Author: Paul Vidal de La Blache
Like coal, natural gas is a resource with a domestic production that nearly meets domestic needs. The United States is currently the world’s largest producer and consumer of natural gas, accounting for a quarter of U.S. energy use and electricity generation. The benefits from the low greenhouse gas emissions of natural gas and of its domestic production are balanced against the environmental impacts of resource extraction, some of which can be significant, particularly with respect to water.
Natural gas is carving an ever greater wedge in the U.S. energy mix: most of the added electricity generation in the last decade has been from natural gas-fired power plants and a record number of drill rigs are active throughout the country. Water use has become a controversial topic with the recent surge in hydraulic fracturing. Water issues are mainly linked to water quality, especially that of potable water resources, although water quantity issues are increasingly important in water-stressed regions.
Unconventional gas is often found deeper than conventional gas sources and requires the use of horizontal drilling techniques. It is also found in less permeable rock formations and requires unconventional techniques such as hydraulic fracturing to improve the gas recovery. On-site drilling and extraction operations associated with horizontal drilling and hydraulic fracturing require varying amounts of water that are typically more than what is needed in conventional wells. Drilling natural gas wells requires water for preparing drilling fluid, which serves various purposes including cleaning and cooling of the drill bit, evacuating drilled rocks and sediments, and providing pressure to prevent a well collapse.
Once the well has been drilled, the shale has to be fractured to increase the permeability of the rock and allow natural gas to begin to flow to the wellhead. Hydraulic fracturing is the main method used, although others are being investigated by the industry, and involves phenomenal amounts of water for every well drilled. The EPA estimates that about 11,000 new wells are hydraulically fractured each year, and a single well requires 1 to 5 million gallons of water.
Although the water intensity of shale gas (gallons of water used per MMBTU of natural gas extracted) depends greatly on the type of shale and the expected productivity of the well, industry values gauge it at 1 to 2 gallons per thousand cubic feet (TCF) of natural gas. This is relatively low compared to oil, which averages over 60 gallons per TCF in the U.S. due to water flooding as a secondary recovery technique. Water flooding consists of injecting water in oil or gas fields to push the resource out. Nevertheless, it is the upfront water cost of a single well that is a problem, particularly in water-stressed regions such the Mountain West and Southern California.
Aside from the water quantity issue, two major problems of water quality arise from this sort of gas development: frac chemicals, injected underground, and minerals, which naturally enter the water in the drilling process. To ensure optimal hydraulic fracturing and continuous production of natural gas, companies inject proppants (sand, ceramic, or silicon pellets), gels, and biocides into the wells. Although natural gas companies insist that frac chemicals account for barely one percent of the volume of water injected for fracking, this nevertheless corresponds to tens of thousands of gallons of chemicals pumped underground in every well. Generally 10-25 percent of the total amount of water comes back to the surface, which has to be disposed. Sometimes this water is reinjected underground or recycled locally, but there have been rising concerns about this degraded water in municipal wastewater systems. Part of the problem is that local wastewater systems and agencies do not know what chemicals they are treating because frac chemicals are considered proprietary information and are not disclosed.
Indeed, the federal Energy Policy Act of 2005, in an effort to promote domestic gas production, exempted hydraulic fracturing from regulations under the federal Safe Water Drinking Act, making fracking only regulated at the state level. As a result, shale gas drillers do not have to disclose what chemicals they use for hydraulic fracturing. This is less formally known as the “Halliburton Loophole,” named after the Halliburton company, formerly run by Dick Cheney, Vice-President under George W. Bush at the time of the Energy Policy Act of 2005. Halliburton was one of the major lobbyists for this exemption. Unsuccessful attempts to repeal this loophole were made in 2009, and again in 2011, with the Fracturing Responsibility and Awareness of Chemicals Act.
In addition to the proprietary frac chemicals, flowback water, or produced water, may also contain high concentrations of salts and heavy metals leached from the subsurface, as well as radionuclides that significantly exceed drinking-water standards. The flowback water could potentially migrate to surface waters or shallow aquifers, contaminating potable water supplies. These high concentrations of inorganics are not usually successfully treated by municipal wastewater facilities and require much more expensive industrial-grade systems. There are also reports that link higher salinity measurement in some Appalachian rivers, to the disposal of processed water from the Marcellus Shale. This leads to lower water quality for potable and agriculture use and endangers freshwater wildlife.
Shale gas represents a tremendous opportunity but an even greater threat for the United States. New natural gas wells have spurred economic development in impoverished rural areas and helped the U.S. reduce gas imports, but the true environmental impact of unconventional gas extraction is unknown. The EPA is currently conducting a multi-year study to evaluate the impacts of hydraulic fracturing on water resources, particularly potable water sources, but widespread support for domestic energy sources may limit impartial examinations of the natural gas industry by government agencies.
Paul Vidal de La Blache is a second-year Environmental Engineering Master’s student at Stanford University. He is a research assistant for the Stanford Water-Energy Nexus Investigation, a Woods Institute for the Environment and Precourt Institute for Energy initiative. He graduated from the Ecole Polytechnique in France.
Kennedy, K. The Role of Natural Gas in America’s Energy Mix. Natural Resources Defense Council, 2012.