Interpretive Glossary of Water-related Terms and Expressions - A

abnormal pore pressure: Any pore pressure that differs from normal pore pressure. It may be higher than normal or lower than normal. Higher than normal pore pressure sometimes is called supernormal pressure, and lower pressure sometimes is called subnormal pressure. How subnormal pore pressures can be created in aquifers, refer to bulk modulus (3). Higher than normal pore pressure is quite common in aquifers. Also, it can be found in true reservoirs, lenses and stratigraphic traps that are surrounded by impermeable barriers that prevent communication and relief for the reservoir fluid and its pressure. Overpressure can result from a number of different conditions.

(1) Relative to aquifers, the abnormal pressure is called artesian head. This is a form of overpressure. It is represented in Darcy’s equation by the term Δp. Here, the difference in water pressure is due to the difference in the elevations between the water table directly under the outcrop of the aquifer and the elevation at the depth where water emerges from the aquifer. These two elevations, of water table at the outcrop and elevation of water egress from the aquifer, can be and probably are separated by a comparatively great distance Δd. That distance Δd is seen within the fraction on the right side of the Darcy equation. It can be seen that as distance from the outcrop increases, the flow rate q decreases, and it continues to decrease with a decreasing Δp over distance until the abnormal pressure ultimately dissipates.

The water pressure in unproduced confined aquifers usually is greater than normal and the excess is called artesian pressure. In turn, artesian pressure suggests a water table higher in elevation directly above the producing aquifer. This is an imaginary water table and invokes another term. This term is the potentiometric surface, defined by O. E. Meinzer, 1923. The water table directly under the outcrop of the aquifer is a real and true potentiometric surface. The potentiometric surface at the water table is projected at decreasing elevations over distance Δd everywhere throughout the aquifer. See artesian head, potentiometic surface, and Darcy's equation.

(2) In other granular sediments as well as aquifers, the most common type of overpressure is caused by overburden. Overburden is supported by both the grain-to-grain framework of rock and the formation pressure within its pores. See compaction (2). When clay materials are deposited, the associated water (see connate water) constitutes a large part of the total volume. While undergoing compaction under the stress of overburden, some of this water is squeezed out of the clay into permeable beds or strata where the expelled water finds relief. The water-filled pore volume remaining in the clay shale is directly related to the stress of overburden to a very high degree and as the burial process continues, the water pressure in the pores increases linearly with the hydrostatic load. A normal pore pressure gradient is established and normal pressure is exhibited. Within this depositional sequence, which can be thousands of feet thick, if there exists a formation or series of beds that have no lateral communicable relief (see lens), then the water in the pores of the clay shales cannot be expelled, attendant porosity will be preserved, and the formation pressure will increase above normal. The formation pressure will continue to increase as overburden and the stress it produces continues to increase. The increased pore pressure relieves some of the load on the surrounding clay shales and grain-to- grain framework of aquifers, and reduces the rate of compaction and compression of pore space. Higher than normal pore pressure in formations can be recognized on petrophysical logs where the porosity of associated clay shales is higher than the normal. Overpressured beds are observed in shallow wells as well as extremely deep wells. See compaction (1) and (2),and normal pore pressure.

absolute permeability: See permeability (1).

absolute water right: A water right, with a specific priority date, that has been placed to beneficial use. CSU.

acre-foot: A measurement of a volume of water (43,560 cubic feet, 325,872 gallons) to cover one acre of land one foot deep.

acid: Any chemical compound, one element of which is hydrogen, that dissociates in solution to produce free hydrogen ions. For example, hydrochloric acid, HCl, dissociates in water to produce hydrogen ions, H+, and chloride ions, Cl-. SPWLA. See also dissociation.

acid stimulation, acid treatment: This is a radical procedure in water wells and aquifers. The acid compound used in these operations must dissociate in solution to release free hydrogen ions so that the solution will attack and dissolve acid-soluble materials that can restrict the flow of fluids. For well treatments, the acid usually is hydrochloric or a mixture of hydrochloric and hydroflouric acids, or other organic acids. When and if the acid treatment is successful, the acid must be backflushed by producing water from the well and the pH must be increased to acceptable levels for potable or domestic water. Acid treatments can be designed to stimulate well production, or injection, where the well performance has been affected by any of the following conditions:

(1) Inside the casing. Scale and other precipitates can obstruct and restrict fluid flow in and on downhole equipment.

(2) ln the casing. Slots or perforations in the casing can become encrusted with scale and other deposits that obstruct and restrict the flow of fluids passing from outside to the inside of the casing.

(3) In the well bore environment. During the drilling process, the hydraulic pressure of the drilling mud 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 water loss of the drilling mud and the permeability of the aquifer. 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 water flow in or out of the formation can be drastically reduced.

(4) In injection wells. The injected water must be free of solid matter and should be free of bacteria, and the injection rate must not be too high. Foreign matter injected into the aquifer can either cause clay solids 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 produced 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.

(5) 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. When the production rate is too high, the force of the flowing water can break off fragile filaments and platelets 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 for the desired production rate at the surface. 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 crystal fragments with the water before they have gathered into brush piles. See brushpile.

adjudication: A judicial process in which a priority is assigned to an appropriation and a court decree is issued defining the water right. Douglas Co.

adverse use: Using decreed water owned by another appropriator. CSU.

aeolian: (1) Wind borne. Sedimentary mineral matter transported by and then deposited by wind to form dunes and other wind-blown features to form porous and permeable strata when buried. From Aeolus, mythological Greek god of the winds. Although this term is entrenched in literature, some consider the spelling of the term obsolete and are changing the spelling to eolian.

(2) In Greek culture. Pertaining to Aeolis or its inhabitants or its language.

aerated zone: The earthen zone at or near the ground surface containing water held by capillarity, and containing air at atmospheric pressure.

aerobic: Typical of microorganisms that thrive in contact with air and absorb oxygen from the air as they feed upon and bring about decomposition of waste organic matter present in water. Compare anaerobic and see biochemical oxygen demand.

alkaline: (1) Having the properties of a base.

(2) Containing ions of one or more alkali metals.

(3) Waters containing more than the average amounts of carbonates of sodium, potassium, magnesium, or calcium. SPWLA.

allogenic: Detrital accumulations of rock constituents and minerals derived from the degeneration and mechanical erosion or weathering of older formations and redeposited. Compare with authigenic.

alluvial: A term that describes a geological formation that consists of gravels, sands and silts that have been transported by rivers and streams and deposited to form porous and permeable strata. The more distant from the water source, or the slower the velocity of the water in the river or stream, the smaller will be the materials that drop out of the water. The Denver Basin consists of a number of alluvial formations. See depositional environment.

alluvial plain: A level, gently sloping, or slightly undulating land surface produced by extensive deposition of alluvium, usually adjacent to a stream that periodically overflows its banks. GWAC.

alluvium: Refers to the unconsolidated sediments such as gravels, sands and silts that have been deposited by rivers and streams.

amorphous: Describes rocks and minerals that have no definite or defined crystalline form.

anaerobic: Typical of microorganisms that thrive where there is no contact with air or free oxygen and obtain oxygen from the decomposition of waste organic matter in water. Compare aerobic and see biochemical oxygen demand.

anhydrite: A common name for the naturally occurring calcium sulfate. Anhydrite is calcium sulfate (CaSO4) existing in the orthorhombic crystalline form. The anhydrite crystal cannot become a gypsum crystal (CaSO4(H2O)2) , the monoclinic form, in a single hydration step. Anhydrite must first become dissociated in solution before the ions can recrystallize as gypsum. Compare gypsum.

anisotropy: The property of rock to exhibit different responses or measurements when stimulated or measured along different axes.

annular space: In a completed water well, it is the space between the casing and the drilled formation wall that is filled with cement during the completion operation. See completion, isolation, channeling, cement bond, and free pipe.

appropriation: The diversion of a certain portion of the waters of the State and the application of same to a beneficial use (CRS 37-92-103). Under certain conditions, an appropriation may be accomplished by the state without the act of diversion and application to beneficial use. Douglas Co.

appropriative rights: See Appropriation Doctrine.

Appropriation Doctrine: The system of water law primarily used in the western United States under which: (1) The right to water is acquired by diverting water and applying to a beneficial use; and (2) a right to water use is superior to a right developed later in time. CSU.

appropriator: The person or persons who put water to beneficial use. CSU.

aquiclude: A layer of soil, sediment, or rock that is incapable of transmitting significant quantities of water under ordinary hydraulic pressure gradients. Often considered to be an impermeable layer or bed that precludes the transmission of water. See pressure gradient and compare aquitard.

aquifer: An underground formation, bed, or stratum of earth, gravel, sand, or other porous rock that contains no liquid other than water, and can transmit mobile water under a sufficient hydraulic pressure gradient. There are a number of different kinds of aquifers, three of which are:

(1) unconfined aquifer. An aquifer in which the water table serves as the upper surface of the water-saturated zone and the aquifer underlies and has direct contact with the aerated zone. This definition would apply to aquifers containing tributary ground water. See water table, drainage (1), and tributary ground water.

(2) confined aquifer. Usually considered to be an aquifer sandwiched between two impermeable rock layers or strata (or aquicludes) at the depth of interest and at the local geographic location. The barriers can be impermeable side beds or any other form of permeability barrier. Might or might not be confined at a remote location, depending on whether or not the water it contains is renewable. This is an aquifer that is confined by impermeable barriers and usually is not considered to have communication with atmospheric pressure except at the distant location where water renewal can take place, such as an aquifer recharge area.

(3) Some confined aquifers are surrounded completely by impervious beds and do not contain renewable water. These are aquifers that contain water related to the water that was deposited simultaneously with the sediments (see connate water). If the aquifer pinches out in all directions and is completely surrounded by impervious strata, then the quantity of water is finite and not renewable. Some aquifers such as these might exhibit higher than normal pore pressure (see abnormal pore pressure), but all will cease water production when the formation pressure or pore pressure dissipates or is reduced to the level that the pressure gradient toward the well bore becomes insufficient to sustain production. Also see lens.

All aquifers containing non-tributary ground water are confined aquifers by the usual definition, but confined aquifers in the same system can be hydraulically connected. See non-tributary ground water and compare tributary ground water.

To help understand aquifers, see drainage, drive mechanism, pressure gradient, and movable water. Also see pass-through aquifer.

aquifer recharge area: (1) The cross-section of the permeable outcrop or exposed ground-level part of the aquifer. The outcrop representing the recharge area might or might not have the characteristics or properties of the buried aquifer. See renewal and recharge.

(2) For renewal of non-tributary water. The location where renewal or natural recharge of a confined aquifer can take place. Usually an outcrop at elevation, where natural recharge can occur by precipitation from the atmosphere; or at depressions where natural recharge of the aquifer can occur from lakes, rivers, streams, or other water concentrations such as runoff.

The flow from a recharge area is from the highest elevation to the lowest elevation. And the water table that underlies the recharge area or outcrop is a potentiometric surface. See detailed discussions under potentiometric surface and hydraulic conductivity.

(3) For renewal of tributary water. The area can be a river, stream, lake, basin, or other depression that can allow water to recharge an unconfined aquifer.

aquifer test: A test involving the withdrawal of measured quantities of water from, or addition of water to, a well or wells and the measurement of resulting changes in head in the aquifer both during and after the period of discharge or addition. GWAC. This is one of a number of different tests that can be made in production wells and monitoring wells for gathering information relative to aquifers. See also biochemical oxygen demand, water analysis, pH, and potentiometric surface.

aquifer yield: The maximum rate of withdrawal that can be sustained by an aquifer. Also see yield.

aquitard: A body of rock or sediment that retards but does not prevent the flow of water to or from an adjacent aquifer. It does not readily yield water to wells or springs but might serve as a storage unit for ground water. NSSH.

Arapahoe aquifer: The Arapahoe formation consists of aquifers that underlie the Denver formation, and is composed primarily of sandstone interlayered with shale.

arch: In unconsolidated sand, sand production can occur with the consequence that the casing can be filled with sand to the level of the perforations. This can reduce water production as each slot or perforation becomes covered. In such formations, if production is brought on very slowly an arch can form on the outside of the casing over each perforation when the sand grains wedge together. Each arch has the chance to remain as long as the production rate is low.

argillaceous: Formations, beds, and strata that contain significant proportions of clay minerals such as found in shales, silts, and slate.

artesian head: The hydrostatic head of water in the well bore of a static, shut in, or non producing well such as a monitoring well. See hydrostatic head (2), back pressure, and potentiometric surface. Also see abnormal pore pressure (1).

artesian pressure: Pressure exhibited by the artesian head. See artesian head.

artesian water: Water from an aquifer, below any form of aquiclude, that has enough hydraulic pressure to force the water level in the well bore to rise above the depth of the aquifer. The water level might or might not rise to the surface of the ground. See hydraulic pressure and back pressure, artesian head, also aquiclude.

artificial: Unnatural. Manmade. Manufactured.

artificial recharge: A method by which water which has been put to prior beneficial use is injected into or returned to aquifers for purposes of replacing water that has been removed.

atmospheric pressure: The pressure of the atmosphere at the surface of the earth, dependent on elevation. At sea level, the pressure is approximately 14.7 psi.

augmentation: An artificial means to add to any groundwater supply or energy downstream from its origin.

augmentation plan: A plan to replace depletions from water pumped from a nonexempt well. Also, a court-approved plan that allows a junior water user to divert water out of priority so long as adequate replacement is made to the affected stream system preventing injury to the water rights of senior users. CSU. Also see non-exempt well.

authigenic: Constituents within the interstices of gravel, sand, or rock that formed or were generated by saturated solutions at the location where they are found. Particularly, crystals of minerals that are found adhering to and coating the walls of the pores within a host rock such as sand. Typical authigenic minerals are quartz, carbonates, and clays. In aquifers, the presence of authigenic crystals, particularly those of clay, can cause flow problems in both injection and production wells. See acid treatment, brushpile, and formation damage. Compare with allogenic.


Compiled and Edited by Robert C. Ransom


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