GSA Critical Issue: Hydraulic Fracturing
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Potential Environmental Issues
|The hydraulic fracturing critical issue will be periodically updated.|
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Fluids used in hydraulic fracturing are a mixture of water, proppant, and chemical additives. Additives typically include gels to carry the proppant into the fractures; surfactants to reduce friction and pipe corrosion; hydrochloric acid to help dissolve minerals and initiate cracks; and scale inhibitors and biocides to limit bacterial growth . The exact mix of additives depends on the formation to be fractured. These chemical additives typically make up about 0.5% by volume of well fracturing fluids, but may be up to 2% [12, 16]. Some potential additives are harmful to human health, even at very low concentrations . Unless diesel is used, the fracturing fluids are not regulated by the Safe Drinking Water Act (SDWA). Underground disposal of oil and gas wastes, however, are regulated by SDWA .
Water cycle in hydraulic fracturing; from U.S. EPA's Study of the Potential Impacts of Hydraulic Fracturing on Drinking Water Resources, Progress Report, 2012.
Diagram of possible fluid migration pathways and other environmental concerns with hydraulic fracturing. Source: Mike Norton, Wikimedia Commons.
Potential pathways for the fracturing fluids to contaminate water include surface spills prior to injection, fluid migration once injected, and surface spills of flowback and produced water (Fig. 9). Because the fluids are injected into the subsurface under high pressure, and because some of the fracturing fluids remain underground, there is concern that this mixture could move through the well bore or fractures created in the reservoir rock by hydraulic pressure and ultimately migrate up and enter shallow formations that are sources of fresh water (aquifers). In addition, there is concern that geologic faults, previously existing fractures, or poorly plugged, abandoned wells could provide conduits for these fluids to move into and contaminate aquifers .
The potential for contaminating groundwater due to hydraulic fracturing is an environmental risk being studied (Fig. 10) [19, 21, 22]. At present, there have been possibly two confirmed cases of groundwater contamination caused directly by the hydraulic fracturing process; in one location the fractured rock is within 420 feet of the aquifer [6, 14, 23]. One challenge is to distinguish natural contaminants that seep into groundwater unrelated to oil and gas development, from contamination due to aspects of drilling unrelated to hydraulic fracturing, and from contamination directly caused by hydraulic fracturing. There often are no water quality samples prior to hydraulic fracturing to provide a baseline comparison [6, 24, 25].
Horizontal Well Construction; from U.S. EPA Study Progress Report, December 2012, modified by Kansas Geological Survey .
For example, methane has been detected in some water wells in areas with oil and gas development [26, 27]. Some researchers suggested hydraulic fracturing may be a mechanism to explain methane in water wells in northeastern Pennsylvania and upstate New York, although methane from leaking well casings was cited as a more likely possibility [22, 25]. Methane can naturally originate from gas-producing rock layers below and close to the aquifer and be unrelated to the deeper fractured zone [14, 24]. Analysis of the gas can be used to identify the origin of gas occurring in groundwater [24, 28].
Figure 12. Groundwater Water Quality Sampling from a small diameter, temporary borehole.
Kansas Department of Health and Environment, 2012.
Figure 13: Measuring groundwater depth before sampling, from a non-pumping well installed to
monitor water quality conditions. Kansas Department of Health and Environment, 2012.
There are confirmed sources of groundwater contamination, however, from improperly constructed oil and gas wells; this contamination is unrelated to the hydraulic fracturing process. To protect groundwater, proper well design, construction, and monitoring are necessary. During well construction, multiple layers of telescoping pipe (or casing) are installed and cemented in place, with the intent to create impermeable barriers between the inside of the well and the surrounding rock . It is also common practice to pressure-test the cement seal between the casing and rock or to otherwise examine the integrity of wells. Wells that extend through a rock formation that contains high-pressure gas require special care in stabilizing the well bore and stabilizing the cement, or their integrity can be damaged .
The physical separation between the relatively shallow freshwater aquifer and the usually much deeper oil- and gas-producing rock layer provides additional protection. Typically there are thousands of feet of mostly low to very low permeability rock layers between an aquifer and oil or gas reservoir rocks that prevent fracturing fluids and naturally migrated hydrocarbons from reaching the aquifer. In areas where there is concern about faults, fractures, or plugged wells, various geophysical methods can be used to locate and avoid faults. There is also renewed interest in the need to locate and plug abandoned or “orphaned” oil and gas wells and unused water wells as a further measure to protect near-surface aquifers. In some regions, identifying and properly plugging all the abandoned wells is a significant undertaking . Proper storage and disposal of fracturing fluids and produced water is important to ensure that both surface water and groundwater are protected. Most fracturing fluids and produced water are re-injected into Class II wells  drilled specifically for deep disposal, treated in wastewater treatment facilities, or recycled . Wastewater treatment facilities, designed primarily for municipal waste, can be overwhelmed with the volume and treatment of fracturing fluids and produced water; a number will not accept it . Disposal wells inject waste water deep into formations that originally produced the oil and gas, or into different formations that generally contain highly saline and otherwise unusable water. Water is generally co-produced in equal or larger volumes than petroleum throughout the life of a well. Fluid handling and disposal are issues for all oil and gas activity, not only activity associated with hydraulic fracturing. Appropriate management practices and regulatory oversight are important to assure that accidental leaks and spills are minimized.
Baseline water-quality testing, carried out prior to oil and gas drilling, helps to document the quality of local natural groundwater and may identify contamination, or lack thereof, before oil and gas activity occurs [34, 35]. Without such pre-drilling baseline testing, it is difficult to know if contamination existed before drilling, occurred naturally, or was the result of oil and gas activity. Many natural contaminants, including methane and elevated chlorides, occur naturally in shallow groundwater in oil- and gas-producing areas and are unrelated to drilling activities . The quality of water in private wells is not regulated at the state or federal level, and many owners do not have their well water tested for contaminants. States handle contamination issues differently. For instance, Colorado requires baseline sampling of wells in oil- and gas-producing regions as part of its regulatory process [16, 34]. Pennsylvania places the presumptive burden of proof on oil and gas companies if groundwater contamination of drinking-water sources is found . New York and West Virginia are considering adopting baseline testing rules. In most states, however, such baseline sampling is only voluntary.
While there is little evidence of groundwater contamination due to hydraulic fracturing itself, there are still many questions about the risks to aquifers with the rapidly expanding industry developing tight oil and gas reserves using modern hydraulic fracturing techniques [6, 14, 19, 20, 21, 23]. There are few long term, peer-reviewed scientific studies. The U.S. Environmental Protection Agency’s Scientific Advisory Board study Potential Impacts of Hydraulic Fracturing on Drinking Water Resources, to be released in 2014, will be an important contribution. Local baseline testing of groundwater quality prior to hydraulic fracturing operations can provide valuable data for assessing the contamination risks.
Contamination risks to surface water are another environmental concern with the development of tight oil and gas reserves. The potential for contamination has led to increased regulations in some U.S. states. Potential pathways for contamination include surface spills, waste disposal, and land spreading of well cuttings. A study of the gas shale development in Pennsylvania documented increased chlorides downstream of the waste treatment plant and elevated total suspended solids downstream of shale gas wells . The elevated suspended solids appear related to the land clearing for the well pad, roads, and related infrastructure.