Activation Log

Key Takeaways

• Carbon-oxygen ratio logging measures the ratio of inelastic gamma ray counts attributable to carbon atoms to those attributable to oxygen atoms within the tool's depth of investigation (approximately 20 to 40 centimetres radius into the formation). Carbon in the formation exists in hydrocarbon molecules (crude oil and natural gas) and in carbonate minerals; oxygen exists in water, silicate minerals, carbonate minerals, and formation water. When the C/O ratio is high, the formation contains more carbon relative to oxygen than the baseline mineralogy alone would produce, indicating the presence of hydrocarbons in the pore space. The C/O log provides a saturation indicator that does not depend on formation-water salinity, unlike resistivity-based methods (which require knowing the water resistivity to compute Archie water saturation). This salinity independence makes C/O logging the preferred cased-hole saturation method in formations where the water salinity is unknown, variable, or changing due to injection of low-salinity water. Interpretation requires a separate silicon-calcium (Si/Ca) ratio measurement to determine whether the formation is sandstone or carbonate, since the background carbon and oxygen signals from mineral matrices differ between siliciclastic and carbonate lithologies.
• Oxygen activation logging exploits the short-lived radioactivity of nitrogen-16 (16N), produced when fast neutrons react with oxygen-16 (16O) by an inelastic scattering reaction: 16O + n → 16N + p. Nitrogen-16 is radioactive with a 7.13-second half-life, decaying back to 16O by emitting a beta particle and a very high-energy gamma ray at 6.13 MeV. When water flows upward in the wellbore or in a behind-casing channel, the water molecules activated near the neutron generator travel with the flowing fluid and arrive at the detector some distance above the generator at a time that depends on flow rate and the 7.13-second decay constant. Mapping the 6.13 MeV count rate along the tool as a function of time while the tool is held stationary (or moving slowly) reveals the velocity and direction of water flow. The technique is sensitive to water flow rates above approximately 1 barrel per minute (0.16 m³/min) and can locate channelling behind casing (water moving between the casing and borehole wall outside the cement) that cannot be detected by any other cased-hole method except noise logging.
• Pulsed neutron capture (PNC) logging is closely related to activation logging and is sometimes discussed together with it. PNC tools fire short bursts of fast neutrons from an electronic neutron generator and measure the rate of thermal neutron decay (capture) in the formation, expressed as the formation sigma (capture cross-section, in capture units or c.u.). Formation sigma depends on formation salinity (chlorine has a very high capture cross-section, approximately 33 c.u. per part-per-thousand NaCl), porosity, and hydrocarbon saturation (hydrocarbons have low sigma, around 20 c.u.; brine has high sigma depending on salinity). By monitoring sigma over time in a producing well, an operator can detect when a saline water flood front arrives at the well (sigma increases sharply) or when gas breaks through (sigma decreases). Unlike C/O ratio logging, PNC sigma interpretation does require knowing the formation water salinity to convert sigma to water saturation, but in a waterflood context where the injected water salinity is known, PNC provides very sensitive flood-front monitoring at a lower cost per logging run than C/O.
• Activation logging tools are run on wireline or, less commonly, on coiled tubing through the production casing or tubing without pulling the completion. This makes activation logging the primary saturation evaluation method for re-evaluating older wells completed with casing where the original open-hole log suite did not include a full formation evaluation. In the WCSB, hundreds of wells drilled through the 1960s to 1980s were evaluated with limited open-hole suites (gamma ray, resistivity, and basic sonic or density) that did not provide unambiguous saturation results in low-salinity or freshwater formation environments. C/O logging through the casing of these old wells allows modern re-evaluation to identify bypassed pay that was not recognized during the original completion, extending the economic life of fields that were otherwise considered fully developed. The technique is also used in horizontal wells to identify which intervals along the lateral are producing water versus oil, guiding workover decisions.
• The depth of investigation of activation logging tools is limited by the fast neutron flux reaching the formation, which decreases exponentially with distance from the neutron generator due to slowing and absorption by the wellbore and casing. Steel casing (with its high iron content) absorbs neutrons efficiently, and a 5.5-inch steel casing string reduces the neutron flux at the formation face by a factor of 3 to 10 compared to an equivalent open-hole measurement, depending on casing weight and wall thickness. Heavier casing (9 and 10 lb/ft wall thickness compared to 5 lb/ft) can reduce the depth of investigation to only 10 to 15 centimetres into the formation beyond the casing OD. Heavy casing is common in HPHT wells in the Alberta Foothills, where casing design requirements push toward thick-wall grades (T-95, Q-125) that severely limit the effectiveness of activation logging tools. In such cases, a larger neutron source (higher output electronic neutron generator, 108 neutrons per second rather than the standard 107) can partially compensate for the casing absorption, at the cost of higher activation of the steel casing itself and more complex background corrections.