An engineer controls a tank at a wastewater treatment plant. By Avatar_023/Shutterstock.com
The rising production of natural gas from hydraulically fractured wells in Appalachia generates along with it contaminated produced water that must be carefully disposed of. Researchers at Pennsylvania State University say that producers would be wise to consider the environmental risks associated with the most commonly used disposal practice of underground injection, and instead adopt more environmentally friendly and sustainable innovations in water filtration.
The study, Sustainability in Marcellus Shale Development, published by Penn State’s College of Engineering in conjunction with Chevron, notes that produced and flowback water from the prolific Marcellus Shale in Pennsylvania is most commonly disposed of through injection into saltwater injection wells drilled far below the deepest known aquifer.
But although this method is the cheapest available and most frequently used, it brings with it the potential for surface spills and casing leaks that can contaminate freshwater, as well as the risk of activating dormant faults and causing earthquakes.
Disposing Fracked Water
“During the hydraulic fracturing process, water and chemicals are used to stimulate the fissures in the rock in order to extract the natural gas. Water is mixed with sand and other chemicals and then injected into the well. After creating cracks in the Marcellus Shale, flowback water, a brine solution with heavy metals and chemicals, quickly comes back. Typically, this flowback water is stored in tanks or pits before treatment, recycling, or disposal,” according to the report, co-written by Kyle Bambu, Mike Spero, and Harry Polychronopoulos.
The most common way to dispose of this produced water is by pumping it into saltwater disposal wells that are drilled hundreds below the deepest known aquifers. But Pennsylvania’s unique geology is not well suited for such wells. At the time the study was published in Fall 2016, there were 144,000 Class II injection wells in the US and only eight of them were Class II salt water disposal wells in Pennsylvania. These eight wells combined accepted 8,667 barrels per day of brine, while similar wells operated in Texas can each dispose of more than 26,000 b/d of brine.
According to the report, the average cost to dispose of one bbl of fluid can range from as low as 25¢/bbl if the oil company operates its own disposal well, to anywhere from 50¢/bbl to $2.50/bbl if a commercial saltwater disposal well is used. The cost of using disposal is further increased by the cost of transportation.
“In northern Pennsylvania, where commercial disposal wells aren’t plentiful, the brine water may have to be transported to Ohio or West Virginia. This can increase costs by $4.00 to $6.00 a barrel, bringing the net cost of disposal in the Marcellus Shale region to $4.50/bbl to $8.50/bbl,” the study said.
The use of underground disposal wells is not without risk, and frequent concerns include the potential for groundwater contamination and induced seismic activity. In Youngstown, Ohio, the researchers noted that a Class II disposal well for fracking wastewater was linked to seismic activity after it activated a previously unknown fault line. That well was blamed for 10 minor earthquakes, the largest of which is a magnitude of 3.9. A spate of earthquakes in Oklahoma in recent years has likewise been linked to the increased injection of water into disposal wells.
The need to dispose of produced water in Pennsylvania has become more pressing in recent years as natural gas production from the prolific Marcellus and neighboring Utica shales has taken off. Data from the federal Energy Information (EIA) Administration show that output from the shale formations more than tripled Appalachian gas production from 7.8 billion cubic feet per day in 2012 to 23.8 Bcf/d in 2017 (EIA). These plays are credited for driving growth in US natural gas production since 2012 and have played a critical role in enabling low domestic prices and increasing exports.
The Water Filtration Alternative
Researchers note that a number of alternatives to disposal wells are emerging at varying levels of cost. These largely involve treating the produced water to remove its various contaminants, which can include radioactive substances, heavy metals, and high concentrations of salt. Traditional wastewater treatment plants cannot be used because they lack the sufficient processes needed to clean this water.
The most cost competitive alternative to underground injection highlighted by researchers is the option of using a membrane to clean the brine produced water. The company Oasys Water offers a system that drives the brine solution through a series of semi-permeable membranes at a cost of nearly $2/bbl of water. The water that emerges from this process is clean enough to be discharged into streams or drainage systems.
Other potential treatments on the horizon that require further research include the option of boiling the water. However, researchers note that the cost of using this process can run upwards of $17/bbl and the heavy salt causes extreme wear and tear to the requisite industrial boilers, resulting in massive equipment replacement costs.
Lastly, the study says the process of electrodialysis could be used to separate water from contaminants. Researchers at the Massachusetts Institute of Technology have found that an electrical current can be used to separate fresh water from a salty solution. Salt is an effective conductor of electricity and successive stages of electrodialysis can remove most contaminates. But this process has not been tested in the oil and gas industry and there are not commercial treatment options available.
Researchers ultimately concluded that while the common practice of injecting produced water into disposal wells is relatively cheap, this practice comes with high environmental risks. These risks include the potential for groundwater contamination that is caused by surface spills or breaks in the tubing for saltwater disposal wells and even induced seismic activity.
At present, the impetus for improving produced water disposal practices is driven primarily by the sustainability practices of each producer and not government regulations. Researchers found that the oil and gas industry is exempt from some of the most stringent federal environmental regulations, like the Safe Drinking Water Act the Clean Water Act, but noted that states have been working to impose their own rules to address areas of concern. For instance, Pennsylvania in recent years adopted new guidelines intended to prevent spills and releases of harmful substances.
Today’s Best Option
The study ultimately recommends Oasys Water’s membrane filtration as the best option for disposing of produced water today. Researchers said that while using this method can result in slightly higher costs for water treatment and transportation, it appears to be the most sustainable solution until other technological advances are advanced in the future.
“This (membrane) system was recommended because of its relatively cheap cost yet adherence to sustainability and environmentally friendly concerns,” the study said.
To read a PDF of the Penn State study, click here.