Neutron Well Logging Elastic Scattering Cross Section Interpretation

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Solution Overview

Problem

Current neutron well logging techniques, such as thermal neutron die-away and neutron porosity measurements, face limitations in accurately determining petrophysical properties of subsurface formations due to sensitivity to hydrogen content, temperature, borehole salinity, and complex lithologies, and are not effective for gas-filled porosity or shale formations.

Innovation Solution

The method involves detecting radiation events from imparting neutrons into the formation at an energy level of at least 1 MeV and determining petrophysical properties using the elastic scattering cross section, which is related to the number of detected radiation events, allowing for more accurate interpretation of neutron porosity measurements by focusing on the elastic scattering cross section rather than inelastic scattering or capture cross sections.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If thermal neutron die-away measurement is used to determine formation thermal neutron capture cross section, then the measurement is simple and direct, but the measurement is sensitive to temperature and borehole salinity

Engineering Contradiction:

Improvemeasurement simplicityVSAvoidmeasurement accuracy under varying conditions

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The patent changes the measurement parameter from thermal neutron capture cross section to elastic scattering cross section by using fast neutrons (1-10 MeV) instead of thermal neutrons. This parameter change makes the measurement less sensitive to temperature and salinity variations while maintaining operational simplicity

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

Instead of measuring thermal neutron capture cross section directly (conventional approach), the patent inverts the approach by measuring fast neutron elastic scattering cross section and using it to determine formation properties. This inversion bypasses the sensitivity issues of thermal neutron measurements

Inventive Principle:

Principle #13The other way round (Inversion)

2Measurement precision

If neutron porosity measurement is used to evaluate hydrogen content, then the measurement is sensitive to water-filled porosity, but it is not accurate for gas-filled porosity or shale formations

Engineering Contradiction:

Improvewater-filled porosity accuracyVSAvoidapplicability to different formation types

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent makes the neutron measurement universally applicable to different formation types (water-filled, gas-filled, shale) by using elastic scattering cross section as the basis for interpretation. The elastic scattering cross section can be related to various formation properties depending on the specific measurement and interpretation method used

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent changes from using thermal neutron capture cross section (sigma) to using fast neutron elastic scattering cross section as the primary measurement parameter. This parameter change enables accurate interpretation for gas-filled and shale formations where traditional thermal neutron methods fail

Inventive Principle:

Principle #35Parameter changes

3Measurement precision

If traditional neutron porosity measurement is used, then the measurement works well for clean single lithology formations, but it requires special treatment for complex lithologies and shale

Engineering Contradiction:

Improveporosity measurement accuracyVSAvoidinterpretation complexity for different formations

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent changes the measurement parameter to fast neutron elastic scattering cross section, which provides a more universal basis for interpretation across different lithologies. This reduces the need for formation-specific calibration and interpretation procedures

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach enables more accurate and reliable determination of petrophysical properties, including porosity and fluid saturation, by leveraging the correlation between elastic scattering cross sections and moderating power, which is less affected by temperature and salinity, and can differentiate between water-filled and gas-filled porosities.

Implementation Method 1

They can be scattered elastically, which means kinetic energy and momentum are conserved

Methodology Applied

Scientific EffectElastic scattering: Scattering

Implementation Method 2

Slowing-Down Length (Ls) is a parameter that describes how far a fast neutron travels on average before it is slowed down to thermal energy

Methodology Applied

Scientific EffectNeutron moderation: Scattering

Implementation Method 3

they can also be captured by a nucleus to form a new nucleus

Methodology Applied

Scientific EffectNeutron capture: Absorption (physical)

Data Source

PatentUS10012756B2Method for using neutron interaction cross section to interpret neutron measurements

Publication Date: 2018.07.03 SCHLUMBERGER TECH CORP

AI summary

A method for determining a petrophysical property of a formation includes detecting radiation events resulting from imparting neutrons into the formation at an energy level of at least 1 MeV. The petrophysical property is determined from an elastic scattering cross section of the formation. The elastic scattering cross section is related to a number of detected radiation events.