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Hydraulic fracturing
Hydraulic fracturing is a process that results in the creation of fractures in rocks. This petroleum engineering method has been used over the past 60 years in more than one million wells by the worldwide natural gas and oil exploration and production industry to create fractures that extend from a wellbore drilled into targeted rock formations to enhance oil and natural gas recovery.

Hydraulic fractures may be natural or man-made and are extended by internal fluid pressure which opens the fracture and causes it to grow into the rock. Man-made fluid-driven fractures are formed at depth in a borehole and extend into targeted rock formations. The fracture width is typically maintained after the injection by introducing a proppant into the injected fluid. Proppant is a material, such as grains of sand, ceramic, or other particulates, that prevent the fractures from closing when the injection is stopped. The method is informally called fracing or hydrofracing. Natural hydraulic fractures include volcanic dikes, sills and fracturing by ice as in frost weathering.
Process
The technique of hydraulic fracturing is used to increase or restore the rate at which fluids, such as oil, gas or water, can be produced from a reservoir, including unconventional reservoirs such as shale rock or coal beds. Environmental concerns regarding hydraulic fracturing techniques include potential for contamination of aquifers with fracturing chemicals or waste fluids. On the other hand, hydraulic fracturing is applied to remediation of environmental waste spills.

Hydraulic fracturing enables the production of natural gas and oil from rock formations deep below the earths surface (generally 5,000-20,000 feet). At such depth, there may not be sufficient porosity and permeability to allow natural gas and oil to flow from the rock into the well bore and be recovered. For example, creating conductive fractures in the rock is essential to produce gas from shale reservoirs because of the extremely low natural permeability of shale, (which is measured in the microdarcy to nanodarcy range). The fracture provides a conductive path connecting a larger area of the reservoir to the well, thereby increasing the area from which natural gas and liquids can be recovered from the targeted formation.

[edit] History
Hydraulic fracturing for stimulation of oil and natural gas wells was first used in the United States in 1947.[2] It was first used commercially in 1949,[2] and because of its success in increasing production from oil wells was quickly adopted, and is now used worldwide in tens of thousands of oil and natural gas wells annually. The first industrial use of hydraulic fracturing was as early as 1903, according to T.L. Watson.[3] Before that date, hydraulic fracturing was used at Mt. Airy Quarry, near Mt Airy, North Carolina where it was (and still is) used to separate granite blocks from bedrock.

[edit] Method
The main industrial use of hydraulic fracturing is in stimulating production from oil and gas wells.[4][5][6] Hydraulic fracturing is also applied to stimulating groundwater wells,[7] preconditioning rock for caving or inducing rock to cave in mining,[8] as a means of enhancing waste remediation processes (usually hydrocarbon waste or spills) [9], to dispose of waste by injection into suitable deep rock formations, and as a method to measure the stress in the earth. Volcanic dikes and sills are examples of natural hydraulic fractures. Hydraulic fracturing incorporates results from the disciplines of fracture mechanics, fluid mechanics, solid mechanics, and porous medium flow.

When applied to stimulation of water injection wells, or oil/gas wells, the objective of hydraulic fracturing is to increase the amount of exposure a well has to the surrounding formation and to provide a conductive channel through which the injected water or produced natural gas or oil can flow easily to the well. A hydraulic fracture is formed by pumping the fracturing fluid into the well bore at a rate sufficient to increase the pressure downhole to a value in excess of the fracture gradient of the formation rock. The pressure then causes the formation to crack which allows the fracturing fluid to enter and extend the crack further into the formation. In order to keep this fracture open after the injection stops, a solid proppant is added to the fracture fluid. The proppant, which is commonly a sieved round sand, is carried into the fracture. This sand is chosen to be higher in permeability than the surrounding formation and the propped hydraulic fracture then becomes a high permeability conduit through which the formation fluids can be produced back to the well.

Drilling a borehole or well involves applying downward pressure to a rotating drill bit. This drilling action produces rock chips and fine rock particles that may enter cracks and pore space at the wellbore wall, resulting in damage to the permeability at and near the wellbore. The damage 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 mitigate this damage.

Hydraulic fracture stimulation is commonly applied to wells drilled in low permeability reservoirs. As estimated 90% of the natural gas wells in the U.S. rely on hydraulic fracturing to produce natural gas at economic rates.

The fracture fluid can be any number of fluids, ranging from water to gels, foams, nitrogen, carbon dioxide or even air in some cases. Various types of proppant are used, including sand, resin-coated sand, and man-made ceramics depending on the type of permeability or grain strength needed. Radioactive sand is sometimes used so that the fracture trace along the wellbore can be measured. The injected fluid mixture is approximately 99.5% water and sand.

Microseismic monitoring is a common method for measuring the orientation and approximate size of a hydraulic fracture. Microseismic activity is measured by placing an array of geophones in a nearby wellbore. By mapping the location of small seismic events that are 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 fracture geometry.

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, high pressure treating iron, a chemical additive unit (used to accurately monitor chemical addition) low pressure pipes and gauges 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 MPa (15,000 psi) and 265 L/s (100 barrels per minute).

The location of fracturing along the length of the borehole can be controlled by inserting tough inflatable plugs, also known as bridge plugs, below and above the region to be fractured. This allows a borehole to be progressively fractured along the length of the bore, without leaking fracture fluid out through previously fractured regions. The plugs are inserted into the bore deflated, then expanded to seal off the borehole into a small working region. Piping through the upper plug admits fracturing fluid and proppant into the working region. This method is commonly referred to as "plug and perf."

Typically, hydraulic fractures are placed in cased wellbores and the reservoir zones to be fractured are accessed by perforating the casing at those locations.

Advances in completion technology have led to the emergence of open hole multi-stage fracturing systems. These systems effectively place fractures in specific places in the wellbore, thus increasing the cumulative production in a shorter time frame.

Certain reservoirs such as the Bakken, Barnett Shale, Montney and Haynesville have proved to be difficult to produce using conventional methods. These formations have begun using high tech completion systems capable of mechanically fracturing at certain intervals. An alternative to the plug and perf method, multi-stage fracturing systems are capable of stimulating several stages in a single day. Compared to the weeks required by the plug and perf method, cost-effective multi-stage completion systems are quickly becoming sought after technology by oil companies.

Source
http://en.wikipedia.org/wiki/Hydraulic_fracturing