The main industrial use of hydraulic fracturing is in stimulating production from oil and gas wells. Hydraulic fracturing is also applied to stimulating groundwater wells, preconditioning rock for caving or inducing rock to cave in mining, 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.

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. 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.

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. 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 quadplex 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 15,000 psi and 100 Barrels per Minute.

Glossary

• Leakoff - loss of fracturing fluid from the fracture channel into the surround permeable rock.
• Proppant - solid round grains of sand or other material to prop open the fracture as it tends to "heal".

References