Question 5 of 10: Types of Neutron Detectors

Question 5: Types of Neutron Detectors (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A neutron tool emits fast (4–14 MeV) neutrons from a chemical source, and those neutrons are slowed by elastic collisions — overwhelmingly with the hydrogen in the pore fluid — until they are captured. Neutron logging tools are classified by WHICH product of that slowing-down and capture sequence their detector responds to, and there are three:

• 1. Capture gamma-ray detectors. The detector responds not to neutrons at all but to the high-energy gamma rays emitted when a thermal neutron is finally captured by a nucleus in the formation or borehole (chlorine and hydrogen dominate). This is the oldest arrangement, used in the original GNT (gamma ray–neutron tool); the detector hardware is a scintillation crystal (NaI(Tl)) with a photomultiplier, or an early Geiger–Müller tube. Its weakness is that the response is contaminated by natural formation gamma radiation and is strongly salinity-dependent, because chlorine is such a powerful capturer.
• 2. Thermal-neutron detectors. The detector counts the neutrons themselves once they have been slowed to thermal energy (about 0.025 eV). This is the basis of the CNL/CNT compensated neutron log, which uses two thermal detectors at different source spacings and takes their count-rate RATIO so that most borehole effects cancel. The hardware is a gas-filled proportional counter — helium-3 (³He + n → ³H + ¹H) in modern tools, boron trifluoride using the ¹⁰B(n,α)⁷Li reaction in older ones. Thermal detectors give the highest count rates (hence the best statistics and fastest logging speeds) but remain sensitive to thermal-neutron absorbers such as chlorine, boron and gadolinium.
• 3. Epithermal-neutron detectors. The detector is wrapped in a thermal-neutron shield (cadmium or boron) so that it counts only neutrons still in the epithermal band (roughly 0.1–100 eV), i.e. BEFORE capture can occur. This is the basis of the SNP sidewall neutron porosity tool. Because epithermal slowing-down is controlled almost purely by hydrogen, the reading is nearly free of the salinity and absorber effects that bias the other two — at the cost of a much lower count rate and therefore poorer statistical precision.

The practical consequence for interpretation is that the three detector types do not give the same apparent porosity in the same rock. A capture-gamma or thermal-neutron reading rises with formation-water salinity (more chlorine, more capture) while an epithermal reading does not, so a shaly or saline zone logged with a thermal tool needs a larger environmental correction than the same zone logged with an SNP. The excavation-effect and matrix corrections applied in Question 7 are likewise tabulated per tool type, which is why a neutron porosity should never be quoted without stating the tool and matrix it was scaled to.