NEUTRON LOG BASICS
In the sidewall neutron log, fast neutrons are emitted and epithermal neutrons are detected, and related to porosity by an algorithm built into the surface recording equipment. The compensated neutron log measures thermal neutrons, but has two detectors so that borehole effects can be derived. Porosity can be displayed as percent or as a decimal fraction.
GRN and CNL log types can be run in air, oil, or mud filled open or cased holes. SNP logs were not calibrated for use in cased holes.
There are three basic neutron logging instruments with a chemical neutron source. Each instrument is classified according to the energy level of the detected particles. Fast neutrons have energies greater than 100 KeV; epithermal neutrons have energies above 0.025 eV up to 100 KeV; thermal neutrons have an energy of approximately 0.025 eV (at 25 C).
One neutron logging system produces a neutron-gamma (N – G) response, and is sensitive to capture gamma rays emitted upon absorption of thermal neutrons by nuclei in the rocks. This class of logging tool is represented by older, single detector equipment, commonly called gamma ray neutron (GRN) logs.
The emitted neutrons come from a radium – beryllium or an americium – beryllium source. Radium and americium are natural alpha particle emitters and the alphas eject fast neutrons from the beryllium. With its 433 years half life, the AmBe source output is considered very stable. Approximately 40 x 10^7 neutrons/sec at 4.5 MeV average energy are emitted by the source.
When the neutrons are sufficiently slowed down by collisions with the formation, they are captured by the nuclei and a high energy gamma ray of capture is emitted. The gamma ray count rate at the detector is inversely proportional to the hydrogen content of the formation, in a semi-logarithmic relationship. Borehole size and mud weight corrections and calibration to porosity was done by the log analyst. Detectors were Geiger-Mueller gamma ray counters.
The source to detector spacing on the older tools was 15.5 inches, giving good statistical accuracy. Longer 18.5” spacing tools became popular because they were more sensitive to porosity but suffered from higher statistical variations. These tools are obsolete and no longer available, but many thousands exist in well files waiting for the serious petrophysicist to use for finding bypassed oil and gas.
A second type of logging system responds primarily to epithermal neutrons and is referred to as a neutron-epithermal (N-EN) log or a Sidewall Epithermal Neutron Log (SNP). The neutron detector counts the slow epithermal neutron density, which is largely determined by the amount of intervening hydrogen between the source and detector. The detector is a lithium iodide crystal with suitable shielding to eliminate thermal neutrons.
Detector count rates from the SNP systems are converted by a computer to porosity units, on a sandstone, limestone, or dolomite scale, depending on the assumed or known mineralogy of the formation. There is little borehole effect because the tool is a sidewall pad device similar to the density log skid. However, it sees a relatively small volume of rock, and has been generally superceded by the compensated neutron log (CNL)..
The most commonly run neutron log today is the compensated neutron log. It is an eccentered dual detector log that can be run in both open and cased boreholes. This log measures the rate of decrease of neutron density with distance from a source and converts it to a calibrated apparent porosity value. The rate of decrease, represented by the ratio of the near to far count rates, is primarily due to the hydrogen content of the formation.
Most CNL tools are neutron – thermal neutron (N – N) tools but some have additional detectors for epithermal neutrons (N – EN) measurements.
Helium-3 detectors are used in the small diameter CNL instrument whereas the large diameter instrument utilizes lithium iodide crystals.
Variations from standard borehole conditions are compensated by means of the dual detector system. The corrected apparent porosity values are derived from the count rate ratio of near and far spaced detectors by a computer program. Additional environmental corrections may be required in hot, high salinity boreholes. The program also compensates for casing and cement thickness in cased hole situations.
The count rates from the two detectors can be displayed and are often used as gas detection indicators.
In clays, micas, and zeolites, the apparent CNL (thermal) porosity is consistently higher than the SNP (or CNL epithermal) porosity. As a result, the CNL thermal measurement shows higher porosities in shales than the SNP tool. The explanation for this effect is that the SNP tool responds only to the slowing down of the neutrons by hydrogen atoms, whereas the CNL measurement is also affected by the neutron capture process, since the tool measures both thermal and epithermal neutrons.
A fourth style of neutron log uses a particle accelerator to create the fast neutrons, instead of a chemical source. The accelerator forces deuterium and tritium collisions at high energy levels to produce the neutrons. This tool is derived from the concept of the pulsed neutron (thermal decay time) tool widely used in cased hole logging to measure reservoir properties. The tool is much safer to run because the neutron source can be turned off while handling the equipment at the surface, and there is much less paperwork if a tool is lost downhole.
Gamma Ray Neutron (GRN)
Sidewall Neutron Porosity Log (SNP)
Compensated Dual-Spacing Neutron Log (CNL)
Integrated Porosity Lithology Tool (IPL)
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