Shale gas development has significant negative and positive environmental impact. Negative environmental impact is due to the process of shale gas development and positive impact is through a substitution of clean natural gas to other energy sources. This paper is organised in three parts. In the first part, shale gas development and potential of shale gas is described. In the second part, environmental challenges in the development of shale gas are described. In the third part, environmental benefits as a result of shale gas are discussed.
Shale gas and its potential
Shale gas is natural gas that is stored in organic rich rocks such as dark-coloured shale, inter bedded with layers of shaley siltstone and sandstone [1]. Shale is a fine-grained rock made up of ancient compacted clays or mud and can be the source, reservoir and the seal for the gas. Shale gas plays are classified as a "continuous" type gas accumulations extending throughout large areas, typically with low permeability [1]. Shale gas is considered as unconventional gas source and same techniques used for conventional shallow gas development may also be used with shale gas drilling [1].
Shale gas is extracted through a process known as hydraulic fracturing which was developed in the United States in 1948 [3]. Hydraulic fracturing involves injecting a mixture of water, sand and chemicals into rock formations to stimulate oil and natural gas production shown in the figure 1.
Figure1: Shale gas extraction process [2]
Hydraulic Fracturing creates fractures that extend from a borehole into shale formations. Hydraulic fracture is formed by pumping a fracturing fluid into the well bore at a rate sufficient to increase the pressure down the bore hole fracturing the surrounding shale rock formation [3]. Solid proppant (like sieved round sand) is added to the fracture fluid to keep the fracture open after the injection stops.
Shale gas potential
Shale gas fields were largely untapped until recently due to the difficulty and cost to extract the gas. As conventional natural gas production has been decreasing, demand for the unconventional natural gas is more than ever now. This fact coupled with improvements in extraction methods has made it possible to explore the shale gas reserves.
Recoverable shale gas reserves increase total gas availability of the world by 40 percent per the report by Energy Information Administration (EIA) of USA. A new EIA-sponsored study reported initial assessments of 5,760 trillion cubic feet of technically recoverable shale gas resources in 32 foreign countries [6], with China topping the list with 1275 trillion cubic feet of shale gas reserves followed by USA with 862 trillion cubic feet [Appendix 1].
In 2010, U.S. shale gas production was 4.87 trillion cubic feet (23 percent of total U.S. natural gas production), compared with 0.39 trillion cubic feet in 2000. EIA predicts that shale gas will account for about 46 percent of U.S. natural gas production by 2035[6]. Rising production from shale gas resources in the U.S. has been credited with both lower natural gas prices and declining dependence on imported natural gas [6]. In most of the developing countries, development of shale gas reserves has not started yet. Energy hungry countries like India and China can greatly benefit from their recoverable shale gas resource as recent oil prices trend shows that oil is no longer a cheap commodity. At the current level of consumption, world may have 250 years supply of natural gas as result of new shale gas reserves around the world as per International Energy Agency (IEA) [7]. Shale gas is "a huge deal" per Shell's CEO Peter Voser. BP's ex CEO Tony Hayward hailed shale gas as "complete game changer" [8]. These comments from the prominent energy experts show the importance of shale gas as future potential energy source.
Figure 2: Estimated global shale gas recoverable resources
As is often the case with any resource development, shale gas production also has raised many environmental concerns, largely on the water and air. 2004 study by the EPA (Environmental Protection Agency) found hydraulic fracturing is harmless and the oil industry has been using similar extraction method since the 1940s. However many residents that are near to the shale gas exploration are claiming water and air quality problems. Potential negative environmental effect in the production of shale was discussed below.
Environmental impact
Environmental impact of shale gas development is not completely understood yet. Shale gas exploration in the province of Quebec has been put on hold until a full environmental study can be done [10]. Environmental Protection Agency (EPA) in U.S. has recently launched a new review on the hydraulic fracturing to understand the consequences. EPA anticipates finishing its latest study of the practice by 2012.
In the shale gas development process there is potential for the injected mixture seeping into aquifers in the event well casings fails even though shale is found thousands of feet below the water table. As the development of shale gas is comparatively recent, regulators don't have sufficient information and data to frame regulations accordingly to make shale gas development sustainable. In the developed countries like Canada and USA, strict regulations and effective enforcement can minimize the environmental impact. However in the developing countries like India and China where preference may be given to the energy than environment, shale gas development can create major problems in the day to day life of the people in terms of water and air quality.
Impact on drinking water
Hydralic fracking involves a lot of water. There are at least 11.5 million litters of water involved in fracking a well [9]. There are fluids (mostly water) that come out along with the natural gas in the shale gas development. These fluids shall be properly treated or pumped back to deep formations to minimize the consequences. In Dimock, Pnsilvania, leaks from badly cased wells contaminated drinking water wells and one even exploded [9]. Houston-based Cabot Oil and Gas spilled fracturing fluid, diesel and other fluids, according to Pennsylvania's Department of Environmental Protection [9]. And other parts of Pennsylvania state, fracturing fluid contamination has been detected in the Monongahela River, which is a source of drinking water. EPA tests have found suspected fracking fluid chemicals in drinking water wells, and a study by the New York State Department of Environmental Conservation identified 260 chemicals used in the fracking process [9]. Companies don't reveal their mixtures that are used in the fracking process as it is a trade secret. New York City analysis of fracking found that a single fractured natural gas well may not do any harm. But hundreds of wells are required to exploit shale gas which can bring an increased level of risk to the water supply.
Figure 3: A glass of water taken from a residential well after the start of natural gas drilling in Dimock, Pennsylvania, March 7, 2009. Photo: Tim Shaffer
Air pollution
A set of seven samples collected throughout Dish (small town in Texas), analyzed for a variety of air pollutants in the August 2010 found that benzene was present at levels as much as 55 times higher than allowed by the Texas Commission on Environmental Quality (TCEQ). Similarly, other chemicals like xylene and carbon disulfide (neurotoxicants), along with naphthalene (a blood poison) and pyridines (potential carcinogens) all exceeded legal limits by as much as 384 times [9].
Many companies distinguish themselves by how they filter out air pollutants. There are vapour recovery units that can put in place to cut out 95 percent of the emissions from a site. Town of Dish conducted air quality test and found toxic mix of air pollution. Texas Health and Human Services Commission is conducting tests on about 30 residents of Dish to see what might be the human health impact of this air pollution exposure [9].
Environmental benefits
If shale gas can be extracted safely, it might go a long way to cut emissions of greenhouse gases around the world from other fossil fuels. Positive environmental effects are through substation effect. As shale gas is abundantly available, more natural gas and less coal may be used to generate electricity. Department of Energy (DoE) in the U.S. expects 21 percent of U.S. electricity to be derived from natural gas by 2035, and by 2034 power plant builder and consulting firm Black & Veatch expects almost 50 percent of all U.S. electricity to come from burning natural gas [9]. Environmental benefits of substituting coal with natural gas cab be understood by studying the emissions of these fuels which is given in figure 4.
Pollutant Natural Gas Coal
Carbon Dioxide 117,000 208,000
Carbon Monoxide 40 208
Nitrogen Oxides 92 457
Sulfur Dioxide 1 2,591
Particulates 7 2,744
Mercury .000 0.016
Figure 4: Emission Levels (Pounds per Billion Btu of Energy Input) [11]
Substitution of oil with natural gas economic and environmental benefits
Natural gas vehicles have many benefits compared to conventional gasoline vehicles. Natural gas vehicle benefits are mainly in fuel cost savings and clean environment. Presently, there are 12 million natural gas vehicles around the world. The International Association of Natural Gas Vehicles (IANGV) is predicting 50 million vehicles by 2020 [15]. Figure 5 shows the natural gas vehicles growth around the world in the last twenty years.
Figure 5: Natural gas vehicle growth [15]
Cost effectiveness of natural gas fuel
Energy content of oil and natural gas is in the ratio of 6: 1. Traditional price ratio of oil to natural gas is in the range of 7: 1 to 10: 1 [13]. However based on the present prices, price ratio is about 25: 1. As the present ratio is relatively very high when compared to the energy content, more consumers are willing to buy natural gas vehicles as an alternative to the gasoline vehicles.
Pricing of natural gas
At present natural gas price costs up to 50 percent less compared to the oil, depending on the country. As more and more people use natural gas vehicles, there will be more and more demand for natural gas, however, because of the availability of shale gas, natural gas prices may not go up significantly.
Figure 6: Average fuel prices comparison
Greenhouse gases and pollutants
Natural gas is environmentally friendly compared to oil. Natural gas requires less processing before use, where as gasoline requires refining. High methane in the natural gas gives octane rating of 120 to 130 [14]. Natural gas emits less green house gases and other pollutants into the environment. Environmental benefits of natural gas compared to oil are shown in figure 7.
Pollutant Natural Gas Oil
Carbon Dioxide 117,000 164,000
Carbon Monoxide 40 33
Nitrogen Oxides 92 448
Sulphur Dioxide 1 1,122
Particulates 7 84
Mercury .000 0.007
Figure 7: Fossil Fuel Emission Levels (Pounds per Billion Btu of Energy Input) [11]
Water intensity and energy efficiency
Water is a very precious resource. Natural gas in its life cycles uses less water compared to any other fossil fuel (Figure 8). Also, energy efficiency of natural is relatively high compared to other energy sources (Figure 10) and energy consumption of natural gas is comparatively less (Figure 9).
Figure 8: Water intensity of transportation fuels [12]
Figure 9: Total Energy consumption of select alternative fuels [12]
British Thermal Unit (BTU)/Mile
Figure 10: Energy efficiency of various alternative fuels [12]
Conclusion
Because of recoverable shale gas reserves, natural gas is abundantly available. Development of shale gas reserves has environmental challenges. If shale gas can be safely developed, it can be used to reduce green house gas emissions and other pollutants and natural gas can be prominent fuel for power generation and personal transportation.
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