GSA Critical Issue: Hydraulic Fracturing
Aquifer: A body of permeable rock or sediment that is saturated with water and yields useful amounts of water.
Biocide: A chemical substance capable of destroying some life forms. In hydraulic fracturing, biocides are used to inhibit growth of bacteria and mold.
Carbonate rock: A rock composed primarily of carbonate minerals (minerals containing the CO3 anionic structure, such as calcite). Common carbonate rocks are limestones and dolomites.
Casing: The hard metal or plastic pipe that lines the well, prevents a borehole from caving in, and provides a barrier to the outside rock and groundwater.
Chloride: A chemical compound with one or more chlorine atoms bonded within the molecule; a salt of hydrochloric acid. Table salt is sodium chloride (NaCl).
Fault: A fracture or fracture zone along which rock layers have moved.
Fine-grained: A geologic term to describe a rock texture, referring to its mineral or rock fragment components.
Formation: A basic unit of rock layers distinctive enough in appearance, composition, and age to be defined in geologic maps and classifications.
Flowback water: The fracturing fluid that returns to the surface through the wellbore during and after a hydraulic treatment.
Fracture: A crack or break in the rock.
Fracturing fluids: The water and chemical additives used to hydraulically fracture the reservoir rock, and proppant (typically sand or ceramic beads) pumped into the fractures to keep them from closing once the pumping pressure is released.
Frictional resistance: The force that inhibits the relative motion of two solid objects in contact. It is usually proportional to the force which presses the surfaces together.
Hydraulic fracturing: A process to propagate fissures in a subsurface rock layer with the injection of pressurized fluid through a wellbore, especially to extract oil or gas.
Hydrocarbon: An organic compound made of carbon and hydrogen, found in coal, crude oil, natural gas and plant life.
Kick-off point: The depth at which the vertical drill hole is deviated for directional drilling so the well bore can enter the target zone roughly horizontal.
Moment magnitude scale: Used by scientists to measure the size of earthquakes in terms of the energy released. The scale was developed in the 1970s to improve upon the Richter magnitude scale, particularly to describe large (M>7) earthquakes and those whose epicenter is over 370 miles away.
Microseismic: A faint earth tremor, typically less than Richter Magnitude zero, which was the detection limit in 1935.
Methane: A colorless, odorless and flammable gaseous hydrocarbon (CH4).
Permeability: The capacity of a rock for transmitting a fluid. Permeability depends on the size and shape of pores in the rock, along with the size, shape, and extent of the connections between pore spaces.
Pore space: The spaces between grains in a rock that are unoccupied by solid material.
Produced water: The naturally occurring fluid in a formation that flows to the surface through the wellbore, throughout the entire lifespan of an oil or gas well. It typically has high levels of total dissolved solids with leached out minerals from the rock.
Proppant: Solid material used in hydraulic fracturing to hold open the cracks made in the reservoir rock after the high pressure of the fracturing fluids is reduced. Sand, ceramic beads, or miniature pellets “prop” open the cracks to allow for freer flow of oil or gas.
Reservoir rock: The oil or gas bearing rock, typically a fractured or porous and permeable rock formation.
Seismic event: An earth vibration, such as an earthquake or tremor.
Shale: A fine-grained sedimentary rock that formed from the compaction of finely layered silt and clay-sized minerals (“mud”).
Shale gas: natural gas locked in tiny bubble-like pockets within shale or other layered, sedimentary rock.
Tight oil or gas reserves: Hydrocarbons dispersed in rocks of low permeability and porosity, which makes it more difficult to recover than conventional hydrocarbon deposits.
Well bore: A hole that is drilled to explore and recover natural resources, such as oil, gas, or water.
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