March 2020 Wireline Workshop Recap
In previous issues of Wireline Workshop, the newsletter discussed the application of neutron logs in sedimentary and hard rock formations. In future issues, the myriad of applications of density logs in mineral exploration will be investigated. For the month of March, they dug deep into the theory of nuclear interactions that allow loggers to use radioactive sources for borehole investigations.
The following is a brief overview of some of the informative topics covered this this months’ Wireline Bulletin:
Neutron Borehole Interactions
In a neutron logging tool, a radioactive AmBe source bombards the target formation with high energy neutrons. These neutrons interact primarily with hydrogen due to their similar size. After enough collisions with hydrogen atoms, these neutrons lose energy and form a cloud where our tool’s detector is positioned at a set distance from the source.
The neutron detector counts thermalized neutrons (free neutrons with kinetic energy of 0.025 eV), and the count rate is primarily controlled by the concentration of hydrogen atoms in the formation. These counts lead to a calculation of the hydrogen/water content and formation porosity. But these results are only valid in sandstone, limestones, dolomites and other clay-free rocks, as clay will affect the hydrogen content.
The count rate can also be decreased by thermal absorption due to the presence of certain elements such as chlorine, manganese, and iron. In mineral logging, the thermal neutron-neutron log is fit for purpose and rarely modified by chlorine, as salty borehole fluid is the exception rather than the norm.
Density Logging Measurements
There are three types of interaction that result from gamma radiation and their probability of occurrence depends on the energy of the gamma rays and the target formation’s average atomic number. At high gamma ray energies, the absorption of a gamma rays near a nucleus results in the creation of an electron/positron pair, known as Pair-Production. However this interaction is not commonly used in well logging applications. At low energies (<100 keV) and for elements with relatively high atomic number, the photo-electric (PE) effect is dominant. This phenomenon is used to estimate rock type as well as gamma detection.
At mid-range energies (~660 keV) and low atomic numbers, Compton Scattering dominates. In this situation, no gamma rays are absorbed, and energy is transferred, like the neutron interaction previously described. Gamma rays emitted by a source interact with the formation and lose part of their energy. The flux in gamma rays from source to detector depends on the density of the electrons in the formation, which is proportional to bulk density.
The distance from source to detector is critical. if the detector is too close, the count rate will be too low, but as we increase the source detector spacing, the count rate will increase up to a certain point then begin to decrease. The ideal detector placement is such that the range of expected densities result in count rates that fall on the slope, not too near the peak.
Although use of gamma ray count moderation by Compton Scattering works well to estimate density, these measurements are not perfectly accurate for two reasons. The first is resolution. At 15 cm source/detector spacing, any 1 cm data point is effectively an average of 15 cm of the formation, and the log can look smeared at boundaries.
The second is the ratio of electron density to bulk density (Z/A), controlled by formation chemistry. For most rock building elements, this ratio is proportional, but there are two exceptions: iron and hydrogen. Iron has a lower Z/A ratio due to its large nucleus and extra neutrons to maintain stability, whereas hydrogen has a higher Z/A ratio since its nucleus has 1 proton and zero neutrons. When logging either iron ore or water-logged porous formations, straight-line corrections need to be applied to account for these chemical variations.
Where Can You Reduce Source/Detector Spacing? A Real World Experiment
Depending on the logging environment, it may be necessary to reduce the source/detector spacing in order to increase resolution. One such example took place in South Africa where the Ventersdorp Contact Reef is mined. Rock bursts are a major hazard that can occur when the hanging wall, composed of Ventersdorp lava, is accidentally drilled or blasted. This boundary is not always obvious because instead of being straight, it rolls in waves.
High resolution density measurements can aid in reef-lava boundary identification in short 2-3 meter length holes. The idea was that an ultra-slim density sonde pushed into jumper holes could activate a red light if the density or count rate crossed a calibrated threshold, thus identifying the reef-lava boundary. Given the small tool diameter and high density of the formation, the source-detector spacing was reduced from the standard 15 cm to 8 cm.
The experiment was carried out in a series of jumper holes, one of which was extended 2 meters upwards to ensure at least one hole intersected the lava. A standard jumper hole was tested first and measured ~1200 counts/sec, and then the extended jumper hole was tested and measured at 1045 counts/sec in the last 40 cm of the hole. This drop clearly showed the boundary between the reef zone and the overhanging lava.
Although this technique was ultimately not adopted, this experiment served as an illustration of how logging tools and measurements could be modified to try novel approaches based on client needs.