To call it a fractious debate is an understatement.
Hydraulic fracturing, or fracking, wrenches open rock deep beneath the Earth's surface, freeing the natural gas that's trapped inside. Proponents argue that fracking-related gas recovery is a game changer, a bridge to the renewable energy landscape of the future. The gas, primarily methane, is cheap and relatively clean. Because America is brimful of the stuff, harvesting the fuel via fracking could provide the country jobs and reduce its dependence on foreign sources of energy.
But along with these promises have come alarming local incidents and national reports of blowouts, contamination and earthquakes. Fracking opponents contend that the process poisons air and drinking water and may make people sick. What's more, they argue, fracking leaks methane, a potent greenhouse gas that can blow up homes, worries highlighted in the controversial 2010 documentary Gasland.
Fears that fracking companies are operating in a Wild West environment with little regulation have prompted political action. In June, the group Don't Frack Ohio led thousands of protesters on a march to the statehouse, where they declared their commitment to halting hydraulic fracturing in the state. Legislation banning the process has been considered but is now on hold in California. New York ? which sits atop a giant natural gas reserve ? has a statewide fracking moratorium; pending policies would allow the process only where local officials support it.
Despite all this activity, not much of the fracking debate has brought scientific evidence into the fold. Yet scientists have been studying the risks posed by fracking operations. Research suggests methane leaks do happen. The millions of gallons of chemical-laden water used to fracture shale deep in the ground has spoiled land and waterways. There's also evidence linking natural gas recovery to earthquakes, but this problem seems to stem primarily from wastewater disposal rather than the fracturing process itself.
While the dangers are real, most problems linked to fracking so far are not specific to the technology but come with many large-scale energy operations employing poor practices with little oversight, scientists contend. Whether the energy payoff can come with an acceptable level of risk remains an open question.
"People want it to be simple on both sides of the ledger, and it's not simple," says environmental scientist Robert Jackson of Duke University. "Our goal is to highlight the problems, so we can understand the problems and do what we can to help."
What is hydraulic fracturing?
Hydraulic fracturing has been cranking up output from gas and other wells for more than 50 years. But not until fracking joined up with another existing technology, horizontal drilling, was the approach used to unlock vast stores of previously inaccessible natural gas. The real fracking boom has kicked off in just the last decade.
Conventionally drilled wells tap easy-to-get-at pockets of natural gas. Such gas heats homes and offices, fuels vehicles and generates electricity. But as easily accessible reserves have been used up, countries seeking a steady supply of domestic energy have turned to natural gas buried in difficult-to-reach places, such as deep layers of shale.
Gas doesn't flow easily through shale or other impermeable rock. Drilling a conventional well into such formations would gather gas only from a small area right around the well. And, for shale in particular, many formations in the United States extend hundreds of kilometers across but are less than 100 meters thick, hardly worth sending a vertical well into.
Combining hydraulic fracturing with horizontal drilling offers a way to wrest gas from these untapped reserves. By drilling sideways into a rock formation and then sending cracks sprawling though the rock, methane can burble into a well from a much larger area.
The drill-frack punch goes something like this: After constructing a drill pad, engineers drill a well straight down, typically for thousands of meters, toward the target bed of rock. Operators then begin "kicking off," turning the drill so it bores into the formation horizontally, forming an L-shape.
After small explosive charges perforate the far end of the well's horizontal portion, called the toe, hydraulic fracturing can begin. Millions of gallons of fracking fluid ? a mixture of water, sand and chemicals ? are pumped into the well at pressures high enough to fracture the shale. Methane within the shale diffuses into these fissures and flows up the well. Along with the gas comes flowback water, which contains fracking fluid and additional water found naturally in the rock.
After the well's toe is fracked, engineers repeat the procedure, moving back along the horizontal portion of the well until its heel is reached. Compared with conventional wells, which may steadily pump out fuel for more than a decade, shale gas extraction is like blasting open a faucet. There's a huge surge in gas, but it may become merely a dribble after a few years. At the end of its life, the well gets plugged.
Today hydraulic fracturing is used in about nine out of 10 onshore oil and gas wells in the United States, with an estimated 11,400 new wells fractured each year. In 2010, about 23 percent of the natural gas consumed in the United States came from shale beds.
While the immediate output is gas, the uptick in this type of extraction has also fueled fears over fracking's potential dangers ? such as drinking water contamination.
Does methane leak into water?
One of the most explosive issues, literally, is whether fracking introduces methane into drinking water wells at levels that can make tap water flammable or can build up in confined spaces and cause home explosions.
Studies are few, but a recent analysis suggests a link. Scientists who sampled groundwater from 60 private water wells in northeastern Pennsylvania and upstate New York found that average methane concentrations in wells near active fracturing operations were 17 times as high as in wells in inactive areas. Methane naturally exists in groundwater ? in fact, the study found methane in 51 of the 60 water wells ? but the higher levels near extracting sites raised eyebrows.
To get at where the methane was coming from, the researchers looked at the gas's carbon, which has different forms depending on where it has been. The carbon's isotopic signature, and the ratio of methane to other hydrocarbons, suggested that methane in water wells near drilling sites did not originate in surface waters but came from deeper down.
But how far down and how the methane traveled aren't clear, says Duke's Jackson, a coauthor of the study, published last year in the Proceedings of the National Academy of Sciences. He proposes four possibilities. The first, most contentious ? and, says Jackson, the least likely ? is that the extraction process opens up fissures that allow methane and other chemicals to migrate to the surface. A second possibility is that the steel tubing lining the gas well, the well casing, weakens in some way. Both scenarios would also allow briny water from the shale and fracking fluid to migrate upward. The well water analysis found no evidence of either.
Newly fracked gas wells could also be intersecting with old, abandoned gas or oil wells, allowing methane from those sites to migrate. "We've punched holes in the ground in Pennsylvania for 150 years," Jackson says. Many old wells have not been shut down properly, he says. "You find ones that people plugged with a tree stump." In some places in Pennsylvania, West Virginia and elsewhere (especially those with existing coal beds), methane turned up in well water long before hydraulic fracturing became widespread.
A fourth possibility, which Jackson thinks is most probable, is that the cement between the well casing and the surrounding rock is not forming a proper seal. Cracking or too little cement could create a passageway allowing methane from an intermediate layer of rock to drift into water sources near the surface. Such cases have been documented. In 2007, for example, the faulty cement seal of a fracked well in Bainbridge, Ohio, allowed gas from a shale layer above the target layer to travel into an underground drinking water source. The methane built up enough to cause an explosion in a homeowner's basement.
Other types of gas and oil wells have similar problems, Jackson says, but fracking's high pressures and the shaking that results may make cement cracks more likely. "Maybe the process itself makes it harder to get good seals," he says. "We need better information."
Accompanying these concerns are worries that methane leaking into the air will have consequences for the climate and human health. Burning methane creates fewer greenhouse gas emissions and smog ingredients than other fossil fuels, so natural gas is considered relatively clean. But evidence suggests that methane frequently escapes into the air during drilling and shipping, where it acts as a greenhouse gas and traps heat. Such leaking undermines the gas's "clean" status.
Methane leaking into the air can also cause ozone to build up locally, leading to worries about headaches, inflammation and other ills among people who live nearby. Scientists in Pennsylvania have proposed a long-term study examining possible links between air pollution from the shale gas boom and human health. A more immediate concern for human health, Jackson and others argue, is exposure to fracking wastewater.
Is fracking fluid hazardous?
A typical fracked well uses between 2 million and 8 million gallons of water. At the high end, that's enough to fill 12 Olympic swimming pools. Companies have their own specific mixes, but generally water makes up about 90 percent of the fracking fluid. About 9 percent is "proppants," stuff such as sand or glass beads that prop open the fissures. The other 1 percent consists of additives, which include chemical compounds and other materials (such as walnut hulls) that prevent bacterial growth, slow corrosion and act as lubricants to make it easier for proppants to get into cracks.
As the gas comes out of a fracked well, a lot of this fluid comes back as waste. Until recently, many companies wouldn't reveal the exact chemical recipes of their fluids, citing trade secrets. A report released in April 2011 by the House Energy and Commerce Committee did provide some chemical data: From 2005 to 2009, 14 major gas and oil companies used 750 different chemicals in their fracking fluids. Twenty-five of these chemicals are listed as hazardous pollutants under the Clean Air Act, nine are regulated under the Safe Drinking Water Act and 14 are known or possible human carcinogens, including naphthalene and benzene.
In addition to the fracking fluid, the flowback contains water from the bowels of the Earth. This "produced" water typically has a lot of salt, along with naturally occurring radioactive material, mercury, arsenic and other heavy metals.
"It's not just what you put into the well. The shale itself has chemicals, some of which are quite nasty," says Raymond Orbach, director of the University of Texas at Austin's Energy Institute. A report analyzing the risks associated with fracking was released by the Energy Institute in February in Vancouver at the meeting of the American Association for the Advancement of Science. (The report is under independent review because one of its authors didn't disclose that he is on the board of a gas-drilling company, but Orbach stands behind the study.)
Wastewater is dealt with in different ways. Sometimes it is stored on-site in lined pits until it is trucked off. When these pits are open to the air, they can release fumes or overflow, with possibly hazardous consequences.
The Energy Institute report cites one case in West Virginia in which about 300,000 gallons of flowback water was intentionally released into a mixed hardwood forest. Trees prematurely shed their leaves, many died over a two-year study period, and ground vegetation suffered. A briefing paper coauthored by geophysicist Mark Zoback of Stanford University points to spills: In 2009, leaky joints in a pipeline carrying wastewater to a disposal site allowed more than 4,000 gallons to spill into Pennsylvania's Cross Creek, killing fish and invertebrates.
For obvious ethical reasons, controlled studies exposing people to fracking fluid don't exist. And long-term population studies comparing pre- and post-fracking health haven't yet been done. But these incidents ? and the known dangers of some of the chemicals used ? raise alarms about the possible consequences of human exposure.
Local geology in some areas may also allow fracking chemicals and produced water to seep up from deep below into water sources. A study published in July in the Proceedings of the National Academy of Sciences found a geochemical fingerprint of briny shale water in some aquifers and wells in Pennsylvania. Local geology probably also played a role in fracking fluid getting into drinking water in Pavillion, Wyo., a site that has been at the heart of the fracking controversy.
Still, several reviews of where fracking chemicals and wastewater have done harm find that the primary exposure risks relate to activities at the surface, including accidents, poor management and illicit dumping.
An accepted disposal route is injecting the water into designated wastewater wells. But that strategy can cause an additional problem: earthquakes.
Does fracking cause earthquakes?
Hydraulic fracturing operations have been linked to some small earthquakes, including a magnitude 2.3 quake near Blackpool, England, last year.
But scientists agree such earthquakes are extremely rare, occurring when a well hits a seismic sweet spot, and are avoidable with monitoring.
Of greater concern are earthquakes associated with the disposal of fracking fluid into wastewater wells. Injected fluid essentially greases the fault, a long-known effect. In the 1960s, a series of Denver earthquakes were linked to wastewater disposal at the Rocky Mountain arsenal, an Army site nearby. Wastewater disposal was also blamed for a magnitude 4.0 quake in Youngstown, Ohio, last New Year's Eve.
A study headed by William Ellsworth of the U.S. Geological Survey in Menlo Park, Calif., documents a dramatic increase in earthquakes in the Midwest coinciding with the start of the fracking boom. From 1970 to 2000, the region experienced about 20 quakes per year measuring at or above magnitude 3.0. Between 2001 and 2008, there were 29 such quakes per year. Then there were 50 in 2009, 87 in 2010 and 134 in 2011.
"The change was really quite pronounced," says Ellsworth. "We do not think it's a purely natural phenomenon." But the earthquakes weren't happening near active drilling ? they seemed to be clustered around wastewater wells.
It's hard to look back without pre-quake data and figure out what triggers a single earthquake, notes Ellsworth. There are several pieces of the geology equation that, if toggled, can tip a fault from stable to unstable.
A recent study examining seismic activity at wastewater injection wells in Texas linked earthquakes with injections of more than 150,000 barrels of water per month. But not every case fit the pattern, suggesting the orientation of deep faults is important.
Ellsworth advises that injection at active faults be avoided. Drill sites should be considered for their geological stability, and seismic information should be collected. Only about 3 percent of the 75,000-odd hydraulic fracturing setups in the United States in 2009 were seismically monitored.
"There are many things we don't understand," says Ellsworth. "We're in ambulance-chasing mode where we're coming in after the fact."
Is it worth it?
That ambulance-chasing mode is what makes current shale gas operations so worrisome to many. If scientists had the data needed to identify problems and find ways to ameliorate or eliminate them, then the current fracas over fracking may have been preempted.
"Transparency has been missing," says Stanford's Zoback. "Then the public gets suspicious and alarmed, and you get misplaced hysteria."
Zoback and other scientists surveying existing data generally have concluded that there are dangers associated with fracking but that existing technologies, regulation and serious enforcement could resolve them. Such regulations would include minimizing the local environmental footprint of setting up the well site and trucking in water and sand, monitoring the integrity of steel casings and cement, swapping out toxic chemicals from the fracking fluid, and collecting seismic and other geologic data.
Like many technologies, fracking comes with promise and with risk, says Zoback. Rules tailored depending on local geology and other factors can mitigate those risks. Consider all the regulations surrounding automobiles. There are seat belts and air bags, emission tests and proper and improper ways to dispose of oil and brake fluid.
Ultimately, unless people are willing to cut way back on their energy use, the risks associated with natural gas recovery have to be weighed against the risks that come with coal, nuclear power and other energy sources.
"It's clear that it's a remarkable resource," Zoback says. "It's abundant, and as a transition fuel between today and the green-energy future, natural gas really is the answer, I'm convinced. But that's not a get-out-of-jail-free card."
Fracking footprint
A typical shale gas drilling site is abuzz with activity. After a well pad is constructed, engineers drill straight down, typically thousands of meters, toward the target shale. Then the well is drilled horizontally. Explosives set off in the horizontal portion create holes in the well?s sides through which millions of gallons of fracking fluid are pumped. The fluid fractures the shale, releasing the trapped gas for recovery. Beyond the rig itself, there are holding tanks and pits, and trucks for pumping in water and carrying away wastewater and gas. Such a big operation leaves a lot of room for error.
View larger image 
Illustration: Nicolle Rager Fuller
Potential hazards
1. Blowout When blowout prevention equipment is absent or fails, pressurized fluid and gas can explode out the wellhead, injuring people and spewing pollutants.
2. Gas leak Methane, the primary gas in natural gas, may be present in layers of rock above the target layer. Cracks in the cement that seal the well to the surrounding rock can provide a path for this methane to travel into the water table.
3. Air pollution Flare pipes that burn methane so it doesn?t build up, diesel truck exhaust and emissions from wastewater evaporation can dirty the air near a drill site. When methane is released without being burned, it acts as a potent greenhouse gas, trapping 20 times as much heat as carbon dioxide.
4. Wastewater overflow Fracking fluid, about 1 percent of which is made up of chemicals (sometimes including carcinogens), is increasingly recycled for use in other wells. But sometimes it is stored in open pits that emit noxious fumes and can overflow with rain.
5. Other leaks There are some worries that local geology in particular areas would allow fracking-produced fluid and methane to travel upward. But most evidence of exposure stems from surface problems such as spills or illicit dumping.
6. Home explosions If methane does get into the water table ? because of cracked cement, local geology or the effects of old wells ? it can build up in homes and lead to explosions.
Source: http://www.sciencenews.org/view/feature/id/343202/title/The_Facts_Behind_the_Frack
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