Quad Neutron Physics Principles

The Quad is a four detector Neutron tool that measures reservoir responses due to Neutron bombardment. Two detectors are Thermal Neutron detectors, but of critical importance, two are Neutron-Gamma detectors.

When bombarded with high energy Neutrons, reservoir rock and fluids not only create Thermal Neutrons, but also create numerous Gamma Rays (N-Gammas) from both Neutron capture and activation.

The Quad Neutron-Gamma detectors measure these responses, and the combination of Thermal Neutron and Neutron-Gamma measurements are very powerful in differentiating many reservoir parameters.

The dilemma for all downhole tool designs is to, not only measure environmental responses including reservoir affects, but to understand and separate/correct all the interfering responses into meaningful singular reservoir characteristics.

Untangling the information hidden in the combination of Quad detector responses requires several techniques, including detector balancing, statistical normalization, and Quad Litho Log Overlay (QLO) methods.  Without these techniques, the responses cannot be turned into singular reservoir parameters, and that is key to why the Quad works where many other tools, such as those that apply spectral techniques and/or decay techniques, may not.

 

To demonstrate the point, consider this simple example:  Let’s measure saturation with a sigma device.  Because sigma is a discrete measurement, saturation determination is critically dependent on somehow gaining an accurate current porosity and then further correcting sigma for a host of other estimated environmental effects, all with variable, and possibly dramatic effect.  Quite distinctly, the Quad uses the difference between a porosity log created from Thermal Neutrons and another created from Neutron Gamma’s.  Because these two measurements were generated from the same tool, with the same statistical variability, on the same pass, with balanced and complementary detectors, the difference is chiefly based on salinity, while almost all other effects are automatically normalized and usually do not need to be considered; both elegant and powerful.

For more information, our petro-physics team is always available.