There many types of neutron logging however thermal neutron porosity tools are perhaps the most widely utilized. This type of Neutron measurement is used primarily to calculate formation porosity when lithology is known. When combined with other
porosity logs, neutron measurements are also used for lithology identification, evaluation of shaly sands, and gas detection in both open and cased hole.
Neutron logs primarily measure hydrogen ion concentration and are, therefore, sensitive to fluid-filled pore space, but are also influenced strongly by clays.
Measurement Principle
Neutrons are electrically neutral particles, each having a mass almost identical to the mass of a hydrogen atom. High-energy (fast) neutrons are continuously emitted from a radioactive source which is mounted in the logging sonde. These neutrons collide with nuclei in the formation. With each collision, the neutron loses energy. The amount of energy lost per collision depends on the relative mass of the nucleus with which the neutron collides. The greatest energy loss occurs when the neutron strikes a nucleus of nearly equal mass. The most common nucleus of this type is that of hydrogen. Collisions with nuclei of dis-similar mass do not slow down the neutron very much. Thus, the slowing-down of neutrons depends primarily on the amount of hydrogen in the formation.
Within a few microseconds, the neutrons have been slowed by successive collisions to epithermal and thermal velocities, corresponding to energies of from 100 down to 0.025 electron volts. They then move (diffuse) randomly, without losing any more energy, until they are captured by the nuclei of receptive atoms such as chlorine, hydrogen, silicon, etc. The capturing nucleus emits a high energy gamma ray. Depending on the type of neutron logging tool, either the gamma rays of capture or the neutron concentrations themselves are counted by one or more detectors in the sonde. The traditional CNL log detects thermal neutrons, however, some newer CNL tools incorporate "dual porosity"
measurements which include both epithermal and thermal detectors.
The Compensated Neutron Tool (CNL)
The measurement configuration of the traditional compensated neutron tool is shown in the above figure. Fast (high energy) neutrons are produced by a source located near the bottom of the tool. The source consists of 16 curies of
americium housed in a beryllium container. The interaction of gamma rays emitted by the americium with the beryllium produces high energy neutrons which radiate into the formation. Two thermal neutron detectors are spaced about 30 and 60 cm. above the source. The ratio of count rates from these near and far detectors are measured and transformed into a value for formation
porosity. The ratio varies with porosity, but there is also significant influence from lithology because the matrix contributes to the slowing down and capture of the neutrons. Therefore, to accurately derive porosity from the near/far count ratio, lithology must be known. The ratio measurement reduces borehole effects, and increases depth of investigation relative to a single detector system.
The CNL tool is de-centralized by means of a bow spring, and standard tool diameter is 3-3/8." It must be run in liquid-filled boreholes, which may be either open or cased. The CNL may be combined with other tools including density, sonic, resistivity, caliper and gamma ray.
Applications of Neutron Porosity
Determine formation porosity in fluid-filled, open or cased boreholes
Identify lithology when combined with other porosity logs
Indicate formation gas
Calculate shale volume
Facilitate inter-well stratigraphic correlation
Borehole and Quality Considerations
If the tool is not eccentered properly, and the tool loses contact with the borehole wall, accuracy is severely affected and porosity reads too high . Hole caves (wash-outs) are the primary cause of tool eccentering problems.
Dry gas substantially reduces apparent porosity.
Porosity values sensitive to many borehole effects , many offsetting if corrects are applied ,all should be applied not just some.
Shale causes greatly increased apparent porosity.
Key Points
Neutron curve is presented in porosity units of some lithology (usually appropriate for the area, i.e. ss for Gulf of Mexico), most charts and many calculations require neutron values in limestone units.
Neutron logs respond to the amount of hydrogen in the formation. Thus, in clean formation, when pores are filled with oil or water, the log reflects porosity.
In the CNL tool, sandstone, limestone and dolomite produce different count rate ratios in rock of identical porosity, making it necessary to know lithology in order to obtain correct porosity.
The Compensated Neutron Log (CNL) was designed to reduce environmental effects inherent in neutron logging, improve performance in washed-out hole, and to be capable of running in combination with other logs.
When formation and borehole conditions differ from calibration conditions (as they almost always do), corrections must be applied to obtain accurate porosity values, however, these are generally small (one to two porosity units). All corrections to apparent neutron porosity are specific to tool type and service company.
The CNL has the deepest depth of investigation of all common neutron tools. At 22 porosity units, the tool sees about 10 inches into the formation (that is, 90% of the signal comes from 10 inches or less).
The depth of investigation decreases with increasing porosity (the opposite of the density log).
Vertical resolution varies with logging speed. At a logging speed of 1800 ft./hr. vertical resolution is about three feet, and may be improved to a maximum resolution of about 1-1/2 ft. by further reducing logging speed.
Neutron porosity is strongly affected (increased) by shale. Correction requires combining with other logs.
Displacement of water or oil by gas or steam will, in general, result in lower apparent measured neutron porosity.
Compensated neutron logs are normally run in combination with density and gamma ray tools.