are now being considered by the US Environmental Protection Agency.
Every aspect of oilfield waste treatment supports a strong need for analytical chemistry to help determine viability of different treatment technologies. Yet, decisions are not made solely based on water quality. An ideal technology has to achieve the desired performance for its purpose, have sufficient throughput to contribute meaningfully to the reduction or redirection of waste streams, and be cheaper than reinjection of the waste into a disposal well. Economics and throughput can be determined in a fairly straightforward fashion. Determining the performance of treatment technologies requires analytical chemistry methodology, to assess the complexity of the initial waste to be treated, as well as the quality of the treated product.
Our group has had the opportunity to work with a number of companies in this capacity. We have evaluated a multimodal treatment technology that combines a variety of traditional treatment steps in an effective arrangement to output clean brine mixtures (13). We have worked with a major membrane technology provider to evaluate the potential for forward osmosis treatment of oilfield wastwater (12). Most recently, we are evaluating combined use of different adsorption media and cavitation. Each of these technologies has its advantages and limitations; realistically, the opportunities for water treatment in the UOG space are so large, there is significant potential for each of these approaches, among others. With the analyses we provide to determine metals, organics, anions, biologicals, and general water quality measures, we can provide accurate determinations of treatment performance and rational recommendations for technological improvement where necessary to address various challenges.
Wastewater treatment in the context of UOG operations is still in its infancy. Unfortunately, because there are so many widgets out there to treat water, UOG operators are wary of adopting technologies that cannot work at scale, cannot achieve the performance purported, are not robust and reliable, or are too costly to implement. Analytical measurements are essential to making informed decisions about technology adoption. The variability and complexity of produced water provides no shortage of challenges for new method development. And, for the United States to maintain its geopolitical position in worldwide petroleum production, more water will be needed to develop hydrocarbon resources. Not only does it make sense to reuse treated wastewater, especially if it is economically viable to do so, but there may also be significant opportunities for reclamation of high value chemicals and metals from these waste streams. Analytical measurements are needed to direct that enterprise, as well.
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Kevin A. Schug is a Full Professor and Shimadzu Distinguished Professor of Analytical Chemistry in the Department of Chemistry & Biochemistry at The University of Texas (UT) at Arlington. He joined the faculty at UT Arlington in 2005 after completing a Ph.D. in Chemistry at Virginia Tech under the direction of Prof. Harold M. McNair and a post-doctoral fellowship at the University of Vienna under Prof. Wolfgang Lindner. Research in the Schug group spans fundamental and applied areas of separation science and mass spectrometry. Schug was named the LCGC Emerging Leader in Chromatography in 2009 and the 2012 American Chemical Society Division of Analytical Chemistry Young Investigator in Separation Science. He is a fellow of both the U.T. Arlington and U.T. System-Wide Academies of Distinguished Teachers.