Oil and Gas Terms Beginning with “U”

In a U-tube manometer, the height of one leg of fluid changed by altering the density of some of the fluid in the other leg. In a well with drillpipe in the hole, the string of drillpipe is one leg and the annulus between the drillpipe and the wellbore is the other. If a denser mud goes into the drillpipe, mud flows up the annulus, and vice versa. The practice of putting a dense slugging pill in the drillpipe in order to pull a dry string makes use of the U-tube effect.

Abbreviation for ultralarge crude carrier.

A grade scale for classifying the diameters of sediments. Particles larger than 64 mm in diameter are classified as cobbles. Smaller particles are pebbles, granules, sand and silt. Those smaller than 0.0039 mm are clay. Several other grain size scales are in use, but the Udden-Wentworth scale (commonly called the Wentworth scale) is the one that is most frequently used in geology.

A worldwide grid system of rectangular map coordinates that uses metric (SI) units. A location is specified on the basis of its location within one of 60 zones worldwide of 6o of longitude and 8o of latitude each that are subdivided into subzones that are 100,000 m [330,000 ft] on each side. Locations consist of a series of numbers and letters that can be accurate to within an area of one square meter. The headquarters of the Geological Society of America are at 13TDQ8743172 (Merrill, 1986). Information about the UTM grid, including grid ticks on quadrangle maps, can be found on most maps produced by the US Geological Survey. Latitude and longitude coordinates, or geographic coordinates, are another means of locating a point at the Earth's surface, but the accuracy, computer compatibility and uniqueness of UTM have resulted in its finding acceptance within the scientific community.

A valve installed above the kelly that can be closed manually to protect the rotary hose from high pressure in the drill string.

(noun) Crude oil with an API gravity of less than 10 degrees and a viscosity typically exceeding 10,000 centipoise at reservoir conditions. Ultra heavy oil does not flow naturally under reservoir pressure and requires thermal recovery methods such as steam-assisted gravity drainage (SAGD) or cyclic steam stimulation for commercial production.

A conventional electrical log with very long electrode spacing. The ultralong-spaced electrical log (ULSEL) is a normal device 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. These objects may be a saltdome or casing, in the case of a relief well.

Referring to any particle in the size range 2 to 44 microns.

A supertanker with 500,000 deadweight tons of capacity or more. The term is commonly abbreviated as ULCC.

Values of interfacial tension (IFT) less than about 10-2 mN/m. Mixed surfactant systems, as used in enhanced oil recovery, adsorb at the oil/water interface and can be designed to generate an interface that is flexible and that has an ultralow IFT. Ultralow IFT implies (1) a significant increase in the capillary number for a given flow velocity and therefore that viscous forces generated during a flood can mobilize additional oil; and (2) a significant increase in the Bond number and therefore that gravitational forces (i.e., buoyancy) can mobilize additional oil.Micellar-polymer and alkaline-surfactant-polymer flooding are techniques used to achieve ultralow IFT.

A device for measuring the internal diameter of a casing, tubing or open borehole using high-frequencyacoustic signals. A transducer (in transmit mode) emits a high-frequency pulse that is reflected by the pipe or borehole wall back to the transducer (in receive mode). The diameter is determined from the time of flight of this echo and the fluid acoustic velocity. The transducer is rotated to produce a cross section of the borehole size and full-coverage images of the borehole wall. The measurement has high resolution and is used to detect deformations, the buildup of scale, or metal loss due to corrosion.The amplitude of the echo from the inner casing surface provides qualitative information on the state of the surface, such as rugosity or corrosion. Casing thickness may also be measured simultaneously, either by analysis of the casing resonance signal, or by detecting separately the echoes from the inner and outer casing surfaces.

In the context of boreholelogging, measurements 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.

The degree to which a data set may be in error or stray from predicted values. Sometimes quantified in terms of variance or standard deviation, uncertainty exists in data because of a variety of problems, such as poor calibration or contamination or damage to rocks prior to measurement. Uncertainty is the cause of many problems, which occasionally can be overcome by normalizing the data.

A quantum-mechanical principle formulated by German physicist Werner Karl Heisenberg. It proposes that measuring either one of two related quantities, such as position and momentum, or energy and time, produces uncertainty in the measurement of the other. The error in measurement of one (for example, position) times the error in measurement of the other (for example, momentum) must be greater or equal to Planck's constant. This principle accounts for bizarre behavior of subatomic particles and implies that we can never know everything exactly.

A geological surface separating older from younger rocks and representing a gap in the geologic record. Such a surface might result from a hiatus in deposition of sediments, possibly in combination with erosion, or deformation such as faulting. An angular unconformity separates younger strata from eroded, dipping older strata. A disconformity represents a time of nondeposition, possibly combined with erosion, and can be difficult to distinguish within a series of parallel strata. A nonconformity separates overlying strata from eroded, older igneous or metamorphic rocks. The study and interpretation of unconformities locally, regionally and globally is the basis of sequencestratigraphy.

A type of hydrocarbon trap whose closure is controlled by the presence of an unconformity. There is disagreement about whether unconformity traps are structural or stratigraphic traps.

An umbrella term for oil and natural gas that is produced by means that do not meet the criteria for conventional production. What has qualified as unconventional at any particular time is a complex function of resource characteristics, the available exploration and production technologies, the economic environment, and the scale, frequency and duration of production from the resource. Perceptions of these factors inevitably change over time and often differ among users of the term. At present, the term is used in reference to oil and gas resources whose porosity, permeability, fluid trapping mechanism, or other characteristics differ from conventional sandstone and carbonate reservoirs. Coalbed methane, gas hydrates, shale gas, fractured reservoirs, and tight gas sands are considered unconventional resources.

The amount of pressure (or force per unit area) exerted on a formation exposed in a wellbore below the internal fluid pressure of that formation. If sufficient porosity and permeability exist, formation fluids enter the wellbore. The drilling rate typically increases as an underbalanced condition is approached.

The uncontrolled flow of reservoir fluids from one reservoir into the wellbore, along the wellbore, and into another reservoir. This crossflow from one zone to another can occur when a high-pressure zone is encountered, the well flows, and the drilling crew reacts properly and closes the blowout preventers (BOPs). Pressure in the annulus then builds up to the point at which a weak zone fractures. Depending on the pressure at which the fracturing occurs, the flowing formation can continue to flow and losses continue to occur in the fractured zone. Underground blowouts are historically the most expensive problem in the drilling arena, eclipsing the costs of even surface blowouts. It may prove necessary to drill a second kill well in order to remedy an underground blowout.

Gas that is being stored in salt domes, salt layers or depleted oil and gas fields.

Porepressure less than normal or hydrostatic pressure. Underpressure, or a zone of underpressure, is common in areas or formations that have had hydrocarbon production.

To enlarge a wellbore past its original drilled size. Underreaming 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. An underreaming operation may also be done if a small additional amount of annular space is desired, as might be the case in running a liner if surge pressures were problematic.

A solution that could contain more solute than is presently dissolved in it. In brines, an undersaturated solution will not form crystals as easily as if it were saturated or supersaturated. In saltwater muds, an under-saturated fluid is used to allow salt to leach into the mud, keeping the hole from closing in on the drilling assembly. For moisture in air, a relative humidity of less than 100% is under-saturated.

A technique for acquisition of seismic data beneath areas that are difficult to access at the surface of the Earth, such as near rivers, drilling rigs, production platforms, environmentally sensitive areas or around seismically problematic features such as salt domes, which introduce uncertainty because of their high velocity. The sources and receivers are located on opposite sides of the feature.

In sucker-rod pumping, a situation that occurs when the stroke length at the downhole pump is shorter than the surface stroke length.

The part of the formation that has not been affected by invasion.

The geological principle formulated by James Hutton in 1795 and publicized by Charles Lyell in 1830 that geological processes occurring today have occurred similarly in the past, often articulated as, "The present is the key to the past."

The combining of multiple wells to produce from a specified reservoir.

Pooled production from wells or a reservoir. The proceeds of this pooled production are distributed to the participants according to an agreed-upon formula.

A constant provided in an equation that applies only for a particular system of units. This is useful for analysts who must work in different units systems to satisfy local preferences.

To initiate flow from a reservoir by removing the column of kill fluid from the wellbore. Several methods of unloading the well are used, including circulation of lower density fluid, nitrogen lifting and swabbing. The method used will depend on the completion design, reservoir characteristics and local availability.

A system that is in a transient state at the time of a measurement. In the case of permeability measurements on core samples, two transient techniques are used. In the pressure falloff method, the sample is at atmospheric pressure, either in a chamber or under a probe. Fluid at a higher pressure is released into one end of the sample. The decay of pressure with time at that end of the sample is recorded and analyzed by techniques similar to those used for transient well tests. In the pulse-decay method, the sample is held in a chamber and connected to two reservoirs, all of which are filled with a fluid at high pressure. The pressure in one of the reservoirs is increased a small amount and then re-connected to the sample. The change in pressure with time of this sample is recorded and analyzed using flow equations.

A mud that contains no commercial weighting material. Native-solids muds are unweighted muds, containing no barite. More solids-control techniques are available for unweighted muds than for weighted muds. In fact, dilution of unweighted muds is highly economical.

Located up the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbon reservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip.

A refinery unit used to improve or upgrade heavy oil to produce higher-quality hydrocarbon liquids or upgraded synthetic crudes. The refining unit may include any combination of the following: hydrogen addition processes, carbon rejection processes or carbon concentration and removal processes.

A part at the end of tubulars, such as drillpipe, casing or other tubing, which has extra thickness and strength to compensate for the loss of metal in the threaded ends.

Pertaining to equipment, facilities or systems located in the wellbore or production train above the surface choke or Christmas tree.

The stage of downhole pumping at which the polished rod is going up and the downhole pump is pumping fluid.

The use of measurements of a field at one elevation, level or surface to determine the values of the field at a higher level. The technique is most often used on potential fields, such as gravity or magnetic fields, to reduce scattered measurements to a common level for a simpler interpretation.

An element with an atomic number of 92. The 238U isotope is radioactive and decays with a half-life of 4.4 * 109 years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of bismuth, 214Bi. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of 238U. The concentration in the Earth?s crust is about 4 ppm by weight.Uranium-bearing minerals are rare. Uranium is a soluble trace element that is transported easily and can be precipitated far from its source. It is most frequently found in carbonates and organic materials. A log of uranium is presented in parts per million, ppm. It is useful for detecting organic material, but is otherwise considered not useful for quantitative evaluation. In the corrected gamma ray log, the uranium contribution is removed to differentiate carbonates from shales.