Neutron density logs are a crucial tool in formation evaluation, providing a direct measurement of total porosity in formations. The neutron log measures the hydrogen index (HI) of the formation, which is proportional to the number of thermal neutrons resulting from collisions with hydrogen. Neutron density 2 is a specific type of neutron log that measures the epithermal neutrons in the formation, allowing for deeper porosity measurements.
Understanding the Principles of Neutron Density 2
Neutron density 2 is a type of neutron log that measures the epithermal neutrons in the formation. Epithermal neutrons are fast neutrons that have lost some, but not all, of their energy after colliding with formation nuclei. These neutrons are more penetrating than thermal neutrons, allowing the tool to measure porosity deeper into the formation.
Neutron Source and Detectors
The technical specifications of neutron density 2 tools vary depending on the manufacturer and the specific model. However, they all share common components, such as a neutron source, detectors, and electronics for processing and recording the data.
The neutron source emits high-energy fast neutrons (on the order of 10^6 eV) from chemical or electronic sources. These neutrons collide with nuclei of formation materials, losing energy and eventually reaching a low energy state (0.025 eV), which are referred to as thermal neutrons.
The detectors in the neutron density 2 tool count the neutrons that have passed through the formation and attained thermal energy levels. The number of thermal neutrons resulting from collisions with hydrogen is proportional to the hydrogen index (HI) of the formation.
Measuring Hydrogen Index (HI)
The HI is derived from the ratio of counts from the two detectors in the neutron density 2 tool. Analysts then apply a lithology-dependent transform to convert the HI to neutron porosity.
The formula for calculating the HI is:
HI = (N_t – N_e) / N_e
Where:
– HI is the hydrogen index
– N_t is the count of thermal neutrons
– N_e is the count of epithermal neutrons
Lithology-Dependent Calibration
Neutron density 2 tools are calibrated to measure porosity in lithology-dependent units, such as limestone porosity units or sandstone porosity units. This is because the response of the tool to the formation is affected by the lithology and the hydrogen index of the formation.
By using lithology-dependent units, analysts can correct for these effects and obtain accurate porosity measurements. For example, the neutron porosity in limestone (NPHI_L) can be calculated as:
NPHI_L = a + b * HI
Where:
– a and b are lithology-dependent constants
Similarly, the neutron porosity in sandstone (NPHI_S) can be calculated as:
NPHI_S = c + d * HI
Where:
– c and d are lithology-dependent constants
Technical Specifications and Capabilities of Neutron Density 2
The technical specifications of neutron density 2 tools can vary depending on the manufacturer and the specific model. However, some common features and capabilities include:
Neutron Source
- Chemical sources: Americium-Beryllium (Am-Be) or Californium-252 (Cf-252)
- Electronic sources: Neutron generators using deuterium-tritium (D-T) fusion reaction
Detector Types
- Thermal neutron detectors: 3He or 10B-lined proportional counters
- Epithermal neutron detectors: 3He or 10B-lined proportional counters
Measurement Ranges
- Porosity range: 0 to 60% (lithology-dependent)
- Depth of investigation: 6 to 18 inches (depending on tool design and formation properties)
Vertical Resolution
- Typically 6 to 24 inches, depending on tool design and logging speed
Accuracy and Precision
- Porosity accuracy: ±2 to 4 porosity units (depending on lithology and other formation properties)
- Porosity precision: ±1 to 2 porosity units (depending on logging speed and other factors)
Environmental Corrections
- Borehole size and fluid
- Formation lithology and fluid
- Temperature and pressure
Applications
- Identification of fluid-filled porosity
- Determination of total porosity
- Differentiation between gas, oil, and water-bearing formations
- Lithology identification (in combination with other logs)
- Hydrocarbon exploration and production
Practical Considerations and Limitations
While neutron density 2 is a powerful tool for formation evaluation, there are some practical considerations and limitations that users should be aware of:
Depth of Investigation
The depth of investigation of the neutron density 2 tool is typically 6 to 18 inches, depending on the tool design and formation properties. This means that the tool may not be able to measure porosity in highly heterogeneous or fractured formations, where the porosity can vary significantly over short distances.
Lithology Dependence
The response of the neutron density 2 tool is highly dependent on the lithology of the formation. This means that the tool must be calibrated for the specific lithology of the formation being evaluated, which can be a time-consuming and complex process.
Environmental Corrections
The neutron density 2 tool is also affected by various environmental factors, such as borehole size and fluid, formation temperature and pressure, and the presence of other elements in the formation. Analysts must apply appropriate environmental corrections to the data to obtain accurate porosity measurements.
Interpretation Challenges
Interpreting the neutron density 2 log can be challenging, as the tool’s response is influenced by a variety of factors, including porosity, fluid type, and lithology. Analysts must have a good understanding of the principles of neutron logging and the specific characteristics of the formation being evaluated to accurately interpret the data.
Conclusion
Neutron density 2 is a powerful tool for formation evaluation, providing a direct measurement of total porosity in formations. By understanding the principles of neutron logging and the technical specifications of the neutron density 2 tool, physics students can effectively use this tool to identify fluid-filled porosity, determine total porosity, and differentiate between gas, oil, and water-bearing formations.
References
- Petrophysics MSc Course Notes, The Neutron Log, Dr. Paul Glover, University of Leeds, https://homepages.see.leeds.ac.uk/~earpwjg/PG_EN/CD%20Contents/GGL-66565%20Petrophysics%20English/Chapter%2015.PDF
- LithoTrak bulk density and neutron porosity | Baker Hughes, https://www.bakerhughes.com/evaluation/loggingwhiledrilling-services/lwd-formation-evaluation/lithotrak-bulk-density-and-neutron-porosity
- Measuring Porosity Downhole, SLB Oilfield Review, 2015-09-09, https://www.slb.com/resource-library/oilfield-review/defining-series/defining-porosity
- Neutron Logging: Principles and Applications, Schlumberger Oilfield Glossary, https://www.glossary.oilfield.slb.com/en/Terms/n/neutron_logging.aspx
- Neutron Porosity Logging, Halliburton, https://www.halliburton.com/en-US/ps/wireline-and-perforating/formation-evaluation/neutron-porosity-logging.html
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