By: John Walton and Arturo Woocay
INTRODUCTION
Hydraulic fracturing occurs when high pressure fluids primarily consisting of water and
sand are pumped at high pressure into subsurface formations, typically shale that
contains natural gas and/or oil. Te high pressure fluid causes the rock to fracture. Te
new fractures increase the surface area of the shale and better interconnect previously
existing fractures, allowing more natural gas and/or oil to be pumped from the
formation. Modern hydraulic fracturing, referred to as “fracking,” is an evolving
technology that largely began after 2000 and has signifcantly increased natural gas
production in the United States in the past fve years with corresponding decreases in
natural gas prices.
Te revolution in hydraulic fracturing has been made possible by technological
advancements in directional drilling. In the past, wells were drilled vertically and
sometimes passed only briefly into the producing formation. Shale is a sedimentary
rock that is initially formed underwater as a horizontal layer containing compacted
mud that is cemented into rock. Intact shale has a low permeability, making fluid
movement slow except along natural or artifcial fractures in the rock. In the case of
the Marcellus Shale in Pennsylvania, the shale is approximately 100 to 250 feet thick.
Te Barnett Shale in Central Texas is between 100 and 500 ft, averaging about
300 ft; Eagle Ford Shale in South Texas is very variable with an average of about
250 ft; Fayetteville Shale in Arkansas is between 60–575 ft, average of about 200 ft;
Haynesville Shale in Northwest Louisiana averages about 250 ft. Tectonic activity
may later deform the initially horizontal layer into different angles and shapes, but the
fundamental problem remains of how to most efciently extract fluids from relatively
thin and deep rock layers that have low permeability. Directional drilling allows a
well to be oriented in a vertical direction until the shale layer is approached and then
turned in to the approximately horizontal direction needed to follow along the shale
layer (Figure 1). Te wells can be turned in any compass direction, allowing multiple
wells from a single pad to reach areas of several square miles in the producing shale
location, thus signifcantly reducing their surface footprint and disturbance when
FIGURE 1. Schematic picture showing two pads and two wells.
compared to vertical drilling. Wells have been drilled more than a mile deep and a
mile in horizontal reach. Furthermore, the Marcellus Shale is underlain by the thicker
Utica Shale, making it likely that the same pads may years later be used to drill wells
into the Utica Shale as well when gas prices rise sufciently to make deeper drilling
cost effective. Information on the extents of the Marcellus and Utica Shale formations
is available in USGS reports listed in their Energy Resources Program (USGS, 2013)
and at the website geology.com (Geology.com, 2013).
Te second key to increased access to energy resources is hydraulic fracturing. Shales
naturally have a low permeability, meaning removal of resources is slow and inefcient.
High pressure fluids containing proppants (e.g., sand), biocides, friction reducers,
corrosion inhibitors, iron control, scale inhibitors, surfactants, and acids are injected
into the wells to cause fracturing of the shale. Te proppants move into the newly
created fractures and expanded natural fractures and then prop the fractures open when
the fluids are removed. Te injected fluid is primarily (∼99.5%) sand and water and
the additive mixtures change by location, company, and as the technology evolves. Much
of the injected water is later extracted from the well during production, along with
formation waters.
Te exact spacing of pads, number of wells per pad, orientation of wells drilled
from each pad, and well distance in the horizontal and vertical directions, depends
upon surface access, drilling rights, formation properties, and economics. Leasing is an
issue. Leases typically expire after a fxed period of time (typically about 5 years) if there
is no drilling activity. Companies can drill but choose not to hydraulically fracture or
produce from the well prior to lease expiration in order to lock up the resources until the
economics of production improve.
Hydraulic fracturing has revolutionized the energy feld, causing the United States
to switch from a need to import natural gas to a situation where export of natural gas is
being considered. Energy independence, low energy costs, and economic development are
clear positives that have been advanced by the revolution in hydraulic fracturing.
Tis paper will summarize some of the environmental impacts associated with
fracking. Te order of subjects is approximately from the most obvious and certain to
more subtle impacts.
LAND USE AND DISTURBANCE
Te Pennsylvania Energy Impacts Assessment (Johnson, 2010) estimated surface impacts in
Pennsylvania as 7,000–16,000 new pads by 2030 with 60,000 new wells. Well pads were
estimated as 3.1 acres and forest cleared for associated infrastructure (roads, pipelines, surface
impoundments) was estimated as 5.7 acres per pad, totaling 8.8 acres per pad, or 138 square
miles of new development in the State of Pennsylvania. Te study estimated that nearly 2/3 of
the new well pads will be in forested areas, including public lands.
Tis is a large environmental impact, particularly as the disturbance is distributed at regular intervals in currently undeveloped areas based upon the pad spacing. Tis contrasts with
urban development that tends to be concentrated around urban areas rather than dispersed.
Construction of the pad and associated infrastructure and access roads can impact the quality of local streams and lakes due to rain runoff containing sediments from the disturbed site.
Typically drilling will take place in rural areas, requiring the construction of new roads whose
associated environmental impacts can only be partly mitigated.
Te impact of land disturbances can be reduced by mitigation strategies based on use
of seeding and flter strips, in addition to terraces, check dams, flter fences, and straw bales.
Proper selection of pad placements such as at the edge of forest patches (Johnson, 2010) can
also reduce land use impacts.
Furthermore, as the technology advances, fracking play lateral reaches will increase thus
allowing for fewer pads to cover the same play footprint and reducing their total impact.
NOISE
Drilling and hydraulic fracturing is a temporary, transient activity typically lasting a few
weeks. Typical noise sources are the drilling rig (diesel generators, braking mechanisms and
drilling pipe handling), large compressor, diesel electric generators, and truck trafc.
Noise nuisance can be mitigated through the installation of mufers, bafes, and silencers on the pad’s installation. Also, sound absorbing buildings can be constructed around compressor stations. A cost effective strategy that can minimize noise from initial operations is
the use of tall plastic sound-barriers (acoustic walls), as these can be moved from site to site as
needed. New more silent drilling rigs have been developed by some companies specifcally to
address noise issues.
TRAFFIC
Te development will increase trafc on roads including truck trafc to the pads. Injected and
produced water is mostly transported to and from the pads by truck. Increased trafc means
more accidents and more localized air pollution. Site operations and the heavy trafc will
increase noise levels. Transient workers can disrupt rural areas.
Pipeline networks facilitate transporting injected and produced water with reduction of
truck trafc, but this increases construction and land disturbance. Flowback water storage, for
its later reuse in other jobs, reduces the amount of water transported, but storage of water on
the surface can lead to leaks with resulting surface and ground water contamination. Treatment of produced water can also reduce the amount of water transport. Reduction of truck
trafc is also in the best economic interests of the operator and is weighed against the cost of
infrastructure construction for transport, reuse, and treatment of fluids. If local sources of
water are available to a pad then truck trafc of injection water can be reduced.
WATER, FLOWBACK WATER, AND SURFACE SPILLS
Hydraulic fracturing requires the injection of water and additives at high pressure to fracture
the rock and then prop open the fractures sufciently to allow the trapped gas to escape.
(Kaufman, Penny, & Paktinat, 2008). Te injected water leads to concerns of water use, especially in semi-arid climates, groundwater contamination, and surface water/shallow groundwater contamination from the water returning up the well. Te natural rock is heterogeneous
and has existing fractures and this means that the path and extent of the induced fractures is
difcult to predict and/or control. A concern is that the newly fractured wells will intersect
with abandoned wells increasing the risk of gas migration and well blowout or decreasing
the effectiveness of fracking operation. Te volume of fluid injected is highly variable but on
the order of a million gallons per well. Hydraulic fracturing may be performed in a series of
6–9 frac stages per well. Subsequent to fracturing some fluid returns up the well immediately
(flowback water) and some is produced with the gas (produced water). Tis is a mixture of the
injected fluid and the formation water and the distinction between flowback and produced
water is somewhat arbitrary.
Te additives include (Kaufman, Penny, & Paktinat, 2008):
• Proppant—Small solids are used to prop open the fractures. Te particles should have
a specifc gravity only slightly above that of water in order to be transported effectively
and have high compressive strength. Sand, treated sand, ceramics, and resin coated
walnut shells are examples.
• Friction reducer—Friction reducers are required to reduce pressure loss while pumping
at high rates (50 to 120 barrels per minute) down the long, narrow (∼6 inch) pipes.
Common friction reducers are anionic, cationic, and nonionic polyacrylamide-based
and applied at the rate of ∼0.025% of the fluid. Tese work by reducing the relative
roughness at the pipe/water interface.
• Biocides—Injection of water containing sulfate (an oxidized compound) will promote
growth of sulfate-reducing bacteria that produce hydrogen sulfde and black iron
sulfde deposits on the pipes. Common biocides are quaternary amines, glutaraldehyde,
tetra-kis-hydroxylmethylphosphonium sulfate, and tetrahydro-3,5-dimethyl-1,3,5-
thiadiazinane-2-thione. Reuse of flowback and produced water is another application
for biocides as the bacteria can attack the friction reducing polymer.
• Scale inhibitors—Scale can form on pipes and above ground equipment, lowering
flow rates and production and sometimes precipitating naturally-occurring
radioactive material (NORM). Most available scale inhibitors are phosponates and
organophosphonates.
• Clay stabilization—Shales are formed from muds that had a high percentage of clay
minerals. Some of the clays will come back with produced water. KCl is sometimes
used for clay stabilization.
• Surfactants—Surfactants promote draining of small fractures because they lower
the surface tension that holds water in small pores and fractures. Formulation of
surfactants in an emulsion promotes the surfactant going to the leading edge of the
fluid where it can be more effective.
• Acids—Acids such as HCl dissolve cementing minerals (e.g., calcium carbonate),
helping to initiate fractures.
Increasingly, due to water supply and environmental concerns, fracturing water coming
from flowback and produced waters is stored and reused in other or the same wells.
Te primary threat to surface water and shallow groundwater from hydraulic fracturing is from spilled or released material on the earth’s surface. Water coming back up the well
can be stored in surface impoundments or tanks, either of which can leak. Water may also be
treated then released to surface water, pumped offsite, or trucked offsite. Spills can occur during the pumping and/or hauling. Chemicals used in hydraulic fracturing must be transported
to and stored at the site, providing more opportunity for spills and accidents. Construction of
new roads and pads increases the potential for erosion and sediment generation from stormwater runoff. Pipes, valves, joints, and tanks can leak. Human errors can and will occur. Accidents will result in localized contamination of surface and shallow groundwater with any of
the chemicals associated with drilling, hydraulic fracturing, and gas production.
Proper road building with adequate culverts and steps toward erosion control can minimize sediment generation. Fracturing fluids, flowback, and produced water can be stored in
tanks with secondary containment. Chemicals can be safely stored and transported.
SUBSURFACE ISSUES
A key issue related to hydraulic fracturing is the potential migration of methane and other chemicals from the wells and shale formations into groundwater aquifers. Wells are normally completed far below the depth of groundwater aquifers by about a factor of 10 (thousands of feet for
gas versus hundreds of feet for useful water). For this reason the greatest threat to groundwater
quality is from spills on the surface that infltrate downward into the groundwater. Water quantity is also a concern given the millions of gallons of water used in the fracking process.
Methane is a common naturally-occurring compound that is frequently present in
groundwater at low concentrations and sometimes at high concentrations. For example, the
USGS (White & Mathes, 2006) found that methane concentrations in West Virginia in
170 wells spread throughout the state ranged from 0 to 68.5 mg/L. Methane was detected
in 131 of the 170 wells sampled and concentrations greater than 28 mg/L—where fre safety
becomes a concern—were found at 13 wells. Te presence or absence of methane (e.g., videos of homeowners lighting their water spigots) does not necessarily indicate anthropogenic
contamination of their groundwater. Methane is a mobile compound in the environment
and migrated prior to the addition of pathways related to drilling.
Methane (and potentially formation fluids and fracking additives) might reach groundwater along the preferential pathways caused by drilling. An ongoing EPA study (US Environmental Protection Agency, 2012) identifed four scenarios for subsurface migration from
below upward from the shale to a groundwater aquifer.
• Scenario A: Defective or inadequate well construction creates a migration pathway
through the cement seals along the well bore and/or in damaged rock near the well
bore, from the gas reservoir upward to the groundwater aquifer.
• Scenario B: Te rocks just above the shale are fractured, allowing upward migration to
the aquifer.
• Scenario C: Sealed or dormant previously existing cracks or faults are activated by the
hydraulic fracturing, creating an upward migration pathway.
• Scenario D: Hydraulic fracturing creates a migration pathway to other existing wells
(perhaps old, poorly-documented wells) that have defective seals along the wellbore.
Osborn (Osborn, Vengosh, Warner, & Jackson, 2011) found isotopic evidence of methane migration into overlying groundwater in Pennsylvania and New York. Tey analyzed 68
wells in northeast Pennsylvania and upstate New York for dissolved constituents and stable
isotopes. Te shallow groundwater in active drilling areas had higher methane concentrations and isotopic signatures consistent with thermogenic (deep) methane. Non-active areas
had lower methane concentrations and isotopic signatures more consistent with biogenic or
mixed methane sources. Te study had a limited number of wells and did not have the type
of before-and-after drilling data that could clearly establish that the elevated methane levels
were caused by hydraulic fracturing related drilling. With respect to abandoned wells, some
authors, such as Subra (Subra, 2010), suggest that regulatory mechanisms be implemented to
identify, evaluate, and track orphaned wells, in order to avoid intersecting these while conducting hydraulically fracturing.
NATURALLY OCCURRING RADIOACTIVE MATERIALS (NORM)
Te shale formations that contain natural gas also contain naturally-occurring radioactive elements including uranium, thorium, radium, 210lead, and their decay products. Tese materials
come up with the produced waters and can be concentrated when minerals present in the produced water precipitate from solution to form pipe scales and sludge. Scale that forms in pipes
and gas dehydrators is composed primarily of low solubility calcium, barium, and strontium
compounds that precipitate from the produced waters. Radioactive radium tends to go into
solid solution with the other precipitating ions and thus becomes concentrated in the scale.
Radon gas decay products polonium-210 and lead-210 may accumulate in the inside surfaces
of plant equipment. If produced waters are treated offsite (e.g., local treatment plant, tanks
used for hauling) then similar concentrative effects could become problematic. Operation and
disposal of equipment can be a concern, particularly if nuclear safety is not considered in
equipment salvaging operations.
Prevention of barium, strontium, and calcium precipitation is a consideration for reuse
of waters. Ideally the divalent cations will be kept in solution so that they, and the radium that
tends to co-precipitate with them, are reinjected.
ACCIDENTS
As much as anything, hydraulic fracturing technology means a large increase in industrial
activity—more wells, pads, trucks, etc.—in many areas that are now rural. Te technology
is new and rapidly evolving. Many of the potential impacts result from upset conditions and
accidents. Te casing or seals fail in the well, leading to groundwater contamination. A pipe,
tank, connector, or valve fails leading to a surface spill of contaminated water. Operator error
might lead to an explosion. Increased truck trafc means more trafc accidents. Most truck
accidents occur at ramps and intersections and lead differentially to deaths and injury of small
vehicle drivers rather than the truck drivers. Te truck accident rate in the rural areas associated with fracking will be higher than numbers based primarily on interstate highway driving.
Te frequency and consequences of these accidents will depend upon the safety standards applied by industry and regulatory agencies, but they will periodically occur given
the high number of wells anticipated and the complex and varied nature of the subsurface
environment. A simple calculation puts this in perspective. Each of the accident types has a
frequency or likelihood.
Using the binomial distribution we calculate the probability of one or more accidents
occurring as a function of the number of wells drilled and the frequency of each type of accident. At the extreme left side of the graph in Figure 2 are rare, potentially serious events (e.g.,
a blowout or explosion) that are assumed to be one in a million events. Moving to the right
are more common accidents such as minor spills or trafc accidents that are assumed to occur
at one out of 1,000 wells drilled. Te different lines represent the number of wells drilled and
operated with lines for 10,000, 50,000, 100,000 and 500,000 wells. As the number of wells
increases, accidents that were infrequent become likely. Te US EPA estimates that, as of
2012, 11,000 hydraulically fractured gas wells are being added each year (US Environmental
Protection Agency, 2013). Hydraulic fracturing, to be effective, requires a large number of
wells. Accident frequency can be lowered by adopting a culture of safety.
ATMOSPHERIC EMISSIONS
Emissions of dust and diesel fumes are generated from truck trafc and construction of roads
and drill pads. Emissions from drilling and fracturing are mostly due to engine exhausts and
fugitive vapor from fracturing fluids, and include nitrous oxides, diesel fumes, volatile organic
compounds (VOC), hazardous air pollutants (HAP, such as methanol), and fne particulates
(PM2.5). VOC emissions due to extraction depend on the composition of the gas in the reservoir and are typically rich in BTEX compounds (benzene, toluene, ethylbenzene and xylene).
Shale gas typically has negligible concentrations of H2S. Emissions from compressor engines
are nitrous oxide, carbon monoxide (CO), PM2.5, as well as VOC, and HAP emissions.
Low emission completion equipment (sometimes called green completions) has been
developed to lower methane emissions from pads and to improve recovery. Te idea behind
green completions is to separate and capture natural gas from the flowback water during
the clean-up stage of a completion. Te EPA natural gas star program (http://www.epa.gov/
gasstar/) represents industry/government collaboration on cost-effective strategies to reduce
methane emissions. Te recommended technologies and practices are organized in categories of compressors/engines, dehydrators, inspection and maintenance, pipelines, pneumatics/
controls, tanks, valves, wells, and “other”.
Fugitive releases of methane, ethane, propane, and other volatile organic compounds
from leaks and pressure-relief venting valves, flowback water, and other production activities
can potentially increase regional problems with ozone. Local factors influencing ozone buildup
are snow cover and temperature inversion in the atmosphere above a site. On April 17, 2012,
the EPA issued new regulations intended to limit air pollution emissions from the oil and gas
industry (US Environmental Protection Agency, 2013). Te regulations require equipment
to separate gas and liquid hydrocarbons from the flowback waters in order to increase energy
efciency and reduce volatile organic compound emissions from the well sites. Te equipment
will also lower emissions of air toxics such as benzene, ethyl benzene and n-hexane.
GLOBAL WARMING
Global warming impacts are a controversial and unresolved aspect of the shale gas revolution. Carbon dioxide and methane are both powerful greenhouse gasses. Burning of methane produces lower carbon dioxide emissions per unit of energy production than coal or oil.
However, since methane is a greenhouse gas itself, fugitive emissions of methane (e.g, from
the wellhead, pipelines, processing centers, homes) could potentially cause natural gas to
have a greater greenhouse impact than coal. Te IPCC (United States Environmental Protection Agency, 2013) (Intergovernmental Panel on Climate Change, 2007) estimates a Global
Warming Potential for greenhouse gases based upon the total energy a gas absorbs over a 100-
year period relative to carbon dioxide. On a mass basis methane has a Global Warming Potential 21 times greater than carbon dioxide, although the half-life of methane in the atmosphere
is a relatively short 12 years versus hundreds of years for carbon dioxide. Depending upon the
fraction of produced methane that escapes as fugitive emissions to the atmosphere, and the
timeframe considered (tens of years or hundreds of years), a switch from coal to natural gas
could either increase or decrease the rate of human-caused global warming.
One study (Howarth, Santoro, & Infraffea, 2011) estimated that 1.9% of the methane
produced from fracking is released as fugitive emissions at the wellhead. Total fugitive emissions were 3.6 to 7.9% of production. Over a 20-year time period the greenhouse effects of
fracked gas exceeded that of coal and over a 100-year time frame were comparable to coal.
In contrast, another study (Jiang, Grifn, Hendrickson, Jaramillo, VanBriesen, & Venkatesh, 2011) compared Marcellus shale gas with electricity from coal and estimated the
greenhouse impacts from natural gas were lower than coal. A major part of the difference was
the estimate of fugitive emissions of methane. Lifecycle impacts of different technologies are
difcult to assess, particularly when hard-to-quantify aspects such as fugitive emissions are of
critical importance. Green completion of wells, where gas and liquid hydrocarbons are separated from flowback, will reduce well-site methane emissions and is now required by the EPA.
Tis shifts the balance back towards gas being preferable to coal.
An alternative perspective is that comparison with coal is inappropriate. By lowering the cost of fossil fuels, hydraulic fracturing technology lowers the economic viability of
renewable energy sources such as solar and wind energy, thus delaying the transition to a
renewable energy economy. By expanding the economic viability and timeline of the carbonbased energy economy, hydraulic fracturing technology increases greenhouse gas emissions
and thus global warming.
CONCLUSION
Key impacts are:
Positive
• Lower-cost natural gas with associated lower electricity rates will improve the standard
of living for nearly everyone in the US
• Te economic activity associated with new oil and gas drilling will create thousands
of new mostly high-paying jobs; many of these jobs do not require a college degree,
meaning that they may also help with income inequality
• Lower energy prices make U.S. industry more competitive worldwide
Negative
• Signifcant land areas, many currently in natural states, will be developed
• Te impacts to natural areas will be dispersed on a grid spacing rather than
concentrated as is the case for most land development
Noise, trafc, and human resource issues will impact rural areas
• Accidents leading to air pollution, water pollution, and loss of life will occur
• Lower energy costs will extend the fossil fuel–based economy, thereby delaying the
transition to renewable energy sources, with consequent increases in greenhouse gas
emissions
Te most signifcant environmental impacts from hydraulic fracturing are a result of the
large degree of industrial activity required, the high number of wells associated with removing
resources from a tight formation, and the distributed nature of the resource. A large number
of well pads must be developed and distributed over a broad area rather than concentrated.
Dispersed, transient, heavy industrial activities have a variety of fairly well understood environmental ramifcations as described in the sections above. Contamination of groundwater
resources by the subsurface operations, while it will sometimes occur, will not be the major
environmental impact of unconventional gas extraction.
The environmental problems related to hydraulic fracturing can be minimized, but
not eliminated, by careful regulation that adheres to current best practices, encourages the
improvement of technology and the further development of better practices.
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Jiang, M., Grifn, W., Hendrickson, C., Jaramillo, P., VanBriesen, J., & Venkatesh, A. (2011). Life cycle greenhouse gas emissions of Marcellus shale gas. Environ. Res. Lett., 6, 9.
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Subra, W. (2010). Methane and Volatile Organic Chemicals in Air in DISH. Washingnon, DC: Earthworks.
United States Environmental Protection Agency. (2013). Retrieved 1 4, 2013, from Greenhouse Gas Emissions:
http://epa.gov/climatechange/ghgemissions/gases/ch4.html
US Environmental Protection Agency. (2012). Study of the potential impacts of hydraulic fracturing on drinking
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US Environmental Protection Agency. (2013, 1 4). Oil and Natural Gas Air Pollution Standards. Retrieved from
http://www.epa.gov/airquality/oilandgas/index.html
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1John Walton, Professor, University of Texas at El Paso, Dept. Civil Engineering, 500 West University Avenue, El Paso, TX,
79968. PhD University of Idaho, 1991, Chemical Engineering. Email: [email protected]. Phone: 915-539-5797.
2Arturo Woocay, Professor, División de Estudios de Posgrado e Investigación, Instituto Tecnológico de Ciudad Juárez, Ave.
Tecnológico 1340, Ciudad Juárez, CHIH 32500, MX. PhD, 2008, University of Texas at El Paso, Environmental Science
and Engineering. Email: [email protected]. Phone: (52-656) 688-2533.
