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Hydraulic fracturing is the propagation of fractures in a rock layer caused by the presence of a pressurized fluid. Some hydraulic fractures form naturally, as in the case of veins or dikes, and are a means by which gas and petroleum from source rocks may migrate to reservoir rocks. Induced hydraulic fracturing or hydrofracking, commonly known as fracking, is a technique used to release petroleum, natural gas (including shale gas, tight gas and coal seam gas), or other substances for extraction.^[a][1] This type of fracturing creates fractures from a wellbore drilled into reservoir rock formations.

Hydraulic fracturing has come under scrutiny internationally owing to concerns over possible chemical contamination of groundwater and air. Some countries have suspended or even banned it. The first use of hydraulic fracturing was in 1947, though the current fracking technique was first used in natural gas shales in the late 1990s in the Barnett Shale in Texas.^[1][2] The energy from the injection of a highly-pressurized fracking fluid creates new channels in the rock which can increase the extraction rates and ultimate recovery of fossil fuels.

Proponents of fracking point to the vast amounts of formerly inaccessible hydrocarbons the process can extract.^[3] Detractors point to potential environmental impacts, including contamination of ground water, risks to air quality, the migration of gases and hydraulic fracturing chemicals to the surface, surface contamination from spills and flowback and the health effects of these.^[4] State and federal regulatory agencies and the industry are working to address these concerns.^[5] The United States Environmental Protection Agency (EPA) is conducting a study, set to be released for peer review at the end of 2012, of hydraulic fracturing's impact on drinking water and ground water resources.^[6]

Schematic depiction of hydraulic fracturing for shale gas, showing potential environmental effects.

Geology[link]

Mechanics[link]

Fracturing in rocks at depth is suppressed by the confining pressure, due to the load caused by the overlying rock strata. This is particularly so in the case of 'tensile' (Mode 1) fractures, which require the walls of the fracture to move apart, working against this confining pressure. Hydraulic fracturing occurs when the effective stress is reduced sufficiently by an increase in the pressure of fluids within the rock, such that the minimum principal stress becomes tensile and exceeds the tensile strength of the material.^[7][8] Fractures formed in this way will typically be oriented perpendicularly to the minimum principal stress and for this reason, induced hydraulic fractures in wellbores are sometimes used to determine stress orientations.^[9] In natural examples, such as dikes or vein-filled fractures, their orientations can be used to infer past stress states.^[10]

Veins[link]

Most vein systems are a result of repeated hydraulic fracturing during periods of relatively high pore fluid pressure. This is particularly clear in the case of 'crack-seal' veins, where the vein material can be seen to have been added in a series of discrete fracturing events, with extra vein material deposited on each occasion.^[11] One mechanism to explain such examples of long-lasting repeated fracturing is the effects of seismic activity, in which the stress levels rise and fall episodically and large volumes of fluid may be expelled from fluid-filled fractures during earthquakes. This process is referred to as 'seismic pumping'.^[12]

Dikes[link]

High-level minor intrusions such as dikes propagate through the crust in the form of fluid-filled cracks, although in this case the fluid is magma. In sedimentary rocks with a significant water content the fluid at the propagating fracture tip will be steam.^[13]

History[link]

Fracturing as a method to stimulate shallow, hard rock oil wells dates back to the 1860s. It was applied by oil industries in Pennsylvania, New York, Kentucky, and West Virginia by using liquid and later also solidified nitroglycerin. Later the same method was applied to water and gas wells. The idea to use acid as a nonexplosive fluid for a well stimulation was introduced in the 1930s. Due to acid etching, created fractures would not close completely and therefore enhanced productivity. The same phenomenon was discovered with water injection and squeeze cementing operations.^[14]

The relationship between well performance and treatment pressures was studied by Floyd Farris of Stanolind Oil and Gas Corporation. This study became a basis of the first hydraulic fracturing experiment, which was conducted in 1947 at the Hugoton gas field in Grant County of southwestern Kansas by Stanolind.^[14][1] For the well treatment 1,000 US gallons (3,800 l; 830 imp gal) of napalm (gelled gasoline) and sand from the Arkansas River was injected into the gas producing limestone formation at 2,400 feet (730 m). The experiment was not very successful as deliverability of the well did not change appreciably. The process was further described by J.B. Clark of Stanolind in his paper published in 1948. A patent on this process was issued in 1949 and an exclusive license was granted to the Halliburton Oil Well Cementing Company. On March 17, 1949, Halliburton performed the first two commercial hydraulic fracturing treatments in Stephens County, Oklahoma, and Archer County, Texas.^[14] Since then, hydraulic fracturing has been used to stimulate approximately a million oil and gas wells.^[15] Significant R&D and technology demonstration were necessary before hydraulic fracturing could be commercially applied to shale gas deposits, due to shale's high porosity and low permeability. In the 1970s the federal government initiated both the Eastern Gas Shales Project, a set of dozens of public-private hydro-fracturing pilot demonstration projects, and the Gas Research Institute, a gas industry research consortium that received approval for research and funding from the Federal Energy Regulatory Commission.^[16] Over this time, Sandia National Laboratories was conducting research into microseismic imaging for use in coalbeds, a geologic mapping technique that would prove crucial for the commercial recovery of natural gas from shale as well as oil from offshore drilling rigs. In the late 1970s, the Department of Energy pioneered massive hydraulic fracturing in tight sandstone formations, a completion technique that would be improved upon for the economic recovery of shale gas in the future.^{[citation needed]}

In the Soviet Union, the first hydraulic proppant fracturing was carried out in 1952. In Western Europe, in 1977–1985 hydraulic fracturing was conducted at Rotliegend and Carboniferous gas-bearing sandstones in Germany, Netherlands onshore and offshore gas fields, and the United Kingdoms sector of the North Sea. Other countries in Europe and Northern Africa included Norway, the Soviet Union, Poland, Czechoslovakia, Yugoslavia, Hungary, Austria, France, Italy, Bulgaria, Romania, Turkey, Tunisia, and Algeria. ^[17]

In 1980s improvements of existing and implementation of new technologies for horizontal drilling increased its application in conventional drilling. Among others these improvements included usage of downhole drilling motors and telemetry equipment.^[15] In 1986, a joint DOE-private venture completed the first successful multi-fracture horizontal well in shale. The Department of Energy later subsidized Mitchell Energy's first successful horizontal drill in the north-Texas Barnett Shale in 1991.^[18] Mitchell Energy engineers would go on to develop the hydraulic fracturing technique known as 'slickwater fracturing' that started the modern shale gas boom.^[19]

In 2011, France became the first nation to ban the hydraulic fracturing.^[20][21]

Induced hydraulic fracturing[link]

Uses[link]

The technique of hydraulic fracturing is used to increase or restore the rate at which fluids, such as petroleum, water, or natural gas can be produced from subterranean natural reservoirs. Reservoirs are typically porous sandstones, limestones or dolomite rocks, but also include 'unconventional reservoirs' such as shale rock or coal beds. Hydraulic fracturing enables the production of natural gas and oil from rock formations deep below the earth's surface (generally 5,000–20,000 feet (1,500–6,100 m)). At such depth, there may not be sufficient permeability or reservoir pressure to allow natural gas and oil to flow from the rock into the wellbore at economic rates. Thus, creating conductive fractures in the rock is essential to extract gas from shale reservoirs because of the extremely low natural permeability of shale, which is measured in the microdarcy to nanodarcy range.^[22] Fractures provide a conductive path connecting a larger volume of the reservoir to the well, thereby increasing the volume from which natural gas and liquids can be recovered from the targeted formation. So-called 'super fracking', which creates cracks deeper in the rock formation to release more oil and gas, will allow companies to frack more efficiently.^[23] The yield for a typical shale gas well generally falls off sharply after the first year or two.^[24]

While the main industrial use of hydraulic fracturing is in stimulating production from oil and gas wells,^[25][26][27] hydraulic fracturing is also applied to:

• Stimulating groundwater wells^[28]
• Preconditioning rock for caving or inducing rock to cave in mining^[29]
• As a means of enhancing waste remediation processes, usually hydrocarbon waste or spills^[30]
• Dispose of waste by injection into deep rock formations
• As a method to measure the stress in the earth
• For heat extraction to produce electricity in an enhanced geothermal systems.^[31]
• To increase injection rates for geologic sequestration of CO₂

Method[link]

A hydraulic fracture is formed by pumping the fracturing fluid into the wellbore at a rate sufficient to increase pressure downhole to exceed that of the fracture gradient (pressure gradient) of the rock.^[32] The fracture gradient is defined as the pressure increase per unit of the depth due to its density and it is usually measured in pounds per square inch per foot or bars per meter. The rock cracks and the fracture fluid continues farther into the rock, extending the crack still farther, and so on. Operators typically try to maintain "fracture width", or slow its decline, following treatment by introducing a proppant—a material such as grains of sand, ceramic, or other particulates, that prevent the fractures from closing when the injection is stopped and the pressure of the fluid is reduced — into the injected fluid. Consideration of proppant strengths and prevention of proppant failure becomes more important at deeper depths where pressure and stresses on fractures are higher. The propped fracture is permeable enough to allow the flow of formation fluids to the well. Formation fluids include gas, oil, salt water, fresh water and fluids introduced to the formation during completion of the well during fracturing.^[32]

During the process fracturing fluid leakoff, loss of fracturing fluid from the fracture channel into the surrounding permeable rock, occurs. If not controlled properly, it can exceed 70% of the injected volume. This may result formation matrix damage, adverse formation fluid interactions, or altered fracture geometry and therefore decrease of production efficiency.^[33]

The location of one or more fractures along the length of the borehole is strictly controlled by various different methods which create or seal-off holes in the side of the wellbore. Typically, hydraulic fracturing is performed in cased wellbores and the zones to be fractured are accessed by perforating the casing at those locations.^[34]

Hydraulic-fracturing equipment used in oil and natural gas fields usually consists of a slurry blender, one or more high-pressure, high-volume fracturing pumps (typically powerful triplex, or quintiplex pumps) and a monitoring unit. Associated equipment includes fracturing tanks, one or more units for storage and handling of proppant, high-pressure treating iron, a chemical additive unit (used to accurately monitor chemical addition), low-pressure flexible hoses, and many gauges and meters for flow rate, fluid density, and treating pressure. Fracturing equipment operates over a range of pressures and injection rates, and can reach up to 100 megapascals (15,000 psi) and 265 litres per second (9.4 cu ft/s) (100 barrels per minute).^{[citation needed]}

Well types[link]

This unreferenced section requires citations to ensure verifiability.

While hydraulic fracturing is many times performed in vertical wells, today it is also performed in horizontal wells. When done in already highly-permeable reservoirs such as sandstone-based wells, the technique is known as "well stimulation".^{[citation needed]}

Horizontal drilling involves wellbores where the terminal drillhole is completed as a 'lateral' that extends parallel with the rock layer containing the substance to be extracted. For example, laterals extend 1,500 to 5,000 feet (460 to 1,500 m) in the Barnett Shale basin in Texas, and up to 10,000 feet (3,000 m) in the Bakken formation in North Dakota. In contrast, a vertical well only accesses the thickness of the rock layer, typically 50–300 feet (15–91 m). Horizontal drilling also reduces surface disruptions as fewer wells are required to access a given volume of reservoir rock. Drilling usually induces damage to the pore space at the wellbore wall, reducing the permeability at and near the wellbore. This reduces flow into the borehole from the surrounding rock formation, and partially seals off the borehole from the surrounding rock. Hydraulic fracturing can be used to restore permeability.^{[citation needed]}

Hydraulic fracturing is commonly applied to wells drilled in low permeability reservoir rock such as tigh sands, coalbeds or shales.^{[citation needed]}

Fracturing fluids[link]

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The two main purpose of fracturing fluid is to extend fractures and to carry proppant (usually a sieved sand or specific size sand) into the formation and stay there with out damaging the formation or production of the well. Two methods of transporting the proppant in the fluid is high-rate and/or high-viscosity. With high-viscosity you tend to have large dominant fractures, while with high-rate you tend to have small spread out micro-fractures.^{[citation needed]}

With high-rate, there can be a job known as a slickwater, where a fracturing fluid is pumped at a high rate to ensure the rate of the fluid carries the proppant down the well, through the perforations, and into the formation. With these types of jobs, there is usually a polymer pumped with the fluid known as a friction reducer. The purpose of a friction reducer is to reduce pressure loss to friction, thus allowing the pumps to pump at a higher rate without having greater pressure on the surface. Rate jobs have trouble at high concentrations of proppant thus more water is required to carry the same amount of proppant. Slickwater is usually the most-desired type of job on shale formations, but this is not always the case.^{[citation needed]}

The fluid injected into the rock is typically a slurry of water, proppants, and chemical additives. Additionally, gels, foams, and compressed gases, including nitrogen, carbon dioxide and air can be injected. According to Kathleen Hartnett White of the Armstrong Center for Energy and the Environment, of the fracking fluid over 99.5% is water and sand, and the chemicals accounts about 0.5%.^[35] According to the study prepared for the United States Department of Energy, the additives in the fracturing fluid accounts 0.5–2% while water accounts 98–99.5%.^[32]

There are more than 50 types of fluids that can potentially be used as fracturing fluids, following are the fracturing fluids used at more than 95% of fracturing jobs world wide:

• Conventional linear gels. These gels are cellulose derivatives (carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl cellulose, methyl hydroxyl ethyl cellulose), guar or its derivatives (hydroxypropyl guar, carboxymethyl hydroxypropyl guar) based, with other chemicals providing the necessary chemistry for the desired results.

• Borate-crosslinked fluids. These are guar based fluids cross-linked with Boron ions (from aqueous borax/boric acid solution). These gels have higher viscosity at pH 9 onwards and are used to carry proppants. After the fracturing job the pH is reduced to 3–4 so that the cross-links are broken and the gel is less viscous and is therefore pumped out.

• Organometallic-crosslinked fluids zirconium, chromium, antimony, titanium salts are known to cross-link the guar based gels. The cross-linking mechanism is not reversible. So once the proppant is pumped down along with the cross-linked gel and the fracturing part is done. The gels are broken down with appropriate breakers.^[36]

• Aluminium phosphate-ester oil gels. Aluminium phosphate and ester oils are slurried to form cross-linked gel. These are one of first known gelling systems. They are very limited in use currently, because of formation damage and the difficulty in clean-up.^{[citation needed]}

Hydraulic fracturing may use between 1.2 and 3.5 million US gallons (4.5 and 13 Ml) of fluid per well, with large projects using up to 5 million US gallons (19 Ml). Additional fluid is used when wells are refractured; this may be done several times.^{[36]:7, 33} Water is by far the largest component of fracking fluids. The initial drilling operation itself may consume from 6,000 to 600,000 US gallons (23,000 to 2,300,000 l; 5,000 to 500,000 imp gal) of fracking fluids. Various types of proppant include silica sand, resin-coated sand, and man-made ceramics. These vary depending on the type of permeability or grain strength needed. The most commonly utilized proppant is silica sand. However, proppants of uniform size and shape, such as a ceramin proppant, is believed to be more effective. Due to a higher porosity within the fracture, a greater amount of oil and natural gas is liberated.^[37] Sand containing naturally radioactive minerals is sometimes used so that the fracture trace along the wellbore can be measured. Chemical additives are applied to tailor the injected material to the specific geological situation, protect the well, and improve its operation, varying slightly based on the type of well. The composition of injected fluid is sometimes changed as the fracturing job proceeds. Often, acid is initially used to scour the perforations and clean up the near-wellbore area. Afterward, high-pressure fracture fluid is injected into the wellbore, with the pressure above the fracture gradient of the rock. This fracture fluid contains water-soluble gelling agents (such as guar gum) which increase viscosity and efficiently deliver the proppant into the formation.^[36] As the fracturing process proceeds, viscosity reducing agents such as oxidizers and enzyme breakers are sometimes then added to the fracturing fluid to deactivate the gelling agents and encourage flowback.^[36] The proppant's purpose is primarily to provide a permeable and permanent filler to fill the void created during the fracturing process so that oil and gas can flow to the wellbore. At the end of the job the well is commonly flushed with water (sometimes blended with a friction reducing chemical) under pressure. Injected fluid is to some degree recovered and is managed by several methods, such as underground injection control, treatment and discharge, recycling, or temporary storage in pits or containers while new technology is being continually being developed and improved to better handle wastewater and improve reusability.^[32] Although the concentrations of the chemical additives are very low, the recovered fluid may be harmful due in part to minerals picked up from the formation.^{[citation needed]}

There are a variety of chemicals that can be used to increase the viscosity. With any viscosity increase, some type of gelling chemical must be used first. One of the most common gelling agents used is guar (commonly used in ice cream as a thickening agent), but there are several others including guar derivatives and cellulose gelling agents. These are known as gel jobs. Oil can also be used to increase the viscosity for formations that are water sensitive, but are not considered gel. All of these are known as gels in the frac industry, but the main thing that links them is that they are long polymer chains, on the chemical level, to increase the viscosity. The rate of viscosity increase for several gelling agents is pH-dependent, and thus occasionally pH modifiers must be added to ensure viscosity of the gel.^{[citation needed]}

If a greater viscosity is needed than a gel job, an ion can be added known as a crosslinker. A crosslinker takes the long chemical chains and connects them, thus the concept of crosslink. There are several types of chemicals that can be used as a crosslinker (some only work with certain gels), boron being the common one used with guar gels. Diesel could be cross-linked since it is a long polymer, and creates a fluid almost identical to napalm (again good for formations sensitive to water or water is detrimental to production). Many crosslink chemicals are only effective in a certain pH range, thus pH modifiers almost always need to be run with crosslinkers or some crosslinker chemicals have a pH modifier premixed. These are known as crosslink jobs. Viscosity is used to carry proppant into the formation, but when a well is being flowed back or produced, it is undesireable to have the fluid pull the proppant out of the formation. For this reason, a chemical known as a breaker is almost always pumped with all gel or crosslinked fluids to reduce the viscosity. This chemical is usually an oxidizer or an enzyme. The oxidizer reacts with the gel to break it down to reduce the fluids viscosity, to ensure no proppant is pulled from the formation. An enzyme acts as a catalyst for the breaking down of the gel. Sometimes pH modifiers are used to breakdown the crosslink at the end of a hydraulic fracture job, since many require a pH buffer system to stay viscous.^{[citation needed]}

CO₂ and nitrogen are also pumped with the fluid occasionally. These are known as energized fluid jobs. These are usually pumped with a foamer to ensure the fluid mixes well with the gas. The quality of the job is the percentage of the gas by volume.

Initially it is common to pump some amount (normally 6000 gallons or less) of HCl (usually 28%-5%), or acetic acid (usually 45% -5%), to clean the perforations or break down the near well bore and ultimately reduce pressure seen on the surface. At this point, the pumps are usually brought up to the rate the job calls for or what can be accomplished without exceeding pressure limits. Then the proppant is started and stepped up in concentration. For slickwater it is common to include sweeps or a reduction in the proppant concentration temporarily to ensure the well is not overwhelmed with proppant causing a screen-off.^{[citation needed]}

Fracture monitoring[link]

Measurements of the pressure and rate during the growth of a hydraulic fracture, as well as knowing the properties of the fluid and proppant being injected into the well provides the most common and simplest method of monitoring a hydraulic fracture treatment. This data, along with knowledge of the underground geology can be used to model information such as length, width and conductivity of a propped fracture.^[32]

Injection of radioactive tracers, along with the other substances in hydraulic-fracturing fluid, is sometimes used to determine the injection profile and location of fractures created by hydraulic fracturing.^[38] Patents describe in detail how several tracers are typically used in the same well. Wells are hydraulically fractured in different stages.^[39] Tracers with different half-lives are used for each stage.^[39][40] Their half-lives range from 40.2 hours (Lanthanum-140) to 5.27 years (Cobalt-60).^[41] Amounts per injection of radionuclide are listed in the The US Nuclear Regulatory Commission (NRC) guidelines.^[42] The NRC guidelines also list a wide range or radioactive materials in solid, liquid and gaseous forms that are used as field flood or enhanced oil and gas recovery study applications tracers used in single and multiple wells.^[42] According to the NRC, some of the most commonly used include Antimony-124, Bromine-82, Iodine-125, Iodine-131, Iridium-192, and Scandium-46.^[42] A 2003 publication by the International Atomic Energy Agency (IAEA) provides a detailed description of tracer use, confirms the frequent use of most of the tracers above, and says that Manganese-56, Sodium-24, Technetium-m, Silver-m, Argon-41, and Xenon-133 are also used extensively because they are easily identified and measured.^[43] Other potentially suitable tracers are named in various patents.^[44][39][40] In terms of quantities used, the NRC gives the following examples: Iodine-131, gas form, 100 millicuries total, not to exceed 20 millicuries per injection; Iodine-131, liquid form, 50 millicuries total, not to exceed 10 millicuries per injection; Iridium-192, "Labeled" frac sand, 200 millicuries total, not to exceed 15 millicuries per injection; Silver-110m, liquid form, 200 millicuries total, not to exceed 20 millicuries per injection[5]

For more advanced applications, Microseismic monitoring is sometimes used to estimate the size and orientation of hydraulically induced fractures. Microseismic activity is measured by placing an array of geophones in a nearby wellbore. By mapping the location of any small seismic events associated with the growing hydraulic fracture, the approximate geometry of the fracture is inferred. Tiltmeter arrays, deployed on the surface or down a well, provide another technology for monitoring the strains produced by hydraulic fracturing.^{[citation needed]}

Horizontal completions[link]

Since the early 2000s, advances in drilling and completion technology have made drilling horizontal wellbores much more economical. Horizontal wellbores allow for far greater exposure to a formation than a conventional vertical wellbore. This is particularly useful in shale formations which do not have sufficient permeability to produce economically with a vertical well. Such wells when drilled onshore are now usually hydraulically fractured in a number of stages, especially in North America. The type of wellbore completion used will affect how many times the formation is fractured, and at what locations along the horizontal section of the wellbore.^[45]

In North America, shale reservoirs such as the Bakken, Barnett, Montney, Haynesville, Marcellus, and most recently the Eagle Ford, Niobrara and Utica shales are drilled, completed and fractured using this method. The method by which the fractures are placed along the wellbore is most commonly achieved by one of two methods, known as 'plug and perf' and 'sliding sleeve'.^{[citation needed]}

The wellbore for a plug and perf job is generally composed of standard joints of steel casing, either cemented or uncemented, which is set in place at the conclusion of the drilling process. Once the drilling rig has been removed, a wireline truck is used to perforate near the end of the well, following which a fracturing job is pumped (commonly called a stage). Once the stage is finished, the wireline truck will set a plug in the well to temporarily seal off that section, and then perforate the next section of the wellbore. Another stage is then pumped, and the process is repeated as necessary along the entire length of the horizontal part of the wellbore.^[46]

The wellbore for the sliding sleeve technique is different in that the sliding sleeves are included at set spacings in the steel casing at the time it is set in place. The sliding sleeves are usually all closed at this time. When the well is ready to be fractured, using one of several activation techniques, the bottom sliding sleeve is opened and the first stage gets pumped. Once finished, the next sleeve is opened which concurrently isolates the first stage, and the process repeats. For the sliding sleeve method, wireline is usually not required.^{[citation needed]}

These completion techniques may allow for more than 30 stages to be pumped into the horizontal section of a single well if required, which is far more than would typically be pumped into a vertical well.^[5]

Chemicals[link]

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A variety of additives are used in hydraulic fracturing fluids. Over the life of a typical gas well, up to 100,000 US gallons (380,000 l; 83,000 imp gal) of chemical additives may be used.^[47] While some of them are common and generally harmless, some are known carcinogens or toxic.^[47] The most common chemical used for hydraulic fracturing in the United States in 2005–2009 was methanol, while some other most widely used chemicals were isopropyl alcohol, 2-butoxyethanol, and ethylene glycol.^[47]

The 2011 US House of Representatives investigative report on the chemicals used in hydraulic fracturing states that out of 2,500 hydraulic fracturing products, "[m]ore than 650 of these products contained chemicals that are known or possible human carcinogens, regulated under the Safe Drinking Water Act, or listed as hazardous air pollutants".^[47] The report also shows that between 2005 and 2009, 279 products had at least one component listed as "proprietary" or "trade secret" on their Occupational Safety and Health Administration (OSHA) required Material Safety Data Sheet (MSDS). The MSDS is a list of chemical components in the products of chemical manufacturers, and according to OSHA, a manufacturer may withhold information designated as "proprietary" from this sheet. When asked to reveal the proprietary components, most companies participating in the investigation were unable to do so, leading the committee to surmise these “companies are injecting fluids containing unknown chemicals about which they may have limited understanding of the potential risks posed to human health and the environment” (12).^[47] Without knowing the identity of the proprietary components, regulators cannot test for their presence. This prevents government regulators from establishing baseline levels of the substances prior to hydraulic fracturing and documenting changes in these levels, thereby making it impossible to prove that hydraulic fracturing is contaminating the environment with these substances.^[48] Third-party laboratories are performing analyses on soil, air, and water near the fracturing sites to measure the level of contamination by some of the known chemicals, but not the proprietary substances, involved in hydraulic fracturing. Each state has a contact person in charge of such regulation.^[49]

Another 2011 study identified 632 chemicals used in natural gas operations. Only 353 of these are well-described in the scientific literature; and of these, more than 75% could affect skin, eyes, respiratory and gastrointestinal systems; roughly 40-50% could affect the brain and nervous, immune and cardiovascular systems and the kidneys; 37% could affect the endocrine system; and 25% were carcinogens and mutagens. The study indicated possible long-term health effects that might not appear immediately. The study recommended full disclosure of all products used, along with extensive air and water monitoring near natural gas operations; it also recommended that fracking's exemption from regulation under the US Safe Drinking Water Act be rescinded.^[50]

Some states have started requiring natural gas companies to "disclose the names of all chemicals to be stored and used at a drilling site," keeping a record on file at the state’s environmental agency, such as the case in Pennsylvania with the Department of Environmental Protection and in New York with the Department of Environmental Conservation.^[51] In addition, seven states now require companies report their chemical use through fracfocus.org. However, the continuing concern of some activists who oppose hydraulic fracturing is the lack of information really provided. According to Weston Wilson in Affirming Gasland, "about 50% or so of these MSDS sheets lack a specific chemical name, and some MSDS sheets simply claim 'proprietary' status and list none of the chemicals in that container."^[52] As a result, some activists are calling for specific disclosure of chemicals used, such as the Chemical Abstract Service (CAS) number and specific chemical formulas, and increased access to such information. In his State of the Union address for 2012, Barack Obama stated his intention to force fracking companies to disclose the chemicals they use,^[53] though the subsequent, proposed guidelines were criticised for failing to specify how drillers will disclose the chemicals they use.^[54]

Economic impact[link]

Hydraulic fracturing has been seen as one key methods of extracting unconventional oil and gas resources. According to the International Energy Agency, the remaining technically recoverable resources of shale gas are estimated to amount to 208 trillion cubic metres (7.3 quadrillion cubic feet), tight gas to 76 trillion cubic metres (2.7 quadrillion cubic feet), and coalbed methane to 47 trillion cubic metres (1.7 quadrillion cubic feet). As a rule, formations of these resources have lower permeability than conventional gas formations and therefore, depending on the geological characteristics of the formation, specific technologies, such as hydraulic fracturing, are required. Although there are also other methods to extract these resources, such as conventional drilling or horizontal drilling, hydraulic fracturing is one of the key methods making their extraction technically visible. The multi-stage fracturing technique has facilitated shale gas and light tight oil production development in the United States and is believed to do so in the other countries with unconventional hydrocarbon resources. Significance of the extraction of unconventional hydrocarbons lies also in the fact that these resources are less concentrated than of conventional oil and gas resources.^[3]

Environmental impact[link]

New York State Assembly members Robert Castelli and Steve Katz call for a moratorium on hydraulic fracturing in the Croton Watershed in October 2010.

Hydraulic fracturing has raised environmental concerns and is challenging the adequacy of existing regulatory regimes.^[55] These concerns have included ground water contamination, risks to air quality, migration of gases and hydraulic fracturing chemicals to the surface, mishandling of waste, and the health effects of all these.^[4][47][32]

A University of Texas study listed water contamination and consumption, blowouts, explosions, spill management, atmospheric emissions, and health effects as associated problems.^[15] The UT study described the environmental impact of each part of the hydraulic fracturing process, which included:^[56][57]

• Drill pad construction and operation
• Construction, integrity, and performance of the wellbores
• Injection of the fluid once it is underground (which proponents consider the actual "fracking")
• Flowback of the fluid back towards the surface
• Blowouts, often unreported, which spew hydraulic fracturing fluid and other byproducts across surrounding area
• Integrity of other pipelines involved
• Disposal of the flowback, including waste water and other waste products

All but the injection stage were reported to be sources of contamination.^[15]

Because hydraulic fracturing originated in the United States,^[58] its history is more extensive there than in other regions. Most environmental impact studies have therefore taken place there.

Research issues[link]

Several organizations, researchers, and media outlets have reported difficulty in conducting and reporting the results of studies on hydraulic fracturing due to industry^[59][60][61] and governmental pressure, and expressed concern over possible censoring of environmental reports.^[62][63][59] Researchers have recommended requiring disclosure of all hydraulic fracturing fluids, testing animals raised near fracturing sites, and closer monitoring of environmental samples.^[64] After court cases concerning contamination from hydraulic fracturing are settled, the documents are sealed. The American Petroleum Institute deny that this practice has hidden problems with gas drilling, while others believe it has and could lead to unnecessary risks to public safety and health.^[65]

One New York Times report claimed that the results of a 2004 EPA study were censored due to political pressure.^[59] An early draft of the study had discussed the possibility of dangerous environmental threats due to fracking, but the final report omitted this.^[59] The study's scope had been narrowed so that it only focused on the injection of fracking fluids, while omitting other aspects of the process.^[66] The 2012 EPA Hydraulic Fracturing Draft Plan was also narrowed thusly.^{[62][60][67][68]}

As of May 2012, the US Institute of Medicine and US National Research Council and were preparing to review the potential human and environmental risks of hydraulic fracturing.^[69][70]

Air[link]

The air emissions from hydraulic fracking are related to methane leaks originating from wells, and emissions from the diesel or natural gas powered equipment such as compressors, drilling rigs, pumps etc.^[32]

Shale gas produced by hydraulic fracturing causes higher well-to-burner emissions than conventional gas. This is mainly due to the gas released during completing wells as some gas returns to the surface, together with the fracturing fluids. Depending on their treatment, the well-to-burner emissions are 3.5% to 12% higher than for conventional gas.^[55] According to a study conducted by professor Robert W. Howarth et al. of Cornell University, "3.6% to 7.9% of the methane from shale-gas production escapes to the atmosphere in venting and leaks over the lifetime of a well." The study claims that this represents a 30-100% increase over conventional gas production.^[71] Methane gradually breaks down in the atmosphere, forming carbon dioxide, which contributes to greenhouse gasses more than coal or oil for timescales of less than fifty years.^[71][72] Several studies suggest that this paper is flawed,^{[73][74][75][76]} and Howarth's colleagues at Cornell agreed.^[77] Howarth et al. responded, "The latest EPA estimate for methane emissions from shale gas falls within the range of our estimates but not those of Cathles et al, which are substantially lower."^[78][79]

In 2008, concentrations near drilling sites in Sublette County, Wyoming were frequently above the National Ambient Air Quality Standards (NAAQS),^[80] though a 2011 study for the city of Fort Worth, Texas "did not reveal any significant health threats" traceable to fracking.^[81][82]

In some areas, elevated air levels of harmful substances have coincided with elevated reports of health problems among the local populations. In DISH, Texas, elevated substance levels were detected and traced to fracking compressor stations,^[83] and people living near shale gas drilling sites complained of health problems;^[84] though a causal relationship to fracking was not established.^[84]

The EPA has proposed new regulations for controlling emissions from upstream oil and gas operations. These regulations would reduce emissions from aspects of the oil and gas production process including completions and various fugitive emissions. The regulations are scheduled to go into effect on April 17, 2012. However, the industry has requested a delay in implementation. ^[85]

Water[link]

Consumption[link]

The large volumes of water required have raised concerns about fracking in arid areas, such as Karoo in South Africa.^[58] During periods of low stream flow it may affect water supplies for municipalities and industries such as power generation, as well as recreation and aquatic life. It may also require water overland piping from distant sources.^[86] Over its lifetime an average well requires 3 to 5 million US gallons (11,000 to 19,000 m³) of water for the initial hydraulic fracturing operation and possible restimulation frac jobs.^[32][86] Using the case of the Marcellus Shale as an example, fracking accounted for 650 million US gallons per year (2,500,000 m³/a) or less than 0.8% of annual water use in the area overlying the Marcellus Shale as of 2010.^[86] To minimize water consumption, recycling is one possible option.^[55]

Waste water[link]

As the fracturing fluid flow back through the well, it consists of spent fluids and may consist of dissolved constituents such as minerals and brine waters. It may account about 30–70% of the original fracture fluid volume. In addition, natural formation waters may flow to the well and need also treatment. These fluids, commonly known as produced water, should be managed by underground injection, wastewater treatment and discharge, or recycling to fracture future wells.^[87] Treatment of produced waters may be feasible through either self‐contained systems at well sites or fields or through municipal waste water treatment plants or commercial treatment facilities.^[87] However, the quantity of waste water needing treatment and the improper configuration of sewage plants have become an issue in some regions of the United States. Much of the wastewater from hydraulic fracturing operations is processed by public sewage treatment plants, which are not equipped to remove radioactive material and are not required to test for it.^[88][89]

To mitigate the impact of hydraulic fracturing to groundwater the well and the shale formation itself should remain hydraulically isolated from other geological formations, especially freshwater aquifers.^[55] Several cases of groundwater contamination due to fracking waste water have been suspected.^[90][91] In 1987, an EPA report was published that indicated fracture fluid invasion into a well in West Virginia. The well, drilled by Kaiser Exploration and Mining Company, was found to have induced fractures that allowed fracture fluid to contaminate groundwater — though the oil and gas industry, as well as the EPA, questioned the accuracy of the report.^[65] In 2006, over 7 million cubic feet (200,000 m³) of methane were released from a blown gas well in Clark, Wyoming and shallow groundwater was found to be contaminated.^[92] Directed by Congress, the EPA announced in March 2010 that it will examine claims of water pollution related to hydraulic fracturing.^[93]

The 2010 film, Gasland, presented claims that chemicals polluted the ground water near well sites in Pennsylvania, Wyoming, and Colorado.^[94] Energy in Depth, an oil and gas industry group, called the film's facts into question;^[51] in response, a detailed rebuttal of the claims of inaccuracy was posted on Gasland's website.^[95]

Methane[link]

Groundwater methane contamination is also a concern as it has adverse impact on water quality and in extreme cases may lead to potential explosion.^[96] However, methane contamination is not always caused by fracking. Drilling for ordinary drinking water wells can also cause methane release. Some studies make use of tests that can distinguish between the deep thermogenic methane released during gas/oil drilling, and the shallower biogenic methane that can be released during water-well drilling. While both forms of methane result from decomposition, thermogenic methane results from geothermal assistance deeper underground.^[97][98]

According to the 2011 study of the MIT Energy Initiative, "there is evidence of natural gas (methane) migration into freshwater zones in some areas, most likely as a result of substandard well completion practices by a few operators."^[99] 2011 studies by the Colorado School of Public Health and Duke University also pointed to methane contamination stemming from fracking or its surrounding process.^[96][98] However, a study by Cabot Oil and Gas that examined the Duke study, and which used a much larger sample size, concluded that its conclusions were premature and unsubstantiated, and that there was no correlation between methane concentrations and proximity to gas production areas. Rather, their data showed that those concentrations were controlled by topography, with the highest readings found in low-lying areas. Using a more precise isotopic analysis, they showed that the methane found in the water wells came not from the Marcellus Shale where fracturing occurred, but from the shallower Upper or Middle Devonian formations.^[97]

Radioactivity[link]

Hydraulic fracturing fluid might release heavy metals and radioactive materials from the deposit which may reflow to the surface by the flowback.^{[100][101][102][103]} The New York Times has reported radiation in wastewater from natural gas wells, which releases into Pennsylvania rivers,^[89] and compiled a map of contamination levels,^[104] along with stating that some EPA reports were never made public. The Times' reporting on the issue has come under some criticism though.^[105][106] Recycling the wastewater has been proposed as a solution but has its limitations.^[107]

The EPA has asked the Pennsylvania Department of Environmental Protection to require community water systems in certain locations, and centralized wastewater treatment facilities to conduct testing for radionuclides.^[108][109] Safe drinking water standards have not yet been established to account for possible substances or radioactivity levels known to be in hydraulic fracturing waste water,^[89] and although water suppliers are required to inform citizens of radon and other radionuclides levels in their water,^[110] this doesn't always happen.^[111]

Seismic[link]

Hydraulic fracturing causes induced seismicity called microseismic events or microearthquakes. The magnitude of these events is usually too small to be detected at the surface, although the biggest micro-earthquakes may have the magnitude of about -1.6 (M_w). The injection of waste water from gas operations, including from hydraulic fracturing, into saltwater disposal wells may cause bigger ow-magnitude tremors, being registered up to 3.3 (M_w).^[112]

A report in the UK concluded that fracking was the likely cause of some small tremors that occurred during shale gas drilling.^{[113][114][115]} The United States Geological Survey (USGS) reports that earthquakes induced by human measures, including fracking, have been documented in a few locations;^[116] though the disposal and injection wells referenced are regulated under the Safe Drinking Water Act and UIC laws, and are not wells where hydraulic fracturing is generally performed.^[117]

Several earthquakes, including a light 4.0 magnitude quake on New Year's Eve that hit Youngstown, Ohio throughout 2011 are likely linked to a disposal well for injecting wastewater used in the fracking process, according to seismologists at Columbia University.^[118] Consequently, Ohio has since tightened its rules regarding the wells^[119] and is considering a moratorium on the practice.^[119]

Public policy, information, and relations[link]

Governments are developing legislation related to hydraulic fracturing. The US has the longest history with hydraulic fracturing, so its approaches to hydraulic fracturing may be modeled by other countries.^[58] In the US, some states have introduced legislation that limits the ability of municipalities to use zoning to protect citizens from exposure to pollutants from hydraulic fracturing by protecting residential areas. For instance, In Pennsylvania, The new Marcellus Shale Law (House Bill 1950)^[120] requires all municipalities to allow Marcellus Shale well drilling in all zoning districts, including residential, and allow water and wastewater pits in all zoning district, including residential. Compressor stations must be allowed in industrial and agricultural zoning districts, and gas processing plants allowed in industrial zoning districts. Municipalities are no longer permitted to limit the hours of operation of gas related activities. Gas pipelines must be allowed in all zoning districts.^[120] Similar laws have been created in Ohio,^[121] and New York, Colorado, and Texas are battling over related legislation.^[122] Pennsylvania's Marcellus Shale Law (House Bill 1950) also contains a provision that allows doctors in Pennsylvania access to the list of chemicals in hydraulic fracturing fluid in emergency situations only, and forbids them from ever discussing this information with their patients.^[123]

A number of public information resources have been developed by governments and news sources (see external links section below). The US Environmental Protection Agency has a site labeled Natural Gas Extraction - Hydraulic Fracturing. Other sites include FracFocus, a site indicating the chemical composition of fracking fluid of individual wells and ProPublica's sites containing collected news, commentary and illustration. The Guardian has posted Shale gas and fracking site. The New York Times also has a page dedicated to hydraulic fracturing.

The considerable opposition against fracking activities in local townships has led companies to adopt a variety of public relations measures to assuage fears about fracking, including the admitted use of "mil­i­tary tac­tics to counter drilling oppo­nents". At a conference where public relations measures were discussed, a senior executive at Anadarko Petroleum was recorded on tape saying, "Download the US Army / Marine Corps Counterinsurgency Manual, because we are dealing with an insurgency", while referring to fracking opponents. Matt Pitzarella, spokesman for the most important fracking company in Pennsylvania, Range Resources, also told other conference attendees that Range employed psychological warfare operations veterans. According to Pitzarella, the experience learned in the Middle East has been valuable to Range Resources in Pennsylvania, when dealing with emotionally-charged township meetings and advising townships on zoning and local ordinances dealing with fracking.^[124][125]

See also[link]

• Hydraulic fracturing by country
• Directional drilling
• ExxonMobil Electrofrac
• Cost of electricity by source
• Environmental impact of the oil shale industry
• Environmental impact of petroleum
• Environmental concerns with electricity generation
• FrackNation, a 2012 documentary by Phelim McAleer

Notes[link]

a. ^{^} Also spelled "fraccing"^[126] or "fracing".^[127][128]

References[link]

External links[link]

This article's use of external links may not follow Wikipedia's policies or guidelines. Please improve this article by removing excessive or inappropriate external links, and converting useful links where appropriate into footnote references. (May 2012)

• FracFocus Site indicating chemical composition of fracking fluid of individual wells
• Fracking collected news and commentary at ProPublica
• GasLand,a 2010 documentary by Josh Fox exploring the environmental impacts of hydraulic fracturing
• Hydraulic Fracturing at Earthworks.
• Hydraulic fracturing illustration on ProPublica
• Hydraulic fracturing shale gas extraction at YouTube
• Shale gas and fracking collected news and commentary at The Guardian

In 2006, as a WTCC driver
Nationality	German
24 Hours of Le Mans career
Participating years	1999–2000, 2010 -
Teams	Champion Racing, Dick Barbour Racing, BMW Motorsport
Best finish	15th (2011)
Class wins	0

Dirk Müller (born 18 November 1975 in Burbach) is a German race driver, driving for BMW Team RLL in the 2012 American Le Mans Series. His former Schnitzer BMW team-mate Jörg Müller is unrelated, as is French driver Yvan Muller.

In 1996, Dirk Müller won a Formula 3 race at the Nürburgring. In 1998, after a win in the GT1 class of the 24 Hours of Daytona, he scored his first title, the German Porsche Cup, to be followed by the ALMS GT title in 2000.

From the 2002 to 2005 season, both Dirk and Jörg were driving BMW 3series model E46 touring cars for the Schnitzer-operated works squad Team Deutschland in the FIA European Touring Car Championship (now WTCC). Since 2006, the team uses the new BMW 3series model E90 which is based on the BMW 320si limited edition model that has a 4-cyl engine rather than the usual 6-cyl.

The 2004 24 Hours Nürburgring was won by both Müllers (and Hans-Joachim Stuck) with the BMW M3 GTR V8 that had been raced successfully in the 2001 ALMS. In 2005, the Müllers finished 2nd behind their sister car.

After touring cars he joined to Ferrari AF Corse with Toni Vilander and won the FIA GT Championship in GT2 class. In 2008 raced in the American Le Mans Series in Ferrari F430 GTC with Dominik Farnbacher in the GT2 class.

In 2009, Müller rejoined BMW to spearhead its factory program with Rahal Letterman Racing in the American Le Mans Series. He and Joey Hand won the 2011 American Le Mans Series GT driver's championship following wins at the Twelve Hours of Sebring, Grand Prix of Long Beach and Lime Rock Park.

24 Hours of Le Mans results[link]

Year	Class	No	Tyres	Car	Team	Co-Drivers	Laps	Pos.	Class Pos.
1999	GT	80	P	Porsche 911 GT3-R Porsche 3.6L Flat-6	Champion Racing	Bob Wollek Bernd Mayländer	292	19th	2nd
2000	GT	83	M	Porsche 911 GT3-R Porsche 3.6L Flat-6	Dick Barbour Racing	Lucas Luhr Bob Wollek	319	DSQ	DSQ
2010	GT2	79	D	BMW M3 GT2 BMW 4.0L V8	BMW Motorsport	Andy Priaulx Dirk Werner	320	19th	6th
2011	GTE Pro	56	D	BMW M3 GT2 BMW 4.0L V8	BMW Motorsport	Andy Priaulx Joey Hand	313	15th	3rd

External links[link]

• Dirk Müller
• Profile - by FIA GT Championship official website

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