Hydraulic fracturing, more commonly referred to as “fracking” in the media, is the fracturing of rock by a pressurised liquid. Some hydraulic fractures form naturally – certain veins or dikes are examples. However, induced hydraulic fracturing or hydro-fracturing is also a long tried-and-tested mining technique that has been most controversial recently… But let’s not panic!
Fracturing in rocks at depth tends to be suppressed by the pressure of the overlying rock stratas weight, and the cementation of the formation. However, fracturing does occur naturally when effective stress is overcome by the pressure of fluids within the rock.
A Geological Aside on Nature’s Own “Fracking”
Natural examples of ‘fracking’ include:
dikes (or dykes) – When magma freezes upwards along a vertical fissure or crack in volcanic rock, it forms a vertical curtain of intrusive igneous rock called a dike.
vein-filled fractures – A vein is a rock fracture that has become filled by minerals (typically quartz or calcite) precipitated from solution, and sometimes with inclusion of ore minerals.
Dikes record tension in the Earth’s crust at the time of their formation.
Most mineral vein systems are a result of repeated natural fracturing during periods of relatively high pore fluid pressure. This 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.
An 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 – a process that is referred to as “seismic pumping”.
The orientations of such natural structures can be used to infer the past states of stress of our environment. The fractures formed are generally oriented in a plane perpendicular to the minimum principal stress.
Now, we get back to mining and human-made fracturing…
The Shale Gas Revolution
Shale gas is one of a number of unconventional sources of natural gas, with others including coal bed methane, tight sandstones, and methane hydrates – a type of natural gas that can be extracted from shale rock deposits.
Shale is a fine-grained, clastic (i.e. composed of fragments, or clasts, of pre-existing minerals and rock) sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. The ratio of clay to other minerals can vary.
Shale is characterised by breaks along thin laminae or parallel layering or bedding less than one centimetre in thickness, called fissility.
Mudstones are similar in composition, but crucially, they do not show fissility.
Ordinarily, shale deposits have insufficient permeability to allow significant fluid flow to a well bore. Thus, most shales are not viable commercial sources of natural gas.
Shales that host economic quantities of gas have a number of common properties. Shale deposits are rich in organic material (0.5% to 25%), and are usually mature petroleum source rocks in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas.
It is worth noting that far from being a new energy resource, shale gas has in fact been produced for years, collected from shales with natural fractures. Increasingly, shale has become an important source of natural gas in the United States since the start of this century, and interest has spread to potential gas shales in the rest of the World.
In 2000, shale gas provided only 1% of U.S. natural gas production.
By 2010, shale gas made up over 20% of the total production.
The United States government’s Energy Information Administration predicts that by 2035, 46% of the United States’ natural gas supply will come from shale gas.
A Proven Mining Technique
Geo-physically speaking, shale gas areas are often known as resource plays (as opposed to exploration plays). This means that the geological risk of not finding gas is low in resource plays and that the potential profits per successful well are usually also lower.
Shale gas was first extracted as an energy resource in 1821, in Fredonia, New York, from shallow, low-pressure fractures. Horizontal drilling began back in the 1930s, and in 1947 a well was first fracked in the United States.
Shale has a low matrix permeability. As a result, shale gas production in commercial quantities requires fractures to provide permeability.
The first experimental use of hydraulic fracturing dates back to 1947.
The first commercially successful applications were in 1949.
As of 2012, 2.5 million hydraulic fracturing jobs have been performed on oil and gas wells worldwide – more than one million of them in the United States alone.
The Uranium Energy Corporation is also planning to use hydraulic fracturing to mine uranium. Fracking for uranium involves injecting oxygenated water to dissolve the uranium, then pumping the solution back up to the surface.
Induced Hydraulic Fracturing or Hydrofracturing
The shale gas boom in recent years has relied on modern technology improvements in hydraulic fracturing to create extensive artificial fractures around well bores.
Induced hydraulic fracturing or hydro-fracturing is a technique in which water is mixed with sand and chemicals, and the mixture is injected at high pressure into a wellbore to create very small fractures (typically less than 1 millimetre), along which fluids such as gas, petroleum, uranium-bearing solution, and brine water may migrate to the well. Once the rock achieves equilibrium, hydraulic pressure is removed from the well, then small grains of proppant (sand and/or aluminium oxide) hold these fractures open.
This well stimulation is usually conducted once in the exploitation lifetime of the well and greatly enhances fluid removal and well productivity, but there has been an increasing trend towards multiple hydraulic fracturing as production declines.
The process is commonly known in the media as “fracking“, but within the industry the term “frac” is preferred to frack. Fracturing would be used rather than frac-ing.
The technique is very common in wells for
Shale gas – natural gas that is found trapped within shale formations,
Tight gas – a type of natural gas that is produced from reservoir rocks with low permeability, most commonly sandstone, but sometimes limestone
Tight (or shale) oil – petroleum that consists of light crude oil, contained in petroleum-bearing formations of low permeability, either shale or tight sandstone
Coalbed methane or coal seam gas (CSG) – a form of natural gas, mostly methane, extracted from coal beds, well known from its occurrence in underground coal mining, where it presents a serious safety risk,
and in hard rock wells.
Horizontal drilling is often used with shale gas wells, with lateral lengths up to 3,000 metres (10,000 feet) within the shale, to create maximum borehole surface area in contact with the shale.
Lifecycle of a Hydraulic Fracturing Well
The drilling and hydraulic fracturing phases of opening a well for the recovery of oil and natural gas trapped inside shale rock accounts for a mere fraction in the total lifetime of a well.
Drilling and Well Construction 50-100 Days
Oil and gas wells in shale formations are drilled to reach far below the Earth’s surface. These wells are built with redundant layers of steel casing that are cemented into place.
Hydraulic Fracturing 2-5 Days
Operators pump a mixture of water, sand and chemicals down the well at high pressure to create paper-thin cracks in dense shale rock, freeing oil and natural gas trapped inside. This process is called hydraulic fracturing, or ‘fracking’ for short.
Energy Production 20-40 Years
When the well is complete, reclamation efforts reduce the work area to about the size of a two-car garage. The well will supply energy to consumers for decades to come.
Hydraulic fracturing is a well tried-and tested technique. The technique has been broadly used by the United States energy industry since the mid-1960s.
Massive hydraulic fracturing is a technique first applied by Pan American Petroleum in Stephens County, Oklahoma in 1968. The attribute “massive” refers to well treatments that involve injecting greater amount than about 150 short tons, or approximately 136 metric tonnes (300,000 pounds), of proppant.
Microseismic imaging, a crucial input to both hydraulic fracturing in shale and offshore oil drilling, originated from coalbeds research at Sandia National Laboratories.
Increasingly, American geologists became aware that there were huge volumes of gas-saturated sandstones with permeability too low (less than 0.1 millidarcy) to recover the gas economically. Starting in 1973, massive hydraulic fracturing was used in thousands of gas wells and other rock formations.
Massive hydraulic fracturing quickly spread in the late 1970s to western Canada, Rotliegend and Carboniferous gas-bearing sandstones in Germany, The Netherlands (onshore and offshore gas fields), and the United Kingdom in the North Sea.
Horizontal oil or gas wells were unusual until the late 1980s.
Again, hydraulic fracturing is not a new technology.
What changes is the upward trend of multiple hydraulic fracturing as the well production declines.
Earthquake swarms in Oklahoma
Researchers say that there has been a 40-fold increase in the rate of earthquakes in the U.S. state of Oklahoma between 2008 and 2013.
“Fracking” involves blasting water, sand and chemicals at very high pressure into shale rock formations to release the gas and oil inside. That water cannot be treated and it cannot be released into nearby rivers. The only solution is to re-inject it underground.
But now, there is increasing evidence that massive injections of this waste water from the oil and gas extraction industry are likely to be a trigger in the significant rise in earthquakes in recent years in the American states of Arkansas, Texas, Ohio, and Oklahoma.
The scientists have found that the disposal water in four high-volume wells could well be responsible for a swarm of tremors up to a distance of 35 kilometres away.
According to a report on seismicity induced by waste-water disposal by the U.S. Geological Survey, more than 2,500 earthquakes greater than magnitude 3.0 have occurred around the small town of Jones since 2008. About 20% of the total number of earthquakes in the central and western United States for this time period.
The researchers correlate this increase to the almost doubling of volumes of waste water that were disposed of in central Oklahoma between the years 2004 and 2008.
To determine the impact of this water, scientists developed a model that could calculate the way the underground wave of pressure spread out from four of the largest wells in Oklahoma (Sweetheart, Chambers, Flower Power and Deep Throat) that are under the ownership of a company called New Dominion. These extraction sites have been pumping around 4 million barrels of water a month to a depth of 3.5 kilometres beneath the surface, although the company insisted they were operating safely and within permitted parameters.
The research team led by Dr Karen Keranen say they are uncertain about the potential for large-scale disposal of wastewater to trigger events of larger magnitudes. Comparing their computerised data with the seismic data recorded during the Jones cluster, the injection of wastewater is “likely responsible” for the quake swarm.
Investigators were able to link it to the injection of wastewater from the oil industry, because pressure appears to have risen in places where the earthquakes are occurring.
Such a pressure increase is exactly what happens in natural triggering. Therefore, if a fault is already getting close to failure point, and the amount of pressure goes up at specific locations in the model, this is enough to push them over the edge and trigger an earthquake tremor.
The scientists highlight an incident that occurred in 2010: an earthquake ruptured part of a seven-kilometre long fault. The report’s authors write that “if the entire fault had gone, it could have led to a magnitude 6.0 tremor”. Larger earthquakes are often seen when we notice a lot of smaller ones occurring…
Protecting Groundwater during Hydraulic Fracturing
Water is never far away in the energy extraction process. Be it for hydraulic fracturing or to squeeze out more oil from conventional wells. Of course, the hydraulic fracturing business model involves the disposal of very large volumes of saline water.
Large volumes of naturally occurring water are often released with oil and gas. This liquid must be separated from the fossil fuels in a process called dewatering. There is a high ratio of water to oil, particularly at the beginning of the well. However, each well is different. The typical nationwide ratio is five to one.
The produced water brought to the surface as a by-product of gas extraction varies greatly in quality from area to area, but may contain undesirable concentrations of dissolved substances such as salts, naturally occurring chemicals, as well as heavy metals and radionuclides.
According to the U.S. Geological Survey specialists, the high price of oil has driven this water-based approach. Nevertheless, the law says that drinking water must be protected from the briny flow.
The United Kingdom’s Reserves
For the first time in six years, “fracking” licences can now be issued for beauty spots according to new rules issued by the UK government. The regulations for the new bidding round are stricter than ever. Energy companies applying for drilling in Areas of Outstanding Natural Beauty, will only be able to do so in “exceptional circumstances”, and they will have additional obligations.
Environmental campaigners say these rules don’t go far enough. Planning permission may now indeed be granted, even in National Parks, the Broads, and Areas of Outstanding Natural Beauty if “it can be demonstrated they are in the public interest”. The National Trust gave the move a cautious welcome. Forty percent of the National Trust’s land is in National Parks and it owns large areas of land in other Areas of Outstanding Natural Beauty.
In the United Kingdom, test drilling has already taken place in Lancashire, and West Sussex where thousands protested at a site operated by energy company Cuadrilla. According to the British Geological Survey, the north of England is the area containing the largest shale reserves. South east Scotland also hosts significant shale gas resources.
Drilling for Answers
But American experience shows that no more than around 5% of shale gas can be reclaimed. And we must bear in mind that the British Isles are also notably more crowded than the United States’ vast expanses.
Although these are early days for the science, hopes of Britain being able to copy America’s shale gas revolution may prove unrealistic.
Energy security is a must.
But there is fierce opposition across America and Europe.
Environmentalists argue that the process can cause contamination of the water supply and earth tremors. Professionals of the industry reject those criticisms and say the process can be carried out safely. The truth is there are legitimate concerns that need to be addressed before extraction permits are granted.
Most people agree that we need to ensure the country’s future energy security. With 80% of our heating coming from natural gas and declining North Sea reserves, shale and other unconventional gas extraction could form a part of our everyday energy mix… at least until renewable energy capacity can be realistically scaled up.
In times of international political uncertainty, we must realise that ensuring energy security by securing a home-grown alternative source to Russian gas may not be such a crazy idea…