Discover the IHRDC Energy Glossary_beta

Search or select a letter above to browse the glossary.

U

U chart

An attribute control chart that is used to plot the average number of defects per item.

U-tube effect

The tendency of fluid columns of different density in the drillstring and annulus to equalize, so mud keeps flowing through the bit after the pumps stop.

A type of shell and tube heat exchanger that has U-shaped tubes connected to a tube sheet at one end of the heat exchanger. The end of the heat exchanger is divided to separate the fluid entering the tubes from the fluid leaving the tubes.

U-tube manometer

A type of instrument used to measure and indicate pressure; has two legs formed into the shape of a "U".

Udden-Wentworth scale

The standard grain size classification, each class twice the diameter of the one below — clay, silt, sand, granule, pebble, cobble, boulder.

ugs facility

An underground gas storage facility, buffering seasonal demand.

ullage

The empty space above liquid in a tank, measured to work out how much is in it.

ultra low sulfur

Meeting the strictest modern sulfur limits for diesel and gasoline.

ultra-long spaced electrical log

A conventional electrical log with very long electrode spacing (ULSEL), with a distance between the current-emitting electrode and the measure electrode of between about 75 ft (23 m) and 1000 ft (305 m). The long spacings are designed to locate objects within tens of feet of the borehole, such as a salt dome or casing in the case of a relief well.

ultrafine

Describing particles in the 2 to 44 micron size range.

ultraheavy oil

Crude of 10 degrees API gravity or less, under the US Department of Energy classification.

ultralarge crude carrier

A tanker of 500,000 deadweight tons or more, abbreviated ULCC.

ultralow interfacial tension

Interfacial tension below about 0.01 millinewton per meter, some four orders of magnitude under an untreated oil-water interface.

ultrapure water

Water stripped of ions and organics, the feed quality most electrolyzers require to protect the stack.

ultrasonic

An acoustic method with a frequency above the human ear's limit of approximately 20,000 Hertz (the SI unit of frequency, equal to one cycle per second).

ultrasonic (sound waves)

Sound waves of a higher frequency than can be heard.

ultrasonic caliper

A tool measuring internal diameter from the travel time of high-frequency pulses reflected off the wall.

ultrasonic imaging

In borehole logging, ultrasonic imaging is the measurement of acoustic signals that are in the hundreds of kilohertz to the low-megahertz range. Such ultrasonic instruments are mostly of the pulse-echo type, and are used in the borehole televiewer and in various cased hole devices to determine corrosion and cement bond quality.

ultrasonic imaging, ultrasonic

ultrasonic inspection

A non-destructive test that directs high-frequency sound into a component and reads the echoes to find flaws and measure wall thickness.

ultrasonic measurement

A borehole measurement using acoustic frequencies from hundreds of kilohertz upward, far above the sonic log's band.

ultraviolet light

Ultraviolet light is electromagnetic radiation with wavelengths shorter than visible light.

UN Sustainable Development Goals (SDGs)

The seventeen global goals companies map their social contribution against.

unburned fuel

Fuel leaving the furnace without combusting, which wastes heat and can ignite downstream.

uncased

Without casing, the open section of the hole.

uncemented

Left without cement behind it, as a liner set but not cemented in place.

uncemented liner

A liner hung in the hole without cement, used where the formation must remain open.

uncemented liner completion

A liner set across the reservoir without cement, leaving the formation open.

uncertainty

The band around a measured or estimated value inside which the true value probably falls, quoted with a stated likelihood.

uncertainty and risk

Not knowing the outcome at all, against knowing the possible outcomes and their odds.

uncertainty management

Explicitly carrying and narrowing what is not known, rather than proceeding on single-point assumptions.

uncertainty principle

Heisenberg's principle that certain pairs of quantities cannot both be known precisely at the same time.

uncompleted well

A well drilled but not yet perforated and equipped to produce.

uncondensed vapor

Vapor that has passed through the condenser without condensing.

unconfined compressive strength (UCS)

The axial stress a rock sample fails at with no confining pressure applied, abbreviated UCS.

unconformity

An unconformity is a surface of contact between two groups of unconformable strata.

unconformity trap

A trap in which the seal is provided by rock lying across an unconformity that truncates or overlies the reservoir.

unconscious bias

Automatic associations that influence judgment without the person being aware.

unconsolidated

Loose and uncemented, so the formation produces sand unless the completion controls it.

unconsolidated formation

Loose rock that will produce sand unless the completion controls it.

unconsolidated sand

Loose sand that will produce with the fluid unless sand control is installed.

unconsolidated sediments

Loose materials, ranging from clay to sand to gravel.

uncontrolled hazardous waste site

An area identified as an uncontrolled hazardous waste site by a governmental body, whether Federal, state, local or other where an accumulation of hazardous substances creates a threat to the health and safety of individuals or the environment or both.

unconventional gas

Gas from shale, tight rock or coal, requiring stimulation to flow.

unconventional reservoir

Reservoir needing stimulation or heat because its permeability is far too low to flow naturally.

unconventional reservoirs

Reservoirs with permeability so low (generally less than 0.1 millidarcy) that horizontal hydraulically fractured stimulated wells or other advanced completion techniques must be utilized to extract hydrocarbons at commercial rates. Shale reservoirs such as the Eagle Ford and Barnett, as well as tight reservoirs like the Bakken and Three Forks, both are examples of unconventional reservoirs.

unconventional resource

Oil or gas held in rock too tight to produce at commercial rates without hydraulic fracturing or another stimulation.

unconventional resources

Petroleum accumulations that are pervasive throughout a large area and not significantly affected by hydrodynamic influences. Examples include coal bed methane (CBM)/coal seam gas (CSG), basin-centered gas, shale gas, gas hydrate, natural bitumen, tar sands, and oil shale deposits.

under ream, underream

Under ream is to enlarge a wellbore past its original drilled size. Under reaming is sometimes done for safety or efficiency reasons. Some well planners believe it is safer to drill unknown shallow formations with a small-diameter bit, and if no gas is encountered, to then enlarge the pilot hole.

under- and over-representation

A group present in smaller or larger proportion than in the relevant population.

under-ream

Under-ream is to enlarge a wellbore past its original drilled size. Under-reaming is sometimes done for safety or efficiency reasons. Some well planners believe it is safer to drill unknown shallow formations with a small-diameter bit, and if no gas is encountered, to then enlarge the pilot hole.

underbalance

The amount by which formation pressure exceeds wellbore pressure at the face of the formation.

underbalanced

Said of a well in which the fluid pressure in the borehole is lower than the pressure in the formation.

underbalanced condition

Wellbore pressure below formation pressure, so the well flows while drilling.

underbalanced drilling

Drilling with wellbore pressure below formation pressure to avoid damaging the reservoir.

underdetermined system

An underdetermined system is the situation when there are less equations than the number of unknowns.

underground blowout

Uncontrolled flow from one formation into the wellbore and out into another, with nothing showing at surface.

underground cavern

A solution-mined or excavated void used for storage.

underground gas storage

Naturally or artificially developed containments in subsurface geological strata used for the storage of natural gas.

underlying interests

The needs and concerns behind a stated position, where agreement usually becomes possible.

underpressure

Pore pressure below the normal hydrostatic gradient for the depth.

underpressured

Said of a formation whose pore pressure is below the normal hydrostatic gradient for its depth.

undersaturated

Holding less dissolved gas than it could at that pressure, so no free gas is present.

undersaturated fluid

A solution holding less dissolved material than it could at its temperature and pressure.

undershooting

Acquiring seismic data beneath an obstruction by putting the source on one side of it and the receivers on the other.

undertravel

The condition where a rod pump's downhole stroke is shorter than the stroke at surface.

undervoltage

Lower than normal voltage.

undeveloped acreage

Acreage on which wells have not been drilled or completed to a point that would permit the production of commercial quantities of oil and gas regardless of whether or not the acreage contains proved reserves.

undiscounted future net cash flows

Future revenue less future cost with no discounting, the basis of standardized reserve disclosures.

undisturbed zone

Formation beyond the reach of mud filtrate, still holding its original fluids.

undisturbed zone, uninvaded zone

The undisturbed, or uninvaded zone, is the formation in the vicinity of the borehole which has not been affected by mud filtrate invasion.

undrained test

A rock mechanics test in which pore fluid cannot escape, so pore pressure changes with the applied load.

undrilled

Not yet tested by a well.

undue hardship

The threshold of cost or disruption beyond which an accommodation need not be made.

unequal spread method

A method of offsetting parallel lines in which the travel pieces of the pipes are closer together than the pieces before or after the offset.

uneven stream of payments

Cash flows that differ period to period, requiring each to be discounted separately.

UNFCCC (UN Framework Convention on Climate Change)

The treaty framework under which international climate negotiations are conducted.

uniaxial

Having, or relating to, a single axis.

uniaxial compressive strength

The axial stress at which a cylindrical rock sample fails with no confining pressure, abbreviated UCS.

unidirectional

Working in one direction only.

unidirectional prover

A physical prover in which a fixed volume is determined by the displacer making a one way trip.

uniform hazardous waste manifest

A document containing information about hazardous waste shipments that allows authorities to keep track of them.

uniformitarianism

The principle that the processes shaping the Earth today are the same ones that operated in the past.

unijunction transistor (UJT)

A specialized transistor with the unique characteristic of being either on or off, depending on the polarity of its emitter with respect to base one.

unilateral change

A state altering agreed terms without the investor's consent.

uninvaded

The part of the formation drilling fluid filtrate has not reached.

union certification

The process by which a union becomes the recognized bargaining representative.

unit

In petroleum, the joining of interests in a reservoir or field to provide for development and operations without regard to separate property interests; also the area covered by a unitization agreement. In power generation, a generator and the equipment it requires to make electricity. In process engineering, a group of systems that work together to produce one or more products.

unit trip

A condition in which the generator, the turbine, and the boiler in a generating unit are tripped.

unitization

The process of combining multiple oil and gas leases or properties into a single development unit, managed by one operator, to optimize resource recovery and reduce costs. This approach is used when a reservoir spans across different ownership boundaries. The production is then shared among the owners according to an agreed-upon formula, ensuring efficient operation and fair distribution of resources based on each party's contribution.

unitized

Operated as a single unit across ownership boundaries so a field is developed optimally.

unitized production

Production from a reservoir operated as a single unit across separate leases, and allocated to the owners by agreed participation.

units conversion factor

The numerical constant that makes an equation dimensionally correct for one particular set of units.

unity gain amplifier

An amplifier that has a gain of 1.

unity gain bandwidth

The range of frequencies within which the open-loop voltage amplification is greater than unity.

unity power factor

The ideal condition in a system, when apparent power is the same as working power; expressed as a power factor of 1, or as a 100% power factor.

Universal Transverse Mercator grid (UTM)

A worldwide rectangular coordinate system dividing the Earth into 60 six-degree zones, each with its own projection, in meters.

unknown

A quantity whose value is not currently known, but can be found if enough information is given.

unload

To remove the heavy fluid column from a well so the reservoir can flow.

unloader

The device relieving a compressor's load so it can start or run at reduced capacity.

unloaders

Devices on the valves which allow the capacity of a compressor to be changed.

unloading tank

Emptying a tank, or the tank being emptied.

unloading valve

A gas lift valve set above the operating valve, used only to lift the kill fluid out of the well during startup.

unmigrated

Seismic before migration, in which dipping reflectors are mispositioned.

unproved properties valuation

Assessing whether such acreage is still worth its carrying value.

unproved property

Acreage held without booked reserves, carried and tested for impairment rather than depleted.

unproved reserves

Quantities not meeting the certainty test for proven, reported separately.

unsaturated molecule

A molecule whose structure allows additional atoms to be added across a double bond.

unstabilized cat-cracked gasoline

Cat-cracked gasoline that contains light end products.

unstable (reactive) chemicals

A chemical which is the pure state, or as produced or transported, will vigorously polymerize, decompose, condense, or will become self-reactive under conditions of shocks, pressure or temperature.

unstable state

A condition in multivibrator circuits in which an amplifier stage does not remain in a conducting or cutoff state.

unsteady state

A flow condition in which pressure is still changing with time because boundaries have not yet been felt.

unsupervised learning

Training that finds structure in data without labelled examples to learn from.

unweighted

Drilling fluid containing no added weighting material.

unweighted mud

A mud carrying no barite or other weighting material, so its density comes only from the base fluid and drilled solids.

update

To modify a system or process by entering current data.

updip

In the direction of increasing elevation along a dipping surface.

upgoing

Traveling upward, as the reflected wavefield recorded in a vertical seismic profile.

upgrader

A plant that converts heavy oil or bitumen into lighter synthetic crude by adding hydrogen or removing carbon.

upgrading

Converting heavy, low-value streams into lighter products worth more per barrel.

uphole

Toward the surface end of the wellbore, or a shallow hole drilled to measure near-surface velocity.

uphole time

The uphole time is the time it takes for a direct wave from the subsurface source to reach the first receiver at the surface.

upload

Allen-Bradley's term for copying a PLC project from a PLC to a PC.

upper casing

The upper half of a split machine casing.

upper explosive limit (UEL)

The highest concentration of a flammable vapor or gas in the air above which a flame will not be created in the presence of an ignition source; usually expressed as the percent by volume of vapor in an air and vapor mixture.

uppermost

The highest in a stratigraphic succession.

UPS

An acronym for Uninterruptible Power Supply; a device that provides emergency backup power to network devices during a power outage.

upscaling

Reservoir geomodels frequently involve multi-million cell models which are too large to carry out flow simulations. Upscaling is the process to reduce the number of cells.

upscaling, upscale

Upscaling, or homogenization, is substituting a heterogeneous property region consisting of fine grid cells with an equivalent homogeneous region made up of a single coarse grid cell with an effective property value.

upset

Any change in the set point or process variable that increases the error.

upset failure

An upset failure is when ESD damages a device or component but not enough to cause complete failure.

upset trays

An abnormal condition that occurs in a distillation tower when extreme pressure surges physically move trays out of position.

upstream

A term used for anything located in front of the orifice plate or metering device in a meter tube, before the fluid enters.

upstream fiscal regime

The whole package of royalty, tax, sharing and bonuses that determines how the take is divided.

upstream oil

The exploration and production end of the oil business.

upstream sector

Exploration and production, the part of the industry that finds and lifts hydrocarbons.

upstream side

The side before the element.

upstroke

The upward movement of the sucker rod string.

upthrown

On the side of a fault that has moved up.

uptime

The proportion of time a facility is available and producing.

upward continuation

Computing what a potential field would look like at a higher level from measurements made at a lower one.

upwelling

The rise of nutrient-rich deep water, which drives the productivity that makes source rock.

urals

Russia's export blend, a medium sour crude whose differential to Brent is closely watched.

uranium

A chemical element with symbol U and atomic number 92. Uranium deposits occur in many different rock types, from sedimentary to volcanic. Uranium is remobilized from one area and re-precipitated in a host rock where chemical conditions are conducive to concentrating it in higher amounts, or re-deposited due to water actions.

U