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natural gamma ray log See gamma ray log.
natural gamma ray spectrometry log

The natural gamma spectrometry (NGS) tool uses five-window spectroscopy to resolve total natural gamma ray spectra into the three most common components of naturally occurring radiation – potassium, thorium, and uranium. NGS is a mark of Schlumberger.

Potassium, Thorium, and Uranium Response Curves (Nal Crystal Detector)

natural gas

A highly compressible, highly expansible mixture ot hydrocarbons having a low specific gravity and occurring naturally in a gaseous form. The principal component gases of natural gas, with typical percentages, are –

methane 80.0%
ethane 7.0%
propane 6.0%
isobutane 1.5%
butane 2.5%
pentane plus 3.0%

In addition to these gases, natural gas may contain appreciable quantities of nitrogen, helium, carbon dioxide, and contaminants (as hydrogen sulfide and water vapor). Although gaseous at normal temperatures and pressures, certain of the gases comprising the mixture that is natural gas are variable in form and may be found either as gases or as liquids under suitable conditions of temperature and pressure.

negative separation A term usually used in reference to microlog curves to describe the condition where the longer spaced resistivity curve reads a lower value than the shorter spaced curve. Compare positive separation.
N electrode The potential-measuring electrode most distant from the current A electrode in the electrode configuration of electrical resistivity-measuring devices. See also normal and lateral devices.
net hydrocarbon porosity feet Hydrocarbon porosity feet after applying appropriate cutoffs. See hydrocarbon porosity feet.
net overburden

That part of the overburden (geostatic load) which is supported by grain to-grain contact of the rock. Net overburden usually is less than (total) overburden because of two reasons:

(1) Pore pressure within the bed of interest supports the weight of the column of formation fluid above it. If the pore pressure is higher than normal, then it supports part of the lithostatic load.

(2) The stress on the rock concentrated at the grain-to-grain contact may be less than that caused by the weight of overlying dry rock, because of the buoyant effect on the porous rock column produced by the liquid in the pore volumes.

net overburden pressure Effective stress exerted by porous rock. That support by the grain-to-grain contact of the rock which when combined with the formation pressure is assumed to equal the stress due to overburden.
net pay Within the limitations of given cutoffs for porosity, water saturation, etc., it is that portion of reservoir rock which will produce commercial quantities of hydrocarbon.
net sand thickness The accumulated thickness of sandstone of a specified quality which is found within a specific interval of formation.
neutron

An electrically neutral, elementary nuclear particle having a rest mass of 1.674×10–24 gram or an atomic mass of 1.00898. (i.e., very nearly the same as that of a proton), which exists in all nuclei except that of hydrogen.

Neutrons exhibit a broad variation of kinetic energies ranging from as little as 0.025 eV to as much as 50 MeV. Neutrons are used in logging as a means of measuring the quantity of hydrogen important in the moderation process (energy transfer) or as a means to induce radiation in stable isotopes.

neutron activation All stable isotopes are capable of capturing thermal neutrons. In well logging, neutron bombardment of a formation and the subsequent capture of thermal neutrons causes excitation of certain elements. Following the capture of a thermal neutron by a stable isotope, the compound nucleus de-excites by the prompt emission of one or more gamma-ray protons. If the resulting product nucleus is a radioisotope its later decay to a stable state can be detected and the energy level of emitted gamma rays is characteristic of the specific element. The analysis of the energies of the decay gamma rays is neutron activation analysis.
neutron capture cross section See capture cross section.
neutron detector See scintillation counter and proportional counter.
neutron generator An electromechanical device operating at high voltage (125-l30,000 volts DC) which focuses a beam of high-energy deuterons on a target surface containing tritium. Nuclear fusion of the deuteron ions and target atoms produces high-energy (14-MeV) neutrons. The neutron radiation can be controlled in precise cyclic bursts or pulses, with time in between pulses for the measurement of induced-radioactivity and decay-time schemes.
neutron interactions with matter The three possible processes are elastic scattering, inelastic scattering, and absorption. The latter includes all processes in which the neutron becomes part of the absorbing nucleus. Also see neutron activation.
Neutron Lifetime Log

NLL. The Neutron Lifetime logging technique employs a pulsed neutron source which is periodically actuated to produce short bursts of neutrons and is quiescent between bursts. During the interval between bursts, the neutrons (as well as the various types of radiation which always result from neutron interactions) die away. Their average lifetime can be measured by measuring the length of time required for the neutron population at a particular instant to die away to half value. The radiation intensity is measured in each of two preselected intervals and, by intercomparing these measurements, determine thc rate of neutron die-away. This measured rate has been shown both by theory and experiment to be a measure of the thermal-neutron capture cross section of the medium in which the neutrons are captured. The thermal-neutron capture cross section per unit of volume of formation material is referred to as S. It is related to L, termed the lifetime of neutrons in a material, by the equation

[formula]

Thermal neutrons are captured mainly by the chlorine present. Hence the tool responds to the amount of salt in formation waters. Hydrocarbons result in longer lifetimes than salt water. Tool measurements are porosity-dependent and sensitive to clay content. Can be used in cased holes where resistivity logs cannot be run or to monitor reservoir changes to opt-imize production. Resembles a resistivity log with which it is generally correlatable. Neutron Lifetime Log is a Dresser Atlas registered trademark.

Measuring Sigma by the Dual Gate System

neutron log

A log of a response primarily related to hydrogen concentration but also affected by mineralogy and borehole effects. The neutron log does not distinguish between the hydrogen in the pore fluids (i.e., water, oil, gas), in water of crystallization, or water bound to solid surfaces. In clean oil-filled or water-filled formations the apparent porosity reading ot the neutron log reflects the amount of liquid-filled pore volume. Used with other porosity information. the neutron log is useful to ascertain the presence of gas and determine mineralogy and shaliness.

The tool contains a continuously emitting neutron source and either a neutron- (n-n tool) or a gamma-ray detector (n-g tool). High energy neutrons from the source are slowed down by collisions with atomic nuclei. The hydrogen atoms are by far the most effective in the slowing down process because their mass is nearly equal to that of the neutron. Thus, the distribution of the neutrons at the time of detection is primarily determined by the hydrogen concentration. Depending on the tool type, detection is made of either (1) thermal neutrons; (2) gamma rays, generated when thermal neutrons are captured by thermal-neutron absorbers in the formation (primarily chlorine); or (3) epithermal neutrons (neutrons having energies higher than thermal).

Neutron curves are scaled in API units or in terms of apparent porosity. The neutron log can be recorded in open or cased liquid-filled well bores. There is a maximum hole size limitation in empty holes for running tools in which the detector does not contact the formation wall. See also sidewall neutron log and compensated neutron log.

neutron source

(1) An encapsulated radioactive material which produces neutrons for neutron logging. The neutrons usually are produced in alpha-berillium reactions. The alpha particle producing element may be americium, plutonium, or sometimes radium. Californium-252. which is sometimes used in special applications, is an intense source of 2.3-MeV neutrons but has a short half life of 2.65 years.

(2) A neutron generator. An electromechanical device which emits high-energy (14-MeV) neutrons in controlled cyclic pulses. Pulsed neutron radiation is required in Thermal Decay Time Logging, Neutron Lifetime Logging, carbon-oxygen logging, and activation logging instruments.

Neutron Source Materials

noise

(1) Sudden spurious readings on a curve. These may be random events or caused by instrumentation problems.

(2) A sound. In well logglng, a sound related to some source in the wellbore environment, e.g., fluid flow, the production of formation fluids through perforations, leakage of fluid through apertures, etc.

noise logging Audio logging. A logging process for measuring the amplitude of background noise in the wellbore environment, for specific frequencies in the audible range, at selected stations in the hole. Moving fluids, liquids or gases, generate characteristic sounds having frequency spectra and amplitudes which can be interpreted. The signal amplitude is proportional to the amount of work performed by the fluids in motion and to the location of the tool with respect to the level from which noise emanates. Can be useful in ascertaining fluid-movements behind tubing or casing.
nonconformity Where the older rocks were metamorphosed and exposed by profound erosion before the strata were laid down on them, there is a major unconformity, representing a hiatus of great duration. To distinguish unconformities of this significance, the term "nonconformity" is used.
normal curve

A symmetrical resistivity curve recorded by a normal device.

Normal Curves

normal device

A resistivity-measuring system using a "normal" electrode configuration. A constant current is passed between a current electrode on the sonde (A electrode) and one at the surface (B electrode) while the potential difference is measured between another on the sonde (M electrode) and a reference electrode (N electrode). The "spacing" is the diference between the A and M electrodes. Usually spacing of about 16 inches is used for a the short normal and 64 inches for the medium or long normal. The measure point is midway between the A and M electrodes.

A normal device has a depth of investigation said to be about twice the AM spacing. The normal is an unfocused device which produces a symmetrical curve which has been particularly useful in correlation and indetermination of lithology. Formation detail can be increased by decreasing the AM spacing, but depth of investigation suffers.

Normal Device

normal formation pressure See normal pore pressure.
normalize

(1) To adjust two log curves (or any other pairs of data) for environmental differences in order that one value may be compared with others.

(2) To adjust two log curves or similar data to the same, or equivalent, units so that the data values can be compared.

normal pore pressure

In a reservoir rock it is the hydrostatic pressure resulting from the head of water solution (formation water) filling the pores of the rock in communication with the mean water table or sea surface.

Normal Pressure Trend

normal (pore) pressure gradient The amount of change in pore pressure per unit change in depth when the pores of the rock exhibit normal pore pressure. In parts of the Gulf Coast. the normal pressure gradient has been found to correspond to 0.465 psi/ft change in depth (equivalent to weight of water containing about 100,000 ppm total solids in solution).
nuclear Of or pertaining to the reactions involving atomic nuclei and their transformations, and to the well-logging operations dependent on such reactions. See nuclear log and radioactivity log.
nuclear cement log A well log of scattered gamma rays, differing from the density log in that the gamma-ray source and detector are so spaced as to be sensitive to the density of material in the annulus between casing and formation. Used for distinguishing between cement and fluids behind casing. Can be run in liquid-filled or empty holes.
nuclear cross section See capture cross section.
nuclear flow log A record of borehole-fluid flow rate. See radioactive tracer log and fluid travel log. Compare flowmeter.
nuclear log A well log of some parameter in the well bore environment derived from techniques utilizing nuclear reactions taking place in the downhole logging tool and/or in the formation. Nuclear logs usually are well logs obtained by using radiation sources in the logging tool.
nuclear magnetic resonance A phenomenon exhibited by atomic nuclei which is based on the existence of nuclear magnetic moments associated with quantized nuclear spins. In well logging, it pertains to the measurement of properties related to the nuclear spin states of hydrogen nuclei. See nuclear magnetism log.
nuclear magnetic resonance log See nuclear magnetism log.
nuclear magnetism log A free fluid log. A well log that is dependent on the alignment of the magnetic moment of protons (hydrogen nuclei) with an impressed magnetic field. Protons tend to align themselves with the magnetic field; and when it is removed, they precess in the earth's magnetic field and gradually return to their original state. Proton precession in free fluid produces a radio frequency signal. The amplitude of this radio frequency signal is measured in the nuclear magnetism log as the free fluid index. The rate of decay of the precession signal depends on interactions with neighboring atoms and hence on the nature of the molecule of which the proton is a part. The signal from the borehole fluid decays very rapidly when disseminated iron is present (artificially introduced, or from steel worn from drill pipe and bits). By slightly delaying the time of measuring, the hole signal is minimized. Fluids bound to surfaces (as water adsorbed to clays and silts) and dead oil do not give appreciable response. Thus, the free fluid index indicates the free fluid (the hydrogen in free-fluid hydrocarbons and water). Gas gives a low signal because of its low hydrogen content.
nucleus The central, dense, positively-charged core of an atom. The nucleus makes up practically the entire mass of an atom. The nucleus of an atom is composed of one or more protons and one or more neutrons; except in the case of the hydrogen atom, which has only one proton as its nucleus (in its most common isotope).
nuclide A species of atom characterized by the number of neutrons and protons in its nucleus. An isotope. The atom must be capable of existing for a measurable lifetime, generally greater than 10–10 second.