Hydraulic Fracking Glossary


Acidize: a well stimulation technique used to increase the flow of oil and gas from a well.


Annular valve: a valve on the above ground well that is connected to the space between the tubing and the casing.


Annuli pressures: in a well (well bore) the pressure between the tubing and casing.


Annulus: the space around a pipe in a well bore, sometimes termed the annular space.


API: American Petroleum Institute.  API is a national trade association that represents all aspects of the oil and natural gas industry. API develops industry standards, conducts and sponsors research, and organizes seminars, workshops, conferences and symposia on public policy issues.  http://www.api.org


API rated minimum internal yield: the burst pressure of a casing, tubing or drill pipe calculated by a formula based upon the yield strength of the metal, diameter of and wall thickness of the casing, tubing or drill pipe.


API well number: a unique number assigned to each well in the United States.  An API Number is comprised of four parts: state code, county code, well code and wellbore code.  An optional extension code may be attached. 

Authorized vs unauthorized release: authorized means intentional and with permission; unauthorized means either unintentional or without permission.

Carrier fluidis a base fluid into which additives, such as proppant, gel, and chemicals, are mixed to form a hydraulic fracturing fluid.


CAS: Chemical Abstracts Service is a division of the American Chemical Society, whose objective is to find, collect and organize publicly disclosed substance information.


CAS number: a unique number assigned by the CAS that identifies a chemical substance or molecular structure.


Casing: pipe placed in an oil or gas well to (1) prevent the wall of the hole from caving in, (2) to prevent movement of fluids from one geologic formation to another and (3) provide a means of maintaining control of formation fluids and pressure as the well is drilled.


Casing shoe: a tapered, often bullet-nosed piece of equipment often found on the bottom of a casing string. The device guides the casing toward the center of the hole and minimizes problems associated with hitting rock ledges or washouts in the wellbore as the casing is lowered into the well. (Also known as a “guide shoe.”)


Casing string: pipe that lines a well after it has been drilled. It is formed from sections of tube that have been screwed together.


Collapse strength of casing: is the pressure necessary to collapse a well casing, tubing or drill pipe inside a well. The collapse strength of casing can be calculated from the yield strength of the metal and diameter of and wall thickness of the casing, tubing or drill pipe.


Conductor casing: generally, the first string of casing in a well. It may be lowered into a hole drilled into the formations near the surface and cemented in place; it may be driven into the ground by a special pile driver. Its purpose is to prevent the soft formations near the surface from caving in and to conduct drilling mud from the bottom of the hole to the surface when drilling starts. (Also known as “conductor pipe” and “drive pipe.”)


Corrosive zone: a geological formation whose fluid is capable of corroding wellbore casing, tubing or other metal objects in a well.


Downstream: occurs later on in the production sequence.  Follows in the direction of fluid movement.


Faults active/inactive: Active faults are those that have moved in recent geologic time and are currently under stress; inactive faults have not moved in the recent past and are under little or no stress.


Fluid flowback volume: the amount of fluid that returns to the surface of a well after hydraulic fracture stimulation. The flowback can occur for days and weeks after hydraulic fracture stimulation.


Fluid rate: the rate fluid flows into or out of a well; measured in units of volume or velocity per unit of time.


Geographic Coordinate System (GCS): uses degrees of latitude and longitude to describe a location on the earth’s surface.


Guide shoe: a tapered, often bullet-nosed piece of equipment often found on the bottom of a casing string. The device guides the casing toward the center of the hole and minimizes problems associated with hitting rock ledges or washouts in the wellbore as the casing is lowered into the well. (Also known as a “casing shoe.”)


Intermediate casing: provides protection against caving in of weak or abnormally pressured formations and enables the use of drilling fluids used to drill into lower formations.


Intermediate casing string: the string of casing set in a well after the surface casing but before production casing is set to keep the hole from caving in and to seal off formations. In deep wells, one or more intermediate strings may be required.


Mechanical integrity: the measure of a well’s casing, tubing, packer and cement to contain fluids traveling up and down the well without the fluids leaking into surrounding geologic formations. 

NAD83 (North American Datum of 1983): is the adopted reference coordinate system in the United States when using latitude and longitude to identify a specific location.


Non-freshwater fluids: water with total dissolved solids (TDS) of greater than 3,000 parts-per-million (ppm) or any other fluid used in oil and gas production including hydraulic fracturing fluids.


Other cement evaluation method: can be used to determine the quality of the cement bond between a well’s casing and the geologic formations surrounding a wellbore. 


Perforate: to pierce holes in the casing and cement in a well to allow formation fluids, such as oil and gas, to enter into a well and to allow fluids to be injected into a geologic formation. Perforating is accomplished using a perforating gun, or perforator.


Perforated interval: a section of casing that has been perforated during hydraulic fracturing operations.


Permeability: the quantification of how easily fluids, such as oil, gas, and water, flow through the pore spaces in a geologic formation and into the wellbore.


Poisson’s Ratio: a mechanical property that can be used to predict the direction in which fractures will occur in a given geologic formation.  When a geologic formation is compressed in one direction, it tends to expand in the other two directions perpendicular to the direction of compression. This phenomenon is called the Poisson effect.


Production casing: the last casing set in a well though which oil and gas are extracted.


Productive horizon: formation layers known to contain oil or gas in quantity great enough for commercial production.


Projected fracture height growth: the projected length of fractures in a given formation that occur when a formation is hydraulically fractured.


Proppant: a granular substance (sand grains, crushed walnut shells, aluminum pellets, or other material) that is carried in suspension by the fracturing fluid. Proppant keeps fractures open in a formation when fracturing fluid is withdrawn after a fracture treatment.


Proppant concentration: The concentration of the propping agent per volume of fluid.


Propping agent: see proppant.


Radial cement evaluation log: a continuous log (graph) created by running a sonic transmitter and receiver down a well to determine the quality of the cement bond between the casing and the geologic formations surrounding a well.


Reverse engineering: the reproduction of a product, through analysis of its structure, function, and operation.


Shall be rigged up as designed: equipment to be used in the hydraulic fracturing process is to be set up as the design in the approved proposal.


Shut in: to close the valves on a well so that it ceases production of oil and gas or the injection of fluids into the surrounding geologic formation.


Slurry: a mixture of cement and water that is pumped into a well to harden. There it supports the casing and provides a seal in the wellbore to prevent migration of underground fluids. Slurry can also be a mixture in which solids are suspended in a liquid.


Slurry rate: the injection rate of volume of slurry per time.


Surface casing: isolates freshwater zones during the drilling and oil production phases of a well.


Surface injection pressure: The pressure of a fluid being injected into a well, measured at the surface.


Surface pipe: the first string of casing (after the conductor casing) that is set in a well. It varies in length from a few hundred to several thousand feet.


Total dissolved solids (TDS): the total amount of solids, such as minerals, salts, or metals that are dissolved in a given volume of water.


True vertical depth: The vertical distance from a point in the well (usually the current or final depth) to the surface.


Tubing: a small-diameter pipe that is run inside well casing to serve as a conduit for the passage of oil and gas to the surface. Tubing can be a permanent or temporary part of the well bore. 


Tubing strings: the entire length of tubing in a well.


Well: the hole made by the drilling bit, which can be open, cased, or both. Also called borehole, hole, or well bore.


Well bore: a borehole; the hole drilled by a drill bit; also called a borehole or hole.


Well stimulation: any of several operations used to increase production by increasing the permeability of an oil or gas bearing formation, such as acidizing or hydraulic fracturing.


Young’s Modulus: used in drilling, it is a measure of the stiffness of elastic of a geologic formation, defined as the ratio of the stress along an axis over the strain along that axis. Young’s Modulus can help determine how wide fractures are likely to be in a formation that will be hydraulically fractured.