Borehole geophysical logging employs different techniques in exploring detailed information of the subsurface. However when radiation technique is utilized this log is known as Radiation (nuclear) logging. Today I want to let you know in brief through this post about this type of borehole geophysical technique.
What does Radiometric Logging involve?
What is Radiometric Logging?
radiometric logging
Radiometric Logging, measures the total intensity of natural gamma radiation that presents in rocks and sediments.
Radiometric Logging is primarily based on the principle of Radioactivity as the part of nuclear physics and this is why it also known as Radiation (Nuclear) Logging
It is related to radioactivity of radionuclides such as Potassium (40K), Thorium (232Th) and Uranium (238U).
Basically radiation (radiometric) logging techniques are of two types, which are passive and active. Further more, these two (2) basic categories can be subdivided into various types. This post will describe the three (3) related types, as here below,
1. Natural Gamma Logging
This uses a gamma ray counter to measure the concentration rates of radionuclides present in natural materials. You can refer the post of how the radiation counter works to get more details regarding gamma ray counter.
Natural Gamma Logging, records total natural gamma radiation primarily from potassium (K-40), Uranium (U -238), and Thorium (Th-232) from a borehole that is within a selected energy range.
Regarding Gamma Spectrometry at which the amount and energy level of gamma photons are recorded either on a continuous basis or at selected depths with a stationary probe, types and amounts of radioisotopes can be measured
If the count rates is higher, means the higher concentration of radionuclides, which represent the fine grained deposits such as clay, example micaceous sand (see figure 1).
If the count rate is lower, it means the lower concentration of radionuclides, which represent the very coarse grained deposits (sand). You can see the figure 1 here below, to have a good insight.
Figure 1: Gamma ray Log showing Lithology, sand to shale based on content potassium (K) contents (Rider, 2002).
2. Density (Gamma - gamma ) Logging
Density Logging records the radiation at a detector from a gamma source in the probe after it is attenuated and backscattered in the borehole and surrounding rock.
When the gamma radiation from the source (Cs-137) is sent to formation and interacts with the electrons within it, the same as Compton scattering, (see figure 2). The backscattered electrons are sent to the receiver (detector) at which its density (electron density) can be obtained and then used to calculate the bulk density of the formation.
The high value of backscattered electrons (gamma rays intensity) indicates low electron density and hence low formation density
You can use the following expression to calculate the bulk density,
De = 2Bd (z/A)
Where Bd - Bulk density, De - electron density, A - Atomic number and z - is the atomic mass.
In most cases the ratio z/A is 1/2, except for some element like hydrogen at which this ratio is unit (1).
Bulk density is the density which includes minerals and water contents within pore spaces of a material.
Since the count rate of the backscattered gamma rays can be related to the electron density of the material that is in turn proportional to the bulk density of the material. It means that If we know the fluid and grain densities, then we can calculate the porosity (P) as,
P = (grain density - bulk density)/(grain density - fluid density).
Figure 2: Schematic diagram for density (gamma - gamma) Logging ( Schmitt et al. 2003)
The density logs can be useful for indicating Lithology, especially for either high-density ore minerals or low-density coal seams.
Figure 3: Simple representation of a density Log
In real practicality the depth (m/ft) must increase towards downwards from the ground surface.
3. Neutron Logging
Neutron Logging utilizes neutron probe source and detectors that record neutron interactions in the vicinity of the borehole.
Neutron Logs are frequently plotted in API units which are based on measurement in a standard neutron pit where 19% porosity water filled limestone is defined as 1,000 API neutron units. Another units is in Limestone Porosity which assumes that the matrix is limestone (Telford et al, 1990)
Neutron Logging can further be described as
(i) Neutron Activation (Thermal neutron)
Neutron Activation (Thermal neutron) uses neutrons to activate stable isotopes in the borehole and identify the activated element by measuring the amount and energy level of emissions using the same way as gamma spectrometry.
(ii) Neutron Lifetime (pulsed-neutron decay)
Neutron Lifetime (pulsed-neutron decay) uses a pulsed-neutron generator source at which a burst of high neutrons are emitted while the thermal energies are captured at a fixed interval of time where synchronously gated neutron detector are used to measure the rate of decrease (thermal decay) of neutron population.
Pulsed neutron decay Logs are used to determine the amount of chlorine in saline water percent.
Radioactive Sources
As we have discussed above that, active nuclear logging utilizes artificial sources of radiation. There are many radioactive source materials, however for gamma logging and neutron logging the common source elements which are frequently utilized are Caesium (Cs - 137), Beryllium (Be) and an alpha source such as Americium (Am) respectively. They are often utilized in combined state example Americium- Beryllium (AmBe) which has about 460 years half - Life with a 16 curie source produces about 4 × 107 neutrons/s
Applications of Radiation (Nuclear) Logging.
There are many applications regarding nuclear (radiation) Logging ,however some of useful and frequently uses are described here below,
Lithological indication
Nuclear Logs are used to indicate the types of subsurface rocks (lithology), for example density Log which is used to indicate Lithology based on their density such as high-density ore minerals and low-density coal seams.
Another example gamma ray Log which is used to distinguish between mudrocks which have high natural radioactivity due to presence of high potassium content and sandstones and limestone which have lower radioactivity due to presence of low potassium contents which in turn will give a sand : shale ratio parameter. as shown in the figure 1 above.
Note: Mudrocks are often termed as Shales for petrophysical purposes.
Porosity Indication
Porosity is the measure of presence of void spaces in the Formation (rock), Porosity is always given as a ratio of volume of voids spaces to that of total volume of the rock (formation). The Nuclear Log such as Neutron Log is most commonly utilized in determining the porosity of the Formation.
However in clays formation the neutron Log does not provide a reliable indicator of the porosity. In such cases the density Log is useful for indication of porosity using bulk density if the lithology of the Formation is known
Advantages of Radiation (Radioactivity) logging over electrical Logging.
- It may be made in cased holes filled with any fluid or even in dry holes.
- It can identify formation boundaries between units.
- It can correlate rock units in different boreholes. Example regional correlation.
- It can determine clay contents units
N.B: In radiation logging lowering radioactivity, leads the Log trace to deflect to the left, while increasing radioactivity leads the Log trace to deflect to the right at which each trace has a different interpretation depending on type of radiation logging applied.
Radiation (Radiometric) Logging can provide useful information about subsurfaces, However in these days the best way to use them is by combining them with other Logging tools so as to provide an integrated output that indicate important parameters such as sand: mud ratio, porosity, permeability and hydrocarbon saturation.
Remember that, you have to strictly adhere to safety and health rules when working with this technique, because artificial gamma rays are toxic and may lead you into harmful effects.
References
Ellis, D.V. (1990). Neutron and Gamma Ray Scattering Measurements for Subsurface Geochemistry Science 250 (October 5): 82-87. [gamma-gamma, gamma spectrometry, neutron]
Ellis, D.V and Singer, J.M (2007), Well Logging for Earth Scientists (2nd edition), Wilet, Chichester, 692 pp.
Markstrom, A. (1992). The Use of Natural Radiation Detection in Determining Clay Content of Complex Glacial Deposits, Ground Water Management 11:689-699 (Proc. of the 6th NOAC). [gamma log].
Rider, M.H (2002), The Geological interpretation of Well Logs (2nd edition), Whittles Publishing. Caithness.
Schlumberger, (1987), Log interpretation Principles / Applications, Houston, Schlumberger Educational Services.
Telford, W.M..Geldart, L..P, and Sheriff, R. E. (1990). Applied Geophysics, Cambridge: Cambridge University Press.
Thornhill, J.T. and B.G. Benefield. (1992). Detecting Water Flow Behind Pipe in Injection Wells. Kerr Environmental Research Laboratory, Ada, OK. [neutron lifetime log].
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