Running Pulse Neutron logs in Malaysia has previously been plagued by high uncertainties, especially in brown fields with complex multi stacked clastic reservoirs. Together with a wide range of porosities and permeabilities, the acquired logs quite often than not, tend to yield inconclusive results. In addition, the relatively fresh aquifer water (where salinity varies from 5k to 40k ppm) makes reservoir fluid typing and distinguishing between oil and water even more challenging. Again, the inconsistencies and uncertainties of the results tends to leave more questions than answers. Confidence in utilizing pulse neutron logging, especially to validate fluid contacts for updating static and dynamic reservoir models decreased to very low levels within the study teams. Due to this fact, the Petrophysics team took the initiative to conduct a 3-tool log-off in one of their wells with the objective of making a detailed comparison of 3 pulse neutron tools in Malaysia's market today. The main criteria selected for comparisons were consistency of the data, repeatability and statistical variations.
With recent advancement in Pulse Neutron (Multi Detector) tool technology, newer tools are being equipped with more efficient scintillation crystals, improving the repeatability of the measurements as well as the number of Gamma Ray (GR) count rates associated with the neutron interactions. In addition, the newer tools have now up to 5 detectors per tool, with the farthest detector from the supposedly being able to "see" deeper into the formation, albeit at a lower resolution. With these new features in mind, the log-off was conducted in a single well with a relatively simple completion string (single tubing, single casing), logged during shut-in conditions only, and the logs were acquired directly one after the other (back-to-back) to avoid bias to any particular tool.
Both Sigma and Spectroscopy measurements acquired to compare the capabilities of each tool. Due to the relatively fresh water salinity, the Carbon-Oxygen ratio from the Spectroscopy measurements used to identify the remaining oil located in the reservoirs, while the Sigma measurements determine the gas-oil or gas-water contact, if present.
This paper will illustrate the steps taken by Petronas Carigali Sdn Bhd (PCSB) to compare the raw data and interpreted results from the 3 pulse neutron tools. A comparison from all the tools is discussed in length, and consequently compared to the current understanding of the reservoir assessed. The points from these comparisons will then show why one of the tools is favored than the rest.