Oil and Gas Terms Beginning with “N”

Abbreviation for natural gas liquids.

The procedure in seismicprocessing that compensates for the effects of the separation between seismic sources and receivers in the case of a horizontal reflector.

Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible.

An analytical tool used in forecasting the performance of the various elements comprising the completion and production system. NODAL* analysis is used to optimize the completion design to suit the reservoir deliverability, identify restrictions or limits present in the production system and identify any means of improving production efficiency.*NODAL (production system analysis) is a mark of Schlumberger.

Abbreviation for "National Pollutant Discharge Elimination System." The US Congress passed this Clean Water Act to control discharges of contaminants. Discharges are allowed in to US water only by NPDES permits. Drilling fluids, drill cuttings, produced water, drilling rig deck drainage and blowout preventor fluids are covered specifically. Barite must be monitored for heavy metals to allow mud discharges. Oils are banned from discharge.

The magnetization retained by rocks from previous magnetic fields,abbreviated NRM. NRM is a record of the Earth's magnetic field as it existed at the time that the rock formed, such as when magnetic crystals in igneous rocks solidified (also known as chemical remanent magnetism, CRM) or at the time of deposition of sedimentary rocks (known as depositional remanent magnetism, DRM). During deposition of sediments that become sedimentary rock, magnetized particles can settle with their magnetic pole aligned with that of the Earth at that time.

Conventional marineseismic data acquired using a single vessel to tow one or two seismic source arrays in front of a receiverspread. The resulting angle between the source and receivers, is about 20°.

A fluid that has a constant viscosity at all shear rates at a constant temperature and pressure, and can be described by a one-parameterrheologicalmodel. An equation describing a Newtonian fluid is given below. Water, sugar solutions, glycerin, silicone oils, light-hydrocarbon oils, air and other gases are Newtonian fluids. Most drilling fluids are non-Newtonian.

Crude oil containing asphaltic materials but very little or no paraffin wax. This type of oil is suitable for making gasoline, lubricating oil and asphalt. It is also called asphalt-base crude.

A type of organic compound of carbon and hydrogen that contains one or more saturated cyclic (ring) structures, or contains such structures as a major portion of the molecule. The general formula is CnH2n. Naphthenic compounds are sometimes called naphthenes, cycloparaffins or hydrogenated benzenes. Naphtha is a refined petroleum fraction that contains a high percentage of these types of hydrocarbons. In drilling fluids, particularly oil-base muds, the amounts and types of hydrocarbons in the mud can be an important parameter in the overall performance of the mud.

Clays incorporated into a so-called native-solids mud when drilling shallow formations. Native clays are undesirable in muds that are (or will be) weighted with barite. The viscosity of weighted fluids can rise quickly with added native clays, making it difficult to control and pump the mud. Better mud properties result when the drilled solids level, including the level of native clays, is kept low.

A core taken so as to preserve the in-situ water saturation of the rock. A native-state core is usually drilled with oil-base mud or crude oil from the same reservoir.

A mud in which the suspended solids are dispersed clays, sand, chert and other rock that originated from formations being drilled. A spud mud is commonly a type of native-solids mud. Native muds can be economically diluted with water and passed through banks of desilters and desanders to keep solids down. No expensive weighting materials are being discarded and replaced in such a process. At the depth that higher density is required, native mud is usually totally or partially discarded and new mud is made using commercially prepared mud additives and barite.

A completion system designed to utilize the natural flow capability of the reservoir.

The frequency of the normal, free oscillation or vibration of an entity or a system, such as the vibration of a tuning fork when struck or the open string of a musical instrument when plucked. A system oscillating at its natural frequency is said to resonate.

The technique of measuring the spectrum, or number and energy, of gamma rays emitted as natural radioactivity by the formation. There are three sources of natural radioactivity in the Earth: 40K, 232Th and 238U, or potassium, thorium and uranium. These radioactive isotopes emit gamma rays that have characteristic energy levels. The quantity and energy of these gamma rays can be measured in a scintillation detector. A log of natural gamma ray spectroscopy is usually presented as a total gamma ray log and the weight fraction of potassium (%), thorium (ppm) and uranium (ppm). The primary standards for the weight fractions are formations with known quantities of the three isotopes. Natural gamma ray spectroscopy logs were introduced in the early 1970s, although they had been studied from the 1950s.

What Is Natural Gas? Natural gas is a gaseous mixture of hydrocarbons, predominantly methane (CH4), extracted from geological formations and used as fuel for power generation, heating, and industrial processes, and as a petrochemical feedstock worldwide. It occurs in conventional reservoirs, tight sandstones, shale source rocks, and coal seams, and is traded globally in pipeline networks and as liquefied natural gas (LNG) shipped by specialized tankers between producing and consuming nations. Key Takeaways Natural gas is composed primarily of methane (CH4, 70-99% by volume) with varying proportions of ethane, propane, butane, and heavier hydrocarbons, plus non-hydrocarbon impurities including CO2, H2S, nitrogen, and water vapor. Gas types are classified as dry (less than 0.1 gal/Mcf of natural gas liquids), wet (more than 0.3 gal/Mcf NGLs), sweet (low H2S), or sour (H2S exceeding 1 grain per 100 standard cubic feet), with each type requiring different processing before transport. Unconventional sources including shale gas (Montney, Marcellus, Haynesville), tight gas, and coal bed methane now supply the majority of North American production and are growing globally through the application of hydraulic fracturing and horizontal drilling. Gas is measured in thousand cubic feet (Mcf), billion cubic feet (Bcf), and trillion cubic feet (Tcf) under US standards or in cubic metres (m³) and billion cubic metres (Bcm) under metric standards, with all volumes reported at defined standard conditions of temperature and pressure that differ between jurisdictions. Global benchmark prices include Henry Hub (United States), AECO (Alberta), NBP (UK), TTF (Netherlands), and JKM (Asian LNG spot), each reflecting regional supply-demand balances and infrastructure constraints. How Natural Gas Forms and Is Produced Natural gas forms through two principal mechanisms. Biogenic gas originates from the microbial decomposition of organic matter at shallow depths and low temperatures, typically producing nearly pure methane with very low concentrations of heavier hydrocarbons. Thermogenic gas forms at greater depths and higher temperatures as kerogen in source rocks thermally cracks into hydrocarbon molecules; at intermediate thermal maturity, liquids-rich wet gas dominates; at high thermal maturity, dry gas with high methane content prevails; and in overmature shale formations, the gas stream may contain appreciable nitrogen. The distinction between gas types matters operationally because wet gas requires NGL extraction before pipeline injection, while dry gas may enter the transmission system with minimal processing beyond dehydration and compression. Conventional gas accumulates when buoyant gas migrates upward from a source rock and becomes trapped beneath an impermeable seal, commonly a carbonate cap rock or evaporite, in a structural or stratigraphic closure. Reservoir pressure in a conventional gas reservoir typically follows a hydrostatic or overpressured gradient, and the well produces by expansion of the gas cap as pressure draws down. Recovery factors in conventional gas reservoirs commonly reach 70-90% of original gas in place (OGIP) with adequate compression infrastructure. Unconventional gas reservoirs retain their gas within the source rock itself or in adjacent tight formations; they require artificial stimulation via hydraulic fracturing to release gas from the low-permeability matrix. Recovery factors in unconventional plays typically range from 15-35% of OGIP, though EUR (estimated ultimate recovery) per well continues to improve with advancing completion technology. Surface production begins at the wellhead, where the gas stream passes through a separator to remove free liquids and produced water. The gas then enters field gathering lines operating at low pressure, typically 350-1,400 kPa (50-200 PSI), before reaching a compressor station that boosts pressure for delivery to a gas processing plant or directly into a regional transmission pipeline. At the gas processing plant, dehydration removes water vapor using glycol contactors to prevent hydrate formation downstream; sweetening using amine units (mono-ethanolamine, di-ethanolamine, or MDEA) removes H2S and CO2; and NGL extraction via a cryogenic expander plant or lean oil absorption separates ethane, propane, and heavier components from the dry gas residue stream. The treated, dry, sweet gas enters the transmission pipeline at delivery pressure, typically 7,000-10,000 kPa (1,000-1,500 PSI). Natural Gas Across International Jurisdictions Canada (Western Canadian Sedimentary Basin): Canada ranks among the world's top ten natural gas producers, with the Western Canadian Sedimentary Basin (WCSB) supplying nearly all domestic production. The Montney Trend of northeastern British Columbia and northwestern Alberta has emerged as one of the most prolific natural gas plays globally, with the Alberta Energy Regulator (AER) and BC Oil and Gas Commission estimating Montney OGIP at more than 450 Tcf (12,740 Bcm). The Duvernay shale of Alberta is another major emerging gas and liquids-rich play. Canadian gas volumes are reported at Canadian standard conditions of 15°C (59°F) and 101.325 kPa (14.696 PSI), while US volumes use 60°F (15.6°C) and 14.73 PSI (101.5 kPa). This difference of approximately 1.0009 in conversion factor is small but must be noted when comparing cross-border statistics. The national pipeline system, historically anchored by TC Energy's NOVA Gas Transmission Ltd. (NGTL) network and Mainline, connects WCSB supply to eastern Canadian and US export markets. LNG Canada's Phase 1 project near Kitimat, British Columbia, represents Canada's first large-scale LNG export terminal, targeting Asian Pacific markets. United States: The United States is the world's largest natural gas producer, with the Energy Information Administration (EIA) reporting dry gas production of approximately 103 Bcf/d (2,917 Mcm/d) in recent years. The Appalachian Basin, encompassing the Marcellus and Utica shales of Pennsylvania, West Virginia, and Ohio, is the single largest producing region. The Permian Basin of West Texas and southeastern New Mexico produces large volumes of associated gas alongside crude oil, creating recurring infrastructure constraints that periodically result in gas flaring or curtailment. The Haynesville shale of Louisiana and Texas is a major dry gas play supplying Gulf Coast LNG export facilities at Sabine Pass, Freeport, and Corpus Christi. US gas reserves and production are reported under Securities and Exchange Commission (SEC) Rule 4-10 for public companies, and the EIA publishes monthly statistics using 14.73 PSI and 60°F standard conditions. Henry Hub in Erath, Louisiana, is the physical delivery point for the NYMEX natural gas futures contract, making it the dominant North American price benchmark. Australia: Australia is among the world's top LNG exporters, with a combined nameplate liquefaction capacity exceeding 87 million tonnes per annum (Mtpa) from facilities on the Northwest Shelf and in Queensland. Key projects include the Gorgon LNG project (15.6 Mtpa) on Barrow Island, the Ichthys LNG project (8.9 Mtpa) near Darwin drawing from the Browse Basin, the Prelude floating LNG (FLNG) facility in the Browse Basin, and the North West Shelf LNG venture near Karratha. The Australian Petroleum Production and Exploration Association (APPEA) publishes annual industry statistics. Offshore facilities are regulated by NOPSEMA; onshore operations fall under state and territory regulators such as the Department of Energy, Mines, Industry Regulation and Safety (DEMIRS) in Western Australia. Australia's LNG exports are predominantly sold under long-term oil-linked contracts to Japanese, South Korean, and Chinese buyers, though spot LNG (priced at JKM) has grown as a share of trade. Middle East: Qatar holds the world's largest non-associated natural gas reservoir, the North Field, which extends offshore in the Arabian Gulf and shares a geological structure with Iran's South Pars field. The North Field contains estimated reserves of approximately 1,760 Tcf (49,840 Bcm) of gas in place, making it a dominant force in global LNG markets. QatarEnergy operates six LNG trains at Ras Laffan Industrial City with a combined capacity of 77 Mtpa, a figure set to expand with the North Field Expansion Project targeting 126 Mtpa by 2027. Saudi Aramco produces gas from the Khuff carbonate formation, a deep, HPHT reservoir that supplies the Master Gas System (MGS) for domestic power generation, petrochemicals, and reinjection into oil reservoirs for pressure maintenance. UAE Das Island has historically served as a key NGL fractionation and export hub for associated gas from Abu Dhabi offshore fields. Gas from the region fuels some of the world's largest ethylene and methanol complexes at Jubail, Yanbu, and Ruwais. Norway and the North Sea: Norway is Europe's largest gas supplier and one of the world's top gas exporters by pipeline. Equinor and its partners produce gas from multiple fields on the Norwegian Continental Shelf (NCS), with the Troll field in the North Sea holding approximately 1,330 Bcm (47 Tcf) of recoverable gas reserves, making it one of the largest gas fields outside the Middle East and Russia. The Asgard field on the Haltenbanken and several Barents Sea fields (Snohvit, which feeds the Hammerfest LNG plant) round out the Norwegian portfolio. Sodir (Norwegian Offshore Directorate) publishes comprehensive production statistics via its publicly accessible FactPages database. Norwegian gas reaches European consumers via sub-sea pipeline systems including the Europipe I and II, Norpipe, and Langeled pipelines connecting to Germany, the UK, Belgium, and France. Following the 2021-2022 European energy crisis, Norwegian gas export volumes increased substantially as Europe sought to reduce dependence on Russian supply. The UK's National Balancing Point (NBP) and the Dutch Title Transfer Facility (TTF) are the two primary European gas benchmark prices. Fast Facts Methane content of pipeline-quality gas: 95-99% CH4 by volume LNG storage temperature: -161°C (-258°F) at atmospheric pressure; volume reduction ratio approximately 600:1 versus gaseous state at standard conditions Higher heating value (HHV): approximately 1,020-1,050 BTU per standard cubic foot (38-39 MJ/m³) World's largest gas reservoir: Qatar/Iran shared North Field / South Pars, approximately 1,760+ Tcf (49,840+ Bcm) OGIP Global proven natural gas reserves: approximately 7,100 Tcf (201,000 Bcm) per EIA estimates (subject to revision)

Components of natural gas that are liquid at surface in field facilities or in gas-processing plants. Natural gas liquids can be classified according to their vapor pressures as low (condensate), intermediate (natural gasoline) and high (liquefied petroleum gas) vapor pressure.Natural gas liquids include propane, butane, pentane, hexane and heptane, but not methane and ethane, since these hydrocarbons need refrigeration to be liquefied. The term is commonly abbreviated as NGL.

A natural gas liquid with a vapor pressure intermediate between condensate and liquefied petroleum gas. This liquid hydrocarbon mixture is recovered at normal pressure and temperature and is much more volatile and unstable than commercial gasoline.

The magnetization retained by rocks from previous magnetic fields,abbreviated NRM. NRM is a record of the Earth's magnetic field as it existed at the time that the rock formed, such as when magnetic crystals in igneous rocks solidified (also known as chemical remanent magnetism, CRM) or at the time of deposition of sedimentary rocks (known as depositional remanent magnetism, DRM). During deposition of sediments that become sedimentary rock, magnetized particles can settle with their magnetic pole aligned with that of the Earth at that time.

A well in which the formation pressure is sufficient to produce oil at a commercial rate without requiring a pump. Most reservoirs are initially at pressures high enough to allow a well to flow naturally.

Materials typically found in certain types of barium or strontium scales that may be deposited in the wellbore or productiontubulars. Any attempt to remove and dispose of NORM materials should be performed according to the legislation and policies associated with such potentially hazardous materials.

Cement that has no additives to modify its setting time or rheological properties.

Describing the environment and conditions of the marine zone between low tide and the edge of the continental shelf, a depth of roughly 200 m [656 ft]. A neritic environment supports marine organisms, also described as neritic, that are capable of surviving in shallow water with moderate exposure to sunlight.

A geophone array. Nests can contain numerous closely spaced geophones.

Any model that incorporates more than one variable that is represented by fractal geometry or a fractal function. These models can become very complex if the variables are interdependent.

The volume of gas produced less gas injected.

The volume of oil produced less oil injected. In hydraulic pumping, the oil injected is known as power oil.

A share of net proceeds from production paid solely from the working interest owners share. It is sometimes granted in lieu of a royalty interest.

A share of production after all burdens, such as royalty and overriding royalty, have been deducted from the working interest. It is the percentage of production that each party actually receives.

A concept for advanced computer calculations developed by Alan Turing to mimic some of the operations of the neurons in a brain. Memory elements (neurons) are conceptually interconnected by multiple paths connected with on-off switches to emulate the synapses of the brain. The original intent was to build a data-processing machine.Modern applications reduce the concept to structured digital software processing models. Repeated processing through a neural network allows the network to learn from the data it processes. The learned process obtained from a set of training data with solutions can then be applied to other data sets for which no solution exists. An oilfield example includes training a network with wireline log and core data and then using the network to interpret further log data in terms of the core data. Neural networks are also being used in seismic processing, geological mapping and petrophysical analysis.

The point on a string of tubulars at which there are neither tension nor compression forces present. Below the neutral point, there will be compression forces that build toward the bottom of the wellbore. Above the neutral point, tensile forces build to a maximum applied at the hanger or as hook load.

A chemical reaction between an acid and a base to form a salt and water. Neutralization is used in the manufacture of mud products, removal of acidic contaminants from muds and formation of emulsifiers in oil mud. Neutralization is used in the test for alkalinity of mud and mud filtrate.

A fluid prepared to counteract the corrosive effect of acids or acidic treatment fluids. Neutralizing solutions generally are used when the components to be protected cannot be adequately flushed or when there is a risk that residual fluids may cause problems through prolonged exposure. Neutralizing solutions are commonly formulated with soda ash to provide an inexpensive, nondamaging alkaline fluid that does not create excessive disposal difficulties.

A neutron interaction in which the neutron is absorbed by the target nucleus, producing an isotope in an excited state. The activated isotope de-excites instantly through the emission of characteristic gamma rays. Neutron capture, also called thermal capture, usually occurs at low thermal energies at which the neutrons have about the same energy as the surrounding matter, typically below 0.4 eV (0.025 eV at room temperature). Some elements are better thermal absorbers than others. Neutron capture is an important principle behind the pulsed neutron capture log, the elemental capture spectroscopy log, the pulsed neutron spectroscopy log and the thermal neutron porosity measurement.

A device for producing high-energy neutrons by using a charged particle accelerator. Neutron generators are used in various pulsed neutron devices and some neutron porosity measurements. In a typical device, deuterium (2D) and tritium (3T) ions are accelerated towards a target also containing the same isotopes. When 2D and 3T collide, they react to produce a neutron with an energy of about 14.1 MeV. The first neutron generators were built in the late 1950s and soon led to the first pulsed neutron capture log.

Phenomena involving the transfer of energy from neutrons to nuclei. The reaction rate of neutrons with matter depends on the density of neutrons, their velocity, the nuclear density and the particular interaction cross section. There are four principal neutron interactions that affect formation evaluation: elastic neutron scattering, inelastic neutron scattering, fast-neutron reactions and neutron capture.

Normally synonymous with a neutron porosity log. However, the term is sometimes broadened to include an activation log.

What Is a Neutron Porosity Log? A neutron porosity log bombards the formation with fast neutrons from an AmBe or Cf-252 source and measures the flux of slowed (epithermal) or fully thermalised (thermal) neutrons returning to near and far detectors, with the near/far ratio converting to apparent porosity units calibrated to limestone because neutron deceleration reflects the formation's hydrogen content, making the tool a direct measurement of fluid-filled porosity widely used in combination with the density log to identify gas, assess matrix mineralogy, and evaluate effective porosity in reservoir formations. Key Takeaways The neutron log responds primarily to hydrogen content in the formation; because hydrocarbons and water both contain hydrogen, the tool estimates fluid-filled porosity without directly measuring the rock matrix. All commercial neutron logs are calibrated to a freshwater-saturated limestone formation, so readings in sandstone and dolomite require matrix corrections that shift the apparent limestone porosity to true porosity for that lithology. Gas in the pore space causes the neutron log to read abnormally low (gas has far less hydrogen per unit volume than liquid water or oil), creating the classic "neutron-density crossover" gas flag on the log display. Shale contains bound water in its clay mineral lattice, which donates hydrogen to the measurement without contributing to producible pore volume, causing the neutron log to read artificially high in shaly intervals. The compensated neutron log (CNL) uses a ratio of near to far detector counts to cancel borehole size and mud salinity effects, giving a more formation-responsive measurement than single-detector tools. How the Neutron Porosity Log Works The neutron source emits high-energy (fast) neutrons at roughly 5 million electron volts (MeV). These neutrons collide with nuclei in the formation, losing energy with each collision. The most efficient moderator is hydrogen, whose nucleus (a single proton) is essentially the same mass as the neutron and therefore maximally effective in transferring kinetic energy. Heavier nuclei (carbon, oxygen, silicon, calcium) absorb much less energy per collision. Consequently, the average distance a neutron travels before thermalisation depends almost entirely on the hydrogen concentration in the formation: hydrogen-rich formations (high porosity, water or oil) slow neutrons over short distances; hydrogen-poor formations (low porosity, gas, or tight carbonates) allow neutrons to travel much farther before thermalisation. The near and far detectors count returning neutrons at fixed spacings from the source (typically 14 and 23 inches, or 36 and 58 cm). The ratio of near to far counts decreases as hydrogen index increases (more porosity), because more neutrons are stopped before reaching the far detector. This ratio is converted to apparent porosity through a calibration established at the API pit in Houston, Texas, using water-saturated limestone blocks of known porosity (typically 0, 19.0, and 26.0 percent porosity blocks). Two detector designs are in commercial use. Thermal neutron tools (CNL: Compensated Neutron Log by SLB; CNS: Compensated Neutron Sonde) detect fully thermalised neutrons, which are highly sensitive to thermal neutron absorbers such as chlorine and boron. In saline formation water, the high chlorine concentration absorbs thermal neutrons and reduces the count rate, causing the tool to read anomalously low porosity in saline environments unless a salinity correction is applied. Epithermal neutron tools (EPSN, HNGS) detect neutrons before they thermalise, at a higher energy level where chlorine absorption is negligible, making them more robust in saline environments and in formations containing boron or gadolinium. Epithermal tools have shallower investigation depth (approximately 6-8 inches, or 15-20 cm) compared to thermal tools (approximately 12-14 inches, or 30-35 cm), making them more susceptible to borehole fluid effects but less influenced by saline formation fluids. The compensated neutron log (CNL) runs as part of the triple-combo suite alongside the density log and a gamma ray / resistivity combination. It measures at approximately 1,500-1,800 ft/hour (457-549 m/hour) logging speed, generating data at 0.5 ft (15 cm) depth resolution. Environmental corrections applied at the wellsite address borehole diameter (using caliper input), mud weight, mud salinity, formation temperature, standoff between tool and borehole wall, and casing thickness (for cased-hole runs). Modern LWD neutron tools carry the source and detectors in the drill collar, providing near-real-time porosity measurements while drilling that support geosteering decisions. LWD neutron tools suffer more borehole influence than wireline tools due to larger standoff variations in a rotating drill string, but newer nuclear measurement-while-drilling (NMD) tools partially compensate with real-time azimuthal corrections. Neutron Porosity Log Across International Jurisdictions In Canada, the neutron porosity log is a required component of the AER's minimum log suite for wells penetrating potential pay zones under Directive 044. In the Montney formation of northwest Alberta and northeast British Columbia, the neutron-density combination is used extensively to discriminate between gas-saturated tight rock (neutron-density crossover present) and liquids-rich intervals where the crossover is absent or reversed. The Matrix of Montney tight siltstone (quartz-dolomite-clay mixture) requires careful matrix correction: treating the Montney as pure limestone overestimates porosity by 2-5 porosity units compared to the true mineralogical mix. Operators in the Duvernay and Cardium plays similarly use neutron-density crossplots to guide completions decisions by identifying gas-charged brittle intervals optimal for hydraulic fracturing. In the United States, the Gulf of Mexico deepwater plays use the neutron-density combination across thick sand packages in Miocene and Pliocene reservoirs. Clean, well-sorted deepwater turbidite sands with high porosity (25-35 percent) show clear neutron-density agreement in oil and water zones and pronounced crossover in gas-charged sands. The Bureau of Safety and Environmental Enforcement (BSEE) mandates log submission including neutron porosity for all wells on the OCS. In shale gas plays such as the Barnett (Texas), Haynesville (Louisiana/Texas), and Marcellus (Pennsylvania/West Virginia), the neutron log reads high in organic-rich shale because kerogen contains significant hydrogen in its molecular structure, causing neutron porosity overestimation in source rock intervals. Operators correct for this using a kerogen hydrogen index factor derived from programmed pyrolysis data. In Norway, Sodir requires neutron porosity logging in all exploration wells on the NCS. The Brent Group sandstones of the northern Viking Graben have neutron-density porosity typically in the 18-28 percent range, with excellent tool response in clean, well-sorted Jurassic sands. The Chalk fields of the southern North Sea (Ekofisk, Valhall) present a challenging case for the neutron log: microcrystalline calcite with very fine pore throats gives neutron porosity of 30-48 percent in the chalk matrix, but the producible effective porosity may be only 10-20 percent because capillary-bound water in micropores contributes hydrogen to the neutron measurement without being producible. NMR porosity (which measures only free-fluid pore space) is often run alongside the neutron log in Chalk wells to disaggregate total from moveable porosity. In the Middle East, carbonate reservoirs in the Arab and Khuff formations show strong neutron-density agreement in water zones and measurable crossover in gas-cap intervals, with tool calibration verified at Saudi Aramco's in-country calibration facility and ADNOC's Abu Dhabi facility. Fast Facts Gas has a hydrogen index of approximately 0.3-0.5 relative to pure water at typical reservoir pressures and temperatures, compared to oil at 0.9-1.0 and brine at 1.0. This difference is large enough that even a modest gas saturation of 25 percent reduces the neutron porosity reading by 5-8 porosity units, creating a highly visible crossover with the density log that can be detected even in intervals with only 15 percent total gas saturation. Matrix Corrections and Lithology Effects All commercial neutron logs are calibrated to freshwater-saturated limestone with a grain density of 2.710 g/cm3 (2,710 kg/m3). When the formation is not limestone, the apparent limestone porosity must be corrected to true porosity using crossplot charts that apply matrix corrections derived from laboratory measurements of pure mineral end-members. For sandstone (quartz matrix, grain density 2.648 g/cm3 or 2,648 kg/m3), the neutron porosity in a clean, freshwater-saturated sandstone reads approximately 4-6 porosity units higher than the true porosity, because quartz has a small amount of hydroxyl groups in its surface structure. The matrix correction chart shifts the neutron reading from apparent limestone units to sandstone units. For dolomite (grain density 2.876 g/cm3 or 2,876 kg/m3), the apparent limestone neutron porosity is approximately 2-4 units lower than the true dolomite porosity at the same true porosity value. The neutron-density crossplot is the primary tool for simultaneous matrix identification and porosity determination. On a plot of neutron porosity (horizontal axis, limestone units) versus density porosity (vertical axis, limestone grain density), pure water-saturated minerals plot at specific crossplot values. Limestone plots near the origin (zero-zero). Sandstone plots slightly above and to the left of the limestone point. Dolomite plots below the limestone line. A formation's crossplot point that falls between the limestone and sandstone lines indicates a limestone-sandstone mixture; a point below the limestone line indicates dolomite or anhydrite content. This lithology discrimination is essential in complex carbonate-evaporite sequences of the Middle East and in mixed-lithology tight gas plays. The gas effect on the neutron log is one of its most valuable diagnostic signatures. Gas has a hydrogen index of approximately 0.3-0.5 compared to 1.0 for water at reservoir conditions of 2,000-4,000 psi (13.8-27.6 MPa) and 120-180 degrees Fahrenheit (49-82 degrees Celsius). When gas fills pore space, the apparent neutron porosity drops well below the true gas-saturated porosity because gas does not slow neutrons effectively. Simultaneously, gas has very low density (0.1-0.3 g/cm3 or 100-300 kg/m3 at reservoir conditions), causing the density log to read very low bulk density and thus very high density porosity. The net result is that the neutron curve reads lower than the density curve in a gas zone, creating the crossover pattern where the two curves switch sides of the display track. The magnitude of this crossover scales with gas saturation and gas density: high-pressure, deep gas reservoirs show smaller crossover than shallow gas zones because gas density at depth approaches liquid density. Tip: When reviewing neutron-density log pairs in a potential gas reservoir, always note the actual separation in porosity units rather than just whether crossover exists. A 5-unit crossover in a 25-percent-porosity sand indicates approximately 50-70 percent gas saturation, implying a potentially commercial column. A 1-2 unit separation may reflect residual gas or lithology effects rather than a live gas zone. Confirming gas with independent resistivity evidence (high deep resistivity) and pressure data from a wireline formation tester before committing capital to completion is best practice and avoids the costly mistake of perforating a residual gas zone that will not flow at commercial rates.

A record of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. In the context of productionlogging, the term normally refers to the activation of silicon and aluminum to determine the quality of a gravel pack. Silicon and aluminum are activated by a neutron source to produce isotopes that decay with a half-life of 2.3 minutes emitting a 1.78 MeV gamma ray. These gamma rays are counted in a detector placed below the source, with a high count indicating a high quantity of silicon in a sand pack, or aluminum in a bauxite pack. The log is run slowly so that oxygen and other activated elements have decayed before the detector crosses the activated interval.The carbon-oxygen log, elemental-capture spectroscopy log, pulsed-neutron spectroscopy log, aluminum-activation log and the oxygen-activation log are also examples of neutron-activation logs.

What Is a Nipple? A nipple, formally called a landing nipple, is a short, heavy-wall steel sub machined into a production tubing string during completion to provide a polished sealbore and a locking profile. Wireline-deployed or coiled tubing-deployed tools lock into this profile, enabling operators to install and retrieve downhole devices without pulling the full tubing string. Key Takeaways A landing nipple is a machined sub in the production tubing string that provides a polished sealbore and locking groove for wireline-deployed tools and plugs. The two primary nipple types are the no-go nipple, which stops tools at a specific depth via an internal shoulder, and the selective nipple, which allows tools to pass through unless specifically engaged. API 11D1 profiles (X, XN, R, RN) define interchangeable locking and seal geometries across multiple manufacturers, ensuring cross-vendor compatibility. Subsurface safety valves (SSSVs) installed in nipples are mandatory under regulations including AER Directive 036, BSEE 30 CFR Part 250, and NORSOK D-010, with maximum setting depth requirements that directly control nipple placement. Material selection for nipples in sour service and high-CO2 environments must comply with NACE MR0175 / ISO 15156, requiring specific hardness limits and corrosion-resistant alloys. How a Nipple Works The nipple is threaded into the production tubing string at predetermined depths during the completion design phase. Its internal bore is machined and honed to a tight dimensional tolerance, typically within ±0.002 inches (±0.05 mm), creating a polished sealbore that a matching elastomeric or metal seal assembly on a locking mandrel will engage. Above the sealbore, the bore is machined with a locking groove, a circumferential channel into which spring-loaded collet fingers or lock dogs on the locking mandrel snap when the mandrel is rotated or set. Once engaged, the locked mandrel cannot be moved upward or downward without using the correct wireline running and pulling tools. The no-go nipple incorporates an additional feature: a reduced-diameter no-go shoulder below the sealbore. This shoulder physically stops any tool with an outer diameter equal to or larger than the no-go ID from traveling below that point. The operator designs the completion so that the largest-OD wireline tool runs to the shallowest nipple that accommodates it, while smaller tools can pass through to deeper nipples. A selective nipple lacks the no-go shoulder, meaning tools can pass through freely until a selective locking mandrel is mechanically activated at the correct depth using a wireline locating collar count or gamma-ray depth correlation. This allows multiple selective nipples of the same size to be stacked in a single string and accessed independently, which is especially useful in multi-zone completions on the same tubing string. Nipple spacing along the tubing string follows engineering guidelines to maintain a minimum vertical distance between adjacent nipples, typically 2 to 4 metres (6.5 to 13 ft), so that the body of one locked tool does not interfere with the tool below. When a subsurface safety valve is the primary device installed in the shallowest nipple, API Standard 11D1 governs the safety valve type, the sealbore geometry, and the pressure rating. The nipple also has a maximum pressure rating, expressed in psi or MPa, that must exceed the anticipated wellbore pressure under shut-in conditions and the maximum differential pressure the sealed plug will see during production. Nipple Across International Jurisdictions Canada (Alberta and British Columbia): The Alberta Energy Regulator (AER) governs subsurface safety valves under Directive 036, which mandates the installation of a surface-controlled subsurface safety valve (SCSSV) in all oil sands and deep sour-gas wells above a specified H2S partial pressure threshold. The nipple that houses the safety valve must be set within 100 metres (328 ft) of the surface, measured along the wellbore, placing a hard engineering constraint on the tubing string design. Montney tight-gas completions in northeast British Columbia routinely incorporate multiple selective nipples to allow blanking plugs to be set for zone isolation without milling out perforations. The BC Energy Regulator (BCER) references API 11D1 for safety valve qualification in its Well Authorization Manual. United States (Offshore and Onshore): The Bureau of Safety and Environmental Enforcement (BSEE) regulates offshore subsurface safety valves under 30 CFR Part 250, Subpart H. The regulation requires a SCSSV or storm choke in every offshore production well, with the valve set at or below the ocean floor but no deeper than 100 ft (30 m) below the mudline on most platforms. The nipple housing the valve must be rated for the maximum anticipated shut-in tubing pressure (SITP), and the operator must document the nipple design, profile type, setting depth, and pressure rating in the well's Safety and Environmental Management System (SEMS) file. Onshore unconventional wells in Texas and North Dakota typically use nipple designs per API 11D1 with no regulatory mandate for safety valves unless H2S exceeds threshold concentrations under EPA and OSHA rules. Norway and the North Sea: NORSOK Standard D-010, "Well Integrity in Drilling and Well Operations," classifies a properly seated subsurface safety valve as a well barrier element (WBE). The nipple that accepts the safety valve is itself a critical component of the primary well barrier, and its design, material certification, and inspection records must be documented in the well barrier schematic submitted to the Petroleum Safety Authority Norway (PSA). NORSOK D-010 requires that each WBE be tested to demonstrate pressure integrity at installation. The Norwegian Continental Shelf (NCS) also requires dual barrier philosophy in HPHT wells, sometimes necessitating a second safety valve in a deeper nipple as the secondary barrier element. Australia: The National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA) requires surface-controlled subsurface safety valves in all offshore production wells under the Offshore Petroleum and Greenhouse Gas Storage (Safety) Regulations. NOPSEMA's Well Operations Management Plan (WOMP) framework requires operators to document nipple profile type, setting depth, maximum allowable working pressure (MAWP), and testing intervals. Browse Basin and Carnarvon Basin high-pressure gas wells use nipples manufactured to API 11D1 with supplemental material qualifications for high-CO2 service, as Gorgon and Ichthys reservoir gases contain CO2 concentrations between 3% and 14%. Middle East: Saudi Aramco Engineering Standards (SAES-E-201 and SAES-E-402) specify nipple profile geometry, material grades, and SSSV qualification requirements for Ghawar, Shaybah, and offshore Safaniya field completions. Saudi Aramco requires the nipple and any installed safety valve to pass a function test and pressure test witnessed by an Aramco representative before the well is handed over to production operations. Some offshore wells in the Arabian Gulf with elevated H2S require dual SCSSV installations, with the upper valve in a no-go nipple providing depth assurance and the lower valve in a selective nipple providing redundancy. Fast Facts Typical nipple OD: Matched to the tubing string; most common sizes are 2-3/8 in (60.3 mm), 2-7/8 in (73 mm), 3-1/2 in (88.9 mm), and 4-1/2 in (114.3 mm) nominal tubing OD. Sealbore tolerance: ±0.002 in (±0.05 mm) honed bore ID to ensure reliable sealing across operating temperature range. Pressure ratings: Standard API 11D1 profiles are rated at 5,000 psi (34.5 MPa), 10,000 psi (69 MPa), and 15,000 psi (103.4 MPa) working pressure classes. SSSV setting depth requirement: API 11D1 and most national regulations require the safety valve nipple to be placed within 100 m (328 ft) of the wellhead, measured along the wellbore. Minimum nipple-to-nipple spacing: Typically 2 m (6.5 ft) to prevent tool interference between adjacent locked mandrels. H2S sour service limit: NACE MR0175 / ISO 15156 limits nipple steel hardness to 22 HRC maximum for wetted surfaces in sour environments. API profiles: X (selective), XN (no-go), R (selective), RN (no-go) are the four standard locking profiles defined in API 11D1.

The process of disassembling well-control or pressure-control equipment on the wellhead. Depending on the configuration of the wellhead and casing strings, it may be necessary to nipple-down and nipple-up the blowout preventer (BOP) system as each casing string is run.

The process of assembling well-control or pressure-control equipment on the wellhead.

A multiphase fluid incorporating a liquid base and gaseous nitrogen. Nitrified fluids are frequently used in stimulation treatments to enhance the performance of the treatment fluid and improve the cleanup process following the treatment.

A column of high-pressure nitrogen typically applied to a tubing string in preparation for drillstem testing or perforating operations in which the reservoir formation is to be opened to the tubing string. The nitrogen cushion allows a precise pressure differential to be applied before opening flow from the reservoir. Once flow begins, the nitrogen cushion pressure can be easily and safely bled down to flow formation fluids under a high degree of control.

A process whereby nitrogen gas is injected into an oil reservoir to increase the oil recovery factor. Below the minimum miscibility pressure (MMP), this is an immiscible process in which recovery is increased by oil swelling, viscosity reduction and limited crude-oil vaporization. Above the MMP, nitrogen injection is a miscible vaporizing drive. Miscibility of nitrogen can be achieved only with light oils that are at high pressures; therefore, the miscible method is suitable only in deep reservoirs.

(noun) A well activation technique in which nitrogen gas is injected into the tubing or annulus to lighten the hydrostatic column and reduce bottomhole pressure, enabling reservoir fluids to flow to the surface. Nitrogen kickoff is commonly used to unload kill fluid or completion fluid after workover operations.

The use of nitrogen gas circulated into the production conduit to displace liquids and reduce the hydrostatic pressure created by the fluid column. Nitrogen lifting is a common technique used to initiate production on a well following workover or overbalanced completion. A coiled tubing string is generally used to apply the treatment, which involves running to depth while pumping high-pressure nitrogen gas. Once the kill-fluid column is unloaded and the well is capable of natural flow, the coiled tubing string is removed and the well is prepared for production.

The injection of nitrogen into the fluid column within the production conduit to initiate fluid flow from the wellbore and production from the reservoir. Nitrogen lifting through a coiled tubing string is a common technique used in well kickoff.

A high-pressure pump or compressor unit capable of delivering high-purity nitrogen gas for use in oil or gas wells. Two basic types of unit are commonly available: a nitrogen converter unit that pumps liquid nitrogen at high pressure through a heat exchanger or converter to deliver high-pressure gas at ambient temperature, and a nitrogen generator unit that compresses and separates air to provide a supply of high-pressure nitrogen gas.

A nipple that incorporates a reduced diameter internal profile that provides a positive indication of seating by preventing the tool or device to be set from passing through the nipple. In many completions, a no-go landing nipple is preferred for the deepest nipple location, providing a no-go barrier to protect against a tool string being run or dropped below the tubing string.

Anything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy, such as shot generation ground roll, surface waves, multiples, effects of weather and human activity, or random occurrences in the Earth. Noise can be minimized by using source and receiver arrays, generating minimal noise during acquisition and by filtering and stacking data during processing.

A record of the sound measured at different positions in the borehole. Since fluid turbulence generates sound, high noise amplitudes indicate locations of greater turbulence such as leaks, channels and perforations. Noise logging is used primarily for channel detection, but has also been used to measure flow rates, identify open perforations, detect sandproduction and locate gas-liquid interfaces. The log may be either a continuous record against depth or a series of stationary readings. The log may indicate the total signal over all frequencies, the signal at a single frequency, or consist of a set of logs for different frequency ranges. Different frequency ranges can be tied to different sources of noise or different flow regimes.Although first introduced around 1955, the technique was not used commercially until after laboratory studies in the early 1970s.

A classification of filter used in the cleaning and treatment of brines and solids-free fluids. Nominal filters trap or remove most particles of equal or larger size than the given filter specification.

Fluid flow that deviates from Darcy's law, which assumes laminar flow in the formation. Non-Darcy flow is typically observed in high-rate gas wells when the flow converging to the wellbore reaches flow velocities exceeding the Reynolds number for laminar or Darcy flow, and results in turbulent flow. Since most of the turbulent flow takes place near the wellbore in producing formations, the effect of non-Darcy flow is a rate-dependent skin effect.

A fluid whose viscosity is not constant at all shear rates and does not behave like a Newtonian fluid. Most successful drilling fluids are non-Newtonian. Within that group are several general types and rheological mathematical models to describe them. Pseudoplastic is a general type of shear-thinning, non-Newtonian behavior that is desirable for drilling fluids. Bingham plastic and power-law models describe a psuedoplastic behavior using only two measurements (two parameters). The Herschel-Bulkley model is a three-parameter rheological model.

A mud that does not conduct electricity sufficiently well to allow spontaneous potential (SP) logging or resistivity logging. Oil- and synthetic-base muds are nonconductive drilling fluids. Water muds are not in this category.

A geological surface that separates younger overlying sedimentarystrata from eroded igneous or metamorphic rocks and represents a large gap in the geologic record.

Contaminants such as hydrogen sulfide [H2S], carbon dioxide [CO2], nitrogen [N2], and water, which are commonly associated with oil and gas production.

A gas described by an equation of state of the form pV = znRT, where z is the gas deviation factor dependent on pressure, temperature and gas composition.

A percentage share of production, or the value derived from production, which is free of the costs of drilling and producing, created by the lessor or royalty owner and borne by the lessor or royalty owner out of the lessor royalty. This royalty is paid to nonparticipating interest holders who do not share or participate in bonus or rentals, or a right to explore, or a right to execute oil and gas leases.

Ownership in a share of production, paid to an owner who does not share in the right to explore or develop a lease, or receive bonus or rental payments. It is free of the cost of production, and is deducted from the royalty interest.

Referring to a type of conventional electrical log in which the current emitting electrode (A) and the measure electrode (M) are placed close together on the sonde, and the current return electrode (B) and the measure reference electrode (N) far away. The response is determined mainly by the distance between A and M. The larger AM, the deeper the measurement, but the poorer the bed boundary response. Although many distances have been used, the most common are 16 in. [40 cm], known as the short normal, and 64 in. [162 cm], known as the long normal.

A type of fault in which the hanging wall moves down relative to the footwall, and the fault surface dips steeply, commonly from 50o to 90o. Groups of normal faults can produce horst and graben topography, or a series of relatively high- and low-standing fault blocks, as seen in areas where the crust is rifting or being pulled apart by plate tectonic activity. A growth fault is a type of normal fault that forms during sedimentation and typically has thicker strata on the downthrown hanging wall than the footwall.

The case in which a wavefront is parallel to an interface and its raypath is perpendicular, or normal, to the interface as the wave impinges upon the interface.

A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Normal modes are propagated as reflections off the borehole wall, and exist only in hard rock. They are highly dispersive, starting with the formationshear velocity at a certain cutoff frequency and decreasing at high frequencies to the borehole fluid velocity. Below the cutoff frequency, they do not exist. Normal mode #0 is often considered to be the tube wave and starts at zero frequency. Normal mode #1 is called the pseudoRayleigh, and starts at around 5 kHz. The other normal modes start at increasingly higher frequencies.

The procedure in seismic processing that compensates for the effects of the separation between seismic sources and receivers in the case of a horizontal reflector.

The pore pressure of rocks that is considered normal in areas in which the change in pressure per unit of depth is equivalent to hydrostatic pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 pounds per square inch per foot (psi/ft), or 9.792 kilopascals per meter (kPa/m), and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m.

A function of time and offset that can be used in seismic processing to compensate for the effects of normal moveout, or the delay in reflectionarrival times when geophones and shotpoints are offset from each other.

A unit of concentration for solutions of reagent chemicals used in testing mud chemistry. Normality provides a simple relationship between the volume in cm3 of reagent added during a titration and the chemical equivalents of a material with which the reagent reacts. A one-normal (1N) solution contains the equivalent weight in grams dissolved in one liter of solution.

A device for measuring the density of fluids in a completed well, using a radioactive source of gamma rays and a detector. In most instruments, a 137Cs (cesium) or 241Am (americium) source is used to induce Compton scattering, as in the openholedensity measurement, except that the device is unfocused. The count rate at the detector then depends primarily on the density of the fluids in the well. In some devices, the fluids pass through an open space in the body of the tool within which the measurement is made. The results then reflect the density of the fluids passing through the tool. In other devices, the source and detector are isolated so that the gamma rays pass outside the tool. The results then reflect some average density of all the fluids within the well. In smaller casings, some formationsignal may contaminate the measurement.Compared with a gradiomanometer, the nuclear fluid densimeter is a less direct measurement of density, and has a statistical uncertainty and less resolution. On the other hand, it is not affected by well deviation, friction or kinetic effects.

Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible.

A measurement of the nuclear magnetic resonance (NMR) properties of hydrogen in the formation. There are two phases to the measurement: polarization and acquisition. First, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). This polarization takes a characteristic time T1. Second, the hydrogen atoms are tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The frequency of oscillation is the Larmor frequency. The precession of the hydrogen atoms induces a signal in the antenna. The decay of this signal with time is caused by transverse relaxation and is measured by the CPMG pulse sequence. The decay is the sum of different decay times, called T2. The T2distribution is the basic output of a NMR measurement.The NMR measurement made by both a laboratory instrument and a logging tool follow the same principles very closely. An important feature of the NMR measurement is the time needed to acquire it. In the laboratory, time presents no difficulty. In a log, there is a trade-off between the time needed for polarization and acquisition, logging speed and frequency of sampling. The longer the polarization and acquisition, the more complete the measurement. However, the longer times require either lower logging speed or less frequent samples.

Mathematical methods that require iterative processing of data rather than applying deterministic equations. Some relationships can be solved only by numerical methods, including most integration problems, some differentials and some statistical processes.

A rendering of a model of a reservoir or field in entirely numerical formats. Numerical models, once built, may be used to perform many mathematical operations, including calculations of available reserves and simulations of the behavior of the reservoir.

The mathematical simulation of a numerical model of a reservoir's petrophysical characteristics to analyze and predict fluid behavior in the reservoir over time.