Injection/Pulsed Neutron Log

An injection/pulsed neutron log (PNL) is a through-casing formation evaluation technique in which a downhole tool emits short bursts (pulses) of high-energy neutrons (14 MeV) from a pulsed neutron generator (PNG) into the formation through the casing, and measures the time-dependent gamma ray flux from neutron capture reactions — particularly the gamma ray activity immediately after each neutron burst (the capture spectrum) and the subsequent decay of thermal neutron population — providing measurements of formation and fluid properties including water saturation, gas identification, lithology, and casing corrosion that can be acquired through steel casing without perforating or removing the casing string.

Key Takeaways

The sigma (Σ) measurement from pulsed neutron logging is the formation macroscopic thermal neutron capture cross-section in capture units (c.u.), calculated from the slope of the thermal neutron population decay after each pulse — saline formation water has high Σ (50 to 120 c.u. depending on chlorinity) because chlorine has a very large thermal neutron capture cross-section, while hydrocarbons and fresh water have low Σ (10 to 30 c.u.), making Σ a direct indicator of brine saturation in the invaded formation and a powerful water saturation measurement in wells with known saline formation water.
Pulsed neutron logs are the primary surveillance tool for monitoring waterflood fronts and identifying fluid contacts behind casing in producing wells — by comparing repeat PNL surveys acquired at intervals during production to the initial baseline log, changes in Σ (increasing Σ where water has replaced oil, decreasing Σ where gas has replaced water) identify where waterflood water or gas has reached the formation, guiding perforation strategy, shut-off programs, and conformance control for improved recovery.
Carbon-oxygen (C/O) logging is a pulsed neutron technique that measures the ratio of inelastic scattering gamma rays from carbon versus oxygen nuclei in the formation — carbon is concentrated in oil (all hydrocarbons), while oxygen is concentrated in water, calcite, and silica (non-hydrocarbon phases) — providing an oil saturation indicator that is independent of formation water salinity, enabling hydrocarbon identification in fresh water formations or in CO2 EOR floods where Σ cannot distinguish between hydrocarbons and the non-saline phases present.
Casing inspection and casing corrosion monitoring is an additional application of pulsed neutron tools in which the iron content of the casing is quantified from the iron neutron capture gamma ray signature — thinning of the casing wall from corrosion reduces the iron gamma ray count rate, providing a corrosion indicator that is qualitative but useful for prioritizing casing inspection and remediation in older wells where casing integrity is a concern for continued production or well abandonment.
The depth of investigation of pulsed neutron logs is limited to approximately 20 to 40 cm into the formation from the borehole wall (shallow compared to resistivity logging), and the measurement is dominated by the borehole and casing annulus near the tool — cemented annulus (good cement) versus free-fluid annulus (poor cement or channels) significantly affects the PNL response and must be accounted for in interpretation; good cement provides better formation coupling while borehole fluid in a channeled annulus dilutes the formation signal with borehole fluid chemistry.

Fast Facts

Pulsed neutron logging was commercialized in the 1950s and 1960s as electronic neutron generators (compact particle accelerators using D-T fusion reactions to produce 14 MeV neutrons) became practical for downhole deployment. The original thermal decay time (TDT) tool from Schlumberger and the neutron lifetime log (NLL) from Dresser Atlas (later Baker Atlas, now Baker Hughes) were the first commercial PNL tools, and the technology has evolved through multiple generations to the current spectroscopy-capable tools (Schlumberger's TPHL and PNX, Baker Hughes' RCXTM, Halliburton's DSNT) that simultaneously measure Σ, C/O ratio, and geochemical elemental capture spectra. Modern PNL tools can be conveyed on wireline, slickline, or coiled tubing in producing wells under pressure (live well intervention) using lubricator or pressure control equipment, making them deployable without well killing or workover operations.

What Is an Injection/Pulsed Neutron Log?

After a well has been cased and cemented, the steel casing that protects the wellbore also blocks many of the conventional formation evaluation tools that require direct electrical contact with the formation or that operate in the open borehole. A conventional resistivity log cannot be run through steel casing because the highly conductive steel bypasses the formation current path. Conventional gamma ray and density logs can be run through casing but provide limited formation evaluation capability. Pulsed neutron logging was developed specifically to overcome this limitation — it uses high-energy neutron radiation that can penetrate casing steel, interact with formation atoms, and generate measurable gamma ray signals that carry information about formation composition and fluid content.

The pulsed neutron generator (PNG) in a downhole PNL tool is a miniature particle accelerator that fuses deuterium and tritium nuclei (D-T fusion) to produce 14 MeV neutrons in bursts of 10 to 100 microseconds. These fast neutrons emit from the tool, travel through the casing, cement, and into the formation where they slow down (moderate) through elastic collisions with light nuclei (hydrogen) and ultimately reach thermal energies where they are captured by various nuclei and emit characteristic gamma rays. The detectors in the PNL tool record the gamma ray flux at multiple time windows after each neutron burst, capturing both the prompt inelastic scattering gamma rays (from fast neutron interactions) and the delayed capture gamma rays (from thermal neutron capture).

The word "injection" in the term injection/pulsed neutron log refers to the tool's application for monitoring injection wells — water injection wells in waterflood operations and gas injection wells in pressure maintenance programs — where the injected fluid's distinctive Σ signature (high for saline water, low for fresh water or gas) allows the PNL to track whether injection fluid is entering the intended formation interval or bypassing through channels in the cement or into unintended formations. This surveillance application, combined with the broader production monitoring and behind-casing evaluation capabilities, makes PNL the most versatile cased-hole formation evaluation tool available.

PNL Applications in Production Monitoring

Waterflood monitoring using repeat PNL surveys is the highest-value commercial application of pulsed neutron logging. The Σ measurement tracks the salinity distribution in the formation around the wellbore — a zone filled with saline formation water has high Σ, a zone swept by fresh injection water or hydrocarbon has low Σ. By comparing Σ logs from the initial cased-hole baseline (before flood injection begins) to repeat surveys (at 6 to 24-month intervals during flood operations), the progress of the waterflood front can be tracked vertically and the water-swept volume estimated. Zones with high Σ that were low Σ at baseline have been flooded; zones still showing low Σ are either not swept or contain remaining oil — guiding decisions to perforate additional intervals, to squeeze cement channels that are bypassing injection fluid, or to convert production wells to injection in unflooded areas.

Gas identification from PNL provides detection of gas accumulation behind casing in both producer surveillance and in gas injection monitoring. Gas has very low Σ (1 to 5 c.u., near the pure formation water signal for fresh water) and distinctive C/O characteristics — gas-bearing intervals show low Σ combined with low C/O (low carbon from methane per unit volume) that is distinguishable from oil (low Σ but higher C/O from liquid hydrocarbon carbon content). In gas storage wells and gas injection schemes, PNL surveys confirm that injected gas has filled the intended storage interval and has not migrated into other formations.

The C/O log addresses the fundamental limitation of Σ-based interpretation: it cannot distinguish fresh water from hydrocarbons because both have low Σ. In freshwater formations or in CO2 EOR floods where CO2 displaces both oil and saline water, Σ cannot track oil saturation changes. C/O logging measures the inelastic gamma ray ratio from carbon (present in oil at high concentration — approximately 85% carbon by weight in most crude oils) versus oxygen (present in water, calcite, and quartz — formation matrix without oil). The C/O ratio increases as oil saturation increases, providing an oil saturation indicator that works in low-salinity or fresh water environments where Σ is insensitive to oil versus water differences.

PNL Across International Jurisdictions

Canada (AER / WCSB): WCSB waterflood surveillance programs in Pembina, Lloydminster, and other mature pool developments use repeat PNL surveys to monitor waterflood conformance in cased-hole producing and injection wells. AER surveillance requirements for approved waterflood schemes encourage operators to demonstrate injection fluid conformance, for which PNL is the practical tool when open-hole access is not available. WCSB heavy oil thermal recovery (SAGD, CSS) programs use PNL to evaluate residual oil saturation in the steam-swept zone — the steam chamber reduces Σ (steam replaces high-Σ brine) and the C/O signature changes as bitumen is mobilized, providing a behind-casing view of the SAGD process without perforating the thermal well casing.

United States (API / BSEE): Enhanced oil recovery surveillance in Permian Basin, Gulf Coast, and Midcontinent fields uses PNL to monitor CO2 EOR floods (C/O logging essential since CO2 displaces Σ sensitivity), polymer floods, and ASP floods where injection fluid conformance determines recovery efficiency. BSEE offshore well surveillance regulations for Gulf of Mexico development wells encourage cased-hole formation evaluation for well integrity and reservoir surveillance, with PNL being the primary cased-hole saturation monitoring tool for offshore producers. API RP 39 (Recommended Practices for Standard Calibration and Format for Nuclear Logs) provides calibration standards for pulsed neutron tools applicable in the US market.

Norway (Sodir / NORSOK): NCS field management programs at Ekofisk, Gullfaks, and Statfjord use PNL surveillance for waterflood monitoring and identification of casing integrity issues in mature fields where production optimization and eventual abandonment planning require knowledge of remaining saturation distribution behind casing. Sodir's production license reporting requirements include reservoir management activities including cased-hole saturation monitoring programs, with PNL data submitted to the Diskos national database as part of well intervention and workover records. NORSOK D-007 (Wireline Intervention) covers live well intervention operations applicable to PNL deployment in producing NCS wells.

Middle East (Saudi Aramco): Saudi Aramco deploys PNL on a massive scale for waterflood surveillance in Arab Formation producers — with hundreds of producers per major field and waterflood rates of millions of barrels per day, systematic PNL surveillance is essential for identifying wells ready for production optimization and conformance correction. Aramco's use of segmented completion technology (intelligent well completions with interval control valves) is complemented by PNL saturation monitoring to verify that the interval control valves are directing water injection into the correct intervals as the flood progresses. PNL survey data feeds into Aramco's integrated reservoir simulation models that guide the daily operational decisions for the world's largest oilfield production network.

Pulsed neutron log (PNL) is also called the thermal decay time (TDT) log (Schlumberger's original product name), neutron lifetime log (NLL), or capture cross-section log. The measurement outputs are often identified by their physical quantities: Sigma (Σ) or TPHI (total porosity from hydrogen index) for the capture cross-section; C/O or OIL for the carbon-oxygen measurement; IRON or FE for the iron capture spectroscopy. Related terms include sigma (thermal neutron capture cross-section), carbon-oxygen log, through-casing evaluation, waterflood surveillance, saturation monitoring, pulsed neutron generator, and cased-hole logging. The Sigma tool should not be confused with the neutron porosity tool — both involve neutrons, but the porosity tool measures hydrogen index from slowed neutron detection while the Sigma tool measures thermal neutron capture cross-section from the time decay of the thermal neutron population.