MDN Glossary of Shale Gas Drilling
(courtesy of the EPA and other sources)

Abandoned well: A well that is no longer in use, whether dry, inoperable, or no longer productive.

Aerobic: Life or processes that require, or are not destroyed by, the presence of oxygen.

Anaerobic: A life or process that occurs in, or is not destroyed by, the absence of oxygen.

Analyte: A substance or chemical constituent being analyzed.

Aquiclude: An impermeable body of rock that may absorb water slowly, but does not transmit it.

Aquifer: An underground geological formation, or group of formations, containing water. A source of ground water for wells and springs.

Aquitard: A geological formation that may contain ground water but is not capable of transmitting significant quantities of it under normal hydraulic gradients.

Assay: A test for a specific chemical, microbe, or effect.

Biocide: Any substance the kills or retards the growth of microorganisms.

Biodegradation: The chemical breakdown of materials under natural conditions.

Casing: Pipe cemented in the well to seal off formation fluids and to keep the hole from caving in.

Coalbed: A geological layer or stratum of coal parallel to the rock stratification.

Coalbed methane (CBM): A form of natural gas extracted from coal beds. In recent decades it has become an important source of energy in United States, Canada, and other countries.

Flowback water: After the hydraulic fracturing procedure is completed and pressure is released, the direction of fluid flow reverses, and water and excess proppant flow up through the wellbore to the surface. Both the process and the returned water are commonly referred to as “flowback.”

Fluid leakoff: The process by which injected fracturing fluid migrates from the created fractures to other areas within the hydrocarbon-containing formation.

Formation: A geological formation is a body of earth material with distinctive and characteristic properties and a degree of homogeneity in its physical properties.

Ground water: The supply of fresh water found beneath the Earth’s surface, usually in aquifers, which supply wells and springs. It provides a major source of drinking water.

Horizontal drilling: Drilling a portion of a well horizontally to expose more of the formation surface area to the wellbore.

Hydraulic fracturing: The process of using high pressure to pump sand-laden gelled fluid into subsurface rock formations in order to improve flow into a wellbore.

Hydraulic fracturing water lifecycle: The lifecycle of water in the hydraulic fracturing process, encompassing the acquisition of water, chemical mixing of the fracturing fluid, injection of the fluid into the formation, the production and management of flowback and produced water, and the ultimate treatment and disposal of hydraulic fracturing wastewaters.

Impoundment: A body of water or sludge confined by a dam, dike, floodgate, or other barrier.

Mechanical integrity: An injection well has mechanical integrity if: (1) there is no significant leak in the casing, tubing, or packer (internal mechanical integrity) and (2) there is no significant fluid movement into an underground source of drinking water through vertical channels adjacent to the injection wellbore (external mechanical integrity).

Natural gas or gas: A naturally occurring mixture of hydrocarbon and non-hydrocarbon gases in porous formations beneath the Earth’s surface, often in association with petroleum. The principal constituent is methane.

Naturally occurring radioactive materials: All radioactive elements found in the environment, including long-lived radioactive elements such as uranium, thorium, and potassium and any of their decay products, such as radium and radon.

Play: A set of oil or gas accumulations sharing similar geologic and geographic properties, such as source rock, hydrocarbon type, and migration pathways.

Produced water: After the drilling and fracturing of the well are completed, water is produced along with the natural gas. Some of this water is returned fracturing fluid and some is natural formation water. These produced waters move back through the wellhead with the gas.

Proppant/propping agent: A granular substance (sand grains, aluminum pellets, or other material) that is carried in suspension by the fracturing fluid and that serves to keep the cracks open when fracturing fluid is withdrawn after a fracture treatment.

Public water system: A system for providing the public with water for human consumption (through pipes or other constructed conveyances) that has at least 15 service connections or regularly serves at least 25 individuals.

Redox (oxidation-reduction) reaction: A chemical reaction involving transfer or electrons from one element to another.

Residential well: A pumping well that serves one home or is maintained by a private owner.

Shale: A fine-grained sedimentary rock composed mostly of consolidated clay or mud. Shale is the most frequently occurring sedimentary rock.

Shale cuttings: Small pieces of rock that break away during the drilling process. Cuttings are screened out of the liquid mud by using shale shakers, or screens that allow the liquid to pass through but filter out the bits of rock.

Shale gas: Natural gas produced from shale. Shale gas has become an increasingly important source of natural gas in the United States over the past decade, and interest has spread to potential gas shales in Canada, Europe, Asia, and Australia.

Source water: Operators may withdraw water from surface or ground water sources themselves or may purchase it from suppliers.

Subsurface: Earth material (as rock) near but not exposed at the surface of the ground.

Surface water: All water naturally open to the atmosphere (rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries, etc.).

Tight gas: Natural gas found in reservoirs with low porosity and low permeability. It can be compared to drilling a hole into a concrete driveway–the rock layers that hold the natural gas are very dense, therefore the gas doesn’t flow easily.

Tight sands: A geological formation consisting of a matrix of typically impermeable, non-porous tight sands.

Total dissolved solids (TDS): All material that passes the standard glass river filter; also called total filterable residue. Term is used to reflect salinity.

Turbidity: A cloudy condition in water due to suspended silt or organic matter.

Unconventional gas: Natural gas resources which require greater than industry-standard levels of technology or investment to harvest. The three most common types of unconventional gas resources are tight sands, coalbed methane (CBM), and shale gas.

Underground injection well: A steel- and concrete-encased shaft into which hazardous waste is deposited by force and under pressure.

Underground source of drinking water (USDW): An aquifers currently being used as a source of drinking water or capable of supplying a public water system. USDWs have a TDS content of 10,000 milligrams per liter or less, and are not “exempted aquifers.”

Vadose zone: The zone between land surface and the water table within which the moisture content is less than saturation (except in the capillary fringe) and pressure is less than atmospheric. Soil pore space also typically contains air or other gases. The capillary fringe is included in the vadose zone.

Water table: The level of ground water.