Compiled and Edited by Robert C. Ransom
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darcy, darcys, darcies: The accepted unit of measure for permeability as proposed by Darcy. The permeability of a medium to allow the flow of one milliliter per second of fluid of one centipoise viscosity through one square centimeter under a pressure gradient of one atmosphere per centimeter.
(1) A simplified version of Darcy’s linear equation is
q = (kA/μ ) × (Δp/Δd )
where, in aquifers: q = fluid flow rate of ground water, k = absolute permeability of the aquifer to fresh water, A = cross-sectional area of the cumulative flow paths within the aquifer, μ = viscosity of fresh water, Δp = initial pressure difference that provides the energy for ground water to flow across distance Δd. This is an equation that is commonly applied to liquid flow rates in linear lengths of rock; short, long, vertical, horizontal, and pipelines.
It might be easier to understand this equation if it is rewritten as:
q = (kA/μΔd ) × Δp
In this rewritten version, the total quantity within the parentheses constitutes a fraction with a value lower than 1.0. And, the fraction contains all the conversion units to convert flow rate to pounds pressure per square inch, or the reverse conversion from pressure to flow rate. The A and the μ essentially are constant. The k can vary throughout the aquifer as the quality of the aquifer changes. The expression k/μ is mobility. It can be seen that as distance Δd increases from some starting point that the fraction within the parentheses becomes smaller. Also, it can be seen that the fraction decreases linearly with distance and might have variations in linearity when and if there are variations in k. As distance from a water source increases, the pressure remaining to push water into the borehole decreases, as does the flow rate. In the use of the equation, the term Δp represents the original energy to cause water to flow and is determined by the difference in pressure between two different elevations. One is the elevation of the water table at the recharge area (source) and the second is the elevation within the aquifer where water emerges. But, the initial Δp is a gravity drainage potential. After the original Δp has been reduced over distance Δd a new value for Δp is the result. This is the artesian pressure between the elevation of the potentiometric surface at the well site and the elevation of water emergence from the aquifer. The water drive now has only a fraction of the energy of the initial gravity drive.
When the potentiometric surface falls, the Δp diminishes, and when the potentiometric surface falls to the depth where water emerges from the aquifer, the Δp diminishes to zero. The flow rate of ground water diminishes right along with the Δp.
q = (kh/μ ) × (Δp/ln(re /rw ))
where: q = fluid flow rate, k = absolute permeability to water, h = aquifer thickness, μ = viscosity of fresh water, Δp = pressure difference across the drainage radius of the formation to the well bore, re = effective drainage radius, rw = effective well bore radius (gravel packed radius or fractured radius as applies). This is an equation that is commonly applied to liquid flow rates in producing wells.
(3) There are at least two forms of Darcy’s equation. The two most recognized forms are seen in parts (1) and (2) above. Part(1) pertains to linear distances in aquifers or pipelines over distances of from inches to many hundreds of feet or more. Also applies to vertical distances as well as horizontal distances. For water to flow, it will obey whichever relationship fittingly applies.
The flow rate of water is directly related to the pressure that drives the water. And the driving pressure is a function of two expressions seen on the right side of each equation. Both expressions are based on an energy source that creates an initial total Δp. Because it takes part of that total energy to move water within an aquifer from one place to another over distance Δd, not all the initial energy is available for water production. Darcy’s equation(s) modifies that total pressure difference to a net available pressure difference that results in a lower calculated rate of flow. Although Darcy’s equation has many applications over distances or lengths short and long, horizontal and vertical, one significant function of the equation is that it accounts for the pressure loss in moving water from a distant source to the site of the unopposing borehole. After the pressure loss, there is a decreased net pressure drive resulting in a decreased potential flow rate. Once the opposition to water movement has been overcome, if no net pressure drive remains, then there will be no water production. The same logic also applies to short distances from points inside the aquifer to a point inside the well bore of a producing well. It also applies to different forms of drainage. See resistance to flow and drainage (2)(a) in particular. It is a very useful equation to demonstrate the factors that influence liquid flow.
Not a part of Darcy’s linear relationship, but a factor in determining the rate that water emerges from the drilled face of the aquifer is backpressure, which is a function of drawdown. Back pressure is a part of Darcy’s radial relationship. See also pumping process and withdrawal process.
In a word statement relative to water, Darcy’s equations read:
The flow rate of water is a function of the total energy available to move that water, decreased by the fraction of that same energy required to overcome the opposition to the movement of that water.
For more information about Darcy’s equation and the individual hydrogeological terms appearing within Darcy’s equation, see the detailed discussion under hydraulic conductivity.
Henry Darcy developed his equation ca. 1856. His equation has become law and provides the foundation for work in hydrogeology and petroleum engineering.
Dawson aquifer: The uppermost formation of the Denver Basin aquifer system. The upper part of the Dawson formation is considered to be tributary. The Dawson aquifers are composed of sandstone and conglomerate. Douglas Co.
decree: An official document issued by the Court defining the priority, amount, use and location of a water right or plan of augmentation. When issued, the decree serves as a mandate to the state engineer to administer the water rights involved in accordance with the decree. Douglas Co.
decreed water right: A court decision placed on a water right that is then administered by Colorado’s Water Resources Department. CSU.
deflection: The angle in degrees, or deviation, from vertical in a slant hole or in a directionally drilled hole.
density: Mass per unit volume. Units are either g/cm3 or mg/liter depending on application. Compare specific gravity.
Denver aquifer: The Denver formation underlies the Dawson formation and the aquifers are composed of interbedded sandstone, siltstone, and shale; and contains some coals. Douglas Co.
Denver Basin: A large groundwater basin of sedimentary rock formations containing four principal aquifers, the Dawson, Denver, Arapahoe and Laramie-Fox Hills. The water basin extends from Greeley on the north to Colorado Springs on the south, Limon on the east to the foothills on the west. Ground water contained within the Denver Basin aquifers is considered to be either nontributary or not-nontributary under the Colorado Water Rights System. Douglas Co.
deplete: To use, exhaust, exploit, or consume to excess, usually at a rate faster than the water is replenished. In an aquifer, in compliance with Pascal's Law of pressure transmission, a decrease in pressure at one place of withdrawal can be transmitted to all other parts of the aquifer system, subject to communication and permeability.
depleted: (1) In aquifers immediately underlying the zone of aeration (unconfined aquifers), it is the condition that exists when gravity drainage causes the air-water interface to drop to the level of water emergence from the aquifer. Water no longer can be made to flow into the well bore under this condition. This aquifer is depleted of mobile water above the level of the air-water interface, and the total water volume in place has been reduced. Also see aerated zone, drainage (1), aquifer (1) and irreducible water.
(2) Relative to aquifers that have no communication with the aerated zone (confined aquifers), it is the condition that exists when water pressure in the aquifer has been dissipated or exhausted to the extent that water in the aquifer no longer can be made to overcome both back pressure and resistance to flow out of the rock and into the well bore. This aquifer is pressure depleted, but water saturation is unchanged and the total water volume in place has not diminished, and is the same as before production began. Relative to depleting confined aquifers, see the discussion on flooding by the injection of compressed air under drainage (2). Also, see Darcy’s equation which determines available pressure, and see back pressure, in place, resistance to flow, movable water, drainage, and aquifer (2).
depletion rate: The true depletion rate is the net rate of water use, over time, from a stream or aquifer.
(1) Of a water well. Production volume minus upstream, natural or unnatural recharge volume over a specific time period.
(2) For irrigation and municipal uses, the depletion is headgate or wellhead diversion less return flow to the same stream or aquifer.
depletion time: Time indicating how long it would take the watershed or the groundwater system to dry out if surface runoff or groundwater replenishment (recharge) were stopped from an instant onward, and if outflow was maintained at the rate it had at that instant. Depletion times of surficial water usually are of the order of hours or weeks. They may run into months or years if the river basin includes large lakes. Depletion times of aquifers are usually of the order of tens to hundreds of years. As a consequence, rivers react quickly to precipitation and to the extraction of water, whereas groundwater systems react very sluggishly to these events. GWAC.
deposit: (1) An accumulation of sediment of any material and from any source to form a bed or layer of rock.
(2) To put, drop, lay down, or leave in place.
depositional environment: The environment in which sediment is deposited. The five categories are: marine (from seas or ocean), aeolian or eolian (wind borne), alluvial (river borne), deltaic (formed at the terminus of a river), and interdeltaic (formed between river deltas).
depth datum: The location at the surface of the ground or above the surface of the ground from which all depth measurements are referenced in a drilled hole.
depth of invasion: This is the radial depth within the formation, bed, or stratum that drilling-mud filtrate has penetrated.
depth to base: The depth to the base of an aquifer is the vertical distance from the ground surface to the base of the aquifer. Douglas Co.
depth to water: The depth from the ground surface to the water table. See water table.
designated ground water: Ground water which, in its natural course, is not available to or required for the fulfillment of decreed surface rights, and which is within the geographic boundaries of a designated ground water basin. CSU.
designated ground water basins: Those areas of the state established by the Ground Water Commission located in the Front Range and Eastern Colorado. CSU.
detrital clay: Clay particles or crystals that occupy space within the pores of a rock. Can be both authigenic and non-authigenic types. Detrital clay often is dissolved in interstitial waters and later the ions recombine to form crystal growths on the grain surfaces at the same location or at a different location. See authigenic.
detrital sediment, detritus: A term for rock and mineral fragments, accumulating in sediments, that have been detached or removed by mechanical erosion or mechanical weathering of previously existing rocks.
developed water: Water that has been brought into a water system by manmade means, where it would not have entered the water system by natural means.
diagenesis: The chemical, biological, or physical changes that sediments undergo to become consolidated rock, or in some cases to create secondary porosity. Such changes result from compaction, cementation, recrystallization, dissolution, or replacement.
1. The process of enlarging existing pores or creation of additional porosity, cavities or vugs by further dissolution of rock.
2. Creation, by growth, of constituents within the interstices of gravel, sand, or rock, that were formed, or were generated, from saturated solutions. Particularly, crystals of minerals that are found adhering to and coating the walls of the pores within a host rock such as sand. Typical diagenetic minerals are quartz, carbonates, and clays. In aquifers, the presence of diagenetic crystals, particularly those of clay, can cause flow problems in both injection and production wells. Authigenic crystals are diagenetic crystals that have been formed by dissolution of detrital sediment and re-crystallization at the same location where they are found. In aquifers, the presence of authigenic crystals, particularly those of clay, can cause flow problems in both injection and production wells.
dip: The angle that a bedding plane makes with the horizontal.
directional drilling, directional hole: See deviated hole.
dirty: Describes the presence of appreciable amounts of clay, shale, or silt which is different in mineral nature from that of the host rock. Can describe gravel or sand with appreciable amounts of clay and shale.
(3) To force out of, empty, expel, eject sometimes with explosive force. Erupt.
(4) See also discharge area.
discharge area: An area where ground water is lost naturally from an aquifer through springs. The water leaving the aquifer is called discharge. GWAC.
discontinuity: (1) Any form of interruption or break in the depositional sequence or depositional pattern of a sediment. See unconformity.
disinfectant: An agent or chemical solution, such as chlorine or iodine, or ozone, or ultraviolet light that kills disease-causing microorganisms. Sometimes the chemical disinfectant (chlorine bleach solution) is very corrosive to the casing and downhole hardware.
disinfection by-products: Chemicals, such as total trihalomethanes, formed from naturally occurring humic or fulvic acids and the disinfectant used in treating water. GWAC.
dissociation: The breaking up of a compound into its simpler components such as molecules, atoms, or ions. Results from the action of some form of energy on gases and from the action of solvents on substances in solutions. SPWLA.
diversion records: Records of the daily flow in cubic feet per second for a ditch or other diversion structure during the irrigation season, compiled by the District Water Commissioner. Diversion records are on file and available for review by the public at the State Engineer's Office in Denver, Colorado.
diversion, divert: Removing water from its natural course or location, or controlling water in its natural course or location.
(1) On the surface of the ground, by means of a ditch, canal, flume, reservoir, bypass, pipeline, conduit, pump, or other structure or device.
(2) In an aquifer, by barriers produced by decreased porosity or permeability, increased local water pressure, or by stratigraphy.
divide, drainage divide: The boundary between one drainage basin and another. GWAC.
division engineer: The state engineer’s principal water official in each of the seven water divisions. CSU.
(2) A sedimentary rock consisting primarily of the mineral, calcium magnesium carbonate.
domestic water use, domestic well use: Water used for drinking and other purposes by a household, such as from a rural well. Domestic use permits normally allow limited irrigation and outside watering uses. GWAC.
downhole: A term to describe the location in the well bore of certain activities or equipment.
drainage: Relative to aquifers and reservoirs. When water (also oil or gas) is produced from a formation the space vacated by the produced water must be re-occupied otherwise the pressure gradient between the water in the well bore and the water in the formation will rise sharply depending on existing formation pressure. The increased pressure gradient might cause coning to take place with damaging finality. (Side note. The water-well pump cannot draw water to the well bore by suction. A vacuum, even if it could be created, cannot drive water to the well bore because a vacuum is neither a source of energy nor a form of force. See suction, pumping process and withdrawal process.)
Drainage is the means by which water, in the case of aquifers, is driven out of the formation into the well bore. Drainage cannot be sustained without reoccupation of the space vacated by produced water. Air is what re-occupies the space vacated by produced ground water. Without reoccupation, water production will become sporadic and ultimately will cease. The combined processes of producing water from the formation and reoccupation of the vacated space is called drainage; and, it is how reoccupation takes place that defines or distinguishes the kind of drainage.
There are a number of different methods to produce water from aquifers and reservoirs. Drainage pertains to the type of force or method, usually at the location of a producing well, that causes water to flow to the well bore. All flow rates are subject to Darcy’s equation. See also recovery.
Different mechanisms for drainage can be:
(1) Gravity drainage. This is a local form of drainage that can take place in aquifers underlying and adjacent to the aerated zone. Here, air at atmospheric pressure replaces the water that has been produced and, without replenishment, the water table will be seen to drop or decline as water production continues. Gravity drainage can take place only where there is a difference between the densities of the produced fluid and the replacing fluid. The driving force is gravity. The energy to force the water into the well bore comes from the height of the water table above the level of water emergence from the aquifer (hydrostatic head). Buoyancy moderates the effectiveness of gravity. Gravity drainage commonly occurs in unconfined aquifers. See aquifer (1), buoyancy, and water table.
(2) Other drainage methods result from any of the natural driving forces occurring within confined aquifers. Such force can be: (a) hydraulic pressure derived from a remote hydrostatic load, and transmitted hydraulically, where the pressure gradient toward the borehole is maintained by the height of the water table (at the remote location) above the level of water emergence from the formation, and produced water is continually replaced by renewal water; (b) augmentation of water supply by expansion of water (see bulk modulus) and through compaction of clay shales in overburden (see compaction (2)), both resulting from depletion of pore pressure by water production (also see subsidence); (c) dissolved gases coming out of solution as pressure is decreased, and produced water is replaced by expanding gases; (d) expanding gas that has accumulated (by virtue of its low density) in a stratigraphic trap and produced liquids are replaced by the expanding gas; (e) abnormal pore pressure that drives water to the well bore as long as the pore pressure exceeds back pressure; (f) by the unnatural process of recharging, to artificially replenish the produced water or its pressure by any form of augmentation, or by a flood process wherein a less valuable fluid is injected into the aquifer or reservoir to maintain sufficient pore pressure to increase or sustain production of the more valuable fluid. Flooding is a means of recovering a geofluid that is not recoverable by natural means. In aquifers where water is locked in the rock and will not drain by natural methods, air flooding might be implemented. This is a method where compressed air is injected into the aquifer to increase pore pressure and maintain producing rates. The air occupies pore space vacated by water allowing water production to continue. In aquifers, this would be an air flood operation. In depleted oil reservoirs the injection of air would be a thermal method usually referred to as a fire flood operation. The injection of air into an oil-bearing reservoir supplies the oxygen to start a burn by spontaneous combustion. The resulting hot gases produced would maintain pressure and heat the rock, decreasing the viscosity of the oil and driving the oil ahead of the burn front. See flood and aquifer (2) and (3), and recovery.
drainage area, drainage radius: Production of water by a water-well pump increases drawdown and, therefore, reduces the back pressure in the well bore. This reduction in pressure in the well bore produces pressure gradients in all directions from the well, allowing water pressure in the aquifer to drive water toward the well bore. The effectiveness of drainage and, therefore, the drainage area around the well is related to conditions influenced by a number of factors, among them are: (1) drawdown, (2) resistance to flow, and (3) water pressure in the rock. See drainage, drawdown, pressure gradient, and resistance to flow. See also Darcy’s radial equation.
drainage basin: See basin (2).
(2) The difference in the depths inside the well bore between the water level at static conditions and the water level under producing conditions.
(3) The theoretical maximum drawdown occurs when the water level inside the casing under producing conditions falls to a steady-state level just above the level of the pump intake ports. If the dynamic water level falls to the level of the intake ports, water production will cease. Also compare coning.
(4) Sometimes refers to the difference in depths between the original static water level in a well and the present static water level, thus an indicator related to the decline or rate of decline in formation pressure.
drift: A term sometimes used to describe a flow of ground water caused by hydrostatic force due to a naturally existing regional gradient. Douglas Co.
drilling mud: Rotary rigs utilize a drilling-fluid mixture to facilitate the drilling operation. Drilling mud is pumped into drill pipe and down to the bottom of the borehole where it passes through the drill bit and returns to the surface through the annular space between the drill pipe and the face of the drilled formation. The mud is saved in a mud pit from which it is recycled into the drill pipe.
Drilling mud serves a number of purposes: (1) It cools and lubricates the bit; (2) it removes cuttings from the drilled hole and brings them to the surface where they can be identified and analyzed; (3) with its designed density, it can provide sufficient back pressure to prevent flow of geofluids into the drilled hole (sometimes hydrocarbons can discharge with disastrous consequences); (4) helps to prevent collapse of the drilled hole by the back pressure it produces; (5) can be designed by engineering its chemistry and salinity to minimize the penetration of mud filtrate into the formation, and to minimize the absorption of mud filtrate or water by clays in the formation; and (6) provides the necessary medium for the operation of numerous well-logging instruments.
drill pipe, drill stem: Rotary rigs use drill pipe to direct drilling mud down to and through the drilling bit. The drill pipe consists of numerous joints of pipe screwed tightly together, ending with the appropriate fitting and drill bit for the situation, underground conditions, and formations to be penetrated.
drive, drive mechanism: Provides the natural or unnatural energy that sustains the pore pressure or hydraulic water pressure that forces water in rock to overcome the back pressure, and the resistance to flow within the rock, to flow into the well bore. See drainage, hydrostatic load, hydraulic pressure, and pressure gradient.
drop pipe: See tubing.
dynamic: In a state of motion. In water wells, it means active production or injection is taking place. Water is in motion under hydraulic pressure, either flowing through the pores of an aquifer and into the well bore, or being forced to flow into a formation, bed, or stratum.
dune: A low mound, ridge, bank or hill of loose, windblown, subaerially deposited granular material, generally sand, either barren and capable of movement from place to place, or covered and stabilized with vegetation. NSSH.
duty of water: The total volume of irrigation water required to bring to maturity a particular type of crop. It includes consumptive use, evaporation and seepage from ditches and canals, and the water eventually returned to streams by percolation and surface runoff, usually expressed in acre-feet per acre. Douglas Co.
dynamic water level: In a water well, under producing conditions, it is the depth at the surface of the column of water standing in the well casing. See water level (2).