Hydraulic fracturing or geotechnical fracking is the exploitation of the pressure of a fluid, typically water, to create and then propagate a fracture in a layer of rock in the subsoil. Fracking is carried out after a drilling in a rock formation containing hydrocarbons (oil or natural gas). The objective is to increase the permeability. Improving permeability improves the production of oil or gas contained in the subsoil and increases its recovery rate.
Hydraulic fractures in rocks can be both natural and created by humans; they are created and enlarged by the pressure of the fluid contained in the fracture. The most common natural hydraulic fractures are dykes and strand layers, in addition to cracks caused by ice in areas with cold climates. Man-made fractures are deeply induced at precise levels of rock within the oil and gas fields. They are spread by pumping fluid under pressure, and then kept open by introducing sand, gravel, ceramic microspheres as a permeable filler material. In this way, fractures created can not be closed when water pressure fails.
The technique of hydraulic fracturing is used to increase or restore the speed of extraction of fluids such as oil, gas and water, including unconventional deposits such as coal or bituminous rocks. Fracking allows the extraction of hydrocarbons from permeable rocks (eg, compact limestone, such as cemented sandstone and clay), from which it would otherwise flow in such quantities as to allow extraction at an economically viable rate.
For example, fracturing allows the extraction of natural gas from bituminous rocks, an extremely impermeable material.
The induced fractures increase the permeability of the rock around the well, increasing the flow rate of extraction.
Although the main industrial use of hydraulic fracturing is to stimulate the extraction of fossil fuels oil and natural gas, it is also used:
- in the construction of water wells.
- to prepare the rocks for mining drilling
- make processes to reduce losses (generally hydrocarbon leaks)
- Discard leaks by injecting them into suitable rock formations
- as a method to measure tensions in the earth's crust.
Methodology of hydraulic fracturing
A hydraulic fracture is created by pumping the fracturing fluid into the well, with sufficient pressure to overcome the fracture gradient of the rock. This causes one or more cracks in which the fluid enters, causing an additional extension. To maintain the crack open after the interruption of the pumping fluid, a solid material is added. This material is called a support agent. Normally this material is composed of selected granules of quartz sand or ceramic microspheres. This added material in fractures caused by fracking prevents completely closing the pressure loss, preserving a passage of high permeability for the fossil fuel fluid will be extracted.
Drilling a hole produces rock chips and debris that can slip into cracks and pores in the well wall, partially sealing the pit and reducing permeability: hydraulic fracturing can restore a Adequate flow of extraction from the deposit. For this reason, it is a standard measure adopted in all wells drilled in low permeable rocks, and approximately 90% of all natural gas wells in the United States use hydraulic fracturing to produce gas at a competitive price.
The fluid injected into the fracking wells can be water, gel, foam or compressed gas, such as nitrogen, carbon dioxide or simple air. Various types of solid maintenance material are also used: usually sand, but also sand with resin coatings or ceramic spherules.
To detect the size and orientation of the fractures caused, a microseismic monitoring is carried out during the pumping of the fracturing, the installation of geophones arrays in the adjacent wells. By mapping the microsystems due to growing fractures, we can deduce the approximate geometry of the fractures. Other important information about induced stresses in rocks is obtained by placing inclinometric matrices.
Standard fracture equipment used in oil fields includes a dynamic mixer, one or more high-pressure, high-flow pumps (usually triple or quintuple pumps) and a seismic monitoring unit. Other necessary materials are tanks, high pressure pipes, additive units and manometers to control the pressure, flow and density of the fluid during the injection. The pressure and fluid flow values vary greatly in the various phases: the injection starts with low pressure and flow also of 265 liters per minute. In the stress phase, the pressure increases to 100 MPa and the flow gradually decreases.
Advantages and disadvantages of fracking
One of the great advantages of this technique is that it allows the exploitation of gas reserves that were previously considered unattainable. These sites provide great benefits to countries, local communities and industry. In addition, different countries help achieve greater energy independence with successive geostrategic benefits.
Fight against climate change
Initially, this technique received support from different sectors, including environmental groups. This was due to the fact that gas burning emits less CO2 than burning coal or oil. However, later research questioned the advantages of this technique in the fight against climate change. On the one hand, own natural gas (CH4) is a potent greenhouse gas, its direct filtration into the atmosphere being very dangerous during the extraction process.
On the other hand, the race for the exploitation of these sites may, according to experts' comments, stagnate in the development of truly clean renewable energy sources such as photovoltaic solar energy, wind energy, geothermal energy or hydraulic energy. In addition, other experts argue that, although the burning of gas is cleaner than the burning of traditional fossil fuels, the global increase in energy consumption would inevitably lead to climate change.
Impact on groundwater
One of the problems derived from fracking is the possible contamination of the wells and aquifers that provide drinking water to the population. The industry argues that the cement barriers introduced in the perforations prevent the possible passage of harmful substances to the layers of groundwater where potable water can be found.
However, several investigations by Duke University and the EPA have shown the presence of methane, chemical solvents and other substances in water samples taken near the boreholes.
Impact on surface waters
During the fracking process, a considerable part of the mixture of water, chemicals and sand that is injected into the drilling returns to the surface. In addition, in this process, water transports substances that were trapped in the depth to the surface.
This waste is highly polluting and can cause environmental disasters in case of leaks to rivers and surface water deposits. The classic example of this possible contamination occurred on the Dunkard Creek River in September 2009. An invasion of microscopic algae wiped out much of the river's indigenous life. Subsequently, it was learned that several companies had illegally discharged water from several drilling centers.
One of the proposed alternatives was to treat these waters in traditional treatment plants. However, several experts emphasized that these plants would not be prepared to purify these waters completely, especially with respect to the cleaning of radioactive substances (radio).
History of fracking
The technique of improving the productivity of an oil well by fracking dates from the 1860s, when in Pennsylvania, with the use of nitroglycerin, the production of some wells drilled in solid rocks was improved.
The fracturing technology by applying pressure on the rock using hydraulic fluid, to stimulate the supply of oil from the less productive fields was produced in the United States in 1947 by the Stanolind Oil and Gas Corporation in the field in Hugoton Kansas. The first company to patent a hydraulic fracturing technique was the Haliburton Well Cement Oil Company in 1949. This practice, given the increased production it caused, quickly spread for the first time in the entire US oil industry. UU and then everyone's.
Environmental risks derived from fracking
Hydraulic fracturing is being monitored internationally due to concerns about the risks of chemical contamination of groundwater and air. In some countries, the use of this technique has been suspended or even prohibited.
In March 2014, the journal Endocrinology published an article on mining activities in Colorado, entitled "Activities of estrogen and androgen receptors of hydraulic fracturing chemicals and surfaces and groundwater in a region of high density drilling" ... The researchers analyzed the waters of Garfield County, Colorado, where shale gas shafts are very present. The sampling showed that even the presence of "moderate levels" of chemicals in the fluids used to make fracking had the potential to interfere with normal hormonal functioning. One of the collaborators of the study, Christopher Kassotis, argues that the high levels of alteration in the functioning of the hormones are related to infertility, cancer and damage at birth.
Seismic risks derived from hydraulic fracturing
The techniques of hydraulic micro-fracturing of the sediment can, in some cases, generate a highly localized induced micro-seismicity. The intensity of these microsisms is generally quite limited, but there may be local problems of soil stability when the sediments are shallow.