What is hydraulic fracturing?
Hydraulic fracturing (also called “fracking” or “fracing”) is the process of injecting water and chemical additives into a well at high pressures to fracture targeted reservoir rocks. When the reservoir formation develops fractures, a granular material called “proppant” is then pumped down the well to fill the fracture and thus “propping” it open to allow oil and/or natural gas to flow into the well bore. This process allows the hydrocarbons to flow more freely and can increase production as well as the recovery of oil and gas resources.
Hydraulic fracturing constituents
Proppants can be manufactured (usually ceramic or metal) or natural. Nearly all natural proppant sources are derived from clean sandstones. The size and shape of natural proppant (“frac sand”) generally ranges from 0.106 mm–2.36 mm, is preferentially spherical, and has a high compressive strength in order to resist the forces that want to re-close the rock fractures.
Fresh water is used in fracking to not interact with the chemical additives. Millions of gallons of water can be used on one multi-stage fracked horizontal well. Much of this water returns to the surface as flowback water. The flowback wastewater is then disposed of in injection wells, pits, or treated and discharged into surface waters. It can also be captured and recycled for use on another well.
Chemical additives are added to the fracturing fluids to make the fracking process more efficient and effective. Because unique conditions are encountered with each well, the type and amount of chemical additives varies. According to FracFocus, three to 12 chemicals are typically added to a well’s fracturing fluids. While water comprises 98 percent to 99.2 pecent of the total fracturing fluid volume, chemical additives account for 0.5 percent to 2 percent. The chemicals serve to prevent bacteria growth, scale deposits, and well casing corrosion, control iron precipitation, adjust pH levels, reduce friction, and alter the fluid’s surface tension and viscosity. Other additives include emulsifying/thickening agents, acids that remove drilling mud damage, and gels.
Pie chart showing percentages of different fluids used in
hydraulic fracturing operations. (Click to enlarge)
Some operators are beginning to use propane gel (liquefied propane gas) instead of water to frack wells. The benefits of using propane gel rather than water include a reduction in water use and costs associated with transport and disposal of fracking wastewater, as well as reducing the possibility of soil or groundwater contamination from fracking wastewater. Because the propane gel becomes a gas under the high pressure and temperature in a well, it returns to the surface with the natural gas and can be recaptured, separated, and reused or sold. Finally, the propane gas does not carry fracking chemicals, salts, or natural radioactive material in it when it returns to the surface.
However, the use of propane gel to frack wells also has its disadvantages. It is more expensive than water. Because it is explosive, the propane gel requires special monitoring devices and safety equipment that water does not. Additional special equipment is needed onsite to keep the propane gel at a high pressure and low temperature. It also may not be as effective in deep formations where higher pressures are required to frack a well. Currently, there is a lack of infrastructure for the propane to be recaptured and reused. And because it is a relatively new technology, it has not been entirely proven and accepted by industry.
Many wells, especially horizontal wells, often undergo multiple stages of hydraulic fracturing. Starting at the toe (end) of the well bore, small sections—or stages—of the casing are perforated and subjected to pressurized fracturing fluids. When a stage has been successfully fracked, a temporary plug is set to isolate it from the rest of the well, and the process is repeated for all subsequent stages. Horizontal wells can have up to 100 frac stages, because some horizontal wells are more than 10,000 feet long. When all stages have been fractured, the temporary plugs are removed and natural gas and/or oil flows into and up the well.
Hydraulic Fracturing Topics: