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half-life The length of time required for a radioactive material to lose one-half of its radioactivity by decay (i.e., half of the radioactive element has spontaneously disintegrated).
half thickness Half value thickness. That thickness of an intervening absorbing material which is necessary to reduce the intensity of incident radiation by one-half.
halite Natural salt, sodium chloride. A common mineral of evaporites.
hard rock In the petroleum industry, a hard rock is a cemented, well-consolidated formation.
hardware Physical entities such as computers, electronic circuits and components, peripherals such as tape machines, etc. Contrasted with software.
head

(1) The connector end of the fishing bell. Located at the downhole end of the survey cable. The head consists of insulated electrical connectors or pins and a threaded connector which will attach to various downhole instruments.

(2) The threaded connector end(s) of downhole logging instruments.

heading The form attached to the top of a well log which supplies and documents all vital information and supplementary data regarding the well, the survey, and the wellbore conditions.
heaving shale Shale which becomes dislodged by bit, drill pipe, or jet action. Also shale which becomes hydrated during the drilling process and falls into the well bore. Heaving shale quite often leaves cavities and ledges. Heaving shale is not necessarily related to overpressure. Compare plastic shale.
hertz Hz. A unit of frequency measurement equal to one cycle per second.
high-angle borehole logging system

An expression which applies to a method developed for the purpose of logging highly deviated holes and holes with serious bridging problems. At the present time, systems such as this involve the use of drill pipe to carry specially designed well-logging tools through highly deviated boreholes or past wellbore constrictions to a depth where logging can be performed. A survey cable connects the downhole logging instrument to the surface where the measurements are recorded.

Usually, this is an operation designed for performing openhole surveys below a surface- or protective casing string, but can be used for performing some cased-hole surveys as well. Examples are Slant-hole Express and Toolpusher.

High-Angle Borehole Logging System

high explosives High explosives are unstable chemical compounds that are sensitive to shock. temperature, and their own detonative wave. High explosives detonate with a violent chemical reaction. The detonative wave velocity is a density phenomenon and is in the order of magnitude of 30,000 feet/second. Shaped charges and Primacords are high explosives. Compare low explosives.
high-resolution dipmeter tool A dipmeter tool which records four high-resolution microresistivity curves and has an additional electrode on one pad which yields another curve at displaced depth. The displaced-depth curve is used to correct for variations on sonde speed. Provides not only improved resistivity resolution. but also improved dip resolution or refinement.
high-resolution thermometer A small-diameter, surface recording, fast-response downhole temperature tool for logging open or cased horeholes.
holdup In a producing well, the volume fraction of a specific fluid phase (e.g., water holdup, oil holdup) in the upward moving flow stream. The relative quantities of the fluids produced at the surface are related to the holdup and upward velocity of each phase.
hollow carrier gun A perforating gun consisting of a hollow, thick-walled. cylindrical metal case into which are loaded shaped charges or bullets. Upon firing, debris from the detonated charges falls into the carrier to be retrieved with the reusable gun.
horns Spurious high-resistivity anomalies found on induction log curves inside the upper or lower boundary of a resistive bed in which the apparent resistivity markedly contrasts with that of a conductive adjacent bed. A result of improper boundary compensation for the level of formation resistivity being logged.
horst A block of the earth's crust that has been raised between two faults. The opposite of a graben. See illustration in fault.
hostile environment

Hostile wellbore environment. Any well bore that meets one of the following criteria:

(1) depth greater than 20,000 ft.
(2) pressure greater than 20,000 psi
(3) temperature greater than 325°F
(4) hole deviation greater than 50°
(5) H2S or gas-cut mud

Any one or a combination of these will severely affect or restrict the logging operations.

hot rock Pertains to any rock that is volcanically or radiogenically heated.
hot-water system A system that is dominated by a circulating liquid that transfers most of the heat and largely controls subsurface pressures. Characterized by hot springs that discharge at the surface.
hot wire analyzer A device used to detect hydrocarbon gases returned to the surface by the drilling mud. Basically a Wheatstone bridge, two arms of which are kept at a high temperature. Hydrocarbon gases become oxidized as they pass over one arm, which increases its temperature, changes its resistance, and unbalances the bridge. The hot wire analyzer response is sometimes plotted as a strip log.
housing A cylindrical metal case which protects downhole electrical circuitry. Particularly, a housing which encases the electronic cartridge of the downhole logging instrument. The housing protects the cartridge from damage from pressure and moisture.
huff-and-puff Cyclic steam injection. A form of enhanced oil recovery.
Humble formula

A modified form of Archie's formation factor/porosity relationship.

  F = 0.62f–2.15
where F = formation resistivity factor, and f = porosity.
HVT Half value thickness. See half thickness.
hybrid scale

A resistivity scale used with laterologs and guard logs which compresses high resistivity values. Below midscale. the scale is linear with resistivity. Above midscale, the scale is scaled in resistivity units. but is linear with conductivity (reciprocal of resistivity). The hybrid scale presentation has been replaced by the logarithmic scale on modern laterologs and guard logs.

Laterolog Recorded on Hybrid Scale

hydrate

(1) To combine with water.

(2) A hydrocarbon and water compound that is formed under reduced temperature and pressure in gathering, compression, and transmission facilities for gas. Hydrates often accumulate in troublesome amounts and impede fluid flow. They resemble snow or ice.

hydration

A process by which water molecules become associated (or react) with other substances without destruction of the water molecule.

(1) Water molecules (dipoles) become adsorbed on ions or on solid surfaces exhibiting electrostatic charge.

(2) Water molecules become imbibed as a result of conditions which tend to cause osmotic absorption of water into the space between the crystal surfaces (as in montmorillonite clays).

(3) Water molecules become bound chemically at fixed points as part of the crystal lattice of hydrated substances. The water becomes water of crystallization and loses the properties of the original water.

hydraulic conductivity Ratio of flow velocity to driving force for viscous flow under saturated conditions of a specific liquid in a porous medium.
hydraulic fracturing The breaking or parting of reservoir rock through the use of injected fluids. Hydraulic fracturing is a method of stimulating production or injection at a specific depth in a formation of low permeability by inducing fractures and fissures in the formation by applying high fluid pressure to its face. Fluids (liquids, gases, foams, emulsions) are injected into reservoir rock at pressures which exceed the strength of the rock and overcome internal stresses of the rock. The fluid enters the formation and parts or fractures it. Sand grains, aluminum pellets, glass beads, or similar materials are carried in suspension by the fluid into the fractures. These are called propping agents or proppants. When the pressure is released at the surface, the fracturing fluid returns to the wellbore as the fractures partially close on the proppants, leaving paths with increased permeability for fluid flow.
hydraulic head The pressure caused by the weight of an equivalent column of liquid upon a unit area expressed by the height or distance of the liquid above the point at which the pressure is measured. Although head refers to distance or height, it is used to express the pressure resulting from the weight of a body of liquid since the weight is directly proportional to the height.
hydraulic packing head Replaces stuffing box when high pressure exists at the surface. Grease under pressure hydraulically packs the glands around the cable and provides cable lubrication. Cable can be moved under pressure in the packing head. When glands become cut, the blowout preventer can be closed and glands replaced.
hydraulic pad An articulating, liquid-filled pad used to place current and measure electrodes in direct contact with the drilled formation wall. Used on microresistivity devices (i.e., microlog, microlaterolog, etc.). The liquid-filled pad allows better electrode contact by improving pad conformity with the wall.
hydraulic pressure gradient

(1) As applied to an aquifer, the rate of change of pressure head per unit of distance for flow past a given point and in a given direction.

(2) A vector point function equal to the decrease in hydraulic head per unit distance in direction of greatest decrease in rate.

hydrocarbon porosity feet The product of hydrocarbon saturation and porosity and the number of feet exhibiting those values of saturation and porosity.
hydrocarbon saturation Fraction of the pore volume filled with hydrocarbons (oil or gas).
hydrodynamic pressure Fluid pressure directed laterally along the bed from a higher to a lower pressure head. Compare hydrostatic pressure.
hydrofracture See hydraulic fracturing.
hydrogen embrittlement The corrosive effect produced on steel armor of survey cables and other steel equipment by the penetration of hydrogen released from the decomposition of hydrogen sulfide, H2S. The presence of H2S in wellbore fluids will result in a degree of embrittlement in steel equipment unless the steel has been protected by an inhibitor, or the pH of solutions has been otherwise controlled.
hydrogen index The ratio of the number of hydrogen atoms per unit volume of a material to that number in pure water at 75°F. Neutron logging response depends mainly on the hydrogen index.
hydrogen sulfide A gaseous compound, H2S, of sulfur and hydrogen commonly found in petroleum. which causes the foul smell of sour petroleum fractions. It is extremely poisonous and corrosive. See also hydrogen embrittlement.
hydrostatic head The pressure exerted by a body of water at rest. The hydrostatic head of fresh water is 0.433 psi per foot of height. Those of other liquids may be determined by comparing their gravities with the gravity of water. See pressure gradient.
hydrostatic level That level, which for a given point in an aquifer, passes through the top of a column of water that can be supported by the hydrostatic pressure of the water at that point.
hydrostatic load The weight of formation fluid filling the pores of the rock and in communication with the water table at the well site, or sea surface. Usually the fluid is considered to be formation water.
hydrostatic pressure That pressure due to the weight of a column of liquid (formation water, drilling mud, etc.) extending from the depth of interest to the liquid level which might or might not be at the surface.
hydrothermal Pertains to heated or hot aqueous-rich solutions, to the processes in which they are concerned, and to the rocks, ore deposits, and alteration products produced by them. Hydrothermal solutions are of diverse sources, including magmatic, meteoric, and connate waters.
hydrothermal alteration The phase changes resulting from the interaction of hydrothermal fluids with pre-existing solid phases. Included are the chemical and mineralogical changes in rocks brought about by the addition or removal of materials through the medium of hydrothermal fluids.