Hydraulic fracturing

Hydraulic fracturing is a method used to create fractures that extend from a borehole into rock formations, which are typically maintained by a proppant. The method is informally called fracing. The technique is used to increase or restore the rate which fluids, such as oil, gas or water, can be produced from the formation. By creating or restoring fractures, the surface area of the formation exposed to the borehole is increased, 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.

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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 became a quickly adopted technique that is now used in 1000s of oil and gas wells every year. The first documented industrial use for hydraulic fracturing was as early as 1903, according to Watson.^[6] At or before that date, hydraulic fracturing was being used at Mt. Airy, North Carolina where it was (and still is) used to separate the base of granite blocks from the bedrock in the Mt. Airy quarry.

Method

When applied to stimulation of water or oil and 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 back 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. 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.

Hydraulic fracturing equipment used in the oil field usually consists of a large diesel engine, a fracturing pump (typically a triplex plunger pump) and control unit. Associated equipment includes fracturing tanks, blenders, treating iron, and instrumentation for flow, fluid density, and pressure. Fracturing equipment is available that operates over a range of pressures and injection rates that can reach up to 20,000 PSI or 140 MPa requiring over 2,000 hp.

Glossary

• Leakoff - loss of fracturing fluid from the fracture channel into the surround permeable rock.
• Proppant - solid round grains placed as a slurry into a hydraulic fracture to form a permeable pack that acts to maintain the conductivity of the fracture after the injection is finished and it closes.

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.. "Permeability and stress in crystalline rocks". Terra Nova 8 (3). 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.