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• May 4, 2019
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Pulsed Neutron Physics

This article is second part of article we published last week “Cased hole saturation evaluation” .This article provides brief overview of physics behind pulsed neutron logging.

Basic physics behind pulsed neutron logging can be divided into three parts as below:

1. Physics behind working of pulsed neutron generator
2. Life of neutron after getting released from generator
3. Pulsed neutron detectors (We will discuss it in next article of this series)

Discussion on physics behind pulsed neutron logging:

Physics behind working of pulsed neutron generator: Most of the pulsed neutron generators use 1 to 20 curies source for tritium (³H) atom and deuterium (²H) The generator contains two sections, First section on left side of diagram emits tritium atom which gets accelerated to almost 100 to 110Kev by using high voltage. In the second section, this accelerated tritium atom is made to collide with ionized deuterium atom.

This collision results into reaction which leads to emission of helium also called alpha particle and 14Mev neutron as per reaction below:

D + T = ⁴He + ¹n₀   @14Mev

Figure: Schematic diagram of Pulsed neutron generators

Amount of neutron generated depends on pulse rate of generator. A typical pulse rate of 100/sec generates 10⁸ neutrons per second. Generator offered by major vendors generates up to 3 * 10⁸ neutrons/second while generators from some other vendors generate only 1*10⁸ neutrons/second.

These generators are typically pulsed at different frequencies based on data acquisition requirement. As an example for inelastic data acquisition generator might be pulsed at 8 to 10 Khz frequency on the other hand for capture mode data acquisition even 500 Hz to 1Khz frequency is sufficient

There are certain advantages and disadvantages of using pulsed neutron generator over continuous chemical source as given below.

Advantages of pulsed neutron generator over chemical source:

1. Neutron emission of generator is typically three times the neutron emission from popular chemical source Americium Beryllium ( AmBe).
2. Generator doesn’t have any radioactivity when they are switched off, hence causing less radioactive dose to workers.
3. Tool can be used for versatile measurement as frequency of neutron generated can be regulated by gates and timing.
4. Less handling headaches as compared to chemical source.
5. Less difficult to license.
6. Easy shipping as compared to chemical source

Disadvantages of pulsed neutron generator over chemical source:

1. Limited generator life
2. Expensive as compared to chemical source.

Life of neutron after getting released from generator: Once 14 Mev neutron gets ejected from generator; it goes through two major interactions 1.It interacts with Borehole environment, 2.Interacts with formation. During this interaction neutron goes through three phases:

1. High energy inelastic collision: High energy inelastic collision happens in the energy range of 8 to 14mev during first few initial microseconds of neutron interaction with environment. During high energy inelastic collusion, kinetic energy of system is not conserved hence neutron interaction with nuclide in formation and borehole environment results into rebounding nuclide left into excited state; this nucleus de-excites itself by emitting gamma rays. Released gamma ray has characteristic energy level which depends on atomic number of element. Energy of released gamma ray is plotted against gamma ray intensity to obtain inelastic measurement. As mentioned in previous article this Gamma ray spectroscopy is used to obtain “Carbon-Oxygen ratio, Calcium-Silicon ratio, inelastic count rate and various inelastic elemental yields from both near and far detector .Other measurements are near to far inelastic count ratio, inelastic to capture ratio. Data from this mode is used for oil Saturation evaluation in saline formation water, fresh formation water or in unknown salinity formation water. Depth of investigation in inelastic C/O mode is 6 to 8 inches. To get data with high certainty it is recommended to do 3 to 5 passes at logging speed of 3 ft/min or less if possible in this mode.”
2. Elastic Collision: Elastic collision normally happens below 8 Mev; during elastic collusion kinetic energy of system is conserved. Elastic collision can go on for 1000 microseconds or more. As kinetic energy of system is conserved during elastic collision hence no gamma ray is released during this interaction. Neutron simply slows down during this time till it comes down to thermal level which is 0.025ev at room condition. No data is recorded during this time. How fast this phase will go depends upon amount of hydrogen available in formation and borehole as hydrogen has very high slowdown cross section.
3. Neutron capture decay time collision (Sigma measurement): Once neutron reaches thermal energy level, it gets captured during collision with nuclei. The rate of capture reaction depends upon amount of chlorine and hydrogen available in formation. Chlorine has very high capture cross section and hydrogen in environment has very high slowdown cross-section. As chlorine is present in salt, higher salinity of formation water means higher capture cross-section of formation water. Hydrogen index of Oil and water is very close and hence fresh water and oil has almost same sigma although sigma of oil varies a bit due to changes in hydrogen index as a result of change in gas oil ratio and oil specific gravity. As gas has lower hydrogen index than oil and water depending upon reservoir pressure, sigma recorded against gas is also lower than that of gas and water, how low is gas sigma is a function of reservoir pressure. Sigma of formation is measured as 4550 divided by tau, where tau is time taken in microseconds for gamma ray counts go down to 37% of its original value. Unit of sigma is capture unit (CU).

This article provides the brief overview of physics behind pulsed neutron logging. In the next part of series on Cased hole saturation evaluation, we will discuss about pulsed neutron detectors, environmental corrections and pulsed neutron log interpretation.

Pulsed neutron by itself is a vast subject but we are trying to summarize the subject for readers in very concise articles , we might have missed few parts or few diagrams. To discuss any part of this article in details author can be contacted at support@oilfieldknowledge.com.