A perforation tool is used to puncture the steel well casing within the reservoir, creating pathways for oil or gas to flow to the well. Treatment fluid, about 3 million gallons of fresh water mixed with chemicals and sand, is pumped into the reservoir through the perforations. The treatment fluid causes pressure to build in the reservoir until the rock fractures. Normally, many of these fractures would close again due to natural stresses, but the sand “props” them open, allowing oil or gas to flow to the wellbore. Finally, treatment fluid is allowed to flow back to the surface, where it is collected and may be recycled for use at other wells or disposed of.
Hydraulic fracturing has provided access to oil and gas once considered unobtainable and is largely responsible for the recent development of unconventional reservoirs. These are reservoirs that, in contrast to conventional ones, have low permeability, meaning they lack connection between pore spaces so that without additional means wells will not produce much oil or gas. Hydraulic fracturing increases the connectivity of the reservoir, thereby increasing access to trapped oil or gas. Without the additional oil or gas provided by hydraulic fracturing, unconventional wells would not be profitable or worth drilling.
In many unconventional oil and gas fields, horizontal drilling, which allows a greater length of wellbore to communicate with the reservoir, also helps to make wells profitable. Horizontal drilling and fracturing technologies were essentially developed together, spurred by tax incentives in the late 1980s to develop domestic unconventional gas fields. The tandem technologies proved successful, were commercialized quickly, and together have been responsible for the recent and rapid development of deep shale gas fields, including the Marcellus, Barnett and Haynesville shales that have added decades worth of recoverable natural gas reserves to our energy supply.
Hydraulic fracturing has also earned its reputation as a “game-changing” technology. To date, more than 1 million wells in the United States have been hydraulically fractured. But while rising quickly to popularity, hydraulic fracturing and natural gas drilling in general have also raised a great number of questions concerning their environmental impact.
Natural gas has a reputation as a clean energy option because it produces fewer greenhouse gas emissions than coal or oil when burned, but this fact ignores the overall footprint of unconventional gas production. A recent study from Cornell University suggests the footprint of unconventional production may be equal to or larger than that of other fossil fuels, due to the high global warming potential of “fugitive” methane that may leak to the atmosphere.
Because they require hydraulic fracturing, unconventional wells use millions more gallons of water per well than conventional wells. Using recycling technology, nearly 100 percent of the recovered flow-back fluid can be reused, but only 10 percent to 60 percent of the fluid is recovered, leaving millions of gallons underground.
Another environmental concern is contamination or pollution due to surface spills of fracture treatment fluid. When they occur, they are usually identified and cleaned quickly, but because the water and chemicals are mixed at the well site, spills may contain high concentrations of toxic chemicals, posing a threat to humans, wildlife, or clean water supplies.
Several highly publicized reports from active drilling areas have linked health problems to drinking water contaminated either by methane or fracture treatment fluid. Different types of methane occur naturally and are transported by a variety of mechanisms, making it difficult to confirm that drilling activity is the cause of methane in a water well, especially in the common case that wells were not tested prior to drilling activities.
It may be even more difficult to prove that water is contaminated with fracture treatment fluid because vendors that conduct the treatments are not required to disclose their chemical formulas. This fact has caused concerns, especially among residents of drilling areas who could unknowingly be exposed to hazardous chemicals, and more recently from medical professionals whose ability to treat exposed individuals would be compromised.
The disposal of fracture treatment fluid has also raised heated debates. Under law, treatment fluid may be released into and diluted in surface waters. These waters can be treated at municipal plants and made safe for drinking, but in some places, including parts of New York, where hydraulic fracturing is banned, municipal water comes from naturally clean supplies that are not treated but would need to be if fracturing were allowed.
The other option for disposing of treatment fluid, injection into underground disposal wells, has been linked to increased seismicity and earthquakes. Earthquake swarms — large numbers of earthquakes occurring close together within a short time — have been associated with disposal activities in several drilling areas. They are usually too small to be felt and are detected only with seismic instruments. A few large earthquakes, however, have been linked to disposal activity, including an incident of back-to-back earthquakes felt in Youngstown, Ohio, that gained national attention.
The concern over earthquakes highlights remaining uncertainties of how fluid injection, for fracturing or waste disposal, affects the reservoir and surrounding geology — where the fluid goes and what effects it has. Fracture treatments are often said to be well-controlled with fractures contained to the reservoir and not breaching the seal, an overlying rock that prevents reservoir fluids from migrating upward. But unconventional reservoirs are their own seals, as well as their own sources. This is because their low permeability prevents gas that forms within them from migrating.
Additionally, horizontal wells are drilled to target a zone that often includes rocks other than the reservoir so that fractures made along the wellbore may create connection outside the reservoir. Deep reservoirs often contain small faults, usually assumed to be sealed by intense pressures, but it is not known whether a fault could reopen due to drilling or fracturing activity and, if connected to shallower faults, whether reservoir fluids could migrate to groundwater supplies or to the surface.
Considering the environmental impacts, concerns, and remaining uncertainties associated with hydraulic fracturing, the obvious question many are asking is whether we should abandon the practice. Eliminating hydraulic fracturing would mean eliminating drilling in unconventional areas and, in turn, eliminating reserves that our current and forecasted levels of consumption demand. It is well known that turning off the “tap” of natural gas would increase home heating costs, but, additionally, 25 percent of electricity in the U.S. is sourced from natural gas, and that percentage is on the rise.
Natural gas is a necessity, not an option, and that means hydraulic fracturing must be performed. The concerns, however, are being addressed. Responding to public pressure to increase accountability, several companies have begun volunteering information about the chemicals they use in treatment fluid, a growing number of states are drafting their own disclosure rules, and federal legislation for hydraulic fracturing has been proposed. Investment in research could also prove helpful by studying the impacts of hydraulic fracturing and how to improve the technology.
Ellen Gilliland is a geophysicist and researcher at the Virginia Center for Coal and Energy Research at Virginia Tech. Email: firstname.lastname@example.org.