Fracking (also known as hydraulic fracturing, fracing, hydrofracturing, or hydrofracking) is a well stimulation technique involving the fracturing of formations in bedrock by a pressurized liquid. The process involves the high-pressure injection of "fracking fluid" (primarily water, containing sand or other proppants suspended with the aid of thickening agents) into a wellbore to create cracks in the deep-rock formations through which natural gas, petroleum, and brine will flow more freely. When the hydraulic pressure is removed from the well, small grains of hydraulic fracturing proppants (either sand or aluminium oxide) hold the fractures open.
Fracking, using either hydraulic pressure or acid, is the most common method for well stimulation. Well stimulation techniques help create pathways for oil, gas or water to flow more easily, ultimately increasing the overall production of the well. Both methods of fracking are classed as unconventional, because they aim to permanently enhance, or increase, the permeability of the formation. Thus, the traditional division of hydrocarbon-bearing rocks into source and reservoir no longer holds; the source rock becomes the reservoir after the treatment.
Hydraulic fracking is more familiar to the general public, and is the predominant method used in hydrocarbon exploitation, but acid fracking has a much longer history.Van Dyke JW. 1896. Increasing the flow of oil-wells. Patent No. US 556,651.Grebe JJ and Stoesser SM 1935, Treatment of deep wells. Patent no. US 1,998,756, The hydrocarbon industry tends to use fracturing, although the word fracking now dominates in popular media.
Definition
Hydraulic fracturing (fracking) and acidising (acid fracking) are two of the most common methods for well stimulation. The flow chart shows that hydraulic fracking and acid fracking, highlighted in yellow, are both classified as unconventional hydraulic methods. Acidising is complicated, however, by the fact that matrix acidising, which takes place below the fracture gradient of the rock, is considered a conventional method.
In the UK legislative and hydrocarbon permitting context (see Fracking in the United Kingdom), Adriana Zalucka et al. have reviewed the various definitions, as well as the role of key regulators and authorities, in a peer-reviewed article published in 2021. They have proposed a new robust definition for unconventional well treatments:
The above definition focuses on increasing permeability, rather than on any particular extraction process. It is quantitative, using the generally agreed 0.1 md cut-off value, below which rocks are considered impermeable. It exempts borehole cleaning processes like acid squeeze or acid wash from being classed as unconventional, by using the 1 m radius criterion. It avoids a definition based on, for example, the quantity of water injected, which is controversial, or the injection pressure applied, which determines whether the treatment is above or below the fracture gradient. It also exempts non-hydrocarbon wells from being classed as unconventional.
A low permeability (by consensus defined as less than 0.1 millidarcies) implies that the resource is unconventional, meaning that it requires special methods to extract the resource. Above that value, conventional methods suffice. Unconventional resources are also characterised by being widely distributed, with low energy density (i.e. in a low concentration) and ill-defined in volume. There are no discrete boundaries, in contrast to those bounding a conventional hydrocarbon reservoir.
Although the definition above was developed within the UK context, it is universally applicable, and takes into account the views of the hydrocarbon industry and the US Geological Survey, in particular.
Hydraulic fracking
Hydraulic fracking is the most commonly used well stimulation technique. It involves the fracturing of formations in bedrock by a pressurized liquid. The process involves the high-pressure injection of "fracking fluid" (primarily water, containing sand or other proppants suspended with the aid of thickening agents) into a wellbore to create cracks in the deep-rock formations through which natural gas, petroleum, and brine will flow more freely. When the hydraulic pressure is removed from the well, small grains of hydraulic fracturing proppants (either sand or aluminium oxide) hold the fractures open.
Hydraulic fracking began as an experiment in 1947, and the first commercially successful application followed in 1949. As of 2012, 2.5 million "frac jobs" had been performed worldwide on oil and gas wells, over one million of those within the U.S. Such treatment is generally necessary to achieve adequate flow rates in shale gas, tight gas, tight oil, and coal seam gas wells. Some hydraulic fractures can form naturally in certain veins or dikes. Drilling and hydraulic fracking have made the United States a major crude oil exporter as of 2019, but leakage of methane, a potent greenhouse gas, has dramatically increased. Increased oil and gas production from the decade-long fracking boom has led to lower prices for consumers, with near-record lows of the share of household income going to energy expenditures.
Fracking is highly controversial. Its proponents highlight the economic benefits of more extensively accessible hydrocarbons (such as petroleum and natural gas), the benefits of replacing coal with natural gas, which burns more cleanly and emits less carbon dioxide (CO2), and the benefits of energy independence. Opponents of fracking argue that these are outweighed by the environmental impacts, which include groundwater and surface water contamination, noise and air pollution, the triggering of earthquakes, and the resulting hazards to public health and the environment. Research has found adverse health effects in populations living near hydraulic fracturing sites, including confirmation of chemical, physical, and psychosocial hazards such as pregnancy and birth outcomes, migraine headaches, chronic rhinosinusitis, severe fatigue, asthma exacerbations and psychological stress. Adherence to regulation and safety procedures are required to avoid further negative impacts.
The scale of methane leakage associated with hydraulic fracking is uncertain, and there is some evidence that leakage may cancel out any benefit in terms of greenhouse gas emissions from natural gas relative to other fossil fuels.
Increases in seismic activity following hydraulic fracking along dormant or previously unknown faults are sometimes caused by the deep-injection disposal of fracking flowback fluid (a byproduct of hydraulically fracked wells), and produced formation brine (a byproduct of both fractured and non-fractured oil and gas wells). For these reasons, hydraulic fracturing is under international scrutiny, restricted in some countries, and banned altogether in others. Up until 2014, the European Union was drafting regulations that would permit the controlled application of hydraulic fracturing. However, it has not yet established a complete regulatory framework; some member states, including France, Bulgaria and the Republic of Ireland, have banned it.
Geology
Mechanics Fracturing rocks at great depth frequently become suppressed by pressure due to the weight of the overlying rock strata and the cementation of the formation. This suppression process is particularly significant in "tensile" (Mode 1) fractures which require the walls of the fracture to move against this pressure. Fracturing occurs when effective stress is overcome by the pressure of fluids within the rock. The minimum principal stress becomes tensile and exceeds the tensile strength of the material. Fractures formed in this way are generally oriented in a plane perpendicular to the minimum principal stress, and, for this reason, hydraulic fractures in wellbores can be used to determine the orientation of stresses. In natural examples, such as dikes or vein-filled fractures, the orientations can be used to infer past states of stress.
Veins Most mineral vein systems are a result of repeated natural fracturing during periods of relatively high pore fluid pressure. The effect of high pore fluid pressure on the formation process of mineral vein systems is particularly evident in "crack-seal" veins, where the vein material is part of a series of discrete fracturing events, and extra vein material is deposited on each occasion. One example of long-term repeated natural fracturing is in the effects of seismic activity. Stress levels rise and fall episodically, and earthquakes can cause large volumes of connate water to be expelled from fluid-filled fractures. This process is referred to as "seismic pumping".
Dikes Minor intrusions in the upper part of the crust, such as dikes, propagate in the form of fluid-filled cracks. In such cases, the fluid is magma. In sedimentary rocks with a significant water content, the fluid at the fracture tip will be steam.
History Precursors
Fracking as a method to stimulate shallow, hard rock oil wells dates back to the 1860s, though the general concept of using water pressure to destroy rock was known as early as ancient Rome, in the form of ruina montium. Dynamite or nitroglycerin detonations were used to increase oil and natural gas production from petroleum bearing formations. On 24 April 1865, US Civil War veteran Col. Edward A. L. Roberts received a patent for an "exploding torpedo". It was employed in Pennsylvania, New York, Kentucky, Oklahoma, Texas, and West Virginia by companies like the Lightning Torpedo Company. It used black powder, liquid nitroglycerin, and, later, solidified nitroglycerin. Later still, the same method was applied to water and gas wells. Stimulation of wells with acid, instead of explosive fluids, was introduced in the 1930s. Due to acid etching, fractures would not close completely, resulting in further productivity increase.
20th century applications Harold Hamm, Aubrey McClendon, Tom Ward and George P. Mitchell are each considered to have pioneered hydraulic fracking innovations toward practical applications.
Oil and gas wells The relationship between well performance and treatment pressures was studied by Floyd Farris of Stanolind Oil and Gas Corporation. This study was the basis of the first hydraulic fracturing experiment, conducted by Stanolind in 1947, at the Hugoton gas field in Grant County of southwestern Kansas. For the well treatment, of gelled gasoline (essentially napalm) and sand from the Arkansas River was injected into the gas-producing limestone formation at . The initial experiment was not very successful, as the 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 17 March 1949, Halliburton performed the first two commercial hydraulic fracking treatments in Stephens County, Oklahoma, and Archer County, Texas. Since then, hydraulic fracking has been used to stimulate approximately one million oil and gas wells in various geologic regimes with good success.
In contrast with large-scale hydraulic fracturing used in low-permeability formations, small hydraulic fracturing treatments are commonly used in high-permeability formations to remedy "skin damage", a low-permeability zone that sometimes forms at the rock-borehole interface. In such cases, the fracturing may extend only a few feet from the borehole.
In the Soviet Union, the first hydraulic proppant fracturing was carried out in 1952. Other countries in Europe and Northern Africa subsequently employed hydraulic fracturing techniques, including Norway, Poland, Czechoslovakia (before 1989), Yugoslavia (before 1991), Hungary, Austria, France, Italy, Bulgaria, Romania, Turkey, Tunisia, and Algeria.
Massive fracturing