Neutron technology

A neutron log is a fundamental well-logging tool used in petroleum engineering to measure the porosity of rock formations (phi). It works by bombarding the formation with neutrons and measuring the hydrogen concentration, which is directly related to the amount of fluid-filled pore space.

Salinity: High salinity water has fewer hydrogen atoms than fresh water, which can slightly affect the reading.
Temperature & Pressure: High-pressure gas can increase hydrogen density, making the "gas effect" less obvious.
Borehole Condition: If the borehole is "washed out" (too large), the tool might measure the drilling mud instead of the formation, leading to inaccurate data.
Shaliness: As mentioned, clays have high hydrogen content due to chemical structure, leading to an overestimation of effective porosity.
 

In modern practice, the Neutron Log is rarely used alone. It is the "partnership" between the Neutron and Densitylogs that provides the most accurate calculation of true porosity and fluid type.  

Pulsed Neutron Logging is an advanced and increasingly important application. Unlike standard neutron tools, a pulsed neutron generator can be turned on and off, emitting neutrons in bursts. This allows it to work through steel casing and cement, making it invaluable for evaluating formations long after a well has been cased and completed. Pulsed neutron logs have two primary operating modes:  

Sigma (σ) Mode: Measures the thermal neutron capture cross-section of the formation. Since chlorine (in saline formation water) has a high "sigma," this mode is excellent for determining water saturation in formations where the water is salty. It can also be used to detect gas and estimate gas pressure.

Carbon/Oxygen (C/O) Mode: This mode uses gamma-ray spectroscopy to directly measure the ratio of carbon to oxygen atoms. This is a critical advantage in environments where water salinity is unknown or variable, as it can distinguish oil from water independently of salt content.  

Modern tools use at least two detectors (near and far) to measure the neutron population over time.

• Epithermal Neutrons (The Fast Phase): Measured immediately after the burst. These neutrons are still slowing down and are very sensitive to hydrogen density (porosity). This is the modern equivalent of the traditional porosity measurement, but it's less affected by formation salinity because the neutrons haven't been captured yet.
• Thermal Neutrons (The Capture Phase): Measured after the neutrons have slowed down (a few hundred microseconds later). These thermal neutrons are captured by atomic nuclei, and the rate of this capture is primarily controlled by the thermal neutron capture cross-section, called Sigma (Σ) .

Modern pulsed neutron services also offer additional capabilities:

• Water Flow Logging (WFL): Identifies water movement and entry points behind the casing, which is crucial for diagnosing water production problems in mature fields.
• Gravel Pack Evaluation: Can assess the integrity of sand-control completions by analyzing the activation of elements like silicon, helping operators identify if a gravel pack has deteriorated.
• Geochemical Analysis: Some tools measure natural spectral gamma rays (Potassium, Uranium, Thorium) to enable detailed clay typing and provide a more complete geochemical picture of the reservoir.
   
• Identifying Bypassed Pay: One of the most valuable uses of pulsed neutron logs is finding oil or gas zones that were missed during initial drilling. This is essential for planning workovers and extending the economic life of mature fields.
   
• Monitoring Reservoir Changes Over Time: By running pulsed neutron logs at different times (time-lapse monitoring), engineers can track how fluid saturations change as the reservoir is produced, observing the movement of contacts between oil, water, and gas. This data is key for making informed reservoir management decisions.

Density tools measured at lower gamma-ray energies, this helps identify lithology (e. g., sandstone vs. limestone vs. dolomite).                                           

The Photoelectric Factor (PEF) log is an essential tool in formation evaluation, primarily used to determine the mineralogy or lithology of reservoir rocks. It is often referred to as the "lithology log" because it is largely unaffected by porosity or the fluids within the rock, making it very effective at distinguishing between different matrix types. Here is a detailed breakdown of the PEF log, from its fundamental physics to its practical applications.

In petroleum engineering, you will most frequently use the PEF log in conjunction with the Neutron-Density crossplot to resolve lithology ambiguities.

• If the Neutron-Density separation suggests a lithology, the PEF confirms it.
• To identify a pay zone: First, use the PEF to confirm the rock is a porous sandstone (Pe ~1.8) or a dolomite (Pe ~3.1). Then, use resistivity logs to calculate water saturation.
• To spot a warning sign: A PEF reading that is too high for the suspected lithology suggests the presence of heavy minerals, a barite mudcake, or a complex mineralogy that standard porosity models cannot handle.

The primary use of the PEF log is lithology determination, which is critical for calculating porosity and water saturation accurately.

Differentiating Sandstone from Carbonate: This is the most common application. In a mixed clastic-carbonate environment, the gamma ray log might be unreliable (e.g., radioactive sandstones or clean carbonates). The PEF log clearly delineates the low-PEF sandstone from the high-PEF limestone.
Identifying Complex Lithologies: The PEF is crucial for identifying evaporites (like anhydrite) or specific mineral cements within a reservoir. For example, calcite cement in a sandstone will raise the PEF above the expected value of ~1.8, alerting the analyst to the presence of a mineral that reduces porosity.