RIASSUNTO
ABSTRACT
Recent improvements in neutron detector technology have led to the development of a thermal neutron porosity service for the measurement while drilling (MWD) environment. Porosity logs comparable in quality to the wireline compensated neutron log are obtained by using a dual-spaced detector system that detects both thermal neutrons and neutron capture gamma rays. The borehole compensated thermal neutron measurement provides a high quality porosity log that is free of the many interferences that degrade a capture gamma-ray porosity measurement (e. g. , natural gamma-ray background, tool and formation activation, formation and mud salinity, etc. ). In addition, the thermal neutron measurement exhibits greater porosity sensitivity than the capture gamma-ray measurement. In the present tool, the capture gamma-ray signal is used in conjunction with the thermal neutron response to infer formation salinity. An extensive set of laboratory measurements and mathematical modeling results have been used to quantify environmental effects on both porosity logs and to derive the necessary correction algorithms. Included were effects due to borehole size, tool standoff, mud weight and salinity, formation lithology and pore fluid (i. e. , fresh water, saline water, oil, and gas) . Interpretation of the logs is greatly aided by the understanding of tool response afforded by this comprehensive environmental effects database. Operationally, the tool provides for efficient rig floor handling procedures and continues Schlumberger''s commitment to nuclear source safety. The neutron source is retrievable by conventional wireline/slickline fishing techniques, eliminating the possibility of having to abandon the source downhole in the event that the drill pipe becomes stuck.