Hydraulic Fracturing: importance/impacts/implications
This webpage examines the natural gas sector’s practice of hydraulic fracturing. This practice is also known as fracking, fracing and fraccing. The website is organized into four primary areas, including the importance of unconventional gas reservoirs, the conduct of hydraulic fracturing, its regulation, and potential environmental impacts of fracturing. Empirical data from the Colorado Oil and Gas Conservation Commission , Duke University, and the National Research Council are used to critically assess the risk of potential impacts. Other sources of information from government, industry, environmental organizations are provided for additional inquiry.
What is Hydraulic Fracturing?
As seen in the above photo, the water tanks, sand mover, and chemical trucks contain the principal constituents of hydraulic fracturing fluid. The hydraulic fracturing flow back tank contains so-called produced water; that is, the portion of the hydraulic fracturing fluid injected underground that returns to the surface. Note the private residence in the background, highlighting one environmental controversy associated with hydraulic fracturing. To fully appreciate a hydraulic fracturing operation, however, it is necessary to first understand the development of a natural gas well. Central to a gas well is its wellbore, the conduit through which various fluids and, eventually, gas flow.
The illustration accessed by the link below (Slide 17 of the presentation) shows organization of the vertical section of a wellbore. It is important, first, to appreciate the scale; the producing formations (or production zone) are thousands of feet deep, whereas the groundwater aquifers are typically only hundreds of feet deep. Also note the multiple layers surrounding the wellbore. In addition to the several layers of steel that compose the conductor pipe and surface, intermediate and production zone casings, each of these layers (except for certain sections of the intermediate casing) is encased with cement through the zones that they traverse. If the integrity of these barriers is maintained, wellbore-groundwater aquifer communication is prevented.
A key limitation of a vertical wellbore is its limited contact with the hydrocarbon-bearing, or production, zone. A relatively recent technical advancement, horizontal drilling, has overcome this limitation by permitting greater wellbore-production zone contact, as depicted in the figure at the top of this webpage. This advancement has been augmented by multiple perforations of the production zone casing (and its encasing cement), thereby allowing greater flow of hydrocarbons from the production zone into the wellbore. This hydrocarbon flow results from the pressure difference between the production zone (high) and the wellbore (lower).
The porosity of shale, however, is less than that of sandstone, so hydraulic fracturing is employed to both increase pre-existing pore diameter and create new fractures. These modifications to the shale reservoir improve natural gas flow from the production zone to the wellbore and have proven to be revolutionary. Recovery of available gas has increased from as little as 2% to as much as 50%; 15-35% is the usual recovery range.
To hydraulically fracture an unconventional reservoir, large volumes of fracturing fluid are injected into the production zone via the wellbore at high pressures. Fracturing fluid consists of 3 primary constituents, water, sand and other constituents, in the composition illustrated in the pie chart below. It is the grouping of other constituents that has proven controversial. While some of these constituents are innocuous (e.g., an iron precipitation controller such as citric acid, a common food additive), others are less so (e.g., a scale inhibitor such as ethylene glycol, a principal component of automobile engine coolant). The oil and gas sector has historically regarded the composition of fracturing fluid as proprietary, but has become more transparent of late (refer to For more information section elsewhere on this webpage for link to website disclosing well-specific fracturing fluid composition).
A hydraulic fracturing operation is conducted in three general stages. In the first stage, fracturing fluid without proppant is injected into the wellbore to prime the production zone. Fracturing fluid with proppant follows; the proppant maintains enlarged pre-existing fractures and induced fractures in their open positions. In the final stage, fracturing fluid without proppant is injected to flush fluid with proppant remaining in the wellbore into the production zone. As shown in the illustration accessed by the link below (Slide 11 of the presentation), a multi-layered deposition of proppant in a fracture is best for maintaining it in as open a position as possible and thereby augmenting gas flow from the production zone to the wellbore. While variable, the vertical extent of fractures is usually a few hundred feet; recall that the production zone is thousands of feet below the surface (and below shallow groundwater aquifers from which drinking water is typically obtained).
Much of the injected fracturing fluid returns, via the wellbore, to the surface. In the short-term time frame, 20-40% of the injected fluid returns; longer-term, up to 80% may return. This fluid, produced water, contains not only its original constituents but also constituents acquired during passage through the production zone. These acquired constituents may include some or all of organic and inorganic compounds, metals and naturally-occurring radioactive materials.
Why are Unconventional Gas Reservoirs Important?
Until recently, U.S. oil and gas production occurred exclusively from conventional reservoirs. Conventional reservoirs (or plays, over a large geographic area) have hydrocarbons such as petroleum and natural gas sequestered in a relatively porous rocks such as sandstone. It has long been appreciated that oil and gas are also present in less porous rocks, shale frequently being the host rock. The macro-and microscopic pictures of sandstone and shale below allude to the challenge of extracting hydrocarbons from the less porous shale.
Natural gas resources contained in unconventional plays now comprise approximately 25% of total U.S. natural gas resources. The six principal plays contain about 600 of the 850 trillion cubic feet of unconventional gas resources in the U.S. The map and pie chart below illustrate locations and contributions, respectively, of these six plays: the Marcellus, Haynesville, Barnett, Fayetteville, Woodford, and Eagle Ford Shales. While the Barnett Play is of historical significance, given that economic unconventional gas production was first demonstrated in this play, the Marcellus Play has the greatest potential for gas production.
How is Hydraulic Fracturing Regulated?
At the federal government level, hydraulic fracturing is primarily regulated by the Safe Drinking Water Act of 1974 and its Amendments. Specifically, the Underground Injection Control (UIC) Program protects the quality of underground drinking water sources. While hydraulic fracturing was exempted from this regulatory control (if its sole purpose is to increase gas production) by the Energy Policy Act of 2005, disposal of produced water from hydraulically-fractured gas reservoirs (refer to What is hydraulic fracturing?) into injection wells remains regulated. Many states have assumed primacy for the UIC Program.
At the state government level, in addition to the UIC Program (if assumed by the state), the responsible state agency can impose additional regulations. In Colorado, for example, the Colorado Oil and Gas Conservation Commission has promulgated regulations concerning fracturing fluid composition, well casing and cementing, and disposal of produced water in surface pits. Click here to access those regulations.
Stringency of state regulation of hydraulic fracturing is variable, but most states are increasing stringency of their regulations due to public concerns. A recent report from Resources for the Future, which presents their review of 20 regulatory elements for 31 states in which unconventional gas development is occurring, highlights variability of state regulation. For surface pit liner requirements, of the 6 Western Interstate Energy Board states reviewed, 1 state fell into the pits prohibited class, 3 states into the liner required class, and 2 states into the conditional liner requirement class.
At the local government level, regulation of hydraulic fracturing is upstream of well activities, where the responsible state agency prevails. Roads are an example of a locally-regulated upstream activity.
What Federal Regulatory Actions Have Occurred Recently?
The highest-profile regulatory action taken at the federal level of government in recent years has been the Bureau of Land Management (BLM)’s so-called hydraulic fracturing rule. This rule, finalized and published in early 2015, was promulgated to ensure environmentally-sensitive development of oil and gas resources on both Federal and Indian lands. It updated BLM regulations related to hydraulic fracturing that were 25-30 years old; that is, prior to widespread use of hydraulic fracturing by the oil and gas sector. The rule impacts nearly 50,000 active oil and gas leases and approximately 100,000 wells on public lands.
The hydraulic fracturing rule includes several new requirements. One such requirement concerns steel casings and cement (discussed above) that isolate the wellbore from adjacent groundwater aquifers. Monitoring and remedial action (if monitoring indicates inadequacies) of cementing, as well as a mechanical integrity test of the wellbore prior to commencement of hydraulic fracturing, are now required. Another requirement is that, generally, produced water from hydraulic fracturing must be stored in above-ground storage tanks instead of pits. Furthermore, fracturing fluid composition must be disclosed to BLM and the public; the online FracFocus Chemical Disclosure Registry (see Additional Resources below for link) is a suggested medium for disclosure.
In states in which hydraulic fracturing is regulated, a state or tribe can request a variance from BLM requirements if the state requirements are as or more protective than those of BLM. Several Western U.S. states, including Colorado, New Mexico, Utah and Wyoming, have promulgated hydraulic fracturing regulations in recent years. The BLM rule is intended to provide a backstop level of regulatory stringency.
The BLM’s hydraulic fracturing rule has been the subject of litigation. Soon after the rule’s publication, a pair of oil and gas sector trade associations and groupings of states (Colorado, North Dakota, Utah and Wyoming) and tribes (Southern Utes, Utes) sued the Department of the Interior. The trade associations and states prevailed in the U.S. District Court for the District of Wyoming in June, 2016. BLM has appealed that decision to the U.S. 10th Circuit Court of Appeals; in January, 2017, that federal court delayed oral arguments in the case (originally scheduled for January 17) by two months.
What State/Local Regulatory Actions Have Occurred Recently?
Colorado is the Western state with significant activity on the state/local regulatory front. Several local governments in the state had asserted control over land use permitting via either bans or moratoria on hydraulic fracturing dating back to 2012. The state agency responsible for oil and gas sector oversight (the Colorado Oil and Gas Conservation Commission; COGCC) and a sector trade association countered that bans on hydraulic fracturing effectively constituted production bans, given its importance for economic oil and gas production. In May, 2016, the Colorado Supreme Court sided with the COGCC and trade association on the hydraulic fracturing bans instituted by the cities of Fort Collins (in 2013) and Longmont (in 2012). The decision presumably impacts other local bans or moratoria on hydraulic fracturing in Colorado.
What are The Potential Environmental Impacts of Hydraulic Fracturing?
There are several potential environmental impacts of hydraulic fracturing in addition to those associated with conventional oil and gas exploration and production. Water and air quality impacts will be emphasized here, but seismic events, noise and socioeconomic impacts are additional concerns receiving attention.
Water quality impacts are the most publicized of potential environmental impacts of hydraulic fracturing. Water contamination can occur due to:
- lack of integrity of the steel and cement barriers to wellbore-groundwater aquifer communication (refer to What is hydraulic fracturing?). This possibility can materialize, as the case of groundwater contamination in Pavillion, WY demonstrates.
- propagation of induced fractures from the production zone to groundwater aquifers (refer to What is hydraulic fracturing?). This possibility is less likely to occur because production zones are typically several thousand feet deep, whereas groundwater aquifers are usually located at depths of just a few hundred feet. A 2011 study by investigators from Duke University found no evidence of migration of either fracturing fluid or production zone constituents such as naturally-occurring radioactive materials from the production zone to drinking water wells in several locations in the Marcellus Shale. They did, however, find evidence of migration of methane, the principal component of natural gas, from the production zone to drinking water wells. These investigators speculated that pre-existing fractures in the formations overlying the production zone formation permitted this upward migration of methane and that hydraulic fracturing facilitated this migration. This study is published in the peer-reviewed journal Proceedings of the National Academy of Sciences USA and can be accessed here.
- lack of integrity of surface pits. In addition to disposing of produced water by recycling or injection into UIC Program injection wells, produced water can be held in surface pits to permit evaporation of water (with subsequent treatment of remaining solids at a hazardous waste facility). If the lining of a surface pit is of insufficient integrity, contamination of groundwater aquifers can occur. In the Pavillion, WY case mentioned above, this source of contamination was also implicated.
In December, 2016, the Environmental Protection Agency (EPA) released its long-anticipated report that evaluates impacts on drinking water resources of the hydraulic fracturing water cycle. Potential impacts of this cycle are important, given that nearly 4000 public water systems are located within one mile of a hydraulically-fractured well. The hydraulic fracturing water cycle includes the following activities: water acquisition, chemical mixing, well injection, produced water handling, and wastewater disposal and reuse. For each activity, EPA evaluated potential for impacts on water resources and factors affecting frequency and severity of impacts.
Chemical mixing was found to be one activity with potential for impacts on surface water, or even groundwater, resources. Frequency and severity of impacts were correlated with spill volume. Another activity associated with potential impacts was well injection. Poor wellbore integrity and lesser distances between the targeted rock formation and underground water resources were found to increase impact frequency and severity. Finally, produced water handling and wastewater disposal (particularly in unlined surface pits) was found to impact surface water and groundwater resources.
The EPA study has been, in addition to its lengthiness (ordered by Congress in 2009), controversial. The report, at one point in time, contained the conclusion that hydraulic fracturing has not had “widespread, systemic” impacts on water resources. EPA’s Science Advisory Board advised against such a conclusion, whereas oil and gas sector trade associations disagreed with this advice. Allegations of White House official involvement in the report’s conclusion have also been made.
To access the EPA final report on impacts of hydraulic fracturing on water resources, click here
Water use is also of concern, especially in more arid Western U.S. states. While the volumes of water required for hydraulic fracturing operations are seemingly large (1-5 million gallons), they currently constitute a small fraction of total water use. For example, in Colorado less than 0.1% of state water use in 2010 was for fracturing. Water use projections remain relatively low through 2015, as discussed in the Colorado Oil and Gas Conservation Commission report.
Air quality impacts comprise another category of potential environmental impacts associated with hydraulic fracturing. The additional equipment required to conduct a fracturing operation results in additional emissions of:
- criteria air pollutants such as nitrogen oxides (pollutants designated by the EPA as having to comply with national standards)
- volatile organic compounds
- air toxics such as benzene
Some of these air pollutants also have produced water (refer to What is hydraulic fracturing?) as a source. Although all are of concern due to their association with chronic diseases such as cardiovascular and respiratory disorders and cancer, methane is of particular concern for another reason. Methane is a greenhouse gas of much greater potency than carbon dioxide; currently, much of this air pollutant escapes capture prior to the production phase of a natural gas well. Recently-promulgated regulations by the Environmental Protection Agency are intended to limit emissions of these air pollutants. Click here to access the regulations.
Other potential impacts of hydraulic fracturing include seismic events, noise and socioeconomic impacts. Of these possibilities, seismic events are of perhaps greatest interest. A recent National Research Council report concluded that, in the U.S., no seismic events could be unequivocally linked to fracturing operations. The same report did conclude that two U.K. seismic events, of magnitudes 2.5 and 1.7 on the Richter scale, could be linked to hydraulic fracturing. For reference, the San Francisco Earthquake of 1906 was of magnitude 8-9. The authors of this report postulated that an imbalance between fluid addition and withdrawal may precipitate seismicity; fracturing operations tend to have equal additions and withdrawals of fluid (refer to What is hydraulic fracturing?). Injection of produced water into Class II injection wells would therefore be predicted to have some associated risk of seismic activity. Click here to access the National Research Council report.
WIEB Briefing Paper
Links to other groups
The following websites represent a sampling of the abundant information available on hydraulic fracturing: