Hydraulic fracturing, or fracking, has been a bonanza for oil and natural gas companies. So far fracking has yielded more than 7 billion barrels of oil and 600 trillion cu ft of natural gas in the U.S. alone, according to the American Petroleum Institute. Natural gas in U.S. shale deposits in particular is expected to remain abundant for more than 100 years, and by 2040 fracking is projected to account for half of U.S. natural gas production, compared with about 35% today.
Yet a lot of effort is still needed to get the oil and gas out of the ground. Fracking consumes vast amounts of freshwater, generates more waste than the raw materials it consumes, contributes to Earth’s atmospheric greenhouse gas burden, and in some cases compromises drinking water sources near drilling sites. For many, fracking has become a four-letter word.
Among the unmet needs that could mitigate some of fracking’s environmental downsides and turn that public perception around are safe and sustainable technologies that are more protective of water resources. To that end, scientists and engineers have been pioneering more efficient fracking fluid formulations and new types of tracers to help improve fracking performance and environmental monitoring of fracking chemicals.
“The fracking industry has gone through about eight years of what we might call brute-force fracking—just pump as much fluid as fast as you can, and that would get the gas production results you wanted,” says James Hill, chief executive officer of GasFrac, an energy services company with headquarters in Calgary, Alberta, that is at the forefront of new fracking technology. “But now we are beginning to see a change and are moving toward being more scientific and optimizing how we are doing fracs.”
In the fracking process, a viscous fluid is pumped down a predrilled well hole into shale deposits where oil and gas are trapped. Shale is highly porous, but not permeable, so high pressure generated by the pumped fluid is used to fracture the rock, like cracking a car windshield. The fractures give access to little pockets of hydrocarbons that have been isolated for millions of years.
Fracking fluid is typically about 90% water, about 10% proppant (sand or a granular ceramic material), and less than 1% of an assortment of chemical additives customized for each well to optimize delivery of the proppant to the fracturing zone. Once a fracture is made and the pumping stops, some of the fluid flows back out of the well, and along with it a little extra mineral-laden water bearing toxic heavy metals and naturally occurring radioactive elements. The proppant remains behind, filling the hairline fractures to prop them open and allow the oil and natural gas to seep out, which may take place for 20 to 40 years.
The wells, which routinely stretch more than a mile deep and a mile laterally, are extended by about 200 yards or so between each round of fracking. Each well might be fracked 20 times or more, with each round taking on average several million gallons of water and 300,000 lb of proppant.
Fracking typically uses freshwater, and as a first step industry researchers have been working to perfect fluid-thickening and friction-reducing additives that allow operators to use less water. These chemical agents further provide flexibility in formulating fracking fluid so that recycled fracking water or brine pumped from saline aquifers underground, which contain undesirable dissolved solids, can now be used in place of freshwater.
But even better than reducing water use would be to exclude water from the fracking fluid altogether. That’s where Hill and his team at GasFrac come in. Instead of water, GasFrac uses a gel made from propane, which is a component of natural gas.
Water is used for fracking mainly because it is incompressible—that is, its volume remains nearly the same even when under pressure. It’s therefore very effective in building pressure against and ultimately breaking up rock.
The propane is in liquid form, Hill explains, so it works nearly as well. To create the fracking fluid, GasFrac adds less than 1% concentration of ferric sulfate as an activator to promote gelling and later a similar amount of magnesium oxide as a breaker to disrupt gelling. When ferric sulfate is added, the liquid gels in just a few seconds. As that is happening, the proppant is added.
“We are delivering proppant by viscosity, rather than by velocity, which is the case with a water-based system,” Hill says. “The gel retains proppant better than water and evenly disperses it throughout the well, so it’s possible to get the same results with about 10% as much fracking fluid and to pump it at a slower rate.”
The surface tension of propane is about 10% that of water, Hill adds, enabling nearly all of the fracking fluid to completely flow back out of the well and be recovered. “Because propane is a hydrocarbon, it simply becomes part of the oil or gas production stream, whereas water tends to get stuck in the formation,” he says. GasFrac’s process is generally waste-free, but any natural groundwater that comes out of the well hole must be captured and treated.
As a bonus, the GasFrac process requires fewer chemical additives than water-based fracking. For example, without water the mix doesn’t need a biocide, such as glutaraldehyde, CH2(CH2CHO)2, to prevent growth of bacteria and mold that can clog the well. The biocide is typically the most environmentally problematic component of the fracking fluid additives.
To date, GasFrac’s technology has been employed in some 2,600 fracking operations at 720 wells in the U.S. and Canada using more than 500 million gal of propane and 100 million lb of proppant. Even so, GasFrac’s technology accounts for less than 1% of the North American fracturing market. “We are still a young technology,” Hill says.
Beyond water use issues, the industry’s biggest challenge is satisfying the public’s right to know what fracking is doing to the environment. Residents in some areas where fracking is taking place have encountered methane in their water and worry about what else might be in it.
Because fractures remain separated from freshwater aquifers by several thousand feet of solid rock, fracking fluid should remain trapped deep below ground or should come back out of the well hole where it can be captured, reused, or disposed of as waste. In properly constructed wells made from concentric cement and steel pipe casings, industry experts like to say the chance of groundwater contamination is on the order of “one in a million fractures.”
Hypothetically speaking, most of the pollution risk from fracking stems from how operators handle the fracking fluid and the waste flowback fluid aboveground, not directly from the underground fracking process itself. But not all wells are built properly, nor may they all hold up over time. And accidents happen.
Improved tracers to monitor wayward fracking fluid could provide a level of transparency to allay public concerns. Fracking operators often track short-lived radioactive isotope tracers such as 110Ag or 131I with a scintillation detector lowered into the well hole. They sometimes use chemical tracers such as fluorinated benzoic acids with mass spectrometry detection. The radioactive or chemical tracers are selected to measure proppant progress into the fracture zone and to determine which parts of the well are most productive.
However, the tracers don’t last long enough or they become too dilute to assess whether a well is leaking or if fracking operations are tainting groundwater or surface water. Current environmental monitoring therefore tends to rely on sampling well water, streams, lakes, and wastewater ponds for contaminants.
Recognizing a need for fundamentally new types of tracers, two companies are moving forward with different approaches to frack watching. In one approach, a start-up company called FracEnsure, cofounded by Rice University materials scientist Andrew R. Barron, is developing superparamagnetic metal oxide nanoparticle tracers.
Barron’s group has been developing ceramic fracking proppants, such as strong hollow spheres of aluminum oxide, for more than a decade. These materials are now commercially available from Oxane Materials, another firm Barron cofounded. “With subsequent research projects, we started learning about the mobility of nanoparticles in aqueous systems, which led us to expand our work to tracers,” Barron says.
The Rice researchers make the nanoparticles by thermally decomposing metal acetylacetonate or metal oleate complexes. The nanoparticle surfaces are modified with a proprietary short-chain organic zwitterionic species that acts like a surfactant to help the particles disperse uniformly in fracking fluid.
The particle cores, which have different ratios of iron, manganese, and zinc, or of iron, gadolinium, and aluminum, are designed to exhibit a specific magnetic profile as a result of the unpaired electrons of the metal atoms. The distinctive magnetic signatures of each type of particle can serve to identify a batch of fracking fluid. A few dozen pounds of nanoparticles are needed for each fracking operation, Barron notes.
To monitor fracking fluid, the team uses a ceramic membrane filter developed by Barron’s group for cleaning frack water to concentrate the particles and then isolates them using a magnet. The tracers’ temperature-dependent superparamagnetism allows them to be distinguished from each other and from natural magnetic particles that emerge from the well hole when a test sample is heated in a low magnetic field.
“We are starting to get at how to subtly modify the particles to get more complex information out of a well,” Barron says. As the next step, Texas-based Southwestern Energy has plans to begin testing the nanoparticles in fracking wells later this year. Barron thinks the nanoparticles could first see action monitoring the integrity of injection wells, where industrial wastewater—including fracking fluid—is pumped for permanent storage or used to force out residual oil from partially depleted oil fields.
Taking a different approach to frack tracking, a start-up called BaseTrace located in Research Triangle Park, N.C., is using artificial DNA technology. The company’s cofounder and CEO, Justine Chow, got together with some of her fellow Duke University environmental sciences graduates to introduce the technology.
Each tracer is a strand of DNA, fewer than 200 base pairs long, Chow explains. Because the short DNA sequences can be arranged in millions of different ways, each tracer has a unique signature like a fingerprint or bar code that can be assigned to each individual well or for each separate fracking stage in a well.
The oligonucleotide configuration is designed to withstand the extreme underground conditions associated with fracking, Chow says, including high temperature, high salinity, and shear forces. The DNA can also survive for up to two months when exposed to ultraviolet sunlight levels typical in impoundment ponds where companies store used fracking water for treatment or recycling.
Just a thimbleful of one of the tracers is all that needs to be added to millions of gallons of fracking fluid, Chow notes. Detection is performed by collecting samples, amplifying the DNA present using the polymerase chain reaction, and then determining whether the DNA tracer is present.
“We can detect the DNA in flowback fluid, making it possible to measure the efficiency of different fracturing stages and to trace whether or not that fluid ends up in places it’s not supposed to be,” Chow says.
Like FracEnsure, BaseTrace is still at the product development stage but gearing up to begin field trials. Chow envisions that one day her DNA tracers will be standard ingredients in fracking fluids. But for now she believes they might first be used to keep tabs on radioactive waste storage tanks.
“In the old days, having a classic oil well meant you dug a hole in the ground and stood back as oil gushed all over you, and then you moved to Beverly Hills,” Rice’s Barron muses. “But today’s oil and gas is trapped in rock that is less permeable than concrete, which is why we need fracking. The fracking advances we are seeing now are tackling different problems. But all of them are aimed at making oil and gas extraction processes more efficient and reducing their environmental footprint.”