fault: A discrete break or fracture across strata or a stratum where the rock on one side of the break has been displaced upward, downward, or laterally relative to the other.

federal reserved rights: An implied water right that occurs when the federal government withdraws its land from the public domain and reserves it for a federal purpose, the government, by implication reserves appurtenant water then unappropriated to the extent needed to accomplish the purpose of the reservation. CSU.

field capacity: It is the amount of water remaining in soil or rock against the pull of gravity. It is sometimes limited to a certain drainage period (2 or 3 days) thereby distinguishing it from specific retention, which is not limited by time. GWAC.

fillup: Sand, silt, fines, and other materials that are produced from the aquifer, along with water, that are not pumped to the surface, but fall to the bottom of the cased well bore.

filter: (1) A device to screen or otherwise separate colloidal particles and other sediment, sometimes living microorganisms, from water.

(2) To screen or otherwise separate colloidal particles and other sediment, sometimes living organisms, from water.

(3) Relative to drilling mud, to filter the solids from the drilling mud.

The effluent is a mud filtrate. During the drilling process, drilling mud is filtered by the face of the drilled formation, and drilling mud filtrate penetrates the formation to a radial depth referred to as the depth of invasion.

filter cake: Relative to drilling muds. During the filtration process, mud solids are filtered from the drilling mud, leaving a cake of mud on the face of the filtering medium.

filter loss, filtration loss: The amount of effluent or filtrate that can pass through a filtering medium in a specified length of time.

filtrate: (1) The effluent of a water treatment process.

(2) Mud filtrate. See filter (3).

(3) A percolate. See percolate (2).

fines: A generic term referring to fragments or particles of rock too minute to be categorized, but can accumulate to occupy space in the pores of rock, thus impeding water flow. In a well bore, if the fines are not pumped out with the water, they can settle and accumulate at the bottom of the hole.

firm annual yield: The yearly amount of water that can be dependably supplied from the raw water sources of a given water supply system. CSU.

flaming water: This is not a technical term, but it is a sometime occurrence relative to ground water used for domestic purposes. This term gets its name from well water that flames when a source of ignition is placed near. The water will flame if flammable natural gasses are dissolved in the water and come out of solution when the confining pressure is relieved, such as by turning on a faucet or hydrant. The natural gas that emerges from or with the water is methane or a composite of methane and other gases. Go to www.colorado.gov/cogcc or http://cogcc.state.co.us for further information.

The approximate source of the gas can be determined from the composition of the gas, i.e. biogenic or thermogenic. The COGCC is able to differentiate between biogenic and thermogenic methane using both stable isotope analysis of the methane and compositional analysis of the gas. It is very helpful to know the source of the gas, but it is unlikely that flammable gas will find its way into the produced water unless there is a breakdown in aquifer isolation, or a vertical fracture (natural or manmade) connects the source of the gas to the aquifer. How a breakdown in isolation can occur is explained in completion (6). Also, see isolation, methane, biogenic methane, and thermogenic methane.

flood: (1) An artificial or manmade means to inject fluid (a gas or liquid) into a permeable water-, oil-, or gas-bearing stratum being depleted, to increase or maintain the pore pressure or energy required to drive the natural fluid out of the rock into a remote well bore. Highest efficiency occurs in reservoirs where boundaries are finite and increased pore pressure can be maintained over time. The increased pore pressure finds relief by driving the reservoir fluid to remote producing wells. Lowest efficiency occurs in permeable stratum where any increase in pore pressure can find a natural form of relief and is dissipated. See also discussion of flooding under drainage (2).

(2) An artificial or manmade means to inject fluid into a stratum being depleted to increase the pore pressure and stem subsidence.

(3) At the ground surface. The overflow of rivers and streams or the inundation of lowland by a large amount of water.

flood plain: A low area of nearly level land adjacent to a stream or other water course which is subject to flooding. Often delineated on the basis of the 100 year storm event. CSU.

flow, flow rate: See Darcy’s equation, also see discharge (2).

fluid level: All gases and liquids as they exist at standard conditions of temperature and pressure, or at the usual conditions of temperature and pressure, are fluids. In common usage, in all wells, fluid level means liquid level.

fluid loss: See filter loss.

flushed zone: Relative to the drilling process in an aquifer, mud filtrate penetrates the aquifer and fully displaces the water present in the pores. The zone where the displacement or flushing process has been most efficient is referred to as the flushed zone. Here, all mobile water has been displaced by mud filtrate.

fluvial: Of or pertaining to rivers or streams; produced by river action. Compare alluvial. NSSH.

flume: A sloped channel that is utilized to convey water and is commonly constructed of wood or concrete. Specialized flumes are used to measure flow (e.g. Parshall flume) by means of a calibrated opening or crosssection. GWAC.

flux: Refers to the concentration of flow. It is the quantity of material or energy transferred through a system or a portion of a system in a unit time and is called mass flux. If the moving matter is a fluid, the flux may be measured as volume of fluid moving through a system in a unit time and is called volume flux. For most hydrogeologic applications we desire to know the flux per unit area of a system rather than the flux of the entire system; the flux per unit area is called the flux density. GWAC. Also see hydraulic conductivity and transmissivity.

force: (1) The power or energy required to change the motion of material. Pressure applied over a large area can constitute a driving force. Water pressure in the pores of a rock provides the force necessary to drive water from the higher-pressure environment of the formation to the lower-pressure environment in the well bore. See energy, hydraulic pressure, hydrostatic load, and pressure gradient.

(2) See also gravity as a force.

formation: Often used as a generic term to describe the earthen environment of the borehole without regard to mineral composition, consolidation, permeability, or strata. More specific, it is a named, recognizable and mappable rock division consisting of a distinctive series of similar strata made up of similar minerals, of similar age, from a similar source, and exhibiting similar properties.

formation damage: Damage to a formation or to a specific aquifer can result from any action that disturbs the static equilibrium that exists within the bed. Static equilibrium can and will be disturbed by any dynamic action brought about by drilling the hole, producing water, or injecting water. The damage to aquifers or other beds often occurs where clays are present in their different mineralized forms. Formation damage sometimes can be reversed. See acid stimulation, acid treatment, and brushpiling.

(1) In the well bore environment. During the drilling process, the pressure of the drilling mud usually exceeds the pore pressure within the aquifer. The spurt loss from the drilling mud, and mud filtrate, will invade the aquifer to a specific radial distance depending on the permeability of the formation and the water loss of the drilling mud. The filtrate, and particularly the spurt loss, penetrating the pores produces fluid movement and shear inside the pores, and sometimes forces mud solids into the pores. The shear can weaken and break off fragile filaments from clay crystals that can brushpile at pore throats, and the mud solids can plug pores and pore throats, with the result that subsequent water flow can be drastically reduced.

If lost circulation should occur while drilling, the condition might aggravate the situation just described. See lost circulation.

(2)In the well bore the production of water produces a pressure gradient toward the well bore and this flow of water produces a hydraulic shear within the pores of the aquifer. Injection of water is an even more difficult process. During production of water, the fines and other undesirable constituents are flushed out of the aquifer. During injection, the constituents and obstructive materials are forced into the aquifer. During production, when the production rate is too high, the force of the flowing water can break off fragile filaments of clay crystals, and these in turn can brushpile in the pores and pore throats, thus obstructing flow and reducing the water production rate. Where these clay crystals are found, the number of perforations in the casing must be increased and/or the length of the perforated interval must be increased, thus reducing the flow rate in the near environment of each perforation.

Sometimes, in order to lengthen the perforation interval so that the number of perforations can be increased, a slant hole is drilled, or the hole is drilled directionally, so that the drilled hole will not be perpendicular to the water-bearing bed, but will penetrate at an angle with the bedding plane so that a longer length of perforated casing remains inside the aquifer. This will allow a greater number of slots or perforations, and reduce the flow rate at each perforation for the same production rate at the surface. Wells where this can be a problem must begin production very slowly with a gradually increasing production rate, thus producing the broken filaments with the water before they have collected into brush piles.

(3) In injection wells, the water being injected must match the water in the aquifer as closely as possible. Any change can cause injection or production problems later in the life of the aquifer. The chemical composition, pH, total dissolved solids, salinity, must match the water in the aquifer or formation damage will occur. The injected water must be free of solid matter and should be free of bacteria. In addition, the temperature of the injected water must not be different from the equilibrium temperature of the target injection stratum, and the injection rate must not be too high. A higher temperature than the equilibrium temperature can partially dissolve, weaken or swell clay minerals and their crystals, and also will provide a suitable environment for bacteria to grow. Solid foreign matter injected into the aquifer can either cause clay crystals to loosen from the pore walls, or hydraulic fluid flow can cause sufficient force to break off fragile filaments of clay crystals. The partially dissolved and broken filaments can brushpile in pores and pore throats and any sludge produced by growing bacteria or algae can cause blockage thus reducing the flow rate of injected water. The injected water must be free of contaminants, both inorganic and organic, and the injection rate must begin slowly and gradually be increased to the economical injection rate.

formation evaluation: The analysis and interpretation of well logs and other data obtained from scientific instruments run in the well bore for the purpose of determining the fluid content, properties and characteristics and mineralization of earth formations, beds, and strata. See petrophysical log and well log.

formation pressure: Pore pressure. In aquifers, water pressure.

formation water: Interstitial water. The ionic content of the water in the pores of the rock probably is not the same as that in water that was deposited with the rock originally. Over geological time the ionic content probably has changed numerous times by migration, diffusion, and the dissolution of minerals or by the precipitation of minerals. Compare connate water.

fracturing: See hydraulic fracturing.

free pipe: Free casing. Pipe or casing in the completed well that is free to vibrate or respond to stress. The undesirable condition in a completed well where the completion operation is defective and cement bond with the casing is faulty. Can allow crossflow or contamination of aquifers behind casing. See cement bond, completion, channel, channeling, crossflow, and isolation.

free water: Water in the pores of the rock that will respond to stimulation and has the freedom to flow or to be produced. See mobile water. Compare movable water.

fresh: A non-specific term relative to salinity. (1) Fresh water. Very low in salinity. Often used relative to sea water. Not necessarily potable, but can be potable. Compare sea water.

(2) Fresh drilling mud. Relative to muds with make-up water designed to produce a filtrate that is fresher than the water in the formation to be drilled.

futile call: A situation in which a junior priority will be permitted to continue to divert in spite of demands by a senior appropriator in the same watershed, because to curtail the junior from diversion would not be effective in producing water for beneficial use for the senior. CSU.

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