7.4 Nuclear logging methods

Nuclear logging methods are crucial tools in geophysical well logging. They measure formation properties by analyzing radiation interactions with subsurface materials. These techniques, including gamma ray, neutron, and density logging, provide vital data on lithology, porosity, and fluid content.

Understanding nuclear logging is essential for accurate subsurface characterization. By interpreting gamma ray, neutron, and density logs together, geophysicists can identify potential hydrocarbon-bearing zones, determine formation properties, and make informed decisions about reservoir potential and production strategies.

Nuclear Logging Methods

Types and Applications

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• Nuclear logging methods utilize the interaction of radiation with matter to measure various properties of subsurface formations
  • The main types of nuclear logging are gamma ray, neutron, and density logging
• Gamma ray logging measures the natural radioactivity of formations
  • Primarily used for lithology identification and stratigraphic correlation
• Neutron and density logging involve the use of artificial radiation sources to measure formation properties
  • Used to determine porosity and fluid content in formations
• The choice of nuclear logging method depends on:
  • The specific formation properties being investigated
  • The desired information about the subsurface

Principles and Interactions

• Nuclear logging methods are based on the interaction of radiation with matter
  • Gamma rays interact with electrons in the formation through photoelectric absorption, Compton scattering, and pair production
  • Neutrons interact with the formation primarily through elastic collisions with hydrogen atoms
• The measured radiation response is influenced by various factors, such as:
  • Formation lithology and mineralogy
  • Porosity and fluid content
  • Borehole conditions (diameter, fluid type, and casing)
• Proper calibration and environmental corrections are essential for accurate interpretation of nuclear log data

Gamma Ray Logging for Lithology

Measurement Principles

• Gamma ray logging measures the natural gamma radiation emitted by radioactive isotopes present in the formation
  • Primary radioactive isotopes: potassium-40, uranium, and thorium
• The gamma ray tool contains a scintillation detector that counts the number of gamma rays emitted by the formation over a specified time interval
  • Typically measured in API (American Petroleum Institute) units
• The gamma ray response is influenced by the concentration of radioactive elements in the formation
  • Shale formations tend to have higher gamma ray readings due to their higher content of radioactive elements
  • Sandstone and carbonate formations typically have lower gamma ray readings

Lithology Identification and Stratigraphic Correlation

• Gamma ray logs are used to distinguish between shale and non-shale formations
  • Allows for lithology identification and mapping of shale intervals
• Gamma ray logs are often used as a reference log for:
  • Depth matching other well logs
  • Identifying formation boundaries and marker beds
  • Stratigraphic correlation between wells
• Example: In a clastic sedimentary sequence, gamma ray logs can help identify:
  • Sand-shale alternations (low-high gamma ray readings)
  • Unconformities and sequence boundaries (abrupt changes in gamma ray response)

Neutron Logging for Porosity

Measurement Principles

• Neutron logging involves the use of a neutron source, typically americium-beryllium (Am-Be), which emits high-energy neutrons into the formation
• Neutrons interact with the formation primarily through elastic collisions with hydrogen atoms
  • Neutrons lose energy until they are captured by the nuclei of atoms in the formation
• The neutron tool measures the count rate of either:
  • Slowed-down neutrons (neutron-neutron logging)
  • Gamma rays emitted by the nuclei that captured the neutrons (neutron-gamma logging)
• The neutron log response is primarily influenced by the hydrogen index (HI) of the formation
  • Related to the amount of hydrogen present in the pore spaces

Porosity and Fluid Content Determination

• In clean, water-filled formations, the neutron log provides a measure of the formation porosity
  • Higher neutron porosity values indicate higher hydrogen content and, thus, higher porosity
• The presence of hydrocarbons or gas can affect the neutron log response
  • Hydrocarbons and gas have lower hydrogen content compared to water
  • May result in lower neutron porosity readings compared to the actual formation porosity
• Neutron logs are often used in combination with density logs to:
  • Determine the formation porosity
  • Identify the presence of gas or light hydrocarbons in the pore spaces
• Example: In a gas-bearing sandstone reservoir, the neutron log may show lower porosity values compared to the density log, indicating the presence of gas

Density Logging for Formation Properties

Measurement Principles

• Density logging utilizes a gamma ray source, typically cesium-137 (Cs-137), to emit gamma rays into the formation
  • Gamma rays interact with the electrons in the formation through Compton scattering
• The density tool measures the count rate of the scattered gamma rays that reach the detectors
  • The count rate is related to the electron density of the formation
• The electron density is closely related to the bulk density of the formation
  • Most rock-forming elements have a similar number of electrons per unit mass
• The bulk density of the formation is influenced by:
  • Matrix density
  • Porosity
  • Density of the fluids in the pore spaces

Porosity and Lithology Determination

• In combination with the matrix density (known or assumed), the bulk density measurement can be used to calculate the formation porosity
  • Lower bulk density values indicate higher porosity
• Density logs are often used in conjunction with neutron logs to:
  • Determine the formation porosity
  • Identify the presence of gas or light hydrocarbons (lower densities compared to water or oil)
• Density logs can also provide information about the formation lithology
  • Different rock types have characteristic density ranges (e.g., sandstone: 2.2-2.7 g/cm³, limestone: 2.6-2.8 g/cm³)
• Example: In a carbonate reservoir, density logs can help distinguish between:
  • Dense, low-porosity limestone intervals
  • Porous, high-porosity dolomite intervals

Interpreting Nuclear Log Data

Formation Characterization

• Nuclear logs provide valuable information about the subsurface formations, including:
  • Lithology
  • Porosity
  • Fluid content
• This information can be used to characterize the reservoir properties and assess the storage capacity
• Gamma ray logs are used to identify shale and non-shale formations
  • Allows for the delineation of potential reservoir rocks (sandstone or carbonate intervals)
• Neutron and density logs are used together to determine the formation porosity
  • Porosity is a critical parameter in assessing the storage capacity of a reservoir

Hydrocarbon Identification

• The presence of gas or light hydrocarbons can be inferred from the separation between the neutron and density porosity measurements
  • Known as the "gas effect"
• Zones with high porosity, low gamma ray readings, and a significant separation between neutron and density porosities are often indicative of potential hydrocarbon-bearing intervals
• Example: In a sandstone reservoir, a zone with low gamma ray values (clean sandstone), high neutron porosity, and low density porosity may indicate the presence of gas

Integration with Other Data

• The interpretation of nuclear logs should be integrated with other well log data, such as:
  • Resistivity logs
  • Sonic logs
  • Geological and seismic data
• This integration helps develop a comprehensive understanding of the subsurface formations and identify the most promising hydrocarbon-bearing zones
• Example: A potential hydrocarbon-bearing zone identified from nuclear logs should be corroborated with high resistivity values (indicating hydrocarbons) and consistent seismic reflections (indicating a continuous reservoir)