Environmental impacts of natural gas production
Vigorous debates about the ongoing boom in natural gas production have grabbed a lot of headlines over the last year.
Energy companies are extracting 20 percent more natural gas than they were just five years ago and moving into areas of the country that have not seen drilling operations before. This rapid expansion, coupled with uncertainties about environmental impacts of the extraction process, has triggered considerable concern.
Natural gas is far cleaner than coal when used for electrical power generation and is also a flexible fuel, useful for heating, transportation, and providing a back up for wind and solar energy. Nonetheless, the public is right to be concerned about the development of shale gas resources, says Mark Zoback, professor of geophysics in the Stanford School of Earth Sciences.
Much of the debate has centered on the practice of hydraulic fracturing, often referred to as “fracking”, the process of injecting fluid into a gas well and raising the pressure enough to fracture the rock the gas is trapped in, making extraction possible.
But that process—and unidentified, but potentially hazardous chemicals in the fluid—is what has prompted some to fear that hydraulic fracturing operations could extend far beyond the well, opening up conduits for the hydraulic fluid and its “mystery” chemicals to percolate into groundwater reservoirs, contaminating water used for drinking or irrigation.
“There are a number of potentially detrimental environmental impacts of shale gas development that need to be addressed,” says Professor Zoback, “but, ironically, hydraulic fracturing isn’t one of them.”
Zoback points to the fact that whenever leakage appears to have contaminated subsurface water supplies the reason has been shown to be poor well construction. Proper well construction, along with proper disposal of wastewater from drilling and hydraulic fracturing, are critical concerns in natural gas extraction, he says.
Zoback’s area of expertise is geomechanics—the study of how rocks and other Earth materials behave mechanically on scales from the microscopic to the massive tectonic plates that cover the surface of the Earth. He began studying hydraulic fracturing over 30 years ago as a technique for measuring the forces in the Earth. For the past four years, much of his research has been focused on shale gas development. Twice in the past year he has testified before Congressional committees on development issues.
Last year, Zoback was asked to join a panel put together by Secretary of Energy Steven Chu at the request of President Obama to provide advice on the environmental risks associated with shale gas production. In August 2011, the Shale Gas Production Subcommittee filed a report setting forth 20 recommendations for reducing the environmental impact of shale gas production, including a call for complete “disclosure of all chemicals used in hydraulic fracturing fluids on both public and private lands,” to help allay public concern over the composition of fracking fluids.
Another concern about shale gas development is the potential for the process to trigger earthquakes.
A typical hydraulic fracturing operation involves pressurizing a relatively small volume of rock for a short period of time, typically about two hours, which generates extremely small microseismic events. “The energy released by one of these tiny microseismic events is equivalent to the energy of a gallon of milk hitting the floor after falling off a kitchen counter,” Zoback says. “Needless to say, these events pose no danger to the public.”
In several cases, however, larger, but still very small earthquakes have been associated with hydraulic fracturing operations. Out of the hundreds of thousands of hydraulic fracturing operations carried out over the past few years, there have been only a few reports of triggered earthquakes that might have been large enough to be felt by people living in the region and none were reported to have caused significant damage. Of greater concern are earthquakes triggered by the injection of wastewater from the drilling and fracturing process.
Drilling and hydrofracturing a natural gas well typically takes several million gallons of water. That water is mixed with a number of relatively benign additives, Zoback says. However, when the water is injected into shale during hydraulic fracturing, it picks up a number of naturally occurring chemicals from the shale including arsenic, selenium, iron, and sometimes, radioactive particles. When the hydraulic fracturing water is flowed back to the surface prior to gas production, the highly saline “flowback” water must be properly handled.
Most commonly, the flowback water is injected into disposal wells, licensed by the U.S. Environmental Protection Agency. With proper construction, these disposal wells prevent the injected water from leaking into the environment. But the volumes of water and the pressures involved in wastewater injection sometimes exceed the capacity of the geologic formations being used to store them, and trigger small-to-moderate earthquakes, the largest confirmed one being a magnitude-4.7 event in February in Arkansas.
In an article that appeared in the April 2012 issue of Earth magazine, Zoback pointed out that roughly 150,000 wastewater injection wells have been safely operating in the U.S. for many decades with no earthquakes being triggered. He laid out five simple steps that regulators and energy companies could take to reduce the low risk of triggering earthquakes by wastewater injection even further.
Zoback points out that “the preferable solution is not to inject the flowback water at all, but to reuse it in subsequent drilling and hydraulic fracturing operations.” Increasingly, gas companies in the northeastern U.S. have been taking this approach.
It has long been known that burning natural gas for electricity instead of coal has the potential to significantly reduce greenhouse gas emissions. With the development of hydrofracturing, Zoback says that potential reduction is fast becoming a reality, noting that, “the rapid switch to natural gas from coal in just the past four years has dropped U.S. CO2 emissions to levels not seen for 20 years.”
Shale gas resources are found in many countries around the world, including China, which generates three times as much CO2 as the U.S. by burning coal for electricity.
“Now that the promise of natural gas for reducing greenhouse gas emissions is being realized, it makes the need for environmental protection during shale gas development even more important”, he said.
Zoback is currently serving on a committee advising the Canadian government about shale gas development and environmental protection. Previously he served on the National Academy of Engineering committee established at the request of Interior Secretary Ken Salazar to investigate the Deepwater Horizon disaster in the Gulf of Mexico.