A Comparison Of Logging-While-Drilling And Wireline Nuclear Porosity Logs In Shales From Wells In Brazil

Abstract

Variations between logging-while-drilling (LWD) and wireline neutron logs in shales were studied from six wells in Brazil. Since only small variations were observed in the respective density logs, the neutron differences were not attributed to environmental changes during the time elapsed between LWD and wireline measurements. Monte Carlo simulations for clay minerals show that the large-diameter LWD neutron porosity response is generally lower, and closer to the true hydrogen content, than wireline thermal neutron tools. The LWD and wireline logs confirm this trend in shales. The large amount of steel in the LWD tools reduces the number of detected low-energy (thermal) neutrons and results in the detectors seeing a larger fraction of higher-energy (epithermal) neutrons than conventional wireline tools. Since epithermal neutrons are less sensitive to the strong neutron absorbers found in shales, the LWD tools read lower neutron porosities than wireline in shales. The neutron response in shales is also affected by source-to-detector spacings, and LWD spacings often differ from wireline. Larger LWD neutron tools will read closer to the true hydrogen content of the shales than smaller LWD neutron tools.