Glossary

background radiation	The radiation intensity existing in the environment which is in addition to the specfic radiation under consideration. On the surface, this is the ionizing radiation produced by cosmic irradiation and naturally occurring trace amounts of radioactive elements. In the subsurface, this is the naturally occurring radiation prior to the introduction of radioactive tracer materials or activation.

back-pressure	The pressure maintained on equipment or systems through which a fluid flows.

back-up curve	A curve recorded by a back-up galvanometer.

back-up galvanometer	A galvanometer which begins to record when the primary galvanometer has reached the limit of available track width or goes off scale. The back-up galvaometer will be either less sensitive than the primary galvanometer or have a shifted scale.

ball sealers	Balls made of nylon, hard rubber, or both, and used to shut off perforations through which excessive fluid is lost.

barefoot completion	See openhole completion.

barite	BaSO₄. A natural mineral used as weighting material in drilling mud.

barn	10^–24 cm². A unit of measure for capture cross section.

barrel	Petroleum barrel. A unit of measure for crude oil and oil products equal to 42 U.S. gallons.

basalt	A fine-grained igneous rock dominated by dark-colored minerals, consisting of plagioclase feldspars (a calcium-sodium-aluminum-silicate, usually present in amounts over 50%), and ferromagnesium silicates. Basalts and andesites represent about 98% of all extrusive igneous rocks.

base	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.

base exchange	See ion exchange.

base-line shift	(1) Generally refers to a naturally occurring shift of the base line of any specific curve; e.g., the SP curve. Usually the base line referred to is the shale base line, but could be the sand base line or other base line. (2) Sometimes refers to a manual or electrical shift in the curve produced by the logging engineer. See manual shift.

basement	A complex of undifferentiated rocks that underlies the oldest sedimentary rocks. In many places the rocks of the complex are igneous and metamorphic and of Precambrian age, but in some places they are Paleozoic, Mesozoic, or even Cenozoic. In terms of petroleum exploration, basement is the surface below which there is no current exploration interest.

basin	A synclinal structure in the subsurface, once the bed of an ancient sea. Basins, in which sediments have accumulated. are regarded as good prospects for oil exploration.

bearing	See relative bearing.

bed	A subdivision of a stratified sequence of rocks, lower in rank than a member or formation, internally composed of relatively homogeneous material exhibiting some degree of lithologic unity, and separated from the rocks above and below by visually or physically more or less well defined boundary planes.

bedding planes	In sedimentary or stratified rocks, the division planes that separate the individual layers, beds, or strata.

bedrock	A general term for the rock, usually solid, that underlies soil or other unconsolidated, superficial material.

bed thickness	(1) True bed thickness is the thickness of the stratigraphic unit measured along a line normal to the direction of extension of the unit. True thickness can be derived from information determined by the dipmeter. (2) The bed thickness determined from some well logs is an apparent bed thickness corresponding to the distance the borehole remained in the bed. The borehole may not have penetrated the bed normal to its upper or lower boundary surface because of hole deviation and formation dip. See also true vertical depth.

B electrode	A current-return electrode in the current and measure electrode configuration of a resistivity measuring device. Compare A electrode. See also normal and lateral device.

bentonite	A colloidal clay, composed of montmorillonite, which swells when wet. Because of its gel-forming properties, bentonite is a major component of drilling muds. See gel and mud.

beta particle	A high speed disintegration electron (i.e., negatron or positron) spontaneously emitted from an atomic nucleus as a form of radiation. Electrons (negative or positive) do not exist in nuclei; they are created at the moment of emission and are accompanied by the transition of a neutron into a proton, or a proton into a neutron. The atomic number of the nuclide is changed by a +1 or a –1 with no significant change in atomic weight. A beta particle can penetrate only a few millimeters of rock.

BHT	Bottomhole temperature, usually obtained by maximum reading thermometers.

bi-metallism	Existence of contact between two dissimilar metals in the logging instrument which results in a spurious emf. If electrical leaks exist between the metals and the borehole fluid, a circuit will be completed in the borehole fluid through which electrical current will flow. Opposite resistive beds, the lines of current are distorted and current flow is confined to the borehole. When this current crosses the SP electrode it distorts the SP curve.

bit	A cutting device attached to the bottom end of the drill pipe. Its function is to perform the actual boring or drilling of the rock formations.

blasting cap	Electrical blasting cap, a detonator. An electrically activated explosive device used to detonate high explosives (e.g., shaped charges).

blind ram	An integral part of a blowout preventer, serving as the closing element. Its ends do not fit around the drill pipe but seal against each other and shut off the space below completely. See rams.

blind zone	Shadow zone. Commonly observed on curves recorded by a lateral device. A zone wherein the recorded resistivity is too low. This zone corresponds to the depth interval logged while a thin resistive bed of thickness less than the AO spacing is between the A electrode and the midpoint of the measure electrodes. See also reflection peak.

BLM	See Bureau of Land Management.

blowout	A sudden, uncontrolled flow of drilling fluid, oil, gas, or water from a wellbore when the formation pressure in a permeable formation exceeds the pressure in the borehole. A kick sometimes warns of low pressure in the borehole.

blowout preventer	BOP. (1) A safety device for closing the wellhead, which has rubber rams which can be closed down on the logging cable in the event the well begins to blow out. Blowout preventers may be connected in series for improved control. See illustration at lubricator. See also riser, stuffing box, hydraulic packing head, and flow tube. (2) Equipment installed at the wellhead at surface level on land rigs and on the seafloor of floating offshore rigs to prevent the escape of pressurized fluids either in the annular space between the casing and drill pipe or in an open hole during drilling and completion operations. See also rams.

blue pattern	A convention used in dipmeter interpretation. A blue pattern denotes decreasing formation dip with increasing depth with a uniform azimuth. The blue color is sometimes drafted on the tadpole plot of the computed dipmeter log. Compare green pattern and red pattern. See dip for illustration.

boiling point	The temperature at which the vapor pressure of a liquid becomes equal to the pressure exerted on the liquid by the surrounding atmosphere. The boiling point of water is 212°F or 100°C at atmospheric pressure (14.7 psig).

bomb	A thick-walled container, usually steel. used to hold samples of oil or gas under pressure. See bottomhole pressure.

bond	(1) The state of one material adhering or being joined to another material (as cement to formation). See cement bond. (2) To adhere or to be joined to another material.

bond index	The ratio of attenuation in zone of interest (db/ft) to attenuation in well-cemented section (db/ft). The computed bond index value is an indicator of the quality of cement bond. Both the value of the bond index and the length of cemented interval exhibiting the calculated index value must be considered in estimating the quality of isolation to be expected.

BOP	See blowout preventer.

borehole	The hole made by the drill bit into earthen formations.

borehole camera	See camera.

borehole compensated sonic log	A well log of the interval transit lime: i.e., the time required for a compression wave to travel one foot in the formation (generally in microseconds per foot); the reciprocal of the compressional velocity. The borehole compensated sonic sonde carries two sets of transducers, one with its transmitter above its receiver pair and one with its transmitter below. The transmitters are pulsed alternately, and the alternate measurements are averaged. Spurious effects caused by borehole-size changes, and sonde tilt, which would affect a measurement with a single set of transducers, are thereby substantially reduced.

borehole effect	The spurious influence on a well-logging measurement due to the influence of the borehole environment; e.g., diameter, shape, rugosity of the wall of the borehole, type of borehole fluid, and presence of mud cake.

borehole gravimeter	A gravimeter designed for use in a borehole and equipped for remote leveling and reading at precisely determined well depths. It can be used to determine bulk density deep, laterally within a formation. See gravimeter.

borehole gravity meter	A borehole gravimeter.

borehole log	See well log.

borehole televiewer	A well-logging system wherein a pulsed, narrow acoustic (sonar) beam scans the borehole wall in a tight helix as the tool moves up the borehole. A display of the amplitude of the reflected wave on a cathode ray tube (television screen) is photographed yielding a picture of the borehole wall. The picture is a representation of the wellbore wall as if it were split vertically along magnetic north and laid out flat. Physical discontinuities such as fractures, vugs, etc. are detailed on the log. The tool may be used for casing inspection, without the magnetic orientation for location of perforations or damage.

borehole volume	Volume of the hole resulting from the integrated volume calculated from the hole size recorded on a caliper log.

bottom-hole pressure	The pressure in a well at a point opposite the producing formation as recorded by a pressure measurement device.

Bouguer anomaly	The gravity value existing after the Bouguer corrections to a level datum have been applied.

Bouguer correction	A correction to gravity data because of the attraction of the rock between the station and the elevation of the datum (often sea level). The Bouguer correction is 0.01276rh mgal or 0.4186rh mgal where r is the specific gravity of the intervening rock and h is the difference between the station and datum elevations in feet or meters, respectively.

bound water	(1) Water which has become adsorbed to the surfaces of solid particles or grains. Under natural conditions, this water is viscous like and immobile but might not have lost its electrolytic properties. See also adsorption and water wet. (2) Water which is chemically bound by becoming part of a crystal lattice. This water will have lost its electrolytic properties.

brackish water	Water that contains relatively low concentrations of any soluble salts. Brackish water is saltier than fresh.

break circulation	To start the mud pump to restore circulation of the mud column. Because stagnant drilling fiuid gels during the period of no circulation, a high pump pressure is usually required to break circulation.

breccia	(1) Fragmental rock whose components are angular and therefore, as distinguished from conglomerates, are not waterworn. (2) Rock made up of highly angular coarse fragments that may be sedimentary or formed by the action of crushing or grinding along faults. (3) Volcanic breccia is a more-or-less indurated pyroclastic rock consisting chiefly of accessory and accidental angular ejecta 32 mm or more in diameter lying in a fine tuff matrix.

brick-wall effect	On borehole televiewer logs. An unusual effect produced on the cathode-ray tube display by erratic movement of the tool as it drags along the borehole.

bridge	(1) Constriction in the borehole. (2) Plugging of a perforation or other orifice by debris, cementing materials, or sand grains. Bridging occurs inside the orifice. Compare with arch.

bridge plug	A downhole device composed primarily of slips, a plug mandrel. and a rubber sealing element, that is run and set in casing to isolate a lower zone while testing an upper section.

bridle	(1) The disconnectable, rubber insulation-covered downhole end of a survey cable on which current and measure electrodes (cable electrodes) are mounted for resistivity measuring electrode configurations requiring longer spacings than can be attained on a sonde. The head, to which the sonde is attached, is an integral part of the fishing bell and bridle. (2) For those services which do not require bridle electrodes, it is the short disconnectable length of cable on which the fishing bell and head are attached. The disconnectable feature permits the quick changing of head types and sizes, and easier servicing.

brine	A highly saline solution. A solution containing appreciable amounts of NaCl and other salts.

British thermal unit	BTU. The quantity of heat required to raise the temperature of 1 lb of water 1°F at or near its point of maximum density (equivalent to 252 calories).

bubble flow	A flow regime in which relatively uniform bubbles of gas or oil, approximately homogeneously distributed, flow upward through oil or water at a constant rate. The relative velocity is governed mainly by the difference between the densities of the lighter and heavier phases, and the viscosity of the heavier phase.

bubble point	The temperature and pressure at which part of a liquid begins to convert to gas. For example, if a certain volume of liquid is held at constant pressure, but its temperature is increased, a point is reached at which bubbles of gas begin to form in the liquid. This is the bubble point. Similarly, if a certain volume of liquid is held at a constant temperature but the pressure is reduced, the point at which gas begins to form is the bubble point. Compare dew point.

bubble-point pressure	The pressure at which gas first begins to come out of solution at constant temperature.

bucking electrodes	Bucking-current electrodes. Current electrodes on a laterolog type resistivity measuring system from which bucking current fiows in order to confine the survey current into a thin, horizontal investigative layer. Serve the same purpose as guard electrodes except bucking electrodes are usually rings or point electrodes (e.g., buttons). See laterolog.

bulk density	It is the value of the density of rock as it occurs in nature. In well logging, it is the density of the rock with the pore volume filled with fiuid. Natural density. The equation commonly used to compute porosity from well log derived bulk density is: where f is pore volume filled with a fluid of density r_f, r_b is the well-log-derived bulk density, and r_ma is the density of the rock framework. See density log and Z/A effect.

bulk modulus	See elastic properties of rocks.

bulk volume water	The quantity of formation water present in a unit volume of rock. The product of water saturation and porosity.

bullet	(1) A hollow projectile used for obtaining sidewall cores. (2) A projectile used in gun perforating in order to create holes in casing, cement sheath, and formation so that formation fluids can be produced.

bull plug	A threaded nipple with a rounded, closed end with O-ring seals. Commonly used to plug off the bottom end of downhole logging instruments from borehole fluids. Some logging tools, which can be used jointly in tandem, require a bull plug for a seal when used independently.

Bureau of Land Management	BLM. The Bureau of Land Management was established July 16, 1946, by the consolidation of the General Land Office (created in 1812) and the Grazing Service (formed in 1934). The Bureau is responsible for the total management of 341 million acres of public lands. These lands are located primarily in the far west and Alaska; however, scattered parcels are located in other States. In addition to minerals management responsibilities on the public lands, BLM is also responsible for subsurface resource management of an additional 169 million acres where mineral rights have been reserved to the Federal Government. Resources managed by the Bureau include timber, minerals, oil and gas, geothermal energy, wildlife habitat, endangered plant and animal species, rangeland vegetation, recreation and cultural values, wild and scenic rivers, designated conservation and wilderness areas, and open space. Bureau programs provide for the protection (including fire suppression), orderly development, and use of the public lands and resources under principles of multiple use and sustained yield. Land use plans are developed with public involvement to provide orderly use and development while maintaining and enhancing the quality of the environment. The Bureau also manages watersheds to protect soil and enhance water quality; develops recreational opportunities on public lands; administers programs to protect and manage wild horses and burros; and under certain conditions, makes land available through sale to individuals, organizations, local governments, and other federal agencies when such transfer is in the public interest. Lands may be leased to state and local government agencies and to nonprofit organizations for certain purposes. The Bureau has responsibility to issue rights-of-way, in certain instances, for crossing federal lands under other agencies' jurisdiction. It also has general enforcement authority. The Bureau is responsible for the survey of federal lands and establishes and maintains public land records and records of mining claims. It administers a program of payments in lieu of taxes based on the amount of federally owned lands in counties and other units of local government.

button	A small disc-shaped, button-like electrode used on microresistivity pads.

buoyancy	The apparent loss of weight of an object immersed in a fluid. If the object is floating, the immersed portion displaces a volunme of fluid the weight of which is equal to the weight of the object.