Environmental Impact Associated with Shale Gas Extraction
X N Li
*
, G L Liu, F S Zhang and Y J Lu
Oil Chemistry Key Laboratory, CNPC (Research Institute of Petroleum Exploration
and Development, PetroChina), Beijing, China
Abstract. Countries around the world are now stepping up efforts to develop the extraction of
shale gas resource. Shale gas, as an unconventional energy with huge reserves and broad
development prospects, has an increasing impact on the global energy market. However, the
environmental impact brought by shale gas extraction has also attracted wide attention from
the society. For example, ground water and air pollution and the negative influence on local
ecological system caused by hydraulic fracturing have become the focus of attention from the
society and public opinion. In this paper, the environmental impact caused by hydraulic
fracturing on local water resource, surface water and ground water contamination, air
pollution, and induced earthquakes are summarized and discussed. In the future, this analysis
will have guiding significance for environmental protection in the process of shale gas
extraction.
1. Introduction
Shale gas refers to the unconventional natural gas that is trapped within the reservoir rock series
dominated by rich shale. It is a continuous generation of biochemical gas, thermal genetic gas or the
mixture of the two, which can exist in natural fractures and pores with very low permeability. Since
shale gas is a clean and high efficient energy resource, it is becoming a hotspot in new energy
research area. Shale gas is also regarded as a vital strategic resource, and in recent years, lots of
countries have increased the development strength on shale gas extraction. US energy information
administration (EIA) estimates the technically recoverable shale gas is 1.87×10
14
m
3
in the US,
constituting 26 % of the domestic natural gas resources [1]. China has 1.35×10
14
m
3
shale gas
reserves, mostly in Sichuan and Tarim basin, and the technically recoverable shale gas is 2.5×10
13
m
3
[2].
2. Environmental issues on shale gas extraction
The main environmental risks related to shale gas extraction include water, air quality and habitat
classification. So far, the impact of water and air quality is the most heated debated. The potential
negative environmental impacts are a major obstacle to shale gas development in many parts of the
world [3].
Li, X., Liu, G., Zhang, F. and Lu, Y.
Environmental Impact Associated with Shale Gas Extraction.
In Proceedings of the International Workshop on Environmental Management, Science and Engineering (IWEMSE 2018), pages 89-93
ISBN: 978-989-758-344-5
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
89

2.1. Hydraulic fracturing process
The process of extracting nature gas from shale gas formation is called hydraulic fracturing. The
hydraulic fracturing process utilizes fluid and proppants to generate small fractures in the tight shale
formation, creating pathways for the gases and oils to go from the reservoir to the wellbore. The
fracturing fluid is usually consists of water, proppants, and chemical additives. Specifically, the
fracturing fluid is pumped under very high pressure through the perforations on the horizontal well
[4]. The fracturing fluid and mainly the high pressure help to open up the existing fractures or crack
the shale and create new fractures that extend out into the surrounding rock. Once been cracked up,
these fractures can continue to propagate for hundreds of feet and then be prop up by the proppants.
Thus, the natural gas and the oil that exist in the shale are able to come out to the wellbore. However,
in many cases, the horizontal well is too long to maintain sufficient pressure to fracture the shale
across its entire length. To solve this problem, the well is then divided into several stages separated
by plugs [5].
The hydraulic fracturing starts from the farthest stage to the nearest stage from the wellhead. The
high pressure first comes to the farthest stage of the horizontal well and starts fracturing, after this
stage is finished, this area is locked up with a plug. Then the high pressure comes to the previous
stage, which is nearer to the start of the wellhead and repeats the same technological process. The
plugs of each stage are later drilled through and the well is depressurized. After the pressure is
released, the shale gas and shale oil go into the well driven by the pressure gradient, as well as the
fracturing fluid and formation water [6].
2.2. Water lifecycle for hydraulic fracturing process
Figure 1. Water lifecycle in hydraulic fracturing process [8].
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Shale gas production needs lots of water. Water management is one of the major challenges faced by
the unconventional oil and gas operators. The total cost of water management can be as high as 12 %
of the operation cost of a shale gas extraction well [7].
The specific water lifecycle in hydraulic fracturing process is showed in Figure 1, which includes
water acquisition, chemical mixing, well injection, flowback and produced water storage, and
wastewater treatment and waste disposal. Potential environmental issues caused by each process is
discussed as follow.
2.3. Environmental impact on water resource
Water makes up more than 87 wt% of the hydraulic fracturing fluid. The hydraulic fracturing process
requires large amount of water being injected into the formation in a short period of time. The water
mainly comes from surface water and ground water resource, and sometimes the water comes from
municipal water and reused water. At shale gas extraction places, the large amount of water needed
for hydraulic fracturing process will compete with other water uses and endanger aquatic habitats.
2.4. Surface water and groundwater contamination
During chemical mixing process, a variety of chemicals are used in hydraulic fracturing fluid to
complete the fracturing job. These chemicals are mixed with water and proppants before injection,
and some of them have negative effects on human health as well as the ecosystem [9]. In accidental
events, it is possible for the chemicals to be released into the environment through leaks and spills,
contaminating the surface water, groundwater and soil.
During the well injection process, the main environmental concern is to protect the groundwater
aquifer, which is a major source for drinking water supply. Different problems will be faced
depending on the well completion practices. Well completion needs to be carried out in either typical
deep shale gas formations or shallower shale gas formations. During the well construction, several
layers of casing and cement are supposed to isolate the fluids and gases inside the well from outside
geological formation. Specifically, the surface casing is designed to extend below the base of the
deepest groundwater aquifer and be cemented all the way to the ground surface, which is expected to
effectively isolate groundwater aquifer from the drilling environment [10]. However, failures in
casing and cementing of the well might cause the leak of fracturing fluids or shale gas/oil from the
well into the groundwater aquifer. Drilling through ground water aquifer also might disturb and alter
ground water redox conditions and pressure gradients. At the end of the production life of the well,
the well needs to be properly plugged to prevent fracturing fluid and flowback water migration that
could contaminate soils and ground water [11].
The hydraulic fracturing process also generates a large amount of flowback water. The flowback
water contain various naturally occurring substances in the formation, such as gases, oil and grease,
total suspend solids (TSS), total dissolved solids (TDS), NORM (naturally occurring radioactive
substances) [12]. Additionally, residual of chemical additives used in the fracturing fluid can be
found in the flowback water. The flowback water is usually treated on-site or transported to other
facilities to be treated or disposed of. Inadequate treatment before discharge or disposal or accidental
spill during transportation could lead to contamination of surface and groundwater resource. Once it
is not handled properly, these actions will cause serious environmental contamination, since ground
water aquifer is difficult to recover.
2.5. Cause of air pollution
Most of the shale gas sites are located in arid regions, where water is quite cherished. Thus, reliable
access to water supply is quite difficult to achieve in these areas, especially during drought seasons.
To solve this problem, fresh water is transported to the site by heavy-duty trucks. The diesel powered
heavy-duty trucks cause emissions of pollutants that affect the local air quality [13]. The immediate
Environmental Impact Associated with Shale Gas Extraction
91

pollutants are VOCs, NOx, and PM2.5 and PM10. Moreover, the presence of high level of VOCs
andPM 2.5 and PM 10 is known to harm the human respiratory system [14]. Also, many types of
equipment used for hydraulic fracturing such as compressors and pumps require fuel combustion.
These activities increase atmospheric emission of air pollutants, which affects the local air quality.
During shale gas extraction process, another potential air pollutant is the crystalline silica dust
originated from the silica proppant. It is generated during the transportation and mixing of sand into
the fracturing fluid. The acute toxicity of silica is low to moderate. However, crystalline silica has
chronic effects to cause silicosis, which is a progressive lung disease which may result in lung cancer
in humans. In the United State, the National Institute for Occupational Safety and Health (NIOSH)
conducted a field study of silica exposure in 11 hydraulic fracturing sites in five different states [15].
Among the 116 samples collected, 79% samples have levels of crystalline silica that exceeded
NIOSH recommended exposure limits (REL, 0.05 mg/m
3
). 31% samples showed silica exposures >
10 times higher than REL and one sample is more than 100 times higher. The shocking results
highlighted the high occupational health risk associated with handling silica proppant.
After the hydraulic fracturing process, the fluid that returns to the surface before the well is put in
production is referred to as flowback water. The produced water is the fluid that returns to the surface
after the well is put to production. The flowback water and produced water are usually stored in on-
site tanks or pits before being treated, or transported for treatment and disposal. The storage tanks
and pits need to be properly managed to avoid potential leaks and storm water overflow. In addition
to surface and groundwater contamination, fugitive VOC emission from the flowback and produced
water might also be a concern for air pollution.
2.6. Induced earthquakes
It is well known that earthquakes can be induced by surface and underground mining, impoundment
of reservoirs, and injection or withdrawal of fluids and gases into or from the subsurface formation.
Both hydraulic fracturing and deep well injection of wastewater could result in induced seismicity
[16, 17].
The hydraulic fracturing process may induce a large number of micro-earthquakes which usually
lower than 1.0 magnitude. There are a few incidences when earthquakes large enough to be felt were
attributed to hydraulic fracturing activities; the biggest one happened at the Horn River Basin in
Canada in 2013 with a magnitude of 3.8, which did not pose significant risk. The investigation
carried out by British Columbia Oil and Gas Commission concluded that the event was caused by
fluid injection during hydraulic fracturing in proximity of pre-existing faults [18]. In 2014, a
hydraulic fracturing job was suspended in north England due to a potential link between its activity
and two seismicity reports of 2.3 and 1.5 in magnitude near the well [19].
Deep well injection of shale gas produced water has also been related to earthquakes. In
Youngstown, Ohio, 10 small earthquakes (all < 3.9 in magnitude) were recorded from April to
November, 2016. These earthquakes were linked to the operation of a Class II deep injection well
used to dispose of produced water as the fluid increased pore pressure along a pre-existing subsurface
faults located close to the wellbore [20]. Disposal wells are also linked to the increase in the seismic
activities in Oklahoma, including a 4.0 magnitude earthquake that occurred on October 15, 2013 [21].
3. Discussion and conclusions
Overall, the environmental impact associated with shale gas extraction mainly include habitat
division, fracturing fluid spills and leaks, local air quality, water resource, ground water aquifer, and
seismicity. Among them, the pollution on ground water aquifer and local air quality are the biggest
environmental issue caused by shale gas extraction. These problems should attract the government's
attention, since these environmental damage could harm the health of local residents, and these
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environmental damage is difficult to recover. Thus, the government should strengthen supervision
and make relevant laws to prevent the environmental pollution caused by shale gas extraction.
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