A COMPARISON OF LOGGING-WHILE-DRILLING AND WIRELINE NUCLEAR POROSITY LOGS IN SHALES FROM WELLS IN BRAZIL

摘要

Variations are often seen between logging-while-drilling (LWD) and wireline neutron logs in shales. Sometimes shale hydration or dehydration and the time duration between LWD and wireline can explain these differences. However, even when special drilling muds are used to reduce shale alteration, LWD neutron porosity readings in shales have been observed to be lower than thermal neutron wireline readings. This paper provides an explanation for these differences. Six wells from Brazil were studied for differences between LWD and wireline nuclear porosity logs. The studied intervals were drilled close to vertical with 12.25-inch bits. Only small variations were observed in shales between LWD and wireline density logs, but LWD neutron porosity readings in shales were significantly lower than conventional wireline tools. The LWD nuclear tools for 12.25-inch holes have a steel sleeve with an 11.5-inch outer diameter around the neutron section to reduce borehole effects. The large amount of steel also reduces the number of detected low-energy (thermal) neutrons. This results in the detectors seeing a larger fraction of higher-energy (epithermal) neutrons than conventional wireline tools. Epithermal neutrons are less sensitive to the strong neutron absorbers found in shales. The neutron response in shales is also affected by source-to-detector spacings, and LWD spacings usually differ from wireline. 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 for wireline thermal neutron tools. The LWD and wireline logs confirmed the expected trends in shales.