|Fig. 1: Shale gas extraction typically involves horizontal drilling along the shale formation in combination with hydraulic fracturing to release the trapped gas.|
Newly introduced drilling technologies have recently made the extraction of natural gas deposits from shale formations economically viable. Production of shale gas in the US grew by an average of 17% per year between 2000 and 2006, and by 48% per year between 2006 and 2010.  There exist various reasons for interest in these so-called unconventional gas resources, including claims that they can provide energy independence for the US and can help mitigate global warming.
The shale gas extraction process is characterized by the recent combination of four core techniques, the first of which is directional drilling as shown in Fig. 1.  In contrast to gas extracted by conventional means, shale gas is confined to a region of underground shale that is relatively shallow in extent (not in depth). In addition, the gas being accessed is located throughout the shale deposit, rather than in large pockets as in conventional wells. Thus, to access a significant portion of the stored gas, it is advantageous to drill horizontally along the shale formation. Directional drilling technology makes this possible by allowing a well to be drilled vertically down to the shale, then horizontally along it.
In order to access the gas that is trapped between the layers of shale, a second technique, hydraulic fracture, is used.  In the case of shale, high volumes of hydraulic fracturing fluid (frack fluid), composed primarily of water and sand, are pumped into the well at high pressures. The fluids force their way into the existing fractures in the shale, spreading them and allowing access to the trapped gas. Proppants like sand, so-called because they fill in these fractures and prop them open, are included in the frack fluid to allow the gas to continue to escape after the fluid pressure is relieved.
The advent of a third technology, slickwater, allows the high volumes of frack fluid to do their job more easily, making high volume hydraulic fracture more feasible. Slickwater describes the addition of friction reducers which allow the fluid to be pumped at higher rates while maintaining laminar flow. In addition, other compounds, including biocides, fluid-loss additives, and acid corrosion inhibitors, are added to achieve the desired fluid properties. 
Lastly, shale gas extraction is characterized by multi-well pads. In conventional drilling, a single well is created per pad in order to tap an underground pocket of gas; drilling multiple wells on the same pad would typically end up tapping the same pocket multiple times rather than reaching more gas. The distributed nature of shale gas means that multiple wells drilled in relatively close proximity will reach more of the shale, and therefore more gas. Thus, more wells are drilled to extract the gas.
With the rise in unconventional gas extraction in a short period of time, concerns have arisen about its environmental and health effects. A 2011 study found evidence that linked gas-well drilling and hydraulic fracturing with a concentration of methane in nearby shallow groundwater 17-times greater than that of surrounding aquifers.  The gas discovered in these wells was found to be consistent with that from deeper thermogenic sources such as the Marcellus shale, rather than the biogenic sources sometimes naturally found in groundwater. These increased concentrations of methane present an explosion hazard when the well-water is used by homeowners and well-casing leaks have been linked to explosion deaths in Pennsylvania.  Commercial natural gas has mercaptan added to give it a foul odor; if this mercaptan is not present, the gas becomes more dangerous because it is much more difficult to detect.
Of perhaps greater concern is the contamination of water from hydraulic fracturing fluids. From 2005 to 2009, 866 million gallons of hydraulic fracturing products were used by oil and gas companies. Over 650 of these products "contained chemicals that are known or possible human carcinogens, regulated under the Safe Drinking Water Act, or are listed as hazardous air pollutants."  Though methane has been known to contaminate groundwater aquifers through suspected well-casing leaks, the evidence for direct groundwater contamination from fracking fluid is not as strong.  Gas industry officials and even EPA directors have emphasized this, repeating that there are no documented cases of groundwater having been contaminated by hydraulic fracturing fluid. There was, however, a case of contamination documented in a 1987 EPA report where fluid may have migrated into an abandoned gas well which itself leaked into groundwater.  Other possible cases have been sealed by nondisclosure agreements included in settlements between gas companies and landowners, making investigation difficult. Thus, the extent of aquifer contamination is not wholly known, though it would appear to be quite infrequent. As of December 2011, the EPA has released a draft report of their ground water investigation in Pavillion, Wyoming, which links hydraulic fracturing to the local water contamination.  However, it is important to note that the study has not yet undergone peer review and that the study itself indicates that the gas wells involved were much shallower than most are. Research into these potential contamination issues is ongoing.
Due to the enormous volumes of hydraulic fracturing fluid used in shale gas production, great deals of waste water must be dealt with. Sometimes this fluid is sent to waste water treatment plants to be treated and released back into natural bodies of water. However, wastes often contain significant amounts of radioactive materials collected from deep underground and other contaminants that cannot be adequately filtered by treatment plants, which are then released into rivers.  Testing for radioactivity in sewage is not required by federal law, so not much hard data exists about the outflow of these facilities. It is reported, however, that "in 2009 and 2010, public sewage treatment plants directly upstream from some of these drinking-water intake facilities accepted wastewater that contained radioactivity levels as high as 2,122 times the drinking-water standard."  Drinking water is not the only concern, as consumption of contaminated fish is similarly harmful. Pennsylvania wells alone produced over 1.3 billion gallons of wastewater from 2008-2010, and this number is only expected to rise, despite recycling of some waste in fracking operations.
In addition to contamination, hydraulic fracture practices have other indirect effects. While natural gas derived from unconventional drilling is often cited as having a less detrimental effect on global warming than other energy resources in current use, recent research (as of 2011) has called this assumption into question.  It is true that natural gas, including unconventional gas, produces a smaller quantity of carbon dioxide per megajoule of power provided. However, methane itself is a powerful greenhouse gas, and taking this into account has a considerable effect. Due to release of methane into the atmosphere, shale gas extraction has a higher greenhouse gas footprint than conventional gas and at least 20%, but up to two times as great as that of coal when used for combustion and considered over a 20-year time horizon. The time period considered is important, since methane has a shorter lifetime in the atmosphere than carbon dioxide does. Thus, at the 100-year horizon, the footprint of shale gas is comparable to that for coal. The comparisons to oil at each interval are even less favorable.  The data used for this study are not ideal and have a fair amount of uncertainty, but at the very least indicate the need for future studies to consider fuel production as well as consumption, including the effects of methane, and suggest that shale gas may not in fact have a positive impact in mitigating global warming.
While many, especially industry leaders, tout the potential benefits of shale gas extraction, it is essential to take a measured look at its impact. Much is unknown about the environmental and health effects of unconventional drilling as well as the quantity of gas that it can provide. Estimates of resources and reserves vary considerably and are constantly changing. In its April 2011 Energy Outlook, the EIA estimated that the Marcellus shale formation contained about 400 trillion cubic feet (tcf) of natural gas, around 17 years of US gas consumption at the current rate. [10,11] Several months later, the US Geological Survey report estimated total Marcellus shale resources of about 84 tcf, or about 3.5 years of gas consumption.  These and other environmental and health concerns, combined with supply estimate downgrades call into question the viability of shale gas as a 'bridge' fuel from oil and gas to renewables. If new studies continue to erode the luster of unconventional gas resources, it may become necessary for the United States to draw up plans for a new bridge.
© Nathan Z. Bogdanowicz. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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