GSA Critical Issue: Induced Seismicity

Glossary

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.

Chemical feedstock: The raw material used in the manufacture of chemicals. For example, natural gas methane is a chemical feedstock for ammonia and formaldehyde.

Conventional reservoir: Oil and gas have migrated, often long distances, from its source into permeable layers before being trapped by geologic structures and stratigraphy; these can often can be extracted without stimulated production.

Fault: A fracture or fracture zone along which rock layers have moved.

Flowback water: The fracturing fluid that returns to the surface through the a wellbore during and after a hydraulic treatment.

Formation: A basic unit of rock layers distinctive enough in appearance, composition, and age to be defined in geologic maps and classifications; the identifying characteristics are laterally extensive, perhaps for up to hundreds of miles.

Fracture: A crack or break in the rock.

Hazard: Any sort of potential damage, harm, or adverse impact on something or someone.

Hydraulic fracturing: A process to propagate fractures 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.

Mercalli intensity scale (MI): Used by scientists to measure the size of an earthquake in terms of effects at the earth’s surface (e.g., levels of damage to buildings and their contents).

Moment magnitude scale (Mw or M): 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 with an epicenter is over 370 miles away.

Microseismic: A faint earth tremor, typically less than Richter Magnitude zero, which was the detection limit in 1935.

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.

Pressure front: The leading edge of a zone of high pressure that is moving through a body of rock in the direction of lower pressure.

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.

Reservoir rock: The oil or gas bearing rock, typically a fractured or porous and permeable rock formation.

Richter magnitude scale: A numerical scale previously used by scientists to measure the size of an earthquake, ranging from <0 to >9.

Risk: The chance or probability that a person or property will be harmed if exposed to a hazard.

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”). It typically has low permeability, which makes it difficult to obtain oil or gas using conventional production methods.

Unconventional reservoir: Tight deposits such as shale and other rocks with low porosity and permeability. The gas or oil remains in the layer in which it was created or migrates short distances and requires stimulated production to extract.

Well bore: A hole that is drilled to explore and recover natural resources, such as oil, gas or water.

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