Hydraulic fracturing

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Hydraulic fracturing is a method used to create fractures that extend from a borehole into rock formations, which are typically maintained by a proppant, a material such as grains of sand or other material which prevent the fractures from closing. The method is informally called fracing (pronounced "fracking") or hydrofracing.

The technique of hydraulic fracturing is used to increase or restore the rate which fluids, such as oil, gas or water, can be produced from the formation surrounding the borehole. By creating or restoring fractures, the reservoir surface area exposed to the borehole is increased and the fracture provides a conductive path connecting this reservoir surface area to the well, which effectively increases the rate that fluids can be produced from the reservoir formations.

The main industrial use of hydraulic fracturing is in stimulating production from oil and gas wells.[1][2][3] Hydraulic fracturing is also applied to stimulating groundwater wells,[4] preconditioning rock for caving or inducing rock to cave in mining,[5] as a means of enhancing waste remediation processes (usually hydrocarbon waste or spills), 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.

Contents

[edit] History

Hydraulic fracturing as used today in the oil and gas industry was first developed in the United States in 1948. It was first used commercially in 1949, and because of its success in increasing production from oil wells was quickly adopted, and is now used in thousands of oil and gas wells annually. The first industrial use of hydraulic fracturing was as early as 1903, according to Watson.[6] 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

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 fluid can flow easily to the well. A hydraulic fracture is formed by pumping a 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 seals off the borehole from the surrounding rock. Hydraulic fracturing can be used to bypass this damage.

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.

Hydraulic fracturing equipment used in the oil field usually consists of a Slurry Blender, series of Fracturing Pumps (Typically powerful triplex, or quintiplex pumps) and a monitoring unit. Associated equipment includes fracturing tanks, high pressure treating iron, 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 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.

[edit] Environmental impact and regulation

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Industry and environmental groups dispute whether hydraulic fracturing has a significant environmental impact, with arguments centered around the extent to which fracturing fluid could contaminate water supplies or impact rock shelf causing seismic events. There is concern that the process of fracing itself may destabilize bedrock and cause seismic activity. Reports of minor tremors of no greater than 2.8 on the Richter scale were reported recently as June, 2nd 2009 in Clemburne, TX - the first in the town's 140 year history. [7] Also the fluid or solids injected during hydraulic fracturing may have an impact. Indeed, one use of hydraulic fracturing is to remediate waste spills by injecting bacteria, air or other materials into a subsurface waste spill. In the United States, a 2004 Environmental Protection Agency (EPA) study concluded that the process was safe and didn't warrant further study, because there was "no unequivocal evidence" of health risks, and the fluids were neither necessarily hazardous nor able to travel far underground. The report did find uncertainties in knowledge of how fracturing fluid migrates through rocks, and upon its release service companies voluntarily agreed to stop using diesel fuel as a component of fracturing fluid, due to its potential as a source of benzene contamination. The Energy Policy Act of 2005 further strengthened the industry's regulatory position, specifically exempting hydraulic fracturing from federal regulation under the Safe Drinking Water Act.[8]

Periodic challenges to these conclusions have arisen. A 2008 investigation of benzene contamination in Colorado and Wyoming led some EPA officials to point towards hydraulic fracturing as a culprit, and later EPA statements have stressed that the 2004 report was not intended as a general study of hydraulic fracturing, but only of its use in coalbed methane deposits.[8] As of May 2009, a debate was ongoing in Congress over whether to repeal the 2005 regulatory exemption.[9]

[edit] Terminology

Fracture Gradient
The pressure to fracture the formation at a particular depth divided by the depth. A fracture gradient of 18 kPa/m (0.8 psi/foot) implies that at a depth of 3 km (10,000 feet) a pressure of 54 MPa (8,000 psi) will extend a hydraulic fracture.
ISIP – Instantaneous Shut In Pressure
The pressure measured immediately after injection stops. The ISIP provides a measure of the pressure in the fracture at the wellbore by removing contributions from fluid friction.
Leakoff
loss of fracturing fluid from the fracture channel into the surrounding permeable rock.
Fracturing fluid
The fluid used during a hydraulic fracture treatment of oil, gas or water wells. The fracturing fluid has two major functions 1) Open and extend the fracture; 2) Transport the proppant along the fracture length.

[edit] References

  1. ^ Gidley, J.L. et al. (editors), Recent Advances in Hydraulic Fracturing, SPE Monograph, SPE, Richardson, Texas, 1989.
  2. ^ Yew, C.H., Mechanics of Hydraulic Fracturing, Gulf Publishing Company, Houston, Texas, 1997.
  3. ^ Economides, M.J. and K.G. Nolte (editors), Reservoir Stimulation, John Wiley & Sons, Ltd., New York, 2000.
  4. ^ Banks, David; Odling, N.E., Skarphagen, H., and Rohr-Torp, E. (1996). "Permeability and stress in crystalline rocks". Terra Nova 8 (3): 223–235. doi:10.1111/j.1365-3121.1996.tb00751.x. 
  5. ^ Brown, E.T., Block Caving Geomechanics, JKMRC Monograph 3, JKMRC, Indooroopilly, Queensland, 2003.
  6. ^ Watson, T.L., Granites of the southeastern Atlantic states, U.S. Geological Survey Bulletin 426, 1910.
  7. ^ "Drilling Might Be Culprit Behind Texas Earthquakes". AssociatePress. June v12, 2009. http://hosted.ap.org/dynamic/stories/T/TEXAS_EARTHQUAKES_DRILLING?SITE=NYWNE&SECTION=HOME&TEMPLATE=DEFAULT. 
  8. ^ a b "Does Natural-Gas Drilling Endanger Water Supplies?". BusinessWeek. November 11, 2008. http://www.businessweek.com/magazine/content/08_47/b4109000334640.htm. 
  9. ^ "Industry campaign targets 'hydraulic fracturing' bill". The New York Times. May 7, 2009. http://www.nytimes.com/gwire/2009/05/07/07greenwire-industry-campaign-targets-hydraulic-fracturing-10572.html. 

[edit] External links

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