back pressure: (1) Under static conditions, the hydrostatic pressure in a borehole or other vertical conduits, such as natural fissures or fractures, equal and opposite to the pore pressure in an aquifer, that prevents water from flowing out of the aquifer. Sometimes insufficient in springs and artesian wells to prevent water from flowing to the surface of the ground.
(2) In a water well, back pressure is that positive pressure difference, under producing conditions, between the depth of the surface of the water column inside the well casing and the depth of the producing aquifer. When the level of the water column falls to the depth of the pump, water production will cease. See coning and water level (2). Compare drawdown.
(3) In a producing well where crossflow production occurs, it is the positive difference in pressure between the depth of the surface of the water column inside the well casing and the depth of the aquifer where crossflow originates. This back pressure can be zero if the dynamic water level is at or below the depth of the aquifer producing the crossflow. See thief level, thief zone, also water level (2).
(4) In a drilling well, it is the hydrostatic pressure produced by the engineered drilling mud in the borehole, equal to or greater than the pore pressure in a drilled formation, that prevents any geofluid (water, oil, or gas) from flowing out of the formation. In the case of hydrocarbons, if the head of drilling mud does not balance or overbalance the formation pressure the well can discharge with disastrous consequences. See drilling mud.
(5) With its designed density, the drilling mud can provide sufficient back pressure to help prevent the collapse of the borehole
bacteria: One-celled microorganisms. Some forms are beneficial, others are unsafe for human consumption. Coliform bacteria is an example of harmful bacteria.
bail: To bail. In drilled holes, the mud and water remaining in the well bore after drilling the hole and setting the casing must be bailed out of the hole by a long bucket-like container on the end of a cable before the well can be flushed, disinfected, and put on production.
bank storage: Water contained in an aquifer that is in communication with a stream or lake and capable of supplying water to the stream or lake following a lowering of the open water surface; or, of storing water flowing from the stream or lake following a rise of the open water surface. Douglas Co.
bar: (1) Of a stream. A general term for a ridge-like accumulation of sand, gravel, or other alluvial material formed in the channel, along the banks, or at the mouth of a stream where a decrease in water velocity allows deposition.
(2) Coastal. A generic term for any of various elongate offshore ridges, banks, or mounds of sand, gravel, or other unconsolidated material submerged at least at high tide, and built up by the action of waves or currents, especially at the mouth of a river or estuary, or at a slight distance offshore from the a beach. NSSH.
barrier: Any impermeable bed or strata in contact with an aquifer that prevents the flow of formation water to or from the aquifer. See cap rock.
base: Relative to chemistry, it is a substance capable of combining with charged hydrogen atoms (ions) to form a salt. A typical base is sodium hydroxide (caustic) with the chemical symbol NaOH. SPWLA. Compare acid.
base flow: The amount of water in a stream that results from ground water discharge. CSU.
basement: The undifferentiated rock that underlies sedimentary rock. Mostly of igneous or metamorphic nature.
(2) Drainage basin. The area of land that drains to a specific river or stream.
basin rank: A number used in the State Engineer's tabulations of decreed water rights to indicate the relative seniority of a decreed right as determined by its date of adjudication and the date of appropriation. Douglas Co.
basin yield: The maximum rate of withdrawal that can be sustained by the complete hydrogeologic system in a basin without causing unacceptable declines in hydraulic pressure anywhere in the system or causing detriment to any other component of the hydrologic cycle in the basin. GWAC. See yield.
bed: (1) A lithostratigraphic unit of rock. A homogeneous subdivision of a stratified rock sequence within a formation.
(2) The floor or bottom of lake, ocean, sea, river, or stream.
bedded: Formed, arranged, or deposited in layers or beds, or made up of or occurring in the form of beds; especially said of a layered sedimentary rock, deposit, or formation. NSSH.
bedrock: Solid, unweathered rock below unconsolidated sediments.
beneficial use: The use of water that is reasonable and appropriate under reasonably efficient practices to accomplish, without waste, the purpose for which diversion is lawfully made; and shall include the impoundment of water for recreational purposes, including fishery or wildlife (CRS 37-92-103). Douglas Co.
bentonite: Composed of montmorillonite formed from the decomposition of volcanic ash, a swelling clay when in contact with water. Because of its property to swell, it is a major component of drilling mud for the purpose of minimizing the penetration of mud solids and mud filtrate into the formation during the drilling process. Often used to line ditches, carrying irrigation water, and ponds for the purpose of reducing loss by seepage and percolation.
Best Management Practices: BMPs. Practices that are technically and economically feasible and for which significant water conservation or water quality benefits can be achieved. CSU. See environmental concerns.
biochemical oxygen demand, biological oxygen demand: BOD. Microorganisms consume oxygen as they feed upon and bring about the decomposition of organic waste in samples of water that have been subjected to pollution. BOD is one of the better methods to measure the amount of oxygen, dissolved in water, that is consumed by aerobic microorganisms. BOD is an indirect measure of the amount of pollution. On a numbered scale in units of ppm or mg/liter, the higher the number, the greater is the amount of oxygen required for decomposition; and, therefore, it is inferred the greater is the amount of pollution. BOD levels in the range of 1-2 are very good, those in the range of 3-5 are moderate, 6-9 are fairly polluted, and greater than 10 are very polluted. See also water analysis and pH.
biogenic methane: Biogenic methane is created by the decomposition of organic material through fermentation, as is commonly seen in wetlands, or by the chemical reduction of carbon dioxide. It is found in some shallow, water-bearing geologic formations containing coals, into which water wells sometimes are completed.
The COGCC has consistently found that biogenic gas contains only methane and a very small amount of ethane, while thermogenic gas contains not just methane and ethane but also heavier hydrocarbons such as propane, butane, pentane, and hexanes. COGCC. See thermogenic methane and flaming water.
bond: (1) The state of adherence or joining of one material to another.
(2) In water well completions, the degree of bond can describe the quality of adherence of cement to casing and/or the quality of adherence of cement to the drilled formation wall. See cement bond.
borehole: The cylindrical hole that is drilled into and through earthen formations by a drilling rig. The powered drilling rig rotates a drill bit at the end of a string of drill pipe. See drill pipe.
brackish water: Of intermediate salinity. Saltier than potable water, but not as salty as sea water. Usually considered to have a salinity of from 1,000 to 10,000 ppm of total dissolved solids. See total dissolved solids.
breccia: A coarse-grained, clastic rock composed of angular rock fragments commonly bonded by a mineral cement in a finer-grained matrix of varying composition and origin. The consolidated equivalent of rubble. NSSH.
bridge: (1) Constriction in a drilled hole sometimes caused by swelling clays or plastic shales. See plastic shale.
(2) The plugging, inside the slots or perforations, by cementing materials or sand grains.
brine: A highly saline solution of salt and water.
brushpile, brushpiling: Clay crystals with fragile filaments or platelets sometimes can be weakened or broken by disturbances to the equilibrium and dynamics of their environment. Such disturbances can be brought about by an increase in injection water temperature, change in pressure, change in salinity of injected water, or flow rate of water over and past the crystals. These disturbances can cause swelling, weakening, or shearing of the fragile crystals that are then pushed into smaller pores and pore throats by either the hydraulic pressure required for the injection process or by the formation pressure during the water production process, thus causing brush piles that restrict fluid flow. The propensity to brushpile, and brushpiling, can be inhibited by mitigating the conditions that cause the disturbances, including beginning injection or production slowly and reducing the injection rate or production rate. By starting the injection or production process slowly, the broken fragments can be moved gently out of the pores and through the pore throats without brushpiling.
buildout: Completion of the development or project as it was approved. Douglas Co.
bulk modulus: For water. (1) For most practical purposes, water is incompressible. The bulk modulus of elasticity describing the pressure/strain relationship for water is the reciprocal of the compressibility of water. As the reciprocal of compressibility, it will be recognized that, as the bulk modulus increases, compressibility decreases. The higher the bulk modulus for any material, the more incompressible that material becomes. However, because the bulk modulus for water has a finite value, the waters in the aquifers of the Denver Basin can expand as water usage decreases pore pressure. In the Denver Basin where the total amount of ground water is said to be 467,000,000 acre feet, the expansion does constitute some volume, albeit relatively insignificant. In a simplified calculation, for illustration purposes only, the bulk modulus is represented by
E = (-) (change in pressure)/(relative deformation) = (-) Δp / ( ( Δv ) / v0 )
where E = bulk modulus for water = 3.12 × 105
Δp = pressure change (lb / in2 ), for this illustration, minus 10 psi
Δv = change in water volume (acre feet)
v0 = original water volume = 467 × 106 acre feet
Assuming the total amount of water in the Denver Basin is 467 × 106 acre feet and 100% of the water (or any fraction thereof) is subject to a minus 10 lb / in2 change in pore pressure due to water production, then for every 10 psi reduction in pore pressure, the change in total volume due to expansion is
Δv = (-) (-10 lb / in2 ) (467 × 106 ) / (3.12 × 105 ) = (+) 14,968 acre feet, or 14,968 acre feet expansion in water volume for every 10 lb / in2 decrease in pore pressure.
Water expansion in this illustration, in whatever quantity, contributes to the groundwater supply.
(2) Although minimal in the case of nearly incompressible ground water, expansion is finite and does take place as water is produced from the aquifer and pumped to the ground surface where temperature and pressure change from in situ conditions to surface conditions. This form of expansion upon withdrawal from the well bore also applies to other fluids, such as natural gas and crude oil, where the increase in volume can be very significant.
(3) Differences between the elasticity of rock and water can cause subnormal pressure in aquifers, and water will be imported from a higher pressure environment to a lower pressure environment. Along with increased water production, as the potentiometric surface falls, is a decrease in the hydrostatic head. This reduction in pressure causes the expansion of water as discussed in (1) above. Not only does the decline in pressure allow water to expand, but it allows rock to expand as well. The bulk moduli of rock and water are different, and this difference allows the relative volumes to expand at different rates.
For the same difference in pressure decline, rock will expand more than water. This is due to elasticity. So, as the water pressure declines with excessive water withdrawal, rock will expand and the pore spaces between the grains of the rock will expand to a volume greater than the expanding water. Although the volume of water will increase to fill the expanded pore, its pressure is reduced in order to do so. Water pressure now has been reduced further than from excessive water withdrawal alone. This subnormal water pressure compounds the differential in pressure between the vicinity of the high-volume water-producing well and that inthe environment of distant wells. This negative pressure difference is in addition to the pressure decline due to excessive water production at the producing well site. This elastic behavior in rock and water causes additional water to be imported to the site of the producing well.
The elastic properties of both the rock and the water are reversible. This allows limited rebound. The elastic behavior of rock and water is different from the effects of compaction as seen in the Rubey & Hubbert relationship in compaction(2). The compaction that takes place as a result of the reduction in pressure is not reversible.
(4) For virtually every action there is a reaction. Along with the reduction in pressure described in (3) immediately above, will be additional compaction by the overburden. See compaction(2). This tendency to re-compress the rock and water in the aquifer tends to offset the tendency for elastic expansion to some degree. How much the elastic expansion will be countered will depend on the thickness and weight of the overburden, hence, the depth of the aquifer. Although somewhat attenuated, the compaction force will overwhelm the expansion force, and subsidence will occur at the ground surface.
buoyancy: An upward force tending to counter the downward force of gravity. Buoyancy takes place in the presence of liquids and gases, liquids and solids, and immiscible liquids. Buoyancy depends on the difference between the densities of any two materials, one immersed or suspended in the other; and, in rocks, on which material constitutes the host phase and provides buoyancy to the other. This consideration is based on Archimedes’ Principle where a body immersed in a liquid is buoyed up by a force equal to the weight of the liquid displaced. In the case of the aerated zone containing air and water, the host phase is air at atmospheric pressure and the buoyant force of air is negligible, and the component of water volume supported by buoyancy is insignificant. The full force of gravity is now effective on the water volume occupying the voids within the earthen materials and/or adhering to the surfaces of grains and particles of gravel, sand, or rock. Relative to irreducible water where the host or mobile phase is oil and the density is much greater than that of air, the buoyancy is greater, and the immobile phase of water becomes greater. Also see gravity drainage and other factors related to irreducible water: capillarity, surface tension, and evaporation.
Compiled and Edited by Robert C. Ransom
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