native state core

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.

natural gas

A highly compressible, highly expansible mixture of hydrocarbons having a low specific gravity and occurring naturally in a gaseous form.

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.

neural networks

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.

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 of 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.

nipple

Any short piece of pipe, especially if threaded at both ends with male threads.

nipple-down

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.

nitrogen lifting

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 kick-off.

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.