How do you find the porosity of a neutron log?
How do you find the porosity of a neutron log?
To obtain porosity, read the value directly from the log. If the log is recorded in limestone units and the formation you wish to evaluate is sandstone or dolomite, then correct the log value by using the appropriate chart in a log interpretation chartbook.
How does neutron porosity log work?
The neutron log is sensitive mainly to the amount of hydrogen atoms in a formation. Its main use is in the determination of the porosity of a formation. The tool operates by bombarding the formation with high energy neutrons. The count rate of slow neutrons or capture gamma rays in the tool is therefore higher.
What is porosity logging?
The “Porosity” logs. There are three types of logging tools that are used to estimate the amount of pore space in a rock: the neutron, density, and acoustic velocity (or sonic) tool. So, the log is mainly a measure of hydrogen concentration (mostly contained by the pore fluids of the formation).
What is a compensated neutron log?
Compensated neutron log is a type of well log that is used to determine the porosity of each formation in a cased or open well and is adjusted for irregularities in the wellbore.
How do you find the porosity of a well log?
The most common method of determining porosity is with Well Logs….
- ϕsl is the porosity from the sonic log (log measurement) , fraction.
- Δtsl is value of the acoustic transit time measured by the sonic log, μsec/ft.
- Δtma is value of the acoustic transit time of the rock matrix measured in the laboratory, μsec/ft.
What type of log are used for porosity calculation?
The combination of density and neutron logs is now used commonly as a means to determine porosity that is largely free of lithology effects. Each individual log records an apparent porosity that is only true when the zone lithology matches that used by the logging engineer to scale the log.
Why is neutron porosity low in gas?
The neutron porosity log is strongly affected by clay and gas. Hydrogen occurs in clays and hydrated minerals as well as pore fluids. Gas has a low hydrogen density, so that gas zones have a very low apparent porosity.
What does the compensated neutron tool compensate for?
Neutron-scattering porosity tools A modern, compensated tool actually estimates the size of a cloud of neutrons around a source by measuring the ratio of count rates at two different distances from the source.
What is neutron density?
It is convenient to consider the neutron density, the number of neutrons existing in one cubic centimeter. The symbol n represents the neutron density with units of neutrons/cm3. The neutron flux density value is calculated as the neutron density (n) multiplied by neutron velocity (v).
Why is limestone porous?
Limestone is a sedimentary rock that forms on the sea floor where material rich in calcium carbonate accumulates. Because limestone is formed under the bottom of the sea it’s basically pressurised, which makes it super durable, and also porous.
What is well logging and its purpose?
A well log is a record of the formations and any events that are encountered in the well drilling process. The main purpose of keeping a well log is to determine the suitability of the well for its intended use. Well logs are also used by researchers to explore the underground.
How is oil porosity calculated?
Porosity (ɸ) = (Vp ÷ Vb) × 100
- Direct methods: measurement from core flush by fluid or air.
- Indirect methods: well logging tools (density, neutron, and sonic)
What are compensated neutron-porosity logging tools?
The now-standard compensated neutron-porosity logging (CNL) tool, in common use since the 1970s, is still a very simple tool. Like a density tool, it consists of an isotopic source (now most often americium-beryllium, although at least one tool uses an accelerator source) and two neutron detectors.
What types of neutrons should be counted in a porosity tool?
Ideally, a porosity tool would count only epithermal neutrons because the slower thermal neutron population depends as much on the absorption cross section of the formation as it does on the slowing-down length. To get count rates high enough for statistical accuracy, logging tools typically count all neutrons, epithermal and thermal.
Is there a linear mixing law for thermal neutron logs?
Because most logs encountered are thermal neutron logs, the rest of the discussion will center on thermal neutron interpretation. The linear mixing law implies that matrix and shale effects can be handled by a simple apparent porosity of a 0-p.u. mineral.
How accurate are logging tools for neutron capture?
To get count rates high enough for statistical accuracy, logging tools typically count all neutrons, epithermal and thermal. Thus, the tool reading must be corrected for the effects of neutron capture in the formation.
